Determination of Phosphate in Natural Waters by Activation Analysis of Tungstophosphoric Acid Herbert E. Allen
U S . Bureau of Commercial Fisheries, Biological Laboratory, Ann Arhor. Mich. 48107
Richard B. Hahn Department of Chemistry, Wayne State University, Detroit, Mich. 48202
Activation analysis may be used to determine quantitatively traces of phosphate in natural waters. Methods based on the reaction 31P(n,y)a2Pare subject to interference by sulfur and chlorine which give rise to 32Pthrough n,p and n,a reactions. If the ratio of phosphorus to sulfur or chlorine is small, as it is in most natural waters. accurate analyses by these methods are difficult to achieve. In the activation analysis method, molybdate and tungstate ions are added to samples containing phosphate ion to form tungstomolybdophosphoric acid. The complex is extracted with 2,6-dimethyl4-heptanone. After activation of an aliquot of the organic phase for 1 hour at a flux of 10l3 neutrons per cm.2 per second, the gamma spectrum is essentially that of tungsten187. The induced activity is proportional to the concentration of phosphate in the sample. A test of the method showed i t to give accurate results at concentrations of 4 to at least 200 p.p.b. of phosphorus when an aliquot of 100 pl. was activated. By suitable reagent purification. counting for longer times. and activation of larger aliquots. the detection limit could hc lowered several hundredfold.
P
hosphorus can be determined by neutron activation, accompanied by a chemical separation of the induced 32Pand counting of the 1.701 Mev beta particle of the isotope. This procedure has been applied to natural waters by Blanchard and Leddicotte (1959) and Wayman (1964). One milligram of sulfur gives rise to phosphorus-32 equivalent to 55 l g . of phosphorus (Bowen and Cawse, 1963) due to the fast neutron reaction 32S(n,p)32P. Since the weight ratio of sulfur to phosphorus in some natural waters-the Great Lakes, for example-is greater than 100 (Beeton and Chandler, 1963). phosphorus-32 arising from phosphorus alone would be less than 15% of the total activity. The interference of sulfur can be corrected by determining the difference in activity of cadmium shielded and unshielded samples. A large error arises from the correction for 32P produced from 32Swhen the ratio of sulfur to phosphorus is high and an additional correction must be made for the interference arising from the reaction 35Cl(n,a)32P(Ricci, 1964; Wayman, 1964). The activation analysis of phosphorus by 32Ptherefore has three disadvantages : Emission of beta particles only necessitates a postactivation chemical separation before counting; since 32P has ii long half-life (14.3 days), the sample requires a long activation to produce a sufficient quantity of the radioisotope; and, since sulfur and chlorine give rise to 32P,corrections mu5i be made to minimize their interferences. In the present report these disadvantages have been avoided. To eliminate the necessity for postactivation separation, thc phosphate is complexed as mixed tungstomolybdophosphoric acid which is extracted and activated, and the induced tungsten-1 87 is determined by gamma-ray spectroscopy. Tung844 Environmental Science & Technology
sten-187 has a half-life of 1 day and lSSWhas an activation cross section of 34 barns. In comparison, 32Phas a half-life 14 times greater than lS7Wand 31P has a cross section 200 times less than lgEW. Therefore, a much shorter activation period yields sufficient activity for an analysis if tungsten is used. Because the activity of 32Pis not determined, sulfur and chlorine no longer constitute interferences. Mei/iocls
Samples were irradiated in-pool for 1 hour in an empty fuel clement position in the Ford Nuclear Reactor of the University of Michigan’s Michigan Memorial-Phoenix Project. The thermal neutron flux in this position was approximately 1 X l O I 3 neutrons per cm.?per second. Samples were counted 6 hours after irradiation to permit decay of short half-life isotopes. Gamma activity was measured with a multichannel analyzer equipped with a 3- by 3-inch thallium-activated sodium iodide crystal. Reagent grade chemicals were used to prepare all solutions. Ammonium molybdate and sodium tungstate solutions were stored in polyethylene bottles. The solvent, 2,6-dimethyl-4heptanone, was used because of its low solubility in water and because it is satisfactory for the extraction of phosphomolybdic acid (Wadelin and Mellon, 1953). All-glass apparatus was used to redistill the solvent which was then stored in polyethylene bottles. The redistillation reduced the activity of the 1.38 Mev peak of sodium-24 from 300 to 40 counts per minute per ml. of solvent. Preparation of Samples. After preliminary experimentation the following procedure was developed. To a 100-ml. sample, 4 ml. of sodium tungstate (1.80 grams Na2W04.2H20 per liter, 1 mg. tungsten per ml.), 10 ml. of 7 . 