>A 660/2-73-039
ugust 1974
Environmental Protection Technology Series
Office of Research and
-------
RESEAKCK REPORTING SERIES
Research reports of the Office of Research and
Monitoring, Environmental Protection Agency, have
been grouped into five series. These five broad
categories were established to facilitate further
development and application of environmental
technology. Elimination of traditional grouping
was consciously planned to foster technology
transfer and a maximum interface in related
fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
1. Environmental Monitoring
5. Socioeconomic Environmental studies
This report has been assigned to the ENVIRONMENTAL
PROTECTION TECHNOLOGY ; series. This series
describes research performed to develop and
demonstrate instrumentation, equipment and
methodology to repair or prevent environmental
degradation from point and non-point sources of
pollution. This work provides the new or improved
technology required for the control and treatment
of pollution sources to meet environmental quality
standards.
EPA REVIEW NOTICE
The Office of Research and Development lias reviewed this
report and approved its publication. Mention of trade names
or commercial products does not constitute endorsement or
recommendation for use.
-------
ABSTRACT
This report is concerned with modification of the amperometrie titration
method for measuring chlorine residuals in cooling tower blowdown. This
modified procedure can be applied to other water systems with a concen-
tration of metal ions similar to those found in cooling tower blowdown.
The addition of sodium pyrophosphate as a complexing agent removes the
interferences contributed by Fe (III) and Cu (II) in the water matrix.
Procedure recommendations are made to increase the efficiency of both
sampling and the actual titration procedure in order to allow a residual
determination in the minimum amount of time. Equipment recommendations
along with a design of a biamperometrie end point system which allows
greater titration speed along with portability are described.
111
-------
CONTENTS
Page
Abstract iii
List of Figures v
List of Tables vi
Acknowledgment vii
Sections
I Conclusions 1
II Recommendations 2
III Introduction 3
IV Method Familiarization 7
V Characterization of Interferences 11
VI Biamperometric End Point 30
VII References 37
VIII Appendix 39
iv
-------
FIGURES
No. Page
1. Meter Drift vs Time 17
2. Calibration Curve for Total Chlorine Residual 27
3. Chlorine Probe (DPE) 31
4. Schematic Diagram of DPE Control Box 32
5. Background Current vs Applied Voltage Iron 34
Interference Study
6. Background Current vs Applied Voltage Copper 35
Interference
-------
TABLES
No. Page
1 Recovery of Free Chlorine Residual (pH7) 8
2 Recovery of Free and Combined Chlorine in 10
Chlorine Demand-Free Water
3 Loss of Chlorine in Distilled Water as a 12
Function of Stirring
4 Chlorine Stability in Distilled Water in 14
an Unstirred Sample
5 Chemical Analysis of Slowdown Water 16
6 Effect of Cu (II) on Recovery of 19
Chlorine Residual
7 Effect of Citrate on Copper Interference 19
8 Effect of Citrate on Iron Interference 22
in Determination of Chlorine Residual
9 Recovery of Cl in the Presence of Kaolin 22
10 Recovery of Cl Spikes on Cline's Pond Sediment 24
and Corvallis Clay
11 Recovery of Cl Additions in tKe Presence of a 24
Water Treatment Chemical as a Function of Time
12 Precision of Titration 26
13 Chemical Analysis of Natural Water Samples 29
14 Recovery of Chlorine Spikes on Blowdown 29
VI
-------
ACKNOWLEDGEMENT
The author would like to thank the personnel of the Laboratory Services
Branch at the Pacific Northwest Environmental Research Laboratory for
their assistance to the project. Personal thanks are given to David
Maier who helped extensively in the experimental portion of the project,
Daniel F. Krawczyk, Chief Laboratory Services Branch, for valuable
advice and orientation and John Jacobson for the construction of needed
equipment.
vii
-------
CONCLUSIONS
Experimental results have shown that the analytical difficulties in
the application of the amperometric technique to cooling tower blow-
down can be eliminated by the addition of a pyrophosphate reagent to
the currently used amperometric procedure. Copper and iron in con-
centrations found in typical blowdown do not interfere in the presence
of pyrophosphate.
Suspensions of sediments exhibit stable end points with good recovery
of incremental additions of chlorine once the initial chlorine demand
of the system is satisfied.
Various synthetic blowdowns are evaluated as to stability of the end
point. All of these exhibit stable end points with good recovery of
the chlorine spikes once the demand constituents were satisfied. The
effect of agitation when using a commercial instrument does cause an
appreciable error in results if the titration is not performed rapidly.
The source of the error may be due to actual flashing off the residual
chlorine or enhanced photodecomposition resulting from an increase in
surface area provided by the agitation.
A modified biamperometric titrator is described and evaluated. When
using this titrator, results on the recovery of chlorine along with
precision of the measurement are comparable to that of a commercially
available instrument. The dual electrode system offers numerous ad-
vantages for field operation in that a liquid reference electrode is
no longer needed. The system is battery operated eliminating the need
for an AC power source. The big advantage is the rapid determination
of the end point. The meter response is faster for the biamperometric
method over the commercial instrument. This permits more rapid addition
of titrant and easily meeting the goal of performing a titration in less
than 1.5 minutes.
-------
RECOMMENDATIONS
Further tests of the pyrophosphate system should be made on site on
actual blowdown systems to check its effectiveness in the elimination
of interferences due to metal ions. Feedback from other laboratories
would be helpful in determining whether the pyrophosphate system would
be effective in most blowdown matrixes. Due to the lack of time com-
plete evaluation of the bioamperometric endpoint detector was not made.
Further tests on its application for the analysis of total chlorine
residuals should be made, perhaps by other interested laboratories.
