EPA-600/4-76-033
October 1976
Environmental Monitoring Series
EVALUATION OF A PROTOTYPE INSTRUMENT FOR
DETERMINING PHOSPHORUS IN WATER
Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Athens, Georgia 30601
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. 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
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL MONITORING series.
This series describes research conducted to develop new or improved methods
and instrumentation for the identification and quantification of environmental
pollutants at the lowest conceivably significant concentrations. It also includes
studies to determine the ambient concentrations of pollutants in the environment
and/or the variance of pollutants as a function of time or meteorological factors.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/4-76-033
October 1976
EVALUATION OF A PROTOTYPE INSTRUMENT
FOR DETERMINING PHOSPHORUS IN WATER
by
Thomas B. Hoover
J. MacArthur Long
Environmental Research Laboratory
Athens, Georgia 30601
ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
ATHENS, GEORGIA 30601
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DISCLAIMER
This report has been reviewed by the Athens Environmental
Research Laboratory, U.S. Environmental Protection Agency, and
approved for publication. Mention of trade names or commercial
products does not constitute endorsement or recommendation for
use.
ii
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ABSTRACT
A second-generation flame spectrometer for the direct
determination of phosphorus in water was evaluated. Response to
phosphorus in the form of phosphoric acid was linear from 0.5 to
16 ppm phosphorus. The relative standard deviation was
approximately constant at 20 percent over the range.
River water and municipal sewage effluent were analyzed after
the addition of phosphoric acid (1.8 ppm P) and filtration
through a series of microporous membranes. Recovery of the
added phosphorus averaged 70 percent for the river water and 95
percent for the sewage effluent after treatment with cation
exchange resin. There was no clear relation to filter pore size
in the range 5 to 0.2 micrometers. Analyses of the higher range
EPA Nutrient Reference Samples (approximately 0.5 ppm P) agreed
within one standard deviation with the reference values, both
for inorganic and total phosphorus. The lower concentration
range samples (approximately 0.1 ppm P) gave barely detectable
signals.
Suggestions are given for further development of the instrument.
This report was submitted in fulfillment of ROAP 16ADN, Task 72
by the Environmental Research Laboratory, Athens, Georgia, under
the sponsorship of the Environmental Protection Agency. Work
was completed as of October 1975.
111
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CONTENTS
Sections Page
I Conclusions 1
II Recommendations 2
III Introduction 3
IV Experimenta1 4
V Discussion 16
VI References 18
v
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FIGURES
No. Page
1. Background Response as a Function of Air/Hydrogen 6
Ratios
2. Response to 1.8 ppm P (As Phosphoric Acid) as a 7
Function of Air Inlet Rate
3. Characteristic Response to Phosphoric Acid 9
4. Calibration Curve for Phosphoric Acid 10
5. Effect of Adding Cations 12
TABLES
No. Page
1. Analyses of Natural and Waste Water Filtered 14
Through Microporous Membranes.
2. Analysis of EPA Nutrient Reference Samples. 15
VI
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SECTION I
CONCLUSIONS
Dissolved phosphorus in water, in either organic or inorganic
forms, was determined in the range 0.5 to 16 milligrams
phosphorus per liter.
The relative standard deviation of a determination was about 20%
over the entire analytical range.
No particle size effects, in the range 0.2 to 5 micrometers,
were detectable either in river water or in municipal sewage
effluent.
The instrument requires considerably more modification to
achieve convenient, safe, and reliable operation.
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SECTION II
RECOMMENDATIONS
The following mechanical modifications should be made:
• installation of a fail-safe flame indicator to shut off
hydrogen flow when the burner goes out,
• positive disposal of excess hydrogen from the vent,
• more direct introduction of the water sample,
• easily changeable photoemission tube, and
• output signal logarithmically related to the emission
intensity.
Because of problems inherent in the "cool-flame" emission
method, it is not certain that the instrument can be made to
respond to total phosphorus in water at environmentally
significant concentrations (0.01 to 1.0 mg P/l).
