OXYGEFf DBMJfNBNO. 2
ANALYTICAL R
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WATER OXYGEN DEMAND NO. 2
Study Number 21
Report of a Study Conducted by the
ANALYTICAL REFERENCE SERVICE
TRAINING PROGRAM
U. S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Bureau of State Services
Division of Water Supply and Pollution Control
Robert A. Taft Sanitary Engineering Center
Cincinnati, Ohio
1965
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The ENVIRONMENTAL HEALTH SERIES of reports was
established to report the results of scientific and engineering
studies of man's environment: The community, whether urban,
suburban, or rural, where he lives, works, and plays, the air,
water, and earth he uses and re-uses; and the wastes he pro-
duces and must dispose of in a way that preserves these natural
resources. This SERIES of reports provides for professional
users a central source of information on the intramural research
activities of Divisions and Centers within the Public Health
Service, and on their cooperative activities with State and local
agencies, research institutions, and industrial organizations.
The general subject area of each report is indicated by the two
letters that appear in the publication number, the indicators are
WP - Water Supply
and Pollution Control
AP - Air Pollution
AH - Arctic Health
EE - Environmental Engineering
FP - Food Protection
OH - Occupational Health
RH - Radiological Health
Triplicate tear-out abstract cards are provided with
reports in the SERIES to facilitate information retrieval. Space
is provided on the cards for the user's accession number and
key words.
Reports in the SERIES will be distributed to requesters, as
supplies permit. Requests should be directed to the Division
identified on the title page or to the Publications Office, Robert
A. Taft Sanitary Engineering Center, Cincinnati, Ohio 45226.
Public Health Service Publication No. 999-WP-26
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FOREWORD
The Analytical Reference Service is conducted by the Training
Program of the Robert A. Taft Sanitary Engineering Center for the
evaluation of laboratory methods in the environmental fielu. Coop-
erative studies by member organizations, through analysis of identical
samples and critical review of methodology, provide the mechanism for:
1. Evaluation of analytical procedures, including
precision and accuracy, by comparison of the
procedures andresults reportedbyparticipating
laboratories.
2. Exchange of information regarding method
characteristics.
3. Improvement or replacement of existing methods
by development of more accurate procedures
and by development of new methodology for
determination of new pollutional components.
Samples are designed to contain measured amounts of selected
constituents. Decisions as to qualitative makeup are made by the
ARS staff, the membership, and consultants. Notice of each study
is sent to the entire membership.
A portion of the study sample with accompanying data forms for
reporting numerical values, a critique of the procedures used, comments
on modifications, sources of error, difficulties encountered, or other
pertinent factors, is then shipped to each of those who expresses a
desire to participate. The results and comments of each study are
compiled and a report is prepared.
Initially directed toward examination of water, studies now include
air, milk, and food. Some studies are periodically repeated, for the
advantage of new members, to evaluate new methods or to reevaluate
existing methods.
The selection of studies is guided by the responses to question-
naires periodically circulated among the membership which now
includes 198 federal, state, and municipal agencies; industries; uni-
versities; consulting firms; and foreign agencies.
James P. Sheehy
Director, Training Program
iii
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STUDIES ON WHICH REPORTS HAVE BEEN COMPLETED
Wat e r - Mine r als
Water-Metals
Water-Fluoride
Water- Radioactivity
Water-Surfactant
Calcium, magnesium, hardness, sulfate,
chloride, alkalinity, nitrite, nitrate, sodium,
and potassium. Studies completed in 1956,
1958, and 1961.
Lead, copper, cadmium, aluminum, chromium,
iron, manganese, and zinc. Studies completed
in 1957 and 1962.
Fluoride in the presence and absence of inter-
ferences, with and without distillation using a
specified procedure. Studies completed in
1958 and 1961.
Studies completed in 1959, 1961, and 1963. The
first two studies were designed to determine
gross beta activity, while the third study was
concerned with gross beta and strontium-90
activity.
Surfactant in various waters.
pleted in 1959 and 1963.
Studies com-
Water-Oxygen Demand Biochemical oxygen demand and chemical oxy-
gen demand study completed in 1960; COD
study completed in 1965.
Water- Trace Elements
Freshwater Plankton
Air-Inorganics
Air-Lead
Air-Sulfur Dioxide
Milk-DDT Residue
Arsenic, boron, selenium, and beryllium.
Study completed in 1962.
Evaluation of the precision and accuracy ob-
tainable by the use of various methods of
plankton counting and identification. Study
completed in 1964.
Chloride, sulfate, fluoride, and nitrate in
aqueous solution and on glass fiber Hi-Vol
filter mats. Study completed in 1958.
Filter paper tape impregnated with lead.
Study completed in 1961.
Determination of sulfur dioxide in air using a
specified method. Study completed in 1963.
DDT in milk. Study completed in 1962.
iv
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PARTICIPANTS IN THIS STUDY
Alabama Water Improvements Commission
Alexander Orr, Jr. , Water Treatment Plant, Miami Florida
American Cyanamid Company, Bound Brook, New Jersey
Arizona State Department of Health
British Coke Research Association, Chesterfield, Derbyshire, England
California State Department of Public Health, Berkeley
California State Department of Public Health, Los Angeles
Connecticut State Department of Health
Delaware Water Pollution Commission
Department of Municipal Laboratories, Hamilton, Ontario, Canada
Department of Water Resources, Durham, North Carolina
Dow Chemical Company, Midland, Michigan
Erie County Laboratory, Buffalo, New York
Florida State Board of Health (Division of Sanitary Engineering, Jacksonville)
Florida State Board of Health (Pensacola)
Florida State Board of Health (Winter Haven)
General Electric Company, Louisville, Kentucky
Georgia Institute of Technology, Department of Applied Biology
HALL Laboratories Division, Calgon Corporation, Pittsburgh, Pennsylvania
Hawaii State Department of Health
Health Department, Beaumont, Texas
Idaho Department of Health
Illinois State Water Survey Division
Indiana State Board of Health
Industrial Chemicals, Incorporated, South Bend, Indiana
Kentucky State Department of Health
Los Angeles Department of Public Works, Hyperion Treatment Plant
Los Angeles Department of Water and Power
Louisiana State Board of Health
Louisville Water Company, Incorporated
Maryland State Department of Health
Massachusetts Department of Public Health
Metropolitan Utilities District, Omaha, Nebraska
Metropolitan Water District of Southern California
Michigan Water Resources Commission
Minnesota Department of Health
Missouri Department of Public Health and Welfare
Monsanto Chemical Company, St. Louis, Missouri
Montana State Board of Health
NALCO Chemical Company, Chicago, Illinois
National University of Colombia, Bogota, Colombia, South America
Nebraska State Department of Health
Nevada State Department of Health
New Hampshire Water Pollution Commission
New Jersey State Department of Health
New York State Conservation Department
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North Carolina Department of Water Resources, Raleigh
Ohio State Department of Health
Oregon State Board of Health
Pennsylvania Department of Health
Rensselaer Polytechnic Institute, New York
Roy F. Weston, Incorporated
St. Louis County Water Company
Scientific Research Council, Kingston, Jamaica, West Indies
Sixth U.S. Army Medical Laboratory, Fort Baker, California
Tennessee Valley Authority (Stream Pollution Control) Chattanooga
Texas State Department of Health
2793DU.S. Air Force Hospital, Regional Environmental Health
Laboratory, McClelland Air Force Base, California
2794th U.S. Air Force Dispensary - Class B, Kelly Air Force Base,Texas
United Kingdom Atomic Energy Authority, Didcot, Berks., England
U. S. Army Environmental Hygiene Agency, Edgewood Arsenal, Maryland
U.S. Industrial Chemicals Company, Tuscola, Illinois
University of Beograd, Civil Engineering Faculty, Beograd, Yugoslavia
University of Kansas, School of Engineering and Architecture
University of Leeds, Houldsworth School of Applied Science,
Leeds, England
University of North Carolina, Chapel Hill
Virginia State Water Control Board
Washington State Department of Health
Washington State University, Division of Industrial Research
Water Department, Charlotte, North Carolina
Water Department, Long Beach, California
Water Research Association, Marlow, Buckinghamshire, England
West Virginia State Water Resources Commission
Wisconsin State Board of Health
vl
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CONTENTS
ABSTRACT ix
PURPOSE OF THE STUDY 1
DESIGN OF THE STUDY 1- 2
TREATMENT OF DATA 2- 3
DISCUSSION 3
SAMPLE A (200 mg/liter COD in distilled water) 3- 7
SAMPLE B (160 mg/liter COD in distilled water 8-12
containing 100 mg/liter chloride)
SAMPLE C (150 mg/liter COD in distilled water 13-22
containing 1,000 mg/liter chloride)
SAMPLE D (40 mg/liter COD to be added to water 23-28
sample collected by participant)
COMMENTS OF PARTICIPANTS 29
SUMMARY AND CONCLUSIONS 30-31
LITERATURE REFERENCES 32
APPENDICES 33
A. TABULATION OF RESULTS 34-60
B. MERCURIC SULFATE PROCEDURE 61
C. GLOSSARY OF STATISTICAL TERMS 62-63
D. PROBABILITY EXPLANATION 64-68
E. ANALYTICAL REFERENCE SERVICE MEMBERSHIP 69-74
F. STAFF AND ACKNOWLEDGMENTS 75
vii
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ABSTRACT
This study consistedof four samples which74 participating labora-
tories were instructed to dilute to a specified volume and analyze by both
the Standard Method for Chemical Oxygen Demand and by the Mercuric
Sulfate modification.