If the pyrophosphate system is not adequate in some systems, a mixture
of complexation reagents should be looked into. A precipitation re-
agent could also be used.
-------
INTRODUCTION
The goal of the project is the development of a simple, accurate method
for the analysis of residual chlorine by the quantification or elimi-
nation of certain interferences typically present in cooling tower
blowdown. Suspected interferences are turbidity, copper, iron and the
chlorine reaction with phosphonates and polyesters in the water treat-
ment chemicals used as corrosion inhibitors.
There are basically two classes of chlorine analysis, colorimetric and
titrametric. The problem with colorimetric procedures is the use of
comparison standards along with a spectrophotometer or color comparator.
The colorimetric class of analysis requires the ability to differentiate
the ways light interacts with the colored species in a sample. The
physical characteristic of the blowdown water, having a high degree of
turbidity and suspended solids, does not allow easy discrimination by
an instrument or the human eye. The field stability of colored stan-
dards is also highly unreliable. For these reasons a titration method
is a more reasonable approach in the field than a colorimetric method.
There are at present two titration methods available in Standard
Methods (1) and ASTM (2). The amperometric end point technique is the
Referee Method in ASTM (2). This involves the reduction of an active
halogen species, either hypochlorite ion in the free residual test or
liberated I~ in the combined residual determination, by phenylarsine
oxide (PAO).
-------
CgH5AsO + HOC1 + H20 -» CgH5AsO(OH)2 + HC1
CgH5AsO + I2 + 2H20 -> CgH5AsO(OH)2 + 2HI
By lowering the pH to 4 the addition of iodide produces free I,, in
solution by oxidation of I" by both the free hypochlorite and combined
chlorine not available as chloride. The effectiveness of the phenyl-
arsine oxide as a titrant is pH independent.
Also mentioned is a tentative procedure using ferrous ammonium sulfate
as the titrant with DPD (N,N-diethyl-p-phenylene diamine sulfate) as
an end point indicator. This procedure has certain features which
detract from its desirability in the field application to blowdown
water. One flaw is the chemical instability of the titrant which
necessitates frequent standardization. The second flaw is that in
extremely colored water systems with a high degree of suspended solids
along with high turbidity the end point is not sharp and easily recog-
nized. In clear water systems, however, the DPD titration technique
allows differentiation among all active chlorine forms at the same pH.
This is a big advantage over the amperometric method which requires a
change in pH from 7 to 4 to detect free and combined chlorine residual.
An abrupt change in pH can change the chemical eequilibria thus dis-
torting the true free and combined chlorine fractions (6).
Certain weaknesses of the amperometric titration are mentioned by
Nicolson (4) and Hasselbarth (5). The primary one is the flashing of
chlorine out of solution or enhanced photodecomposition by the stirring
action of the amperometric titrator. Another deals with the incomplete
reaction of the phenylarsine oxide titrant with the chlorine residual.
Hasselbarth suggests the addition of a catalytic quantity of iodide to
allow the titrant to react completely. He notes that monochloramine
does not increase the diffusion current between the electrodes nor does
monochloramine require any titrant after the addition of KI solution.
-------
His work, however, conflicts with the procedure in Standard Methods
for the determination of monochloramine. Standard Methods suggests
the addition of 0.2 ml of a 0.1N KI solution at the end point of the
free residual titration to obtain a monochloramine fraction. The in-
terpretation of the results when KI is added is at this moment in ques-
tion. Since this report deals with only total residual chlorine, it is
not necessary to resolve this question.
Guter and Cooper (6), in a paper evaluating various field test proce-
dures for the determination of free chlorine residual in aqueous solu-
tions, suggest the following improvements in the amperometric procedure:
1. Exposing the samples to as little light as possible.
(VERY IMPORTANT).
2. Adding a considerable quantity (90 percent) of the
titration solution before turning on the cell stirrer.
3. Completing the titration as quickly as possible.
Guter and Cooper (6) also note that the determination of free residual
and combined residual on the same sample gives erratic results on the
free residual determination of subsequent samples. They suggest rinsing
the cell four times with distilled, chlorine demand-free water between
each sample to remove all traces of iodine before another sample is run.
This interference can be overcome by the use of two separate titrators:
one for the free and one for the total residual chlorine fractions.
Thus, taking into account the physical characteristics of cooling tower
blowdown and the desirability of a fast, simple technique for the deter-
mination of chlorine residuals, the amperometric titration procedure
shows the greatest promise. Reviews of alternate methods and their
limitations are given in papers by Marks (7), Nicolson (4) and Pal in (3).
-------
The experimental work of the project is divided into three phases.
Phase one is devoted to method familiarization. Phase two deals with
an overview in which characterization of various waters as to major
cations, anions and nitrogen compounds is made. Characterization
allows prediction of changes in chlorine residual due to interfering
reactions. Phase three involves the development of a fast, accurate
procedure free of the interferences rioted in cooling tower blowdown.
-------
MLTilOL) FAMILIARIZATION
Double distilled water as used in the Pacific tlorthwest Environmental
Research Laboratory had a negligible cnlorine demand as determined by
the amperometric titrator. Tne water was subjected to chemical analysis
weekly and the concentration of the major demand agent for free residual
chlorine, ammonia, was always less than 0.001 mg/1.
Tiie chlorine stock solutions were prepared from commercial sodium
hypochlorite solutions (Clorox-5.25 percent NaOCl).
Each ml of sodium hypochlorite in bleach contained 50 rng/ml of active
chlorine. A stock solution of 100 mg/1 was produced by dilution of 2
ml of bleach solution to one liter with cnlorine demand-free water.
Tne dilute chlorine stock solution was standardized against thiosulfate
as outlined in Standard Methods (8).