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SECTION III
INTRODUCTION
Eutrophication of fresh water lakes and streams deteriorates the
quality of the environment and has been ascribed to increased
inputs of plant nutrients through human activities. Phosphorus
seems most often to be a critical nutrient in this process and
can be controlled in the treatment of municipal wastes.
Consequently, there is increased emphasis on phosphorus removal
in waste treatment and a greater need for a rapid, simple,
analytical method for monitoring phosphorus concentrations
during such processes.
This report evaluates a second-generation prototype instrument
for determining phosphorus in water samples by flame photometry.
The phosphorus hydride oxide radical (HPO) is formed and excited
in a cool, hydrogen-rich flame. The chemiluminescent decay of
the radical emits green light at 525 nm, which is isolated by an
interference filter and measured by a phototube. Construction
(1) and evaluation (2) of the original prototype have been
reported.
The present instrument was very similar in principle of
operation to the original model (3). The entire sample system
(sample inlet, burner, emission tube) was operated below ambient
pressure by an aspirator driven by a compressed air supply.
This feature prevented the inadvertent escape of hydrogen
through leaks, although the final exhaust, containing unburned
hydrogen, was vented to the room.
A major modification was provision for passing the entire
nebulized sample through the burner. Previously, a large excess
of aerosol was directed at the face of the instrument, a portion
being drawn into the burner by the reduced pressure created by
the aspirator. That procedure was considered unacceptable for
use with pathogen-containing water specimens because of the
health risk to the operator from the excess aerosol discharged
to the room.. In the present model a separately controlled air
stream passed through the ultrasonic nebulizer containing the
sample and the resulting aerosol was directed into the burner
inlet, together with some additional room air.
The optical and electronic components were modified to provide
greater freedom from light leaks and to improve the stability of
the amplifier output.
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SECTION IV
EXPERIMENTAL
MODIFICATIONS OF INSTRUMENT
Several changes in the flow controls were made immediately. As
supplied, the instrument had a micrometer-scaled needle valve
control, but no flow meter, in the air line driving the
aspirator; a branch line from the air compressor supplied
cooling air to the phototube housing and contained a flowmeter
with a control valve at the inlet. The hydrogen fuel flow was
also controlled by the inlet valve of its flowmeter. The
manufacturer's recommendation for setting flow rates included
setting the needle-valve fully open and adjusting the aspirator
air indirectly through control of the cooling air.
Since hydrogen flow rate is one of the most critical factors in
the burner operation, the needle valve was moved to the hydrogen
line and a flow meter was placed in the aspirator line for
direct measurement and control. All flow meters had scale
calibrations in standard liters (or cubic centimeters) of air
per minute and were not recalibrated or corrected.
Consequently, the actual flow rates of hydrogen were
considerably greater than the nominal "air" values. The flow
rate for a given meter reading is approximately inversely
proportional to the density of the fluid (H). Consequently,
hydrogen flow rates were approximately four times the rate
indicated for air. Meter readings were used as relative
measures for empirical adjustment of the burner operation.
Optimum burner design is critical to the success of the
instrument and was a major consideration in the original
contract (1). Since the detection limit of the second
generation prototype was poorer than that of the original, some
additional investigations were made. Conflicting requirements
must be compromised: a high flame temperature favors
evaporation of the aerosol droplets and decomposition of the
phosphorus compounds, while a low temperature (5,6) and excess
hydrogen are necessary to excite the chemiluminescence. Among
the modifications tried were:
• sample introduction to an air-rich flame, with secondary
hydrogen admitted just above the orifice.
• axial introduction of the aerosol, with a surrounding
cylindrical jet of hydrogen.
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• auxiliary cooling air blowing on the emission tube.
Some qualitative observations were made with a glass Liebig
condenser, in which the entire flame could be seen. These
suggested that a major function of the orifice is to cool the
flame. None of the modifications substantially improved the
operation of the burner and it was restored to the
manufacturer's configuration, placing the tip of the hydrogen
capillary about 5 mm above the burner orifice.