The results from this study indicate that the two procedures pro-
duce similar precision and accuracy when no interfering materials are
present. When interferences due to high concentrations of chloride are
present, the standard method will produce equal precision and accuracy
only if the appropriate corrective techniques are applied.
The Mercuric Sulfate modification is the method of choice for COD
measurement since with less manipulation it effectively removes the
interference due to chloride oxidation and is less time consuming.
ix
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WATER-OXYGEN DEMAND NUMBER 2
PURPOSE OF THE STUDY
The chemical oxygen demand (COD) of a liquid sample is one of
the oldest analytical parameters of pollution and is used quite fre-
quently. Many oxidants and variations in procedure have been used
in the past, but the dichromate procedure, as described in the llth
edition of Standard Methods for the Examination of Water and Waste-
water, is used by most analysts today.
A brief evaluation of the precision and accuracy of this test was
made in the previous Water-Oxygen Demand study, which involved a
sample containing no interfering material and having an oxidizability
that varied slightly from the theoretical.
It is generally recognized that chloride ions in the sample prevent
a true measurement of the COD. Several remedial measures in-
volving a mathematical correction, either alone or in conjunction with
the use of silver sulfate, are offered in Standard Methods. On the
other hand, the mercuric sulfate modification developed by Dobbs and
Williams, of the Robert A. Taft Sanitary Engineering Center, seemed
to remove effectively interference by chlorides. A comparative evalu-
ation of the two methods, therefore, appeared timely.
DESIGN OF THE STUDY
To achieve a sound evaluation of accuracy, a search was under-
taken for an oxidizable material that would consistently exert a COD
that is 100 % of the theoretical and that would be stable for several
months. Potassium acid phthalate was found to meet these requirements
in the concentrations used for the samples as shipped.
The study consisted of four samples, designated as A, B, C, and D,
which the recipient was instructed to dilute to a specified volume and
analyze in triplicate by both the standard method and the mercuric sul-
fate modification. Instructions for the latter procedure were supplied
to the participants.
Sample A, designed as a control, contained 8. 5020
g/liter potassium acid phthalate in sterile distilled
water with no interference added. When 10 ml of
this sample was diluted to 500 ml, the resulting
solution produced a COD of 200 mg/liter.
Sample B, designed to simulate a fairly average
sample, contained 6. 8016 g/liter potassium acid
phthalate and 8. 2440 g/liter sodium chloride.
When 10 ml of this sample was diluted to 500 ml,
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the resulting solution had a COD of 160 mg/liter
and a chloride concentration of 100 mg/liter.
Sample C, represented the type of sample most
likely to cause erroneous COD values. This
sample contained 6. 3764 g/liter potassium acid
phthalate and 82. 4400 g/liter sodium chloride.
When 10 ml of this sample was diluted to 500 ml,
the theoretical COD of the resulting solution was
150 mg/liter and the chloride content was 1, 000
mg/liter.
Sample D was designed to evaluate the overall
efficiency of the methods in analyzing a variety of
substrates. This sample contained 3. 4008 g/liter
potassium acid phthalate, and 5 ml was to be di-
luted to 500 ml with surface water or wastewater
(Solution DI) collected by the participants. The
resulting solution (D2> would then contain 40
mg/liter COD in addition to the COD of the diluent
Many participants provided additional analytical data on the water-
used as Sample D^.
TREATMENT OF DATA
The mean of the results reported by each participant was plotted
on probability paper to determine the distribution. Values showing a
deviation from the normal distribution line were rejected as non-
representative because of errors in calculation, dilution, or other
indeterminate factors. The rejected values are circled on the proba-
bility plot. These rejected values were not included in the development
of statistical parameters. In several instances in which a mathematical
error was noted, the corrected values were submitted in the interim
report to the participant for his approval. If the corrected values were
approved by the participant, they were then used in the report.
Calculation of the standard deviations was based on the difference
between the average result submitted by each participant and the over-
all mean value reported for each method. The average reported values
are also used in the bar charts.
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The results obtained by use of the standard method were grouped
according to the method employed for correction of chloride interference
and are identified as follows:
Technique (1) Mathematical correction
Technique (2) Silver sulfate added before reflux
Technique (3) Silver sulfate added after 20 minutes boiling
Technique (4) Silver sulfate dissolved in the sulfuric acid
Statistical terms, as used in this report, are defined in the glossary.
DISCUSSION
SAMPLE A (200 mg/liter COD in distilled water)
Sample A provided for evaluation of the precision and accuracy of
the two methods on waters containing no interfering substances. This
sample functioned as a control, and the two methods were expected to
produce results of equal accuracy and precision.
The mercuric sulfate method seems to exhibit slightly more accu-
racy and a little less precision (Table 1) than the standard method,
when the technique for chloride correction is not considered. (See
accuracy and precision in Glossary. ) The laboratories that used the
silver sulfate correction (technique 2) seemed, according to the mean,
to achieve better accuracy than those using the other techniques, but
the precision was adversely affected. Also, the 50 % range shows that
the mean is misleading, since the better half of the results were still
twice as far from the theoretical as the results obtained by the other
chloride correction techniques. The best standard method values were
obtained by the combination of techniques 1 and 3, although no chloride
interference was present. Thus, this sample provided an evaluation
of procedures rather than efficiency of chloride tie-up. In general, the
results indicate that there is very little difference between the two
methods when applied to this type of sample.
Table 1. SUMMARY OF STATISTICAL DATA ON SAMPLE A
Method
,., Standard
Mean , . .
deviation
Mercuric sulfate
Standard method,
Overall
Standard method (1 )
Standard method (2)
Standard method (3)
Standard method (4)
Standard method
(1 +2)
Standard method
(1 +3)
200.
197.
194.
199.
190.
194.
200.
196.
2
5
9
8
0
9
2
0
13.
11.
9.
14.
13.
5.
10.
1
4
2
4
2
8
5
Median 50 % range
198.
198.
195.
199.
196.
198.
200.
9
2
3
2
8
4
4
+ 4.
+ 4.
+ 4.
+ 8.
+ 3.
+ 1.
± 4-
6
8
8
6
2
6
4
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SAMPLE B (160 ing/liter COD in distilled water containing 100 mg/liter
chloride)
Sample B was designed to contain a moderate amount of interference,
such as might be found in many surface waters. This amount of chloride
would tend to produce a high COD result.
The mercuric sulfate method produced good accuracy as shown by
the close agreement (Table 2) between the mean, median, and amount
added. The 50 % range shows that the better half of the results had an
error of 5 mg/liter or less. Surprisingly, the precision as indicated
by the standard deviation was a little better than on Sample A.
The standard method (overall) showed substantially less accuracy.
The mean and median are in close agreement, but are about 8 mg/liter
higher than the amount added. The 50% range of 11 mg/liter also indi-
cates substantial inaccuracy in even the better half of the results. The
results show a normal distribution and a precision nearly as good as on
the control sample A.
It is evident that on this type of sample substantially equal accuracy
and precision can be obtained by use of either the mercuric sulfate
method or the standard method when the mathematical correction is
applied in conjunction with the procedure of refluxing for 20 minutes
before adding silver sulfate (technique 1 and 3).
Table 2. SUMMARY OF STATISTICAL DATA ON SAMPLE B
Method
Mercuric
Standard
Mean
sulfate
method,
159.
167.
1
7
Standard
deviation
10.
12.
4
4
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159.
168.
9
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0
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Number
72
71
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Standard
Standard
Standard
Standard
Standard
(1 +2)
Standard
(1 + 3)
method
method
method
(1)
(2)
(3)
method (4)
method
method
154.
175.
154.
174.
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0
0
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11.
11.
4.
11.
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6
7
8
157.
176.
153.
172.
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SAMPLE C (150 mg/liter COD in distilled water containing 1, 000
mg/liter chloride)
The greatest difficulty with the COD test is experienced in analysis
of waters of high chloride content, such as some rivers in the Southwest
and estuary waters. Sample C was designed to represent a sample of
this type.
The mercuric sulfate method produced results that were normally
distributed and a precision that was nearly the same as on the control
sample A. The values, however, showed a tendency to be slightly high
(Table 3). The mean and median were in close agreement, but were
approximately 6 mg/liter higher than the amount added; therefore,
chloride oxidation may not have been completely inhibited in all cases.
The overall performance of the standard method was dramatically
inferior. The general tendency was toward very high results with a
mean value of 219. 2 mg/liter and a median value of 200. 8 mg/liter.
The precision, as indicated by a standard deviation of 89.0, was poor.