Aliquots of this stock were added to the amperometric titration vessel
containing (2x) distilled water. Reagents were added as outlined in
Standard Method 114B, and tne titration run. The amperometric titrator
used was a Wallace & Tiernan and the procedure employed is outlined in
Standard Methods (9). The results are shown in Table 1.
-------
TALJLL 1
Recovery of Free Chlorine Residual (pH7)
Cone, of Chlorine Chlorine Residual Net Loss of
Added (mg/1) Recovered (mg/1) Chlorine Residual (mg/1)
1.05 0.96 0.09
1.05 0.98 0.07
1.05 0.95 0.10
1.05 0.95 0.10
0.52 0.48 0.04
0.52 0.47 0.05
0.52 0.48 0.04
2.10 1.98 0.12
2.10 1.97 0.13
-------
As indicated in Table 1, tiie free residual as determined by the ampero-
metric procedure in the absence of iodide gives a lower value for the
free residual chlorine on the order of 6-10 percent. Similar low
readings were presented in the Analytical Reference Service Report
Number 40 (10).
Ammonia in tiie form of ammonium chloride was added to known amounts of
a chlorine solution to check on the recovery of free and combined frac-
tions of the residual chlorine. Results are shown in Table 2.
The importance of the data in Table 2 is that the added step to deter-
mine both free and combined residual chlorine (1) caused consistently
lower recovery of total residual chlorine. Tin's lower reading is pro-
bably due to the flashing off of the combined chlorine residual during
the free residual determination. This effect was also noted by Marks
(7). Direct lowering the pH to four from the start and determining
only total residual allows more complete recovery of the chlorine
residual at a low pH. The loss of Io has been reported to be neg-
ligible (7).
The effect of varying the concentration of ammonia and looking at the
free and combined residual fraction was not pursued due to tiie fact
that the project scope was concentrated only to total chlorine resi-
dual. However, the "flashing off" effect was investigated.
-------
TABLE 2
Recovery of Free and Combined Chlorine
in
Chlorine Demand-Free Water
mg/1
Chlorine
Total
Added
2.04
2.04
2.04
2.04
2.04
2.04
2.04
2.04
2.04
2.04
2.04
2.04
2.04
2.04
mg/1
Ammonia
Nitrogen Added
0.00
1.00
1.00
1.00
1.00
1.00
2.00
2.00
1.00
1.00
1.00
1.00
1.00
1.00
mg/1
Free Chlorine
Residual Determined
1.98
__*
*
__*
__*
*
0.74
0.78
1.18
1.20
1.26
1.28
1.26
1.26
mg/1
Total Residual
Chlorine
Determined
*
1.98
1.90
1.88
1.97
1.90
1.80
1.88
1.74
1.83
1.60
1.84
1.81
1.83
*Determination not made
10
-------
CHARACTERIZATION OF INTERFERENCES
TIME INTERFERENCE
Samples of chloramine T were used as a source of combined chlorine resi-
dual. A 10 ml aliquot of a 100 mg/1 solution of chloramine T was allowed
to stand in the titration vessel with stirring. See Table 3.
An assumption was made that the rate of flashing is proportional to the
concentration of the chlorine in the sample vessel. This type of rate
|/4-
law is exponential in nature. The rate law has the form f(t) = be ,
where t = elapsed time before the titration is performed and b is the
initial concentration of total residual chlorine present. Note at time
t=0 for a residual concentration of 1.20 mg/1
f(t) = be"0 = b (1)
thus b = 1.20 mg/1
For t = 5 minutes, f(5) = 0.90 (from Table 3) and
0.90 = 1.20 e+K^5^ (2)
Solving for K yields the following expression for the concentration
as function of time.
fUJ-l^Oe-*0'058** (3)
To test the model another sample was allowed to stir for 10 minutes.
Duplicate samples yielded 0.67 and 0.69 mg/1 with an average value of
0.68 mg/1.
11
-------
TABLE 3
Loss of Chlorine in Distilled Water
as
Function of Stirring
Stirring mg/1
mg/1 Time Chlorine
Chlorine (min) Determined
1.20 0 1.22
1.20 0 1.17
1.20 5 0.92
1.20 5 0.89
1.20 10 0.67
1.20 10 0.69
12
-------
Using the model to predict what the value would be and knowing starting
time yielded 0.67 mg/1.
f(t) =1.20e-<°-058><10>=0.67 (4)
Nicolson (4) states that after 90 seconds of stirring he noted a 5
to 7 percent loss of chlorine. Applying our model at t = 1.5 minutes,
a predicted loss of 8 percent is obtained, which compares quite favorably
to Nicolson values. For a 30-second stirring time before the addition
of titrant a 3 percent error is obtained. This experimental and theo-
retical treatment lends support to the improvement stated by the U.S.
Army on the survey study of field test kits (6). They suggest the
addition of 90 percent of the titrant before stirring action is ini-
tiated. This drops the chlorine or iodine concentration in solution
to 10 percent allowing a slower rate of flashing of the active species
in solution. This, however, is very difficult to do in the field if
the exact end point is not known. In this case the titration should
be carried out as rapidly as possible. Once experience with a given
dynamic system is obtained, addition of 90 percent of the titrant
should be practical.
Another time interference study was made. A chlorine residual stock
solution was prepared and eight samples were poured at once into 250
ml beakers. The beakers were then stored in the dark with one sample
pulled out at 5-minute intervals. See Table 4. Total chlorine residual
determination was run on the samples.
From the data in Table 4, it is apparent that without stirring in the
dark, the loss of chlorine residual is negligible up to at least 35
minutes for chlorine demand-free systems.