The tip of the orifice tended to become quite hot, especially
when the burner was first lighted and operated with excess air.
The temperature was sufficient to decompose the silicone rubber
gasket sealing the top of the emission tube, resulting in air
leaks that quenched the chemiluminescence and in clouding the
emission tube with silica. The gasket and emission tube were
replaced before further quantitative tests. The inside of the
phototube housing was painted flat black to reduce the effect of
scattered light, and the passage between the emission tube and
aspirator was enlarged to 1/4" I.B. to reduce blockage by
condensed moisture (2).
TESTS
Opti mi z ati on
Burner conditions were optimized by systematically varying
hydrogen and inlet air flow rates in order to reduce the
background signal when no sample was present, and to maximize
the net signal for a fixed phosphorus concentration. Figure 1
shows the background signal for various ratios of inlet air to
hydrogen, at two hydrogen flow rates. It will be recalled that
the nominal hydrogen flow rates, and the corresponding ratios,
are based on meter readings of standard cc of air per minute.
The true hydrogen volumetric flow rate is larger by a factor of
approximately four, and the ratio is smaller by the same factor.
Figure 1 shows a sharp increase in background at a nominal ratio
about 1.5. Figure 2 shows the net response to 1.8 ppm P (as
phosphoric acid) as a function of inlet air rate. The sample
was introduced with a carrier air flow rate of 0.28 1/min, about
the maximum for stable flame operation. Figure 2 indicate a
broad maximum net response at an inlet air rate of 0.15 1/min
and hydrogen rate of 300 cc/min, corresponding to a ratio of
1.5. These conditions were adopted for subsequent tests.
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-0.8
-0.6
Ul
to
Z
o
-0.4
— 0.2
rel.
H.
4
I
NOMINAL RATIO
FIGURE 1. Background Response as a Function of Air/Hydrogen Ratios
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1—0.6
o
o
Of
9
'^i
t-0.4
1—0.2
V
0.20_|
oo
•D
O
0.101
05_J
i
o
INLET AIR(|/min)
FIGURE 2. Response to 1.8 ppm P (as Phosphoric Acid) as a
Function of Inlet Air Rate
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Calibration
A series of calibrations of response vs. phosphorus
concentration as H3 POi, was made. Tracings of the characteristic
responses to distilled water, phosphoric acid, and air are shown
in Figure 3. In contrast to a previous report (2) , a small
positive signal was observed for distilled water despite
rigorous cleaning of the sample cup, inlet and burner orifice.
This response was checked frequently during calibrations and
measurements of test solutions and was subtracted as a blank.
The noise level of the recorder response to water and H3PO^ is
clearly much greater in Figure 3 than that of the background
air-hydrogen flame. This noise, or fluctuation in signal, which
probably resulted from individual liquid droplets in the flame,
provided an estimate of variance associated with each
measurement. The extreme range of pen excursions on the chart
record was squared and divided by twice the width of the record
in millimeters, corresponding to a response time equivalent to
0.5 mm chart travel. These estimates of variance for any
calibration series were extremely nonhomogeneous, as estimated
by the chi-square test. Dividing the individual variance
estimates by the square of the mean observation, which is
equivalent to postulating a constant relative standard
deviation, greatly reduced the nonhomogeneity. (For example, x2
reduced from 186 to 66, for 15 degrees of freedom.) In none of
three calibrations could more than two-thirds of the
observations be considered as coming from a homogeneous
population. Nevertheless, each measurement was weighted
inversely by its estimated variance in computing a least-squares
calibration curve, one of which is shown in Figure 4. The
pooled, weighted calibration curve had the equation
Y = (0.012 ± 0.017) + (0.0743 ± 0.0044)X (1)
where Y is the instrument response, in microamperes, to X ppm
phosphorus, as phosphoric acid. The plus-minus values are
standard deviations of the coefficients. The calibrations were
all made at the same five nominal values of X, with four
randomized replications in each series, so that the variance and
relative standard deviation could be calculated for each value
of the abscissa. The average relative standard deviation for
the five points was 20 percent, while the pooled relative
standard deviation for all points, calculated from the noise
level, was 16 percent.