5 z ammonium molybdate (NH4)6Moi0?4.4H20(40.8 mg. Mo per ml.), and 15 ml. of 5 7 x nitric acid were added. The sample was extracted with 3 ml.of solvent, by shaking for at least 1 minute. The organic phase was extracted twice with 15 ml. of 3% nitric acid. A 100-p1. aliquot of the organic phase was transferred to a 4-cm. length of polyethylene tubing (Intratnedic PE-360) which had been heat-sealed at one end. The solvent was removed by vacuum distillation to prevent leakage of the sample due to decomposition of the solvent and subsequent gas evolution. The tubes were placed in snapcap vials which were heat sealed to prevent contamination of the tubing with pool water. Effect of Tungstate Concentration. Preliminary experiments involving complexation of phosphate with analytical reagentgrade ammonium molybdate, extraction, and activation demonstrated that the reagent contained an impurity of tungsten-187. Figure 1 shows a gamma-ray spectrum of a typical complex and a spectrum of a tungsten standard. Since molybdenum and tungsten are both in periodic group V I B, it is not surprising that the ammonium molybdate was contaminated with tungstate. Activation analysis detects much smaller amounts of tungsten than molybdenum (Table
(I. In addition to the greater activation sensitivity of tungsten over molybdenum, the tungsten also is preferentiall) incorporated in the complex. The weight ratio of molybdenum to tungsten in the ammonium molybdate reagent was 4150-to-1, but in the phosphate complex it was 340-to-1, which reprcsented a twelvefold increase of the tungsten-to-molybdenum ratio in the complex relative to the original aqueous solution. Addition of tungsten to the sample was therefore expectcd to increase the sensitivity of the procedure. Various amounts of tungsten were added to samplca containing 10 pg. of phosphorus and 408 mg. of molybdenum. The activity of tungsten-1 87 was proportional to the amount of tungstate added (Figure 2). Tungstate was not added in amounts greater than 5 mg. because higher concentrations often caused precipitation of tungstic acid. Identification of Tungsten. To confirm the presence of ttingsten in the complex, samples of the complex were activated along with sodium tungstate standards and their gamniaspectra were recorded. Figure 1 shows that all peaks present in the sodium tungstate spectra (the upper curve) were present in the complex's spectra. These peaks include the 60
20
v)
-
e
r
I O
3
7
kev Re Kx-ray peak which overlaps a weak 72 kev photopeak, and photopeaks a t 134, 480, 686, and 775 kev. Additional peaks were apparent a t 619 kev, due, in part, to the stiiii effect of the 480 and 134 kev photopeaks; a t 522 kev, also due to the combination of a photopeak a t that energy and i t sum effect of the 480 and 72 kev peaks; and at 206 kev--a sum peak due to the photopeaks at 134 and 72 kev. All thesc peaks are due to tungsten-1 87 (Crouthamel, 1960). To confirm the identity of tungsten-187 further, the halflives of the major peaks in the gamma-ray spectrum were measured. A sample (prepared by the procedure described previously) containing 12.5 pg. of phosphorus was mounted in the analyzer and was not removed during the period in which measurements were made. The calculated half-life of tungsten-187 based on the photopeaks a t 60, 134, 480, and 686 kev was 24.0, 22.8, 23.7, and 22.6 hours, respectively (Figure 3). The accepted value for the half-life is 24.0 hours (Crouthamel, 1960). It was therefore established that thc gamma-ray spectrum of the activated complex was due to thc presence of tungsten-187. Effect of Acid Concentration. To determine the effect of acidity on the formation and extraction of phosphotungstic acid, samples containing 50% nitric acid were prepared. Seventeen milliliters of 5 0 x HNOa is equivalent to the 15 nil. of 57% " O B used in the final procedure. A 9 % deviation relative to the activity of samples prepared with 17 ml. of 5 0 % H N 0 3 was computed for samples prepared with 16 nil. of H N 0 3 . A smaller deviation was computed for the siirnples prepared with 18 nil. of acid. Although the influence of pH is relatively great, the pH of the sample can easily bc adjusted. Effect of Molybdate Concentration. Although the molybdate concentration was chosen on the basis of a published analytical method (Proctor and Hood, 1954) for the formation of the phosphomolybdate complex, the effect of molybdate concentration on the formation of ttingstomolybdophosphoric acid wit5 investigated. Various amounts of molybdenum were