13
-------
TABLE 4
Chlorine Stability in Distilled Water
in an Unstirred Sample
mg/1 Storage mg/1
Chlorine Time Chlorine
Added (min) Determined
0.30 0 0.30
0.30 5 0.30
0.30 10 0.30
0.30 15 0.31
0.30 20 0.30
0.30 25 0.30
0.30 30 0.29
0.30 35 0.30
14
-------
METAL ION INTERFERENCE
It was thought that some interferences to the amperometric procedure
might be due to heavy metal contamination in the blowdown water. Chem-
ical analysis of both the makeup and blowdown water for one cooling
tower gave the results in Table 5. The ratio of the metal ion content
in the blowdown water to that in the makeup water was approximately
10:1 giving 10 cycles of concentration. Metal ions contained in
the blowdown in appreciable concentration are Zn, Mn, Cu, and Fe.
Marks has stated that cadmium, trivalent chromium, divalent nickel
and zinc in concentrations up to 1000 mg/1 had no immediate effect
on the amount of current flowing though the titration cell or poi-
soning the electrode surface 01)- He has also noted that copper in
high concentrations (500 mg/1) poisons the electrode. Marks also
states that manganese in its higher oxidation states does not inter-
fere if the pH is above 3.5.
Based upon the literature data and heavy metal ion content known to
exist in blowdown water, two elements (Fe CHI), Cu (II)) were present
in sufficiently high concentrations to cause a possible interference.
These two metal ions were added to distilled water to investigate their
possible interference. Solutions of copper at concentrations as low as
0.05 mg/1 exhibit a poisoning effect after two titrations. The poi-
soning effect manifested itself by a reduction in sensitivity and in-
stability of the current flowing through the cell. Copper produced a
general drift in the meter reading as a function of time as shown in
Figure 1. Copper also gave a false chlorine residual when none was
present. See Table 6.
The meter deflections exhibited erratic behavior at the end point along
with the deposition of a crust on the platinum electrode. Note that
15
-------
TABLE 5
Chemical Analysis Of Slowdown Water
Cooling Tower
I
Cooling Tower
I-A
Cooling Tower
II
Characteristic
QIC TOC1 (mg/1)
Kjel N2 (mg/1)
Ammonia N (mg/1)
Nitrite N (mg/1)
Nitrate N (mg/1)
Ortho P3 (mg/1)
Chromium (yg/ml)
Copper (yg/ml)
Iron (yg/ml)
Manganese (yg/ml)
Lead (yg/ml)
Zinc (yg/ml)
Makeup
22.4
0.38
0.007
0
0.52
0.011
< 20
20
440
10
Slowdown
87.1
2.4
0.067
0.025
6.0
1.4
< 20
200
6000
220
Makeup
< 0.01
< 0.001
0.003
0.031
10
5
39
4
< 10
68
Slowdown
5
0.4
0.0025
0.0525
0.765
26000
29
39
6
14
40
Bl owdown
0.0065
0.006
0.113
7600
11
240
1.2
240
1200
1 Total organic carbon using Oceanography International Corp. instrument
2 Organic + anmonia nitrogen
3«
3 Phosphorus as PO*
16
-------
100
90
80
70
Q 60
(T
LJ
UJ40
30
20
10
FIGURE I
METER DRIFT VS. TIME
Cu ~ 5mg/l
I I
2530 60 90 120 150 180 210 240
STIRRING TIME (sec.)
17
-------
for a residual chlorine level of 1 mg/1 one series gave low results
and another high results. A cumulative effect was noted in which the
drift increased with each determination.
Iron in concentrations of 5 mg/1 causes a similar loss in sensitivity;
however, no cumulative effect was seen as in the case of copper. Low
recoveries of chlorine were also obtained (approximately 85 percent).
It was concluded that copper and iron did interfere with the chlorine
residual test. Other metal ions present may also interfere. To eli-
minate these interferences a suitable complexing agent was sought which
would form a complexation product that was inert under the chemical
conditions of the amperometric determination. Complexing agents tried
were sodium citrate, NTA, sodium oxalate and sodium pyrophosphate.
Sodium Citrate
A 20 percent solution of sodium citrate was prepared as a stock solution,
Aliquots of this stock solution were added to the titration vessel.
Addition of citrate to a 5 mg/1 copper solution stabilized the drifting
problem. Table 7 shows a data summary of behavior of copper on the
residual chlorine test.
18
-------
TABLE 6
Effect of Cu (II) on Recovery
of Chlorine Residual
Cl Cone.
mg/1
0
0
0
0.98
0.98
0.98
0.98
0.98
0.98
Cu Cone.
mg/1
0.05
0.10
0.15
0.05
0.05
0.10
0.10
0.15
0.15
Residual Chlorine
mg/1
0.01
0.05
0.04
0.96
1.00
0.88
1.07
0.99
1.05
TABLE 7
Effect of Citrate on Copper Interference
Cl Cone. Copper Cone. Cone, of Citrate Residual Cl
mg/1 mg/1 grams/liter mg/1
1 mg/1 0 0 0.99
1 mg/1 5 10 0.99
19
-------
Iron in a concentration of 5 mg/1 gave low recoveries of residual
chlorine. See Table 8.
Iron (Fe) does not seem to form a stable complex sufficient to prevent
interference. The citrate used with the iron and copper solution pro-
duced an incrustation on the platinum electrodes and this was viewed
as undesirable.
NTA
The chlorine demand of NTA (nitrilo triacetate acid sodium salt) was
checked by the addition of 1 gram of NTA to the titration vessel which
contained a chlorine residual of 0.97 mg/1. NTA exhibited a chlorine
demand of 0.06 mg/1. Addition of 5 mg/1 copper and 25 mg/1 iron
yielded the same chlorine demand of 0.06 mg/1.