Examination of the data showed that the results could be divided
into two main groups. One group contained all values to which a math-
ematical correction for chloride interference had been applied - either
combined with the use of silver sulfate added initially, added after
20 minutes of refluxing, or not used at all. The other group of values
then included all results to which no mathematical correction had been
applied, but which had involved the use of silver sulfate in some manner.
Separate bar graphs show the results in these groups. One laboratory
reported that no correction for chloride interference was used; therefore,
this result appears only in the overall presentation. Statistical data
showing further breakdown of the results into six separate groups ac-
cording to the specific technique used for chloride correction appear
in Table 3.
It is evident that, whether or not silver sulfate is used, a math-
ematical correction is mandatory for this level of chloride interference
if the standard method is used. Although two laboratories reported
adding silver sulfate initially, in conjunction with a mathematical cor-
rection (techniques 2 and 1), these two techniques are incompatible
since the silver sulfate would produce a precipitate of silver chloride,
which is only partially oxidized by the procedure, making the standard
mathematical correction inapplicable. In spite of this, the results
submitted by the two participants show much better accuracy than ex-
pected and are tabulated but unexplained.
Again, the technique that produced the best accuracy and precision
for the standard method was the application of a mathematical correction
in conjunction with 20 minutes refluxing before the addition of silver
sulfate (techniques 1 and 3).
13
-------
The use of silver sulfate as a catalyst was not necessary for any
of the samples in this study. As a result, the participants that used
only the mathematical correction achieved essentially the same accu-
racy as the group using both the mathematical correction and the silver
sulfate after 20 minutes reflux. The latter technique, however, would
have been required if the sample had contained materials such as
straight-chain alcohols and acids that require the catalyst for complete
oxidation.
Table 3. SUMMARY OF STATISTICAL DATA ON SAMPLE C
Method
Mean
Mercuric sulfate
Standard method,
155.
219.
9
2
Standard
deviation
13.
89.
9
0
Median
156.
200.
8
8
50 % range
+ 11.
+ 55.
6
3
Number
71
74
Overall "
Standard
Standard
Standard
Standard
Standard
(1 +2)
Standard
(1 + 3)
Standard
+ (1 + 2)
Standard
method
method
method
method
method
method
method
(1)
(2)
(3)
(4)
(1)
147.
282.
221.
233.
1 ^R
1 Oo.
146.
143.
6
8
6
8
7
i
6
0
58.
66.
112.
75.
22.
13.
0
0
9
5
6
8
143.
285.
147.
224.
139.
141.
4
8
9
9
8
2
+ 11.
+ 132.
+ 23.
+ 61.
+ 10.
+ 10.
8
3
0
0
2
3
18
30
5
8
10
26
+ (1 + 3) " ' - - ' -
method
(2)
266.
6
76.
2
270.
9
+ 120.
9
43
+ (3) + (4) -- - - - - -
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SAMPLE D (40 mg/liter COD to be added to water sample collected by
participant)
Sample D was designed to provide an overall estimate of the effi-
ciency of the COD methods in analyzing a variety of collected samples
(D^) containing various amounts of chemically oxidizable material and
interferences.
In preparing sample D2, 5 ml of the sample D concentrate was
made up to 500 ml by adding 495 ml of the collected sample (Dj). Since
before analysis D ^ was not similarly diluted, sample D j contained 1%
more of the collected sample than did D2- That is to say, to make the
samples strictly comparable, 495 ml of the collected sample should
have been diluted with 5 ml of distilled water and this sample then
called Dj .
To minimize confusion, additional manipulations, and possibly
mathematical errors, it was decided to ignore the dilution factor in
the instructions to participants. The submitted results were then
corrected by the Analytical Reference Service staff to compensate for
the dilution. This was done by adding 1 % of the COD value for sample
D ^ to the difference obtained by subtracting the initial COD value of Dj
from the COD value of D? . This, in effect, raised the difference value
to the value that would have been obtained if there had been no dilution
of D} in the preparation of D%. The theoretical value for D2-D^ was
therefore adjusted to 40. 0 mg/liter COD in every case Statistical
calculations were thereby simplified, and presentation of the data in a
manner comparable to that used in the other parts of this study was
made possible.
Since so many participants used a water having a very low COD for
sample Dj, it is unfortunate that the instructions referred only to the
high COD procedures, although many participants did use the low COD,
procedure. Without doubt, many results would have been improved by
using the N/40 reagents rather than the N/4 In addition, the varied
composition of the water used as sample D^ precludes the use of statis-
tical parameters in the usual manner, and they are, therefore, presented
only for the purpose of aiding in the discussion of this sample and should
not be used to predict the precision and accuracy of results that might be
obtained from a different sample.
Many of the participants provided considerable analytical data on
the water used for sample D j. Unfortunately, no significant correlation
was found between the quality of the water and the results obtained.
Errors, such as neglecting to correct for high chlorides in the standard
method, were self-canceling and did not affect the D2-D^ difference.
Difficulties such as an endpoint obscured by color or turbidity un-
doubtedly caused inaccurate results, but were not reported in sufficient
number to evaluate.
23
-------
The data indicate (Table 4) that the mercuric sulfate procedure is
slightly more precise than the standard method, as shown by the standard
deviationsof 15. 5 and 17. 0. The accuracy, however, was less, as shown
by the deviation of the mean from the theoretical. On the other hand, the
50% range shows that the better half of the results and the median of all
of the results obtained by either method are in close agreement. The
differences are not considered significant, and it is the opinion of the
Analytical Reference Service staff that either method would be suitable
for most of the DI samples.
Table 4. SUMMARY OF STATISTICAL DATA ON SAMPLE D
Method Mean Standard Median 50 % range Number
deviation
Mercuric sulfate 35. 4 15. 5 36. 4 + 7. 4 64
Standard method 4Q_ 5 ^_ Q 3?_ 2 ~
Overall
24
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COMMENTS OF PARTICIPANTS
The most frequent comment concerned the precipitate formed when
mercuric sulfate was added to Sample C. A few participants found this
troublesome in detecting the endpoint of the titration, but the majority
experienced no difficulty. Several suggestions were made to use pumice
stone or porcelain chips instead of glass beads to control bumping. The
general opinion of the participants who used a mathematical correction
for chloride in the standard method was that the mercuric sulfate
method was quicker and just as good. Those who did not perform a
chloride analysis found the weighing of mercuric sulfate a time-consuming
extra step. Many suggestions were made to use more than the
recommended 2-3 drops of indicator to improve the color change at
the endpoint of the titration. Some participants took exception to the
statement in the instructions for the mercuric sulfate method that the
COD calculation was the same as that for the standard method.
In preparing the instructions it was assumed that since the mer-
curic sulfate method purports to eliminate chloride interference, it
would be evident that the COD calculation would not include a correction
factor for chloride.
It was suggested that quantitative control of excess mercuric sulfate
might give better results for high chloride samples. This undoubtedly
would be true for extremely saline waters which would require more
than the specified 1 gram of mercuric sulfate to complex the chloride
completely. This type of sample, however, would be a special case
requiring further investigation.
A comment that the sample should be added last to the cooled acid-
dichromate mixture to avoid loss of volatile fractions may have some
virtue in special cases. This technique could then incorporate
another suggestion that the mercuric sulfate be dissolved first in the
sulfuric acid. A sample containing high chlorides and requiring the
catalytic action of silver sulfate might not, however, be completely
oxidized because of some of the silver sulfate precipitating with the
chloride, if this technique were used.
29
-------
SUMMARY AND CONCLUSIONS
The results from this study indicate that the two procedures pro-
duce similar precision and accuracy when no interfering materials are
present. When high concentrations of chloride are present, the mer-
curic sulfate procedure will effectively remove the interference due to
chloride oxidation. The standard method may produce equal precision
and accuracy only if the proper technique is used; namely, the measure-
ment of chloride concentration in order to be able to apply the mathematical
correction (technique 1), and refluxing the sample with the acid- dichromate
mixture for 20 minutes before adding silver sulfate if the catalyst is
required (technique 3). Many inaccurate COD values are entirely due
to inaccurate chloride analysis.
An alarmingly large number of participants were apparently unaware
of the effect of chloride in the measurement of COD by the standard
method. This is evident from the many comments expressing concern
over the great difference in COD values produced by the two methods
in the analysis of Sample C.
Undoubtedly, much of the difficulty is due to a lack of clarity in the
procedure as written in the llth edition of Standard Methods. It is
hoped that the forthcoming 12th edition will be improved in this respect.
The Analytical Reference Service staff members feel that the mer-
curic sulfate modification is the best method for COD measurement
because it is less time-consuming than the correctly performed standard
method and will provide at least equivalent precision and accuracy re-
gardless of interferences present. Additional benefits, described by
Dobbs and Williams, ^ but not evaluated in this study, are the elimination
of inaccuracies due not only to the series of cyclic changes from chlorine
to chloride through the formation of chloramines in wastewater containing
chlorides and a high concentration of ammonia, organic amine, or
nitrogenous matter, but also the reaction of chlorine, produced by the
oxidation of chlorides, with organic matter in the sample. The latter
can materially affect the COD.
A statistical summary of the results is presented in Table 5.