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U«M*
DisHlled Phosphoric Air
Water Acid
0.5 ppm P
FIGURE 3. Characteristic Response to Phosphoric Acid
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10
I
15
PHOSPHORUS (ppmj
FIGURE 4. Calibration Curve for Phosphoric Acid
10
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Cation Interferences
Additions of potassium, calcium, and magnesium, as the
chlorides, were made to solutions of phosphoric acid, containing
9.0 ppm Phosphorus. The results, shown in Figure 5, confirm the
findings of Seitz (2) that Ca and Mg markedly inhibited the
response to inorganic phosphate. Potassium had an insignificant
effect.
Particle Size Effects
The previous inhouse study (2) presented evidence that fine
particulate matter in river water reduced the detection of added
soluble phosphate by the flame detector. It was inferred that
soluble phosphate was closely associated with the particulates
and no longer capable of excitation in the low temperature
flame. To investigate this matter further, both Oconee River
water and effluent from the Athens municipal sewage plant were
filtered through a series of microporous filters. At each stage
of filtration a water sample was analyzed directly and after
addition of a 1.8 ppm spike of phosphoric acid. The analyses
were performed both directly and after a batch treatment with
Amberlite IR-120 cation exchange resin to remove possible
interfering cations. Deionized water blanks were run with each
series.
The detailed procedure was the following: the water was
filtered through course grade analytical filter paper (S&S No.
588). Two portions of filtrate were taken and one was spiked
with 1.8 ppm phosphorus, as phosphoric acid. Each portion was
again divided into two portions, one being analyzed directly.
The other portions, both of original filtrate and of the spiked
sample, were each treated with approximately 1 g ion exchange
resin per 10 ml of water by stirring for several minutes. The
water was decanted into the sample cup of the analyzer. Most of
the filtrate was then filtered through a 5-ym Nuclepore and the
filtrate was analyzed as above. The remaining filtrate from the
5-ym filter was passed through a 1-ym Nuclepore, and
successively, a 0.45-ym Metricel, and a 0.2-ym Metricel. At
each stage four portions of filtrate were analyzed as described.
For each analysis, four measurements of instrument response were
recorded, with intervening records of the air background signal.
The net response above background was corrected by the
corresponding value for deionized water and averaged over the
four readings, weighting each by the reciprocal of its variance.
The corresponding concentration was determined from the
11
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I
K
MOL RATIO
FIGURE 5. Effect of Adding Cations
12
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calibration curve (Equation 1). The fractional recovery of the
spike was calculated from
R = 2 C2/(3.6 + Ci) (2)
where C i and C 2 are the concentrations in ppm of the original
(unspiked) and spiked sample, respectively. The volume of added
spike was equal to the sample volume. Thus, the expected final
concentration, for complete recovery, was
C2 = 1.8 + Cx/2 (3)
Results are shown in Table 1. The ± entries represent estimated
standard deviations, calculated by propagation-of-error formulas
from the variances of the individual chart records. These were
estimated as described under Calibration. It will be observed
from Table 1 that recoveries from sewage effluent were
essentially complete, provided that ion-exchange treatment was
used. On the other hand, recoveries were low (about 70 percent)
in the river water, regardless of ion-exchange treatment. In
neither series was there any clear relation to the pore-size of
the filters.
EPA Nutrient Reference Sample
EPA Nutrient Reference Standards were diluted as directed with
deionized water and analyzed directly. Also, standard additions
of 1.8 ppm P (as phosphoric acid) were made to each solution.
No filtration or ion exchange treatment was used. Each analysis
is the average of four aspirations with intermediate checks of
background, but no replicate solutions were run.