L
O v
21 c
5
.-
>
.-
e 0
a
2
a VI
O 3
I
f
Energy ( k e v )
Figure 1. Gamma-ray spectra of sodium tungstate solution (upper curve) and tungstomolybdophosphoric acid (lower curve) Table I. Physical Constants of Tungsten and Molybdenum Isotopes Important for Activation Analysis Thermal Halt-Lire Neutron Of Cross Z+I Fractional Section, Isotope, Isotope Abundance Barns Hours Moly bdenum-9 8 0.238 0.13 67 Tungsten-186 0.287 36 23
--
.-
I 5 t 480 k e v
0
4
' 5
O
2.0
mg
W
per
K
4.0
Sample
Figure 2. Relation between activity of tungsten-187 and amount of tungsten added to samples containing 10 pg. of phosphorus Voluine 3, Number 9, September 1969 845
added to samples containing 12.5 pg. of phosphorus and 4.0 mg. of tungsten. The data presented in Figure 4 indicate that approximately 200 mg. of molybdenum per sample are required for maximum tungsten incorporation into the complex. The authors prepared samples in the conventional manner, using 32P as a tracer and varying the concentration of molybdate. Constant amounts of 32Pwere extracted except in the absence of added molybdate where no phosphate was extracted. The composition of the complex was determined for a sample containing 12.5 pg. phosphorus and the standard amounts of molybdate and tungstate. The mole ratio of tungsten to phosphorus in the sample was 1.18-to-1. Order of Reagent Addition. Molybdate, tungstate, and acid were added in various orders to standard phosphate samples. The activity of tungsten was similar in all samples except for those in which the order was acid, tungstate, and molybdate. Activity with this order was about 6 9 z as great as that in the other samples. Standard Phosphate Curves. All standard phosphate curves obtained by the procedure outlined above indicate a linear relationship between phosphate concentration and the activity of tungsten-187. Typical curves for the four gamma peaks are shown in Figure 5 . Correlation coefficients based on any of the peaks range from 0.98 to 1.00. Because of variations in neutron flux, counting efficiency, and length of the cooling period from one series of analyses to another, a n absolute value for the slope of the standard curve cannot be given. Approximate values for the slope in counts per minute per pg. of phosphorus for the photopeaks at 60, 134, 480, and 686 kev were 17,000, 2,700. 2,900, and 2,600, respectively, when 100 p l . of the organic phase were used for activation. The sensitivity can be increased tenfold by activating 1 ml. of the organic phase rather than 100 111. or by counting for 10 minutes rather than 1. The high value of the reagent blank limits the sensitivity of this method. For a number of standard curves the blank
1.0
0.5
U
0 *I
'= 0.2 E
C
>
0.1
I-
;1.0 k
u a 0.5
0.2
0.1
H O U R S
Figure 3.
Decay of photopeaks at 60, 134, 480, and 686 kev.
846 Environmental Science & Technology
averaged 0.23 pg. of phosphorus but in one series the value rose to 1.3 pg. We assume that this blank is caused by the phosphorus in the reagents and water used in the standards. If the phosphate in the blank arises from impure reagents, the activity of the blank should be subtracted from both standards and samples in calculating the concentration of phosphorus in natural waters. If, however, the phosphorus in the blank is due to the demineralized water, the activity of the blank should be subtracted only from the standards, and the samples should be corrected only for the fraction of their volume which represents demineralized water. Purification of the acid by redistillation and of the water by redistillation from acidified molybdate should reduce the blank. If these precautions are taken, the detection limit of 1000 counts should be attainable for 4 X 10-9 grams of phosphorus in the original sample, if 1 ml. of the organic phase is activated and counted for 10 minutes. Effect of Type of Acid. To facilitate use of the described method, it was desirable to employ analytical reagent-grade chemicals without further purification. Of all the reagents required in the procedures, we suspected that the acid would be most likely to be contaminated with phosphorus. To determine if another common acid might be less contaminated than nitric acid, samples were prepared with nitric, hydrochloric, sulfuric, and perchloric acids. The amount of phosphorus corresponding to the intercepts based on the standard curves of the 60, 134, 480, and 686 kev gamma peaks were computed for each of the acids. The averages of these values follow: nitric acid, 0.16 pg.; perchloric acid, 0.48 pg.; sulfuric acid, 2.0 pg.; and hydrochloric acid, 7.2 pg. phosphorus per sample. The values indicate that nitric acid was the least contaminated with phosphorus. The