30
-------
Table 5. STATISTICAL SUMMARY
Sample A Sample B Sample C Sample D
Mg/liter COD added 200 160 150 40
Mg/liter chloride added 0 100 1000 0
Mean
Mercuric sulfate 200.2 159.1
Standard method, overall 197. 5 167. 7
Standard method (1) +
(1 + 2) + (1 + 3)
Standard method (2) +
(3) + (4)
Median
Mercuric sulfate 198.9 159.9
Standard method, overall 198.2 168.0
Standard method (1) +
(1 + 2) + (1 + 3)
Standard method (2) +
(3) + (4)
Standard deviation
Mercuric sulfate 13.1 10.4
Standard method, overall 11.4 12.4
Standard method (1 ^ +
(1 + 2) + (1 + 3)
Standard method (2) +
(3) + (4)
50 % range
Mercuric sulfate + 4. 6 + 5. 0
Standard method, overall + 4. 8 +11.0
Standard method (1) +
(1 + 2) + (1 + 3)
Standard method (2) +
(3) + (4)
155.9
219.2
143.0
266.6
156.8
200.8
141.2
270.9
13.9
89. 0
13.8
76.2
+ 11.6
+ 55. 3
+ 10.3
+ 120.9
35.4
40.5
36.4
37.2
15. 5
17.0
+ 7.4
+ 6.9
31
-------
LITERATURE REFERENCES
1. Standard Methods for the Examination of Water and Wastewater
llth edition. APHA, AWWA, WPCF. New York, 1960.
2. Dobbs, R. A. and Williams, R. T. Elimination of Chloride
Interference in the Chemical Oxygen Demand Test. Anal. Che rn.
Vol. 35, p. 1064. 1963.
32
-------
APPENDICES
33
-------
APPENDIX A.
TABULATION OF RESULTS
Table A-l. SAMPLE A (Amount added = 200 mg/liter COD)
Mercuric sulfate method Standard method
Laboratory
number
1114
1211
1312
1314
1322
1415
1426
1511
1611
1624
1911
Results
199. 3
196.9
198.1
177.0
138.0
193.0
196.4
194. 4
196.4
204.0
208. 0
204.0
204.0
196.0
188.0
192.0
192.0
192.0
195.8
195. 8
207.4
174.8
168.8
168.8
201.5
197.5
199.5
Mean Results
205.7
198.1 201.7
204. 5
200.0
169.0 207.0
200.0
181. 2
200.0
195.7 200.0
196.0
2.00. 0
205.0 192.0
196.0
198. 3
228.0
196.0 224.0
224.0
195.2
192.0 199.2
199.2
170.0
199.7 172.8
170.0
182.0
170.8 178.0
178.0
189.0
199.5 191.0
192.9
Chloride
Mean correction
method
204.0 2
202.0 1 and 3
182.0 1
198.7 4
196. 0 none made
191.4 2
225.0 4
197.8 2
170. 9 1 and 3
179.3 2
191.0 2
34
-------
Table A-l SAMPLE A (continued)
Mercuric sulfate method Standard method
Laboratory
number
2111
2124
2144
2211
2311
2411
2513
2526
2611
2811
Results
206. 0
206.0
206. 0
217.1
214.7
215.1
196.8
198. 8
198. 8
185
185
184
199.1
207. 3
199.1
198.8
196. 1
198. 8
192. 0
196.8
193. 6
189.4
198.3
185. 0
192.0
192. 0
192. 0
200. 5
198. 4
197. 8
Mean Results Mean
206.0
206.0 204.0 204.7
204.0
208. 0
215.6 203.2 206.4
208. 0
188.0
198.1 188.0 186.8
184.4
247
185 214 223
207
202. 5
201.8 202.5 199.7
194.2
198.0
197.9 191.4 196.2
199.2
216. 0
194.1 225.6 219.7
217.6
183. 1
190.9 177.0 175.8
167. 2
188.0
192.0 188.0 188.0
188.0
200.5
198.9 198.6 197.9
194. 7
Chloride
correction
method
2
1 and 2
2
2
3
1 and 3
2
1
1
3
35
-------
Table A-l. SAMPLE A (continued)
Mercuric sulfate method Standard method
Laboratory
number
2911
2915
3111
3116
3226
3511
3514
3611
3711
3716
Results
208
204
200
832
816
824
193. 7
195. 6
191. 7
202. 3
202. 3
203. 5
197. 1
199. 0
199.0
217. 6
217.6
214. 4
204. 2
211. 2
204.8
201.1
201. 1
192. 2
198. 6
196. 5
181
181
179
Mean Results
199
204 199
199
188
824 196
188
197. 1
193.7 193.1
189.1
204.9
202.7 203.9
204. 5
199.2
198.4 199.2
199. 2
222. 4
216.5 217.6
222. 4
207.7 212.0
222. 3
211. 4
202. 3 204. 1
204. 1
210.9
195.8 218.6
208. 1
179
180 179
181
Chloride
Mean correction
method
199 1
191 4
193.1 1
204. 4 1 and 3
199.2 2
220.8 2
217.2 2
206.5 1
212.5 2
180 2
36
-------
Table A-l. SAMPLE A (continued)
Mercuric sulfate method
Laboratory
number
3731
3811
3911
3926
4321
4421
4511
4711
4821
Results
216.0
228.0
222. 0
172.6
182. 7
178. 4
185. 8
209.6
209. 6
202. 4
197
196
205
210
213
210
237. 1
246. 2
225. 1
201. 6
201. 6
201. 6
198.8
196. 8
200. 7
195. 9
195. 9
195. 9
Standard method
Chloride
Mean Results Mean correction
method
196.
222. 0 208.
200.
204.
» III:
202.
206.
207. 2 200.
197.
195
199 195
190
196
211 204
210
180.
236.1 189.
175.
198.
201.6 198.
198.
195.
198.8 197.
193.
195.
195.9 195.
195.
0
0 201.0 1
0
0
° 203. 9 1
7
8
1 201. 4 1 and 3
4
193 1 and 3
203 1
2
3 181.5 2
0
4
4 198.4 4
4
2
2 195.2 1
2
9
9 195.9 2
9
37
-------
Table A-l. SAMPLE A (continued)
Mercuric sulfate method Standard method
Laboratory
number
4921
5111
5221
5415
5511
5626
5811
5821
6212
6311
Results
199.7
199.7
199. 7
196. 0
196.0
192.0
206.6
204. 6
202. 6
189. 3
185.4
193.2
197. 2
197.2
197.2
194.9
190.2
190.2
203. 6
203.6
203.6
192.0
200.0
196. 0
230.6
210. 7
218. 7
Mean Results
199. 7
199.7 199.7
199. 7
200.0
194.7 196.0
200.0
186.6
204.6 186.6
186. 6
182.8
189.3 190.3
186. 7
201. 1
197.2 207.1
205. 1
265. 6
199.5
191.8 194.9
194. 9
186. 3
203.6 182.3
182. 3
182.0
196.0 186.0
192. 0
186. 9
220.0 242.5
194. 8
Chloride
Mean correction
method
199.7 2
198.7 2
186.6 2
186.6 4
204. 4 1 and 3
1 98. 2 None
196.1 2
183.6 1
186.6 2
208.1 2
38
GPO 820837-4
-------
Table A-l. SAMPLE A (continued)
Mercuric sulfate method Standard method
Laboratory
6411
6424
6512
6621
6711
6715
6812
6914
7112
7222
Results
226.6
226.6
226.6
194.3
192.4
192.4
199.2
197.2
201.2
169.0
184.3
176. 6
239.2
235.5
239. 2
197.0
197. 0
196.0
169
165
169
199.2
203.2
195
200
196
205.2
199. 1
199. 1
Chloride
Mean Results Mean correction
method
200.4
226.6 196.4 200.4 1 and 3
204.4
192.7
193.0 188.7 191.4 1 and 3
192.7
187. 1
199.2 185.1 185.1 2
183. 1
207.4
176.6 207.4 207.4 2
207.4
187.7
238.0 187.7 187.7 3
187. 7
199.4
196.7 199.4 198.7 1
197.4
169
168 169 168 3
165
198.0
201.2 194.0 196.7 2
198.0
193
197 194 194 1 and 2
194
199. 1
201.1 201.2 198.8 2
199. 1
39
-------
Table A-l. SAMPLE A (continued)
Mercuric sulfate method Standard method
Laboratory
number
7512
7622
7813
7826
7862
8112
8512
9523
9613
9713
1825
Results
208.0
200.0
200.0
208. 8
204.6
204. 6
197.4
199. 5
199.5
202. 0
202.0
202. 0
204.4
205. 2
204.8
192
200
196
202. 0
199. 6
202.0
198. 8
200.7
198. 8
192. 4
193. 9
195.1
200.0
208.0
200.0
226.4
211.7
222. 3
Mean Results
206. 8
202. 7 198. 8
206. 8
204.6
206. 0 200. 4
204. 6
205.8
198.8 205.8
201.6
196. 8
202.0
208. 8
204. 8 226. 3
237. 8
199
196 199
203
210.0
201.2 198.0
190. 5
199.4 195.0
192.6
195. 1
193. 8 194. 7
196.2
200.0
202.7 196.0
192. 0
190. 1
230.1 191.4
191.4
Chloride
Mean correction
method
204. 1 1
203.2 2
204. 4 2
196.8 3
224. 3 2 and 4
200 4
204. 0 1
192. 7 1
195.3 1
196.0 2
191.0 2
40
-------
Table A-2. SAMPLE B
(Amount added = 160 mg/liter .COD + 100 mg/liter chloride)
Mercuric sulfate method
Laboratory
number
1114
1211
1312
1314
1322
1415
1426
1511
1611
1624
1811
Results
151.