Sodium Qxalate
Sodium oxalate exhibited the same type of behavior as NTA. A 1 gram
sample added to a 1 mg/1 solution yielded a chlorine demand of 0.06 mg/1.
Sodium Pyrophosphate
Sodium pyrophosphate was used by Mizuno (12) to mask ferric iron in the
determination of ferrous iron. Since a significant concentration of
iron is present in cooling water blowdown, sodium pyrophosphate should
be successful in eliminating the effects of iron on the residual chlorine
determination.
A 1 gram sample of sodium pyrophosphate was added to the titration vessel,
For a 1 mg/1 chlorine spike all of the chlorine was recovered. Addition
of copper (5 mg/1) and iron (25 mg/1) gave an average chlorine residual
of 0.98 mg/1 or a 2 percent lower value. This, however, is better than
any of the previous complexing agents.
20
-------
The pyrophosphate chelating system gave good stable end points with no
drift. Meter sensitivity was down slightly but tolerable due to the
lack of drift. To quantify the amount of pyrophosphate to effectively
complex interfering ions a 10 percent stock solution of Na. PJ3j lOHLO
was prepared as stated in the appendix; 1.4 ml was added to the titration
vessel. This quantity was found to give stable end points for copper
solutions up to 0.5 mg/1. This quantity of pyrophosphate was sufficient
to complex 10 mg/1 of iron by allowing a stable end point. A composite
solution of 0.5 mg/1 Cu (II) and 10 mg/1 Fe (III) with 1.4 ml of pyro-
phosphate gave a slight drift in the end point. A 2 ml portion of
pyrophosphate however, stabilized the end point. The pyrophosphate
system gave no positive chlorine residual analysis in the presence of
copper. The platinum electrodes remained shiny and clean.
EFFECT OF TURBIDITY
Varying amounts of turbidity were simulated by Kaolin suspensions in
varying concentration. Two mis of a 0.10 mg/ml chlorine stock solution
was added to the titration vessel containing 200 ml of the Kaolin sus-
pension. The Kaolin suspension was prepared by adding 1 g of Kaolin
to one liter of water and performing appropriate dilutions of this
stock. Two mi Hi liters of sodium pyrophosphate were added to the
titration vessel. See Table 9.
Good recovery of the residual was obtained in Kaolin concentrations up
to 500 mg/1. The effects of other solid and colloidal suspensions were
examined. Two samples tested were Corvallis clay from the vicinity of
the Oak Creek bank near 35th street in Corvallis and dried bottom
sediment from Cline's Pond. Both sediments were sieved through 120
mesh screens to separate the fine particles for suspension. Each sam-
ple exhibited an initial chlorine demand, i.e., total recovery of the
chlorine spike was not achieved. Additional spikes of chlorine were
made on the system and good recovery was achieved. See Table 10.
21
-------
TABLE 8
Effect of Citrate on Iron Interference
In Determination of Chlorine Residual
Cl Cone.
mg/1
1
1
1
1
Fe Cone.
mg/1
0
0
5
5
Cone, of Citrate
grams/liter
0
5
5
40
Residual Cl
mg/1
0.99
0.98
0.85
0.95
TABLE 9
Recovery of Cl in Presence of Kaolin
mg/1 Kaolin JTU Cl Residual (mg/1)
0 0.96
25 (25) 0.96
100 (86) 0.96
500 0.94
22
-------
WATER TREATMENT
A 100 mg/1 solution of a phosphonate-type water treatment chemical was
prepared. Solutions with no chlorine added gave no positive indication
of a chlorine residual. Solutions of the water treatment chemical were
spiked with known amounts of chlorine. Chlorine residuals were imme-
diately run. The solutions were allowed to stand with a chlorine resi-
dual run at varying intervals. A data summary appears in Table 11.
From the data it is evident that the water treatment chemical does ex-
hibit a chlorine demand. Additional spikes of chlorine to these sam-
ples allowed recovery of greater than 90 percent.
23
-------
TABLE 10
Recovery of Cl Spikes on Cline's Pond Sediment
And Corvallis Clay
Recovery of Cl Spikes
Spike #1 Spike. #2 Spike #3
Sample Initial mg/1 mg/1 mg/1
Cline's Pond Sediment 0.70 0.97 0.97
(100 mg/1) 0.10* 0.47* 0.43*
Corvallis Clay 0.57 0.95 0.97
(500 mg/1)
Chlorine spikes = 0.98 mg/1
*Chlorine spikes = 0.49 mg/1
TABLE 11
Recovery of Chlorine Additions in the Presence
of a Uater Treatment Chemical as a Function of Time
Contact time
0
5
69
(hours)
Initial Cl #1
Concentrations 1.01
Solution #1
(mg/1) Cl
0.66
0.59
0.38
#2 #3
2.52 5.04
Solution #2
(mg/1) Cl
2.18
0.89
0.00
Solution #3
(mg/1) Cl
4.09
3.33
1.46
24
-------
PRECISION
Twelve replicate runs using the procedure outlined in the appendix
using the Wallace & Tiernan Titrator, were made on each concentration
of residual chlorine. An average value of the chlorine residual along
with the standard deviation and relative standard deviation appear in
Table 12. A calibration curve was plotted for the average values of
Table 12. See Figure 2. A least squares analysis was run with a cor-
relation coefficient of 0.9999.