slopes of the standard curves were approximately the same when nitric or perchloric acids were used to acidify the samples but were less than with sulfuric or hydrochloric acids. Reproducibility of the Procedure. The coefficient of variation in various steps of the method was computed for samples containing 12.5 pg. phosphorus (Table 11). Three aliquots each of four samples were activated and counted in rotation until each sample had been counted six times. Since the coefficient of variation is an additive statistic, the relative importance of the errors may be considered. The largest error was due to aliquoting of the organic phase (with a microsyringe). Relatively small errors were introduced in sample preparation and the positioning of the sample in the analyzer. The latter error is due to the slight change in counting geometry among samples. Inherent counting error cannot be lessened. The total error, + 3 . 8 9 z could be reduced by determining the weight rather than volume of sample. This improvement would reduce the error -1.84z but would require additional time in sample preparation. Extraction Efficiency. The solvent used in the extraction efficiency study, 2,6-dimethyl-4-heptanone, was chosen because of its high extraction efficiency for phosphomolybdic acid and its favorable physical properties. It was necessary to evaluate the extraction efficiency to determine the ratio of tungsten to phosphorus in the complex. The extraction efficiency was determined by complexing and extracting solutions containing phosphorus-32 and inert phosphate by the standard procedure. One milliliter of the organic phase was transferred to a planchet and the sample was evaporated to dryness by the regular vacuum distillation procedure; the sample activity was measured with an end-window Geiger tube. The extraction efficiencies for 2.5 to 15.0 pg. of phosphorus averaged 37.7x and had a standard deviation of +2.4%.
Figure 4. Relation between activity of tungsten-187 (60 kev peak) and amount of molybdenum added to samples containing 12.5 pg. of phosphorus and 40 mg. of tungsten
I
100
I
I
I
I
I
200
300
400
500
600
700
Mo (mg) 500
I
60 kev 400
i /. 4’
300
-
200
-
v)
c
3 c
8
100
w-
0 v)
-u
s c
Figure 5. Relation between activity of tungsten-187 and the concentration of phosphorus. Samples contained 408 mg. of Mo and 4 mg. of tungsten
v)
3 0
f
Y
c .5 .c
8
s g P/liter
Analysis of Phosphorus in Natural Waters. To test the accuracy of the method when applied to samples of natural waters, the phosphorus content of two water samples was analyzed colorimetrically and by activation analysis. One sample was from the municipal water supply of Ann Arbor, Mich., and the other was from a well in Ann Arbor. The procedure of Proctor and Hood (1954) was used for the colorimetric analysis. To correct for interfering radioisotopes produced by activation of the natural water samples, blanks were activated which contained no added molybdate or tungstate. The activity of these blanks was subtracted from the activity of the corresponding tungsten-187 peaks of the samples. For the municipal water the phosphorus concentration was 14.5
Table 11. Coefficients of Variation of Various Steps in the Activation Analysis Procedure for Phosphorus Source Kev Av. of all of error 60 134 480 686 Gamma Peaks Counting Analyzer Geometry Aliquoting Sample Preparation Total
0 . 4 4 1.56
1.20
1.22
1.10
0 . 8 1 0.95 -0.41 1.40 1.06 2.77
-0.04 2.96
0.33 2.05
0.85
0.41 3.89
0 . 2 7 0.78
-0.26
Volume 3, Number 9, September 1969 847
p.p.b. by the colorimetric method and 14.0 p.p.b. by activation analysis. The well water contained 10.0 p.p.b. P according to the colorimetric analysis and 11.4 p.p.b. P by activation analysis. Conclusions The preceding data show that low concentrations of phosphate in natural water may be determined by activation analysis. With the addition of molybdate and tungstate ions to samples containing phosphate, phosphotungstic acid is formed. Approximately 38 of the complex is extracted when 125 ml. of aqueous phase is extracted with 3 ml. of 2,6-dimethyl-4-heptanone. After activation of an aliquot of the organic phase, the gamma spectrum is essentially that of tungsten-187. The induced activity is proportional to the concentration of phosphate in the sample. The method was tested and gave accurate results at concentrations from 4 to at least 200 p.p.b. of phosphorus. By reagent purification, activation of larger samples, and counting for longer times, the detection limit could be lowered several hundredfold. Acknowledgment The assistance of John D. Jones, Robert D. Martin, and the staff of the Michigan Memorial-Phoenix Project for mak-
ing laboratory space and reactor time available acknowledged.