154.
151.
157
168
190
158.
158.
158.
160
160
160
164
156
148
147.
147.
147.
153.
156.
151.
130.
130.
128.
161.
160.
158.
4
2
0
4
4
4
4
4
'4
6
4
7
8
8
8
4
3
6
Standard method
Mean Results
187.
152.2 184.
182.
164
172 167
162
133. 2
173.
158.4 171.
171.
180
160 180
184
161. 1
212
156 212
204
175.
147. 4 179.
169.
160.
153.9 168.
172.
142.
130.1 146.
122.
158.
160.1 159.
158.
2
0
8
9
9
9
5
3
2
8
8
8
0
0
0
1
0
6
Chloride
Mean correction
method
184. 7 2
164 1 and 3
117.3 1
172.6 4
181 none made
174.9 2
209 4
174. 6 2
167. 5 1 and 3
136. 7 2
158. 6 1 and 3
41
-------
Table A-2. SAMPLE B (continued)
Mercuric sulfate method Standard method
Laboratory
number
1911
2111
2124
2144
2211
2311
2411
2513
2526
2611
Results
153. 6
158.4
156. 4
170. 0
170.0
170.0
169. 7
163. 7
167. 7
172. 4
170.4
168. 4
148
157
148
153. 9
150.2
157. 1
156. 6
155.1
156. 6
164.8
172.8
1'68. 0
144.3
150.2
140. 6
152.0
152.0
152.0
Mean Results Mean
176.9
156.1 178.8 178.8
180.8
184. 0
170.0 188.0 184.7
182.0
166. 7
167.0 166.7 167.9
170. 3
183. 6
170.4 183.6 182.9
181.6
174
151 172 174
175
153. 8
153.7 146.0 148.6
146. 0
154.2
156.1 157.0 156.2
157. 3
179.2
168.5 182.4 185.6
195.2
114.8
145.0 127.4 123.0
126. 7
146. 6
152.0 146.6 146.6
146. 6
Chloride
correction
method
2
2
1 and 2
2
2
3
1 and 3
2
1
1
42
-------
Table A-2. SAMPLE B (continued)
Mercuric sulfate method
Laboratory
number
2811
2911
2915
3111
3116
3211
3226
3511
3514
3611
Results
160. 2
160. 5
159.0
164
164
164
166
168
160
156. 1
152.2
152.2
161.2
161.2
161.2
157. 2
155. 3
155. 3
158.0
158. 0
161. 9
176. 6
180. 8
180. 8
174. 6
180. 5
168. 5
162. 5
166.2
169. 9
Standard method
Chloride
Mean Results Mean correction
method
160.
159.9 160.
159.
173
164 173
173
172
165 172
172
151.
153.5 151.
151.
157.
161.2 158.
158.
156.
155.9 158.
160.
173.
159. 3 160.
160.
185.
179.4 185.
180.
189.
174.5 185.
193.
156.
166.2 171.
160.
3
0 159.8 3
2
173 1
172 4
6
6 151.6 1
6
5
3 158. 0 1 and 3
3
2
2 158.2 1
2
7
0 164.6 2
0
6
6 184.0 2
8
5
3 189. 4 2
5
7
4 162.8 1
4
43
-------
Table A-2. SAMPLE B (continued)
Mercuric sult'ate method Standard method
Laboratory
number
3711
3716
3731
3811
3911
3926
4321
4421
4511
4711
Results
156. 3
155. 1
155. 9
140
140
142
165. 0
171.0
112.0
137.0
140. 5
138. 2
165. 2
166. 8
161. 6
163
160
156
174
178
174
213. 9
151. 7
187. 8
173. 4
165. 3
161. 3
161. 4
157. 4
163. 3
Mean Results Mean
187. 1
155.8 190.6 187.7
185. 3
165
141 165 165
164
153. 7
149.0 161.7 171.7
199. 7
148. 9
138.6 150.0 154.9
165. 8
161. 6
164.5 158.0 159.5
158. 8
152
160 158 154
151
171
175 171 174
181
173. 0
184.5 166.7 169.1
167. 5
182. 2
166.7 182.2 178.1
170.0
160.0
160.7 160.0 159.3
158. 0
Chloride
correction
method
2
2
1
1
1 and 3
1 and 3
1
2
4
1
44
-------
Table A-2. SAMPLE B (continued)
Mercuric sulfate method
Laboratory
number
4821
4921
5111
5221
5415
5511
5626
5811
5821
6212
Results
152.2
152.2
152. 2
157.4
161. 3
165. 1
164.0
164.0
156.0
164.4
164. 4
164.4
158. 3
154. 5
158. 3
153. 8
153.8
153. 8
159. 8
159.8
159. 8
163. 9
159.9
155. 9
158. 0
164.0
162.0
Standard method
Mean Results Mean
176.
152.2 172.
168.
176.
161.3 172.
169.
168.
161.3 168.
168.
168.
164.4 168.
169.
164.
157.0 171.
164.
163.
153. 8 161.
159.
236. 1
181.
159.8 181.
181.
146.
159.9 146.
146.
174.
161.4 172.
172.
4
3 172. 3
3
6
8 172.8
0
0
0 168.0
0
3
3 168. 8
3
3
7 166. 8
3
3
3 161.3
3
193. 8
4
4 181.4
4
6
6 146. 6
6
0
0 172. 7
0
Chloride
correction
method
2
2
2
2
4
1 and 3
None
2
1
2
45
-------
Table A-2. SAMPLE B (continued)
Mercuric sulfate method Standard method
Laboratory
number
6311
6411
6424
6512
6621
6711
6715
6812
6914
7112
Results
174. 9
171.0
174. 9
166.2
168. 2
166.2
164. 8
162. 8
160. 8
158. 3
158. 3
158. 3
142. 1
126. 7
134. 4
173.0
173. 0
176.6
159. 3
159. 3
159. 3
137
133
137
161. 4
157. 4
157. 4
152
152
152
Mean Results Mean
174. 9
173.6 167.0 171.0
171.0
160. 0
166.9 160.0 159.3
158.0
162. 6
162.8 158.6 160.6
160. 6
162. 3
158.3 164.3 161.7
158. 3
176. 6
134.4 176.6 176.6
176. 6
139. 8
174.2 139.8 139.8
139. 8
159. 3
159.3 157.5 158.7
159. 3
153
136 153 153
153
178.4
158.7 180.3 179.7
180. 3
154
152 154 154
154
Chloride
correction
method
2
1 and 3
1 and 3
2
2
3
1
3
2
1 and 2
46
-------
Table A-2. SAMPLE B (continued)
Mercuric sulfate method Standard method
Laboratory
number
7222
7512
7622
7813
7826
7862
7866
8112
8512
9523
Results
166. 6
156. 5
160. 5
168. 0
200. 0
224.0
167. 0
179.0
179. 0
159. 9
155. 5
155. 5
171. 3
171. 3
171. 3
162. 4
174. 3
152. 4
165.0
165. 4
152
160
156
155.2
155. 2
159. 4
159. 4
159. 4
Mean Results
180. 8
161.2 178.8
170. 7
159. 0
197.3 159.0
159. 0
185. 0
175.0 196.6
190. 9
182. 8
157.0 184.9
180. 7
165. 2
171.3 177.2
169. 2
178. 6
168.4 177.0
183. 4
165. 0
160.9 161.5
159. 1
177
156 177
180
159. 8
155.2 151.8
152. 1
159.4 159.9
159. 5
Chloride
Mean correction
method
176.8 2
159.0 1
190.8 2
182.8 2
170. 5 3
179. 7 2 and 4
161.9 1
178 4
155.8 1
157.2 1
47
-------
Table A-2. SAMPLE B (continued)
Mercuric sulfate method
Standard method
Laboratory
number
Results
Mean
Results
Mean
Chloride
correction
method
9613
9713
1825
154. 0
155. 1
154.8
172.0
152.0
160.0
157.2
157.2
154. 6
161. 3
157.2
153. 4
153.8
156. 9
168. 0
168. 0
156.0
167. 9
172. 7
154. 7
164. 0
170. 3
48
-------
Table A-3. SAMPLE C (Amount added = 150 mg/liter COD
+ 1000 mg/liter chloride)
Mercuric sulfate method Standard method
Laboratory
number
1114
1211
1312
1314
1322
1415
1426
1511
1611
1624
1811
Results
154.2
153. 8
163.0
182
168
166
160
156.1
154. 1
152.2
184
192
176
128
140
128
137.7
129.9
133.8
163.2
157.4
149. 8
128.8
118.8
126.8
154.6
155.0
156.6
Mean Results
287.4
157.0 291.5
298.3
169
169 147
141
138
145.4
164. 0
154.1 164.0
164.0
352
184 356
356
153.9
252
132 256
252
334.3
133.8 330.5
338.2
195. 0
156.8 203.0
207. 0
164.0
124.8 130.0
190.0
116.0
155.4 118.3
120.0
Chloride
Mean correction
method
292.4 2
149 1 and 3
150.8 1
164.0 4
355 none made
347.3 2
253 4
334.4 2
201.7 landS
161.3
118.1 1 and 3
49
-------
Table A-3. SAMPLE C (continued)
Mercuric sullate method Standard method ,-,, ,
Laboratory _
, J Results
number
1911
2111
2124
2144
2211
2311
2411
2513
2526
2611
144.0
149.9
151.8
166.0
168.0
168.0
170.9
177.3