25
-------
TABLE 12
Precision of Titration
Cl spike
mg/1
Average
mg/1
Standard
deviation
mg/1
Relative
Standard
deviation (%}
0.053
0.53
1.06
2.12
0.047
0.52
1.02
2.05
0.0039
0.009
0.008
0.0263
8.3
1.7
0.7
1.28
26
-------
in
CJ
cvi
10
"-^
CVI
LU
II
it
0<
o:
CDLJ
OO
-J
O
O CD (0 *-
* »
cvi
CVJ O 00 CD Tf CM
"N t-9QOO' OVd JO 'M
8
10
N
CP|
27
-------
RECOVERY RATE OF CHLORINE IN SYNTHETIC AND NATURAL BLOWDOWH
Synthetic blowdown water was prepared by slow evaporation of natural
waters on a hot plate. Solutions were allowed to evaporate to an
eighth of their original volume. This allows an 8-cycle concentration
of cations and anions along with organic compounds. Samples chosen for
synthetic blowdown were from the Buffalo River, Cuyahoga River and
Cline's Pond. Chemical analysis of these sources is shown in Table 13.
All samples were spiked with a chlorine stock solution to a residual
chlorine level of 5 mg/1 and were allowed to stand overnight to satisfy
any long-term chlorine demand. A residual chlorine analysis was run on
each sample. Approximately 0.5 mg/1 was added to another aliquot of
the natural samples and an additional residual chlorine analysis was
run. The data are presented in Table 14. The last two entries to
Table 14 are actual blowdown systems.
Cooling Tower I blowdown in the presence of pyrophosphate allowed a
stable end point to be obtained with no positive indication of chlorine
when none was present.
28
-------
TABLE 13
Chemical Analysis of Natural Water Samples
Characteristic Cline's Pond Cuyahoga River
Buffalo River
QIC TOC (mg/1)
Ammonia N (rng/1)
Nitrite N (mg/1)
Nitrate N (mg/1)
Chromium (yg/1)
Copper (yg/i)
Iron (yg/1)
Lead (yg/1)
Manganese (yg/1)
Zinc (yg/1)
Recovery of
5 10
0.018
0.014
0.79
5 10
26 14
2100 900
20 25
33 210
16
TABLE 14
Chlorine Spikes on Slowdown
Total Chlorine
Residual Chlorine Residual
Source of Chlorine Addition After Spike
Sample mg/1 * mg/1 mg/1
Buffalo River 1.15
Cline's Pond 2.05
Cuyahoga River 1.12
Cooling Tower I 1 .60
Cooling Tower II 1.22
0.49 1.60
0.52 2.55
0.52 1.61
0.49 2.08
0.51 1.71
6
43
1300
31
58
Recovery
of
Addi ti on
mg/1
0.45
0.50
0.49
0.48
0.49
*After satisfaction of initial demand and overnight standing.
29
-------
BIAMPEROMETRIC END POINT
An alternate amperometric titrator was sought which might be more field
adaptable. Marrow (13) described a dual, polarizable electrode (D.P.E.)
method for the determination of the end point. Many advantages have
been cited for this method. The first is simplicity of the electrode
configuration. The electrode consists of two platinum sheets mounted
in plastic, see Figure 3. This arrangement eliminates the need for
maintenance of a reference electrode. The power supply for a D.P.E.
method is much simpler in that an opposition voltage need not be ap-
plied to the electrodes to get the end point on scale as required by
a single polarizable electrode. For a D.P.E., the end point of a
reversible electrochemical system titrated with an irreversible titrant
will cause the end point to occur close to zero microamps (14).
EQUIPMENT
A dual platinum electrode system similar to that, described by Marrow
(13) was constructed. See Figure 3.
The dual polarizable electrode system was designed in the shape of a
probe for simplicity. The stirring system used was a La Pine "Porta
Stirrer." This magnetic stirrer has a rechargable battery pack al-
lowing portable operation.
The schematic diagram of the control unit appears in Figure 4. The
value of the shunt resistor R is chosen such that the 10 yamp meter
scale now reads 100 yamps full-scale.
The shunt system is always in the circuit unless the push button switch
is physically depressed. Removal of the shunt allows full meter sen-
30
-------
FIGURE 3
CHLORINE PROBE(DPE)
Top View
(0 0]
Wire To Control Box
"
Platinum f
Foil IE
Y'
[LI
w
"V
r
j
Icn
>
i.
N
=
S
/
t
-
\
3
Probe Made From
6" x 1" Lucite SI
Connection Wire
IP mm
|C_IIIIII
f ~~~?~~
1 10mm
Bottom View
31
-------
FIGURE 4
SCHEMATIC DIAGRAM OF DPE CONTROL BOX
1.35V Mercury (D Size)
i
1KI1 10Turn Pot.
AAAAA/VW
lOjJdmp
Push Button Switched
For ]0=1 Shunt
R is Chosen For
a lOx Shunt
T To Electrode
32
-------
sitivity which permits a more precise detection of the end point. Ex-
treme care must be exercised to insure that the currents are less than
10 yamps before the shunt is bypassed or meter damage will occur.
Thus, knowing that the end point will .occur at zero current, rapid
addition of the titrant until the vicinity of the end point is possible,
speeding up the titration procedure. The stirring mechanism for the
titrator causes less vigorous agitation than that of the Wallace &
Tiernan titrator minimizing the flashing problem. The stirrer can be
of any configuration and may be battery operated to be field adaptable
along with a battery power supply of the D.P.E.
EFFECT OF COPPER AND IRON ON D.P.E. METHOD
The effect of copper and iron in concentration analogous to those found
in cooling tower blowdown was studied. The interference studied was
the effect of varying the potential between the dual indicator elec-
trodes and measuring the background currents. Plots of the relationship
of background currents vs applied voltage appear in Figure 5 and Figure
6. The iron concentration was 10 mg/1 and copper concentration was 0.5
mg/1. The pH was 4.