In the fourth and fifth columns of Table 2, concentrations were
calculated from the calibration curve and, in column five,
corrected for the standard addition. In the sixth column the
concentrations were calculated by the method of standard
additions, without reference to the calibration curve.
By reference to the nominal values, in the third column of Table
2, it appears that the standard addition method of calibration
is more reliable than the use of the calibration curve at these
low levels, which are close to the detection limit. The error
limits shown in the table were calculated from the noise level
of the response, as described previously, and do not seem
indicative of the accuracy of the results. There is no
significant difference between the inorganic and organic forms
of phosphorus.
13
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Table 1. ANALYSES OF NATURAL AND WASTE WATER FILTERED THROUGH MICROPOROUS MEMBRANES
Pore
Size
Filtered
Filtered
& Resin-
Treated
C,
After
Spike
(1.8 ppm P)
Resin-
treated
After Spike
C2
Percent Recovery of Spike
(Equation 2)
Resin-
Filtered treated
Coarse
Coarse
5
1
0.45
0.2
0.2
0.76
0.36
0.74
0.74
0.72
0.68
0.27
0.82
0.37
0.70
0.78
0.68
0.73
0.25
Oconee River Water
1.75
1,
1,
1.
1.
1.
09
54
48
64
63
1,
1,
1,
1,
1,
85
20
1.76
49
62
38
80 ±
55 ±
71 ±
68
8
9
8
± 8
1.04
1.37
76 ± 8
76 ± 8
54 ± 8
84 ± 8
60 ± 9
82 ± 10
68
76
64
8
8
8
71 ± 11
Coarse
5
1
0.45
0.2
2.31
2.23
1.93
1.97
2.47
Municipal Sewage Effluent
3.43
3.11
3.16
3.09
3.30
2,
2,
1,
2,
08
10
97
04
3
3,
3
2,
98
03
00
95
2.05
3.22
70 ± 8
72 ± 8
71 ± 9
73 ± 8
68 ± 8
113 ± 11
90 ± 8
89 ± 9
88 + 9
93 ± 9
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Table 2. ANALYSIS OF EPA NUTRIENT REFERENCE SAMPLES
Sample
Type
Nominal
Phosphorus Concentration (ppm)
Direct
Calibration
Spike
Calibration
Standard
Addition
Inorganic
Inorganic
(Undiluted)
Organic
(Undiluted)
Organic
0.021
0.393
78.6
0.142
28.4
0.713
0.25 ± 0.07
0.62 ± 0.08
39.2 ± 2.1
0.28 ± 0.07
20.6 ± 1.1
0.76 ± 0.09
0.52 ± 0.03 0.014 ± 0.005
0.21 ± 0.01 0.46 ± 0.04
0.59 ± 0.03
0.48 ± 0.02
0.04 ± 0.03
0.61 ± 0.08
15
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SECTION V
DISCUSSION
This and preceding studies of the flame emission phosphorus
analyzer show that it does respond to low levels of organic and
inorganic phosphorus in water, but with limitations. The
response to inorganic phosphate is strongly inhibited by calcium
and magnesium and, presumably, by other cations that form
insoluble phosphates. These soluble ions can be removed by ion
exchange treatment. River water was shown to contain an
interference that was not removed by ion exchange or by
filtration through 0.2-rym micropores.
A major drawback of the system appears to be the inability of
the low temperature flame to excite all the phosphorus in
particulate or insoluble forms. The same phenomenon probably is
responsible for the noise level for aerosols of soluble
phosphorus samples and the roughly constant relative standard
deviation for such measurements. For a constant relative error,
a logarithmic output would provide more practical (and equally
accurate) means of covering the analytical range than the range-
switching technique used with linear output in the present model.
Several safety features should be incorporated in any future
development of the instrument. At present, excess hydrogen from
the burner is exhausted to the room. Although the flow of
hydrogen is not large, it might be well to provide an
afterburner to dispose of the excess.