168.5
181.2
186.0
184. 4
185
167
167
179.0
179.0
168.0
164.4
164.4
165.6
144.0
137.6
134.4
137.6
121.8
124.3
132.0
132.0
140.0
Mean Results
373.7
148.6 367.7
369.7
200.0
167.3 204.0
212.0
143.7
172.2 143.7
139.7
364.8
183.9 354.4
360.4
423
173 426
426
150.0
175.3 146.0
146. 0
156.9
164.8 156.9
156.9
236. 8
138.7 217.6
228.8
139. 7
127.9 141.2
138.2
119.6
134.7 127.6
119.6
^-moriae
Mean correction
method
370.4 2
205. 3 2
142.4 1 and 2
359. 9 2
425 2
147.3 3
156. 9 1 and 3
227. 7 2
139.7 1
122.3 1
50
-------
Table A-3. SAMPLE C (continued)
Mercuric sulfate method Standard method
Laboratory
number
2811
2911
2915
3111
3116
3211
3226
3511
3514
3611
Results
149.8
150.6
149.8
164
164
164
960
964
960
140.1
140.1
142.-0
159.7
160.9
160.9
133.4
133.4
148.3
148.3
154. 1
158.4
164.8
160.0
179.5
166.7
175.2
173.6
173.6
169.9
Mean Results Mean
144.7
150.1 149.7 147.9
149.3
348
164 348 348
348
228
961 256 243
244
142.9
140.7 142.9 142.9
142.9
158.6
160.5 156.5 157.2
156.5
135.5
133.4 137.5 136.2
135.5
216.7
150.2 216.7 216.5
216. 0
244.8
161.1 244.8 243.7
241.6
273.2
173.8 290.0 282.3
283.8
49.1
172.4 49.1 49.7
50.9
Chloride
correction
method
3
1
4
1
1 and 3
1
2
2
2
1
51
-------
Table A-3. SAMPLE C (continued)
Mercuric sulfate method Standard method
Laboratory
number
3711
3716
3731
3811
3911
3926
4321
4421
4511
4711
Results
148.6
147.8
149.5
179
179
179
168.0
168.0
112.0
143.2
133.9
122.4
129.8
164. 0
160.8
176.0
161
159
155
170
172
168
166,2
177.8
174.2
145.2
149.2
145.2
159.4
161.4
157.4
Mean Results
292.2
148.6 324.1
340.9
290
179 290
290
1 AQ n -
JL^ty, u
130.1
132.3 104.2
108.3
135.8
166.9 134.2
132.6
142
158 138
140
173
170 169
173
225.1
172.7 225.9
227.2
170.0
146.5 174.1
166.0
136.9
159.4 175.9
Chloride
Mean correction
method
319.1 2
290 2
1
114.2 1
134.2 1 and 3
140 1 and 3
172 1
226.1 2
170.0 4
156.4 1
52
-------
Table A-3. SAMPLE C (continued)
Mercuric sulfate method Standard method n,-,^.^
Laboratory
number
4821
4921
5111
5221
5415
5511
5626
5811
5821
6212
6311
Results
150.2
156.2
144.2
153.6
157.4
157.4
152.0
152.0
148. 0
296.4
237.1
256.2
158.3
158.3
166.2
165.6
161. 7
159.7
261.8
257.4
255.3
164.0
184.0
160.0
152.0
148.0
152.0
160. 0
158. 0
164.0
163.0
163.0
163.0
~ - - \-sll J.\J±. J.VJW
Mean Results Mean correction
method
352.2
150.2 277.3 280.0 2
210.5
222. 7
156.1 215.0 222.7 2
230.4
208. 0
150.7 208.0 206.7 2
204.0
325.0
263.2 302.0 318.6 2
328.8
235.2
160.9 242.5 238.8 4
238. 8
138.8
162.3 130.9 134.2 1 and 3
132.9
401.3
258.2 401.3 401.3 4
401.3
320.0
169.3 284.0 289.3 2
264.0
86.2
150.7 78.2 83.5 1
86. 2
304.0
160.7 300.0 300.6 2
298.0
345.9
163.0 330.0 340.6 2
345.9
53
-------
Table A-3. SAMPLE C (continued)
Mercuric sulfate method Standard method
Laboratory
number
6411
6424
6512
6621
6711
6715
6812
6914
7112
7222
Results
162.2
164.2
166.2
158.9
158.9
154.9
163.0
163.0
159.0
142.1
126. 7
134.4
161.9
154.6
154.6
155.3
160.7
161.1
137
133
137
159.4
163.3
161.4
156
152
156
158.5
152.4
152.4
Mean Results Mean
134.6
164.2 134.6 134.6
134.6
141.8
157.6 137.8 139.8
139. 8
355.9
161.7 353.9 355.9
357.8
169.0
134.4 184.3 176.6
176.6
368.0
157.0 375.0 371.7
372.0
145.9
159.0 147.7 146.5
145. 9
314
136 314 314
314
366.5
161.4 362.6 364.6
364. 6
135
155 139 135
131
286.5
154.4 266.2 270.9
260.1
Chloride
correction
method
1 and 3
1 and 3
2
2
3
1
3
2
1 and 2
2
54
GPO 82O-B37-5
-------
Table A-3. SAMPLE C (continued)
Mercuric sulfate method Standard method
Laboratory
number
7512
7622
7622
7813
7826
7862
7866
8112
8512
9523
9613
Results
200.0
184. 0
200.0
150. 2
154.1
158.1
145.0
145.0
147.1
148.0
150.0
144. 0
168.3
147.3
193. 7
154.0
157.1
157.1
148
148
148
153. 6
154.0
162.1
165. 3
157.4
150.1
151.3
153.2
Mean Results Mean
149.4
194.7 165.4 154.7
149. 4
341.8
154.1 335.9 347.1
363.6
164. 0
272. 9
145.7 260.3 275.0
291.8
131.0
147.3 123.0 127.0
127.0
169.8 183.4 189.6
195. 7
135.4
156.1 131.9 133.6
133.4
211
148 211 211
212
145. 4
153.8 142.2 143.8
153.1
161.6 163.9 161.0
165. 9
138.2
151.5 136.7 138.2
139.8
Chloride
correction
method
1
2
1
2
3
2 and 4
1
4
1
1
1
55
-------
Table A - 3. SAMP LE C (continued)
Laboratory
number
9713
Mercuric Sulfate
Results
164. 0
148. 0
152.0
method
Mean
154.6
Standard method
Results Mean
188.0
208.0
204.0
200.0
Chloride
correction
method
2
175.9 233.8
1825 157.2 166.3 225.2 233.9
165.7 242.8
56
-------
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APPENDIX B.
MERCURIC SULFATE PROCEDURE
A. Measure 50 ml of sample or aliquot diluted to 50 ml with dis-
tilled water, and place in a standard reflux flask, then add:
(1) 1 gram mercuric sulfate
(2) 5 ml concentrated H SO - swirl to dissolve mercuric salt
(3) 25 ml 0. 25N K Cr O
£t £1 [
(4) 70 ml concentrated H SO (cautiously)
(5) 0. 75 gram Ag2SO4
(6) Several glass beads or porcelain chips
B. Mix well by swirling flask.
C. Connect flask to condenser and reflux for 2 hours,
D. Wash down the condenser with distilled water and cool to room
temperature.
E. Add 10 drops of o-phenanthroline ferrous indicator, and titrate
to a red endpoint with standardized ferrous ammonium sulfate
(approx. 0. 25N).
NOTE; Reagents, equipment, and calculations are the same as
in Standard Methods.
61
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APPENDIX C.
GLOSSARY OF STATISTICAL TERMS
A glossary of statistical terms with definitions of their meaning
as used in these reports is presented to insure uniform and complete
understanding.
Arithmetic mean
Median
The sum of a series of test results divided
by the number in the series.
The value above and below which an equal
number of observations lie.
Accuracy
Accuracy Data
Precision
Accuracy is the correctness of a measurement,
or the degree of correspondence between the
result and the true value.
Measurements that relate to the difference
between the average test results and the true
result when the latter is known or assumed.
The following measures apply:
50% Range - The maximum deviation from the
true amount for the more accurate half of the
mean results reported.
Average deviation from true concentration -
The average difference without regard to sign
between each laboratory mean and the true
value.