IS
gratefully
Literature Cited Beeton, A. M., Chandler, D. C., in “Limnology in North America,” D. C. Fre). Ed., p. 540, University of Wisconsin Press, Madison, Wis., 1963. Blanchard, R. L., Leddicotte, G. W., USAEC Report ORNL2610, Oak Ridge National Laboratory, pp. 22-23, 43-49, 1959. Bowen, H. J. M., Cawse, P. A., British Report AERE-R/4309, p. 22, May 1963. Crouthamel, C. E., “Applied Gamma-ray Spectrometry,” p. 237, Pergamon Press, New York, 1960. Proctor, C. M., Hood, D. W., J . Mar. Res. 13,122 (1954). Ricci, E., in “Guide to Activation Analysis,” W. S. Lyon, Jr., Ed., p. 127, Van Nostrand, Princeton, N. J., 1964. Wadelin, C., Mellon, M. C., Anal. Chem. 25, 1668 (1953). Wayman, C. H., Anal. Chem. 36, 665 (1964). Receiced for reciew Julji 25, 1968. Accepted May 12, 1969. Portions of this paper were presented at the 155th National Meeting, ACS, Dicision of Water, Air, and Waste Chemistry, San Francisco, April 1968. Taken in part from the M.S. thesis of Herbert E. Allen, W a j x e State University, Detroit, Michigan (August 1967). Rejerence to trade name does not imply endorsement of the product by the Bureau of Commercial Fisheries. Contribution No. 394 of the Ann Arbor Biological Laboratory, U.S . Bureau of Commercial Fisheries.
Volumetric Measurement of Ultraviolet Energy in an Urban Atmosphere John S. Nader and Norman White National Air Pollution Control Administration, U. S. Department of Health, Education, and Welfare, Cincinnati, Ohio 45227
w This report describes a physical sensor developed to measure all of the UV (300 to 380 nm.) radiation incident on a volume in the atmosphere. Preliminary data is presented for Los Angeles during the smog season with the volumetric and the horizontal-plate sensors. For the smoggy atmosphere the volumetric values ranged from about 2.6 to 3.3 times those of the horizontal-plate sensor; for a clean atmosphere the range was from 2.0 to 3.2. The ratio of volumetric to the vertically incident radiation measurements showed an increasing value with increasing solar zenith angle, with a peak in the vicinity of 70 degrees. A linear line of regression is given for the volumetric radiation as a function of the vertically incident radiation for intensities below 30 w./m. The same relationship appears to be applicable for both polluted and unpolluted atmospheres.
T
he literature contains many reports on measurements of ultraviolet radiation energy (UV) by different methods and under a variety of environmental conditions to meet specified objectives (Bener, 1960; Green, 1966; Nader, 1967; Robinson, 1966; Stair, 1966). Most of these reported data that were obtained using sensors that measured either the combined direct-sun and scattered-sky radiation incident on a horizontal plane or the direct-sun radiation incident normal to the surface of a horizontal plane. However, a volume of reacting gases in the atmosphere is subject to UV irradiation from all directions. These data therefore give information on only part of the total energy available for photochemical reactions. In answer to the indicated need of a volumetric sensor of UV radiation, Lindh, Buchberg, et al. 848 Environmental Science & Technology
(1 964) developed an “omni-directional ultraviolet radiometer’’ in support of some controlled experimental air pollution studies on photochemical reactions at University of California at Los Angeles. No data, however, were reported on values of available UV energy that may have been measured in the urban atmosphere. This report describes an ultraviolet instrument developed to measure all the UV (300 to 380 nm.) radiation that is incident on a volume in the atmosphere, presents preliminary data collected in Los Angeles during the smog season with both volumetric and horizontal-plate sensors, presents comparative data for an urban atmosphere during nonsmoggy or relatively clean days, and discusses experimental results in comparison with predicted data based on calculations made from theoretical considerations of a model atmosphere.
Instrumentation A volumetric sensor was developed for the National Air Pollution Control Administration (NAPCA) by the Eppley Laboratories. The sensor is basically a cubical arrangement of six horizontal-plate sensors (Figure 1 less the quartz hemisphere) of the Eppley Laboratory design (Nader, 1967). Each of the six faces of the cube consists of a square aluminum plate (30-cm. X 30-cm.) with a horizontal-plate sensor located in the center. The sensor’s circular quartz diffuser (4.5-cm. diameter) is in the plane of the aluminum plate with the body of the sensor contained within the cube’s volume. The cubical configuration has the same geometry as that used by Lindh, Buchberg, et al. (1964) although the dimensions, mounting arrangements, and sensor design differed. Their sensors used two Corning 7-37 (colored) filters with a photovoltaic detector. The Eppley sensors used a quartz diffuser with an interference filter and a photovoltaic detector. Of