With the use of hydrogen fuel, some sort of fail-safe device
should be provided in case of flame-outs. In the original
prototype, a thermocouple flame sensor activated an igniter when
the flame went out, but operation was not always reliable. The
thermocouple sensor might better be used to activate a solenoid
shut-off valve in the hydrogen supply line. Alternatively,
hydrogen might be generated electrolytically under the control
of the flame sensor.
A major reason for the poorer sensitivity of the present model,
as compared to the earlier prototype, is certainly due to
modification of the sample aerosol introduction. In the present
model all the aerosol is fed to the burner and this results in
less sample reaching the flame than when a large excess of
aerosol was directed against the face of the instrument in the
original design. The modification was made to prevent
dispersion of pathogens to the area around the instrument when,
-«*Y
16
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for example, raw sewage samples are analyzed. The sample
aerosolizing apparatus is still physically independent of the
burner, and therefore, not foolproof. It should preferably be
built into the instrument so that all aerosol is completely
confined or sterilized in the flame.
17
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SECTION VI
REFERENCES
1. Prager, M.J. "Automated water Monitoring Instrument for
Phosphorus contents." Environmental Protection Agency,
Washington, DC. Report EPA-R4-73-026. June, 1973. 25 p.
2. Seitz, W.R. "Evaluation of Flame Emission Determination of
Phosphorus in Water." Environmental Protection Agency,
Corvallis, OR. Report EPA-660/2-73-007. August, 1973. 18
P.
3. Kim, C.H. "Automated Water Monitoring Instrument for
Phosphorus Contents." NUCOR Corporation, Denville, NJ.
Unpublished. Final Report on Contract No. 68-03-0236.
4. Perry, R.H., C.H. Chilton, and S.D. Kirkpatrick, eds.
Chemical Engineers1 Handbook. Fourth edition. New York,
McGraw-Hill Book Co., 1963. p. 5-13.
5. Syty, A. Determination of Phosphorus in Phosphate Rock by
Cool Flame Emission Spectrometry. At. Absorption
Newsletter JJ2:1-2, January-February, 1973.
6. Dagnail, R.M., K.C. Thompson, and T.S. West. Molecular-
Emission Spectroscopy in Cool Flames. Analyst (London)
93:72-78, February 1968.
18
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/4-76-033
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
EVALUATION OF A PROTOTYPE INSTRUMENT FOR
DETERMINING PHOSPHORUS IN WATER
5. REPORT DATE
October 1976 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Thomas B. Hoover
J. MacArthur Long
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORG "\NIZATION NAME AND ADDRESS
Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Athens, Georgia 30601
10. PROGRAM ELEMENT NO.
1BA027; ROAP 16ADN, Task 72
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
Final
Same as above
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A second-generation flame spectrometer for the direct determination
of phosphorus in water was evaluated. Response to phosphorus in the
form of phosphoric acid was linear from 0.5 to 16 ppm phosphorus. The
relative standard deviation was approximately constant at 20 percent
over the range.
River water and municipal sewage effluent were analyzed after
the addition of phosphoric acid (1.8 ppm P) and filtration through a
series of microporous membranes. Recovery of the added phosphorus
averaged 70 percent for the river water and 95 percent for the sewage
effluent after treatment with cation exchange resin. There was no
clear relation to filter pore size in the range 5 to 0.2 micrometers.
Analyses of the higher range EPA Nutrient Reference Samples
(approximately 0.5 ppm P) agreed within one standard deviation with
the reference values, both for inorganic and total phosphorus. The
lower concentration range samples (approximately 0.1 ppm P) gave barely
detectable signals.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
*Phosphorus, *Spectrometers, *Flame
Photometry, *Emission, *Inorganic
Compounds
Flame Emission
07B
8. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)'
UNCLASSIFIED
21. NO. OF PAGES
25
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
19
U.S. GOVERNMENT PRINTING OFFICE:
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