For the iron interference plot note that at applied voltages of less
than 200 mv the pyrophosphate system caused a reduction in the back-
ground current to the blank level. At higher voltages the pyrophos-
phate system greatly reduced the background currents.
For the copper plots the effect of copper upon the background current
is nil up to about 300 mv. Beyond this the pyrophosphate system re-
duces the background currents of copper although not as dramatic as
the iron curves. Note that as long as the applied voltage is kept
less than 200 mv background contribution is nil. This is an indica-
tion that these heavy metals are electrochemically inactive at these
potentials.
33
-------
FIGURE 5
BACKGROUND CURRENT VS APPLIED VOLTAGE
IRON INTERFERENCE STUDY
4.0
3.0
200 400
APPLIED EMFtm.v.)
600
800
34
-------
FIGURE 6
BACKGROUND CURRENT VS APPLIED VOLTAGE
COPPER INTERFERENCE
2.0
o
o:
o
z
Lul
Cu
Od
O
1.0
.Smg/ICu
.5mg/l Cu-f
Pyrophosphate
:er
Blank
200 400
APPLIED E.M.F (m.v.)
600
800
35
-------
For the determination of the end point in a total chlorine residual
analysis an applied voltage of 150 mv is recommended.
The presence of copper and iron did not exhibit the drifting problems
noted for the Wallace & Tiernan titrator. The meter readings at high
currents (7.5 yamps) showed some instability. This is probably due to
nonconstant stirring. At the end point, however, the meter readings
are very stable.
The recovery rate for both forms of the amperometric end point are the
same and the precision comparable for total residual chlorine. The
biamperometric technique offers an advantage by allowing the end point
to be reached much faster.
36
-------
REFERENCES
1. Standard Methods for the Examination of Hater and Haste Mater,
13th ed., American Public Health Association, New York, 1971
p. 110-142.
2. Annual Book of ASTM Standards, Part 23., American Society for
Testing and Materials, Philadelphia, PA, 1972, p. 222-227.
3. Palin, A. T., Chemistry and Control of Modern Chlorination.
LaMotte Chemical Products Company. Chestertown, Maryland
(1973).
4. Ni col son, N. J., "An evaluation of the Methods for Determining
Residual Chlorine in Hater." The Analyst, 90:187-198, April,
1965.
5. Hasselbarth, V., "Bestimmung von freiem und gebundenem warksamen
Chlor." Zeitschrift fur Analytische Chemie. 234:22 1968.
6. Guter, K. J., Cooper, W. J., "Evaluation of Existing Field Test
Kits for Determining Free Chlorine Residuals in Aqueous Solutions."
Army Medical Environmental Engineering Research Unit. Publication
Number AD752440.
7. Marks, H. C., "Residual Chlorine Analysis in Water and Waste Water."
In: Hater and Water Pollution Handbook. Leonard L. Ciacczo (ed.)
Marcel Dekker, Inc., New York, 1972 (Vol. #3).
8. Standard Methods for the Examination of Wastewater, 13th Edition,
American Public Health Association, New York, 1971, Sec. 114A.
37
-------
9. Ibid, 114B.
10. Lishka, R. J., and McFarren, E. F. "Water Chlorine (Residual)",
No. 2 Report Number 40. Environmental Protection Agency,
Cincinnati, OH, 1971.
11. Marks, H. C. "Determination of Residual Chlorine in Metal Finishing
Wastes." Analytical Chemistry 24:1885-1887. 1952.
12. Mizuno, T., Talanta. 19, 369-72 (1972).
13. Marrow, James J. "Residual-Chlorine Determination with Dual
Polarizable Electrodes." Jour. ANNA 58:363-367. 1966.
14. Stock, John T. Amperometric Titrations. New York Interscience
Publishers, 1965. p. 45-89.
38
-------
APPENDIX
39
-------
AMPEROMETRIC TITRATION PROCEDURE
INSTRUMENTATION
The titrator employed in this report was the Wallace & Tiernan Series
A-790 amperometric titrator. The buret furnished with the unit was
substituted with a 5 ml autofilling microburet with calibration marks
down to .01 ml. The flow of titrant was transported to the titrator
through appropriate tygon tubing and tubing adaptors.
S tandard Meth o ds recommends that the volume of titrant used be less
than 2 ml. For determining chlorine residuals in cooling tower blow-
down within 3 minutes, a larger addition must be tolerated, hence,
the use of a 5 ml buret. If chlorine residuals of greater than 5
mg/1 are anticipated, a smaller sample size should be used, i.e., 100
ml instead of 200 ml. If one desires ease of titration of extremely
low residuals, < .2 mg/1, dilution of the titrant would allow greater
refinement of the equivalence of titrant added to the system. A
separate buret system would be needed to not contaminate the stock
(PAO) solution buret (0.00564N). Appropriate concentration factors
must be applied to the buret readings to correlate the results to
mg/1 residual chlorine.
To facilitate rapid addition of these reagents of the sample vessel,
syringes or autopipets are recommended. For measuring out the 200 ml
samples a graduated cylinder sawed off at the 200 ml calibration mark
makes rapid sample measurement easy. A small lip should be fashioned
to eliminate possible spills when transferring samples to the titration
vessels. A 200 ml aliquot of sample is automatically measured by dip-
ping the cylinder into the water sample, thus filling the cylinder to
overflowing.
40
-------
The contents of the cylinder are carefully poured into the titration
vessel without spilling. Between runs, the titration vessel should be
rinsed with chlorine-free water. The sample vessel need not be dried
however, since the graduated cylinder contains the active chlorine
residual. All that is important is that a consistent amount of sample
water is added to the vessel. Dilution by water adhering to the walls
has no effect.