Average percent deviation from established
concentration (or amount added) - The average
of the differences between a laboratory's repli-
" cate results and the established concentration
expressed as percentages of the established
concentration.
Precision is a measure of the reproducibility
of measurements, or the degree to which the
measurements correspond to one another.
62
GPO 8208376
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Precision Data Measurements that relate to the variation among
the test results themselves, i. e. , the scatter or
dispersion of a series of test results without
assumption of any prior information. The following
measures apply:
Variance - The sum of squares of deviations of
the average test results from the mean of the
series divided by one less than the total number
of average test results.
Standard deviation - The square root of the variance.
Coefficient of variation - The standard deviation
of the laboratories' means as a percentage of the
mean of this series.
Range - The difference in magnitude between the
highest and lowest laboratory mean.
Average percent deviation within laboratory -
The average of the differences between a labora-
tory s replicate results and their mean, expressed
as percentages of their mean.
Confidence limits - Limits within which the true
mean of the series will lie with a given probability.
REFERENCES
1, Annual Reviews. Analytical Chemistry, pg. 364R. April 1962.
2. Bennett, C. A. and Franklin, N. L. Statistical Analysis in
Chemistry and the Chemical Industry. John Wiley & Sons, Inc.
New York. 1954.
3. Bauer, E. L. A Statistical Manual for Chemists. Academic
Press. New York. 1960.
4. Dixon, W. J. and Massey, F. J. Introduction to Statistical
Analysis. McGraw-Hill Book Co. , Inc. New York. 1951.
5. Standard Methods for the Examination of Water and Wastewater.
llth edition. APHA, AWWA, WPCF. New York, 1960.
63
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APPENDIX D.
PROBABILITY EXPLANATION
This section deals with the description and use of normal proba-
bility paper. The bell-shaped normal distribution curve can be reduced
to a straight line on probability paper. Obviously, it is easier to deal
with this straight line than with the complicated bell-shaped curve.
Construction of Normal Probability Paper
In an ideal sample, frequencies of the measurements plotted against
their magnitudes give a bell-shaped normal distribution curve.
This curve is symmetric about its mean, 0, and the percentage of
the area lying between any two points on the curve can be found.
For example, 68. 26 % of the area under the curve lies between
+ one standard deviation (Figure D-l).
The first step in the construction of normal probability paper is
the transformation of the bell-shaped distribution curve to the
probability summation curve. This curve gives the summation
of the area from left to right under the bell-shaped curve up to any
deviation. For example, taking the point, 0, on the X-axis of the
summation curve, we read from the Y axis that 50 % of the total
area lies below this zero value, which is the mean. The summation
of area up to a deviation of - lo-below the mean is 50 34. 13, or
15. 87%, while the summation of area up to + l
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of magnitude automatically places them in the order of their posi-
tion on the probability summation scale. This arrangement permits
ready determination of the probability not exceeding a certain
magnitude of measurement.
In the development of the probability paper many difficulties were
encountered in choosing a formula for plotting the position of the
measurements on the probability summation scale (or X-axis),
but the problem was solved in the following way. Ideally, the
plotting position for the mean of any series, regardless of the
number of values, is at 50 %; i. e., half of the values above and
half below. If the mean is considered as part of the series, the
number of plotting positions becomes n + 1. Thus the X-axis,
which represents 100 %, is divided into n + 1 intervals. To plot
the third point, for example, multiply 1 by 3, or 3 / 1
n + 1 ^n +
The formula for the plotting interval, therefore, is m where
n+ 1
m equals the serial number of the measurements arranged in
ascending order of magnitude, and n equals the total number of
measured values to be plotted. Multiplication of the resulting
value by 100 converts the ratio to a percentage.
In summary, the procedure for plotting data on normal probability
paper is as follows:
1. Array data in order of ascending magnitude.
2. Calculate the plotting position of each value by the expression
m(100), which gives the ratio as a percentage. This point
n+ 1
designates the percentage of the values that are equal to or less
than the plotted value.
3. The Y-axis is graduated linearly from the lowest to the highest
reported value, while the X-axis is graduated according to the
probability scale. Place the first (lowest) value above the cal-
culated plotting interval for m = 1 on the X-axis and at the
appropriate value on the Y-axis. Plot the remaining values
in a similar manner.
INTERPRETATION
If a straight line develops in the plotting, the data have a normal
distribution; that is, in accordance with the theory of probability, this
is the expected distribution of results.
67
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If a straight line does not develop in the plotting, a change in the
conditions affecting the observed measurements is suspected. It may
mean, for example, that the same characteristic has not been measured
under the same conditions.
Sometimes the great majority of the data approximate a straight
line, but on the ends some results will be either extremely high or low.
Just as one of these erratic results is far removed from the others on
the bell-shaped normal curve, so it is far removed from the others on the
straight-line curve. When this happens, these erratic results are
presented in the published report on the probability curve, but the
statistics are based on only the normal segment of the distribution.
REFERENCE
Velz, C, J. Graphical Approach to Statistics. Water and Sewage Works.
pp. R106-R135. 1950
68
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APPENDIX E.
ANALYTICAL REFERENCE SERVICE MEMBERSHIP
STATE AGENCIES
Alabama Water Improvement Commission
Arizona State Department of Health
Arkansas State Board of Health
California Department of Water Resources
California State Department of Public Health (Berkeley)
California State Department of Public Health (Los Angeles)
Colorado State Department of Public Health
Connecticut State Department of Health
Delaware Water Pollution Commission
Florida State Board of Health (Bureau of Laboratories, Jacksonville)
Florida State Board of Health (Division of Sanitary Engineering,
Jacksonville)
Florida State Board of Health (Pensacola)
Florida State Board of Health (Winter Haven)
Hawaii State Department of Health
Idaho Department of Health
Illinois State Department of Public Health (Chicago)
Illinois State Department of Public Health (Springfield)
Illinois State Water Survey Division
Indiana State Board of Health
Kansas State Board of Health (Sanitary Engineering Laboratories)
Kentucky State Department of Health
Louisiana State Board of Health
Maryland State Department of Health
Maryland State Water Pollution Control Commission
Massachusetts Department of Public Health
Michigan State Department of Health
Michigan Water Resources Commission
Minnesota State Department of Health
Missouri Department of Public Health and Welfare
Montana State Board of Health
Nassau County Department of Health
Nebraska State Department of Health
Nevada State Department of Health (Las Vegas)
Nevada State Department of Health (Reno)
New Hampshire State Department of Health
New Hampshire Water Pollution Commission
New Jersey State Department of Health
New Mexico State Department of Public Health
New York State Conservation Department
69
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North Carolina Department of Water Resources
New York State Department of Health
North Dakota State Department of Health
Ohio State Department of Health
Oklahoma State Department of Health
Oregon State Board of Health
Pennsylvania Department of Health
Puerto Rico Institute of Health Laboratories
Rhode Island Department of Health
South Carolina Water Pollution Control Authority
South Dakota Department of Health
Tennessee Division of Preventable Diseases
Tennessee Stream Pollution Control Board
Texas State Department of Health
Utah State Department of Health
Vermont State Department of Health
Vermont State Department of Water Resources
Virginia State Department of Health
Virginia State Water Control Board
Washington State Department of Health
West Virginia State Water Resources Commission
Wisconsin State Board of Health
MUNICIPAL AGENCIES
Air Pollution Control District, Pasadena, California
Alexander Orr Jr. , Water Treatment Plant, Miami, Florida
City Department of Health, New York, New York
City Department of Public Health, Pasadena, California
City Department of Water, Dayton, Ohio
City Department of Water Resources, Durham, North Carolina
City Health Department, Baltimore, Maryland
City Health Department, Beaumont, Texas
City Health Department, Houston, Texas
City Water Department, Charlotte, North Carolina
Department of Air Pollution Control, Chicago, Illinois
Department of Public Works and Utilities, Flint Water Plant,
Flint, Michigan
Department of Service and Buildings, Dayton, Ohio
Department of Water and Sewers, South District Filtration Plant,
Chicago, Illinois
Erie County Laboratory, Buffalo, New York
Long Beach Water Department, Long Beach, California
Los Angeles County Flood Control District
Los Angeles Department of Public Works, Hyperion Treatment Plant
Los Angeles Department of Water and Power
70
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Louisville Water Company, Incorporated
Metropolitan Utilities District, Omaha, Nebraska
Metropolitan Water District of Southern California
Philadelphia Department of Public Health
Philadelphia Department of Public Health (Occupational Environment
Section)
Philadelphia Suburban Water Company
Philadelphia Water Department (Belmont Laboratory)
Philadelphia Water Department (Torresdale Laboratory)
St. Louis County Water Company
St. Louis Department of Health and Hospitals
Water Works, Topeka, Kansas
FEDERAL AGENCIES
Associated Universities, Incorporated, Brookhaven National Laboratory
DHEW, PHS, Great Lakes-Illinois River Basins Project
DHEW, PHS, Northeastern Radiological Health Laboratory, Winchester,
Massachusetts
DHEW, PHS, Northeast Shellfish Sanitation Research Center,
Narragansett, Rhode Island
DHEW, PHS, Off-Site Radiological Safety Program, Las Vegas, Nevada
DHEW, PHS, Water Quality Section, Division of Water Supply and
Pollution Control, Cincinnati, Ohio
DHEW, PHS, Water Supply Section, Interstate Carrier Branch,
Washington, D. C.