The titration vessel supplied with the Wallace & Tiernan titrator should
be adequateprovided that a differentiation of free and combined resi-
dual is not made on the same sample. If the titrant volume is kept low,
sample overflow should not be a problem.
A suitable pH meter is required to check the buffering capacity of the
water sample and to check the proper amount of buffering agent required
to bring the sample plus reagents to the appropriate pH.
REAGENTS
All reagents were prepared as in Standard Methods 114.B, 13th (1)
edition.
The standard phenylarsine oxide was purchased from Wallace & Tiernan.
Sodium Pyrophosphate
Dissolve 100 grams of sodium pyrophosphate Na, PpO^lOhLO in 1 liter
of water heated to 50°C. Place mixture on a mechanical stirring device
to bring the salt into solution. Allow the solution to cool. Add 85
percent phosphoric acid to the solution until a pH of 6 is obtained.
For total residual chlorine analysis a low pH of the pyrophosphate
chelate is desirable as not to overwhelm the pH4 buffer system.
41
-------
TITRATION
1. Preliminary Work.
The electrode surface should be sensitized to iodine in the following
procedure:
a. Fill the titration vessel with distilled water.
b. Add 1 ml of 5% KI., 1 ml pH4 Buffer.
c. Add sufficient hypochlorite to cause a yellow to
yellow-brown coloring of the solution. Place
vessel on titrator and allow to stir for 10
minutes.
d. After the time limit direct a stream of distilled
water from a polyethylene wash bottle around the
electrode to remove as much iodine as possible.
e. Fill titration vessel with distilled water and
place on titrator and add with stirring .5 ml of
titrant. This will remove any traces of Ip present.
f. Remove titration vessel and rinse electrodes
thoroughly by directing a stream of distilled
water around the electrode. The titration vessel
containing the sample can be used as a catch for
the rinse water.
While the titrator is stirring in the sensitizing step a 200 ml sample
of water can be obtained with the specially prepared graduated cylinder
as noted in the equipment section. The purpose of this sample is to
42
-------
determine the amount of buffering needed to drop the pH to 4. To the
water sample add 1 ml KI, 2 ml of the 10% pyrophosphate solution. Place
pH probe into sample and add increments of pH4 buffer until pH is in the
range of 4 to 4.2. Add approximately 0.25 ml in excess to compensate
for variations in the buffering capacity of the natural water. Two mis
of a 10 percent pyrophosphate solution should be sufficient for 10 mg/1
iron and .5 mg/1 copper. If metal ions are significantly higher or
lower, adjustment of the amount of pyrophosphate reagent should be made.
A criterion for the proper amount of pyrophosphate is the stability of
the end point (No drift).
If the meter deflections become erratic, this is due to salt build-up
around the electrode sockets. To cure this, the electrode and the front
faceplate must be removed and the salt scrapped away. A stream of dis-
tilled water can be directed at the socket holes to remove the remaining
salt. The entire socket area should then be wiped dry to prevent any
possible leakage currents to flow.
2. Titration Procedure.
During the actual titration run, all steps should be performed with
both speed and good laboratory technique. All reagent syringes should
be calibrated to deliver the proper amount of reagents. Practice with
the sampling graduate should be done such that sampling is rapid and
without spills. Past experience with the system in question would aid
in the prediction of the amount of titrant required to reach 90 percent
of the end point to eliminate loss due to agitation. Sunlight should
be kept to a minimum and the titration done in subdued light.
Syringes, calibrated for proper delivery of appropriate reagents, should
be placed into the opened bottles to act as lids. The potassium iodide
solution should be stored in a brown bottle. Fresh KI should always be
43
-------
used. If any trace of coloration is present, the solution should be
discarded. (If there is any question about the KI, a titration should
be performed on distilled water to which the proper amount of KI was
added. If a positive indication is noted, fresh KI should be substituted.)
The actual titration procedure is similar to that in Standard Methods.
(9, 10) Modification to the procedure involves the preparation and
use of an additional reagent and addition of .1 large increment of
titrant before the agitation starts. The procedure is as follows:
a. Obtain a 200 ml sample in the precalibrated graduated
cylinder and pour without spilling into the titration
vessel rinsed with distilled water.
b. Using the precalibrated syringes add proper amounts of
KI, pH4 buffer and pyrophosphate solution. One ml of
KI solution is sufficient.
c. Place titration vessel on the amperometric titrator and,
if feasible, add approximately 90 percent of the final
titrant volume. Start stirring action.
d. Continue the addition of titrant until the needle no
longer deflects left. After the first titration the
scale sensitivity control should be adjusted until
the meter reads approximately 20. For subsequent
titrations the end point should appear approximately
at the same reading. Early in the titration the
meter will be off scale. Rapid additions of the
titrant can be made until the meter reads on scale.
e. Read the buret for the concentration of the chlorine
residual.
44
-------
BIAMPEROMETRIC TITRATION
The equipment required is a biamperometric probe, control box and mag-
netic stirring device. These pieces of equipment are described in the
main body of this report.
Experimental Procedure Biamperometric End Point
a. The sample is obtained in the same manner as in the
previous procedure. The sample vessel, in this case
is a 400 ml beaker. (Electrodes need not be sensitized
to iodine.)
b. The necessary reagents are added as described earlier.
c. A magnetic stirring bar is added to the beaker and the
beaker placed on the magnetic stirring device.
d. The power is turned on the control box. This can remain
on until the end of the sampling period.
e. The electrode probe is lowered into the solution and the
stirring action started.
f. Addition of titrant is done rapidly until the scale reads
less than 10 yamps. At this point the X10 button is
depressed and titrant added rapidly until the meter reads
close to 0 microamps. Titrant is added in small increments
until the meter no longer deflects left. The last increment
is then subtracted.
45
------- |