Food and Drug Administration, Division of Pharmacology,
Washington, D. C.
Fourth U. S. Army Medical Laboratory, Fort Sam Houston, Texas
Oak Ridge Institute of Nuclear Studies, Oak Ridge, Tennessee
Pearl Harbor Naval Shipyard
Sixth U. S. Army Medical Laboratory, Fort Baker, California
Tennessee Valley Authority, Chattanooga, Tennessee (Stream
Pollution Control)
Tennessee Valley Authority, Wilson Dam, Alabama (Occupational
Health Branch)
2793D U. S. Air Force Hospital, Regional Environmental Health
Laboratory, McClelland Air Force Base, California
2794th U. S. Air Force Dispensary - Class B, Kelly AFB, Texas
U. S. Air Force Radiological Health Laboratory, Wright-Patterson
AFB, Ohio
U. S. Army Environmental Hygiene Agency, Maryland
U. S. Department of the Interior, Bureau of Reclamation, Denver,
Colorado
U. S. Department of the Interior, Geological Survey, Columbus, Ohio
U. S. Department of the Interior, Geological Survey, Denver, Colorado
71
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U. S. Department of the Interior, Geological Survey, Philadelphia,
Pennsylvania
U. S. Department of the Interior, Geological Survey, Sacramento,
California
Walter Reed Army Medical Center, Washington, D. C.
UNIVERSITIES
Case Institute of Technology, Department of Civil and Sanitary
Engineering
Columbia University
University of Florida, Department of Chemistry
Georgia Institute of Technology, Department of Applied Biology
Johns Hopkins University, School of Hygiene and Public Health
University of Kansas, School of Engineering and Architecture
New Mexico State University, Department of Civil Engineering
University of North Carolina, Chapel Hill, North Carolina
University of Pittsburgh, Graduate School of Public Health
Purdue University, Department of Chemistry
Rensselaer Polytechnic Institute, Troy, New York
Rutgers - The State University
Department of Agricultural Chemistry
Department of Environmental Science
Washington State University, Division of Industrial Research
INDUSTRY
American Cyanamid Company, Bound Brook, New Jersey
Anaconda Company, Grants, New Mexico
Atlantic Refining Company, Philadelphia, Pennsylvania
Battelle Memorial Institute, Columbus, Ohio
Bethlehem Steel Company, Bethlehem, Pennsylvania
California Water Service Company, San Jose, California
Culligan, Incorporated, Northbrook, Illinois
Dearborn Chemical Company, Chicago, Illinois
Dow Chemical Company, Midland, Michigan
E. I. du Pont de Nemours and Co., Aiken, South Carolina
El Paso Natural Gas Products Company, El Paso, Texas
Ekroth Laboratories, Incorporated, Brooklyn, New York
Fairbanks, Morse, and Company, Research Center, Beloit, Wisconsin
Food Machinery and Chemical Corporation, Pocatello, Idaho
General Electric Company, Louisville, Kentucky
Goodyear Atomic Corporation, Piketon, Ohio
HALL Laboratories Division, Calgon Corporation, Pittsburgh,
Pennsylvania
72
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Hazleton Nuclear Science Corporation, Palo Alto, California
Industrial Chemicals, Incorporated, South Bend, Indiana
Ionics, Incorporated, Cambridge, Massachusetts
Kennecott Copper Corporation, Salt Lake City, Utah
Midwest Research Institute, Kansas City, Missouri
Minute Maid Company, Anaheim, California
Monsanto Chemical Company, St. Louis, Missouri
NALCO Chemical Company, Chicago, Illinois
Pacific Gas and Electric Company, Emeryville, California
Pacific Gas and Electric Company, San Francisco, California
Pan American World Airways, Patrick Air Force Base, Florida
Pomeroy and Associates, Pasadena, California
Radiation Detection Company, Mountain View, California
Reynolds Electrical and Engineering Company, Las Vegas, Nevada
Roy F. Weston, Incorporated, Newtown Square, Pennsylvania
Sandia Corporation, Sandia Base, Albuquerque, New Mexico
Shell Chemical Company, New York, New York
Tracerlab, Incorporated, Richmond, California
U. S. Industrial Chemicals Company, Tuscola, Illinois
Water Service Laboratories, Incorporated, New York, New York
FOREIGN
British Coke Research Association, Chesterfield, Derbyshire, England
Central Electricity Research Laboratories, Leatherhead, Surrey,
England
Comissao InterMunicipal de Controle da Poluicao das Aguas E Do Ar,
Sao Paulo - Brasil
Department of Health Services and Hospital Insurance, Vancouver,
B. C., Canada
Department of Municipal Laboratories, Hamilton, Ontario, Canada
Department of National Health and Welfare, Ottawa, Ontario, Canada
Department of National Health and Welfare, Occupational Health
Division, Ottawa, Ontario, Canada
Department of National Health and Welfare, Vancouver, B. C. , Canada
Institute Nacional de Obras Sanitarias, Caracas, Venezuela
Metropolitan Water, Sewerage, and Drainage Board, Sydney, Australia
National University of Colombia, Bogota, Colombia, South America
Permutit Company, Limited, London, England
Scientific Research Council, Kingston, Jamaica, West Indies
Taiwan Institute of Environmental Sanitation, PHA, Taiwan
Pingtung Air Pollution Laboratory
Pingtung Organic Waste Laboratory
Taichung Water Laboratory
Tainun Water Laboratory
Taipei Milk Laboratory
73
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Taipei Radiological Health Laboratory
Taipei Water Laboratory
Taitung Water Laboratory
United Kingdom Atomic Energy Authority, Didcot, Berks, England
University of Beograd, Civil Engineering Faculty, Beograd, Yugoslavia
University of Leeds, Houldsworth School of Applied Science,
Leeds, England
Water Research Association, Marlow, Buckinghamshire, England
74
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APPENDIX F.
STAFF AND ACKNOWLEDGMENTS
ANALYTICAL REFERENCE SERVICE
STAFF
E. F. McFarren, Chief
R. J. Lishka, Chemist
R. T. Cope, Statistician
J. M. Matthews, Chemist
P. A. Miller, Secretary
ACKNOWLEDGMENTS
The suggestions and technical review
of the final report by the following
persons are gratefully acknowledged:
D. G. Balllnger, Supervisory Chemist
Technical Advisory and Investigations
Water Supply and Pollution Control
J. M. Cohen, Chemist, In Charge
Engineering Research
R. C. Kroner, In Charge
General Laboratory Services
National Water Quality Network
GPO 820-8377 75
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pr('Ctdures procLu e s miLar precision ai-d Liccur-
ary when no interft'j irg tna'ei'ials ire ])rcdent
When interferences due to high concentrations
of chloride are present, the standard method will
proouce equal precision &nd accuiacy only if the
appropriate corrective techniques -ire applied.
The Mercuric Sulfate mo'lircnticr is the method of
choice for COD measi rcT'tnt --:inc < with less
(over)
BIBLIOGRAPHIC: Robert A. Taft Sanitary Engineering
Center. WATER OXYGEN DEMAND NO. 2. STUDY
NUMBER 21. Public Health Service Publication
No. 999-WP-26. 1965. 75 pp.
ABSTRACT: This study consisted of four samples which
74 participating laboratories were instructed to
dilute to a specified volume and analyze by both
the Standard Method for Chemical Oxygen Demand
and by the Mercuric Sulfate modification.
The results from this study indicate that the two
procedures produce similar precision and accur-
acy when no interfering materials are present.
When interferences due to high concentrations
of chloride are present, the standard method will
produce equal precision and accuracy only if the
appropriate corrective techniques are applied.
The Mercuric Sulfate modification is the method
of choice for COD measurement since with less
(over)
ACCESSION NO.
KEY WORDS:
BIBLIOGRAPHIC: Robert A. Taft Sanitary Engineering
Center. WATER OXYGEN DEMAND NO. 2. STUDY
NUMBER 21. Public Health Service Publication
No. 999-WP-26. 1965. 75 pp.
ABSTRACT: This study consisted of four samples which
74 participating laboratories were instructed to
dilute to a specified volume and analyze by both
the Standard Method for Chemical Oxygen Demand
and by the Mercuric modification.
The results from this study indicate that the two
procedures produce similar precision and accur-
acy when no interfering materials are present.
When interferences due to high concentrations
of chloride are present, the standard method will
produce equal precision and accuracy only if the
appropriate corrective techniques are applied.
The Mercuric Sulfate modification is the method
of choice for COD measurement since with less
(over)
ACCESSION NO.
KEY WORDS:
-------
manipulation it effectively removes the interference
due to chloride oxidation and is less time consu-
ming.
manipulation it effectively removes the interference
due to chloride oxidation and is less time consu-
ming.
manipulation it effectively removes the interference
due to chloride oxidation and is less time consu-
ming.
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