Determination of Phosphate in Water with Tungsten=185 Richard B. Hahn and Thomas M. Schmitt Department of Chemistry, Wayne State University, Detroit, Mich.48202
MANYMETHODS for the determination of inorganic phosphates involve the reaction of molybdate ion with phosphate ion to form molybdophosphoric acid. The amount of molybdophosphate formed is determined by gravimetric or spectrophotometric methods (1). Because molybdophosphoric acid is soluble in various organic solvents, an extraction step is often added to concentrate low levels of phosphate. If the molybdophosphate complex is reduced to the heteropoly blue before being determined spectrophotometrically, accurate measurements can be made down to 10 ppb of phosphate (2). Indirect instrumental methods have been developed which measure the amount of molybdenum extracted from the aqueous phase in order to determine phosphate. A recent example is the atomic absorption method of Zaugg and Knox (3). Previous work in this laboratory has shown when tungstate ion is added to a solution of molybdophosphoric acid, it will become part of the molybdophosphate complex in such a manner that the amount of tungsten extracted with the heteropoly complex is proportional to the amount of the heteropoly present, and hence proportional to the phosphate concentration ( 4 ) . In light of this, it was decided to develop a method for determining phosphate in water samples employing radioactive tracer by forming the 185-tungstomolybdophosphoric acid complex, extracting it into an organic solvent, and then counting the extracted tungsten activity and relating the activity to the original phosphate concentration. Tungsten185, the radioisotope employed, emits a beta of 0.430 MeV energy and decays with a half life of 75.8 days. A description of this method follows. EXPERIMENTAL Reagents. MOLYBDIC ACID SOLUTION.Forty grams of reagent grade ammonium molybdate was dissolved in 500 ml of distilled water, 127 ml of concentrated nitric acid was added, and the solution was diluted to 1 liter. This gave a nitric acid concentration of 2N. PHOSPHATE STANDARD SOLUTIONSwere prepared from reagent grade KH?P04dissolved and diluted to appropriate concentrations with distilled water. T U N G S T E N -was ~ ~ ~obtained from Oak Ridge National Laboratory and diluted to a working concentration of 0.1 pCijl00 pl (the normal spiking volume). ~-OCTANONE (B.P. 171-3 “C) was obtained from Matheson, Coleman and Bell and used without further purification. Apparatus. Samples were counted in a 2-pi gas-flow proportional counter Model PCC-1OA with a Decade Scaler Model DS-1A, both manufactured by Nuclear Measurements. Q-gas (98.7z, He, 1 . 3 z butane) was used at a (1) I. M. Kolthoff, P. J. Elving, and E. B. Sandell, “Treatise on Analytical Chemistry,” Part 11, Vol. 5, Interscience, New York, 1961. (2) American Public Health Association, “Standard Methods for the Examination of Water and Wastewater,” 11th ed., New York, 1960, pp 198-204 (3) W S. Zaugg and R. J. Knox, ANAL.CHEM., 38, 1759 (1966). (4) H. E. Allen, “Determination of Phosphate in Natural Waters by Activation Analysis of Phosphotungstic Acid,” Masters Th., Wayne State University, Detroit, Mich., 1967
counting voltage of 1150 V. The planchets used were of 2 X 1/8-inch stainless steel. The following procedure was developed: A 100-ml water sample of near-neutral pH was placed in a 125-ml separatory funnel. Five milliliters of molybdic acid solution and 100 pl of 185W tracer were added and the solution was then shaken for 1 min. Three milliliters of 2-octanone were added, the mixture was shaken for 1 min, then the layers were allowed to separate for 20 min. The aqueous layer was drawn off and discarded. The entire organic phase was transferred into a planchet. The planchet was carefully evaporated to dryness under a heat lamp and counted in a proportional counter. Phosphate concentration was then read from a calibration curve. The above procedure was tested by adding known amounts of phosphate ion to 100-ml samples of distilled water. The results are presented in Table I together with results from a standard spectrophotometric method (2). DISCUSSION
A plot of phosphate concentration us. extracted ls5W activity was found to be linear over the region studied. The lower limit of detection is determined by the blank reading which was found to be quite high, most probably caused by the phosphate present in the reagents and also from the finite solubility of water (containing le5Wtracer solution) in 2-octanone. Experiments showed that approximately 1% of the total added ls5Wactivity is extracted into the octanone. The limit of detection is about 10 ppb phosphate. The average deviation for a single determination is 4 %. Because the day-to-day reproducibility of readings is poor, standards should be run with each day’s determinations to adjust the calibration curve. In this respect, this method is no improvement over the standard spectrophotometric method (2). Several methods of counting the 185W activity were investigated. At the ppm level of phosphate, G-M counting gave good results. If evaporation of the 2-octanone is undesirable, the samples can be successfully counted with somewhat impaired efficiency at the ppm level in a well-type sodium iodide, gamma scintillation counter. Best counting efficiency was obtained with a gas-flow proportional counter with Q gas. Liquid scintillation counting gave comparable results, but was not used in obtaining the data presented here. Many solvents have been used to extract molybdophosphoric acid from aqueous solutions. Among these are vari-
Table I.
Comparison of the Method with the Standard Spectrophotometric Method
Phosphate reported Phosphate by Phosphate spectroadded, photometric pprn method, ppm Error, pprn 0.00 0.25 0.50
0.02 0.17 0.46
+0.02 -0.08 -0.04
by
ISSW
method, ppm
Error, pprn
0.00 0.26 0.54
0 +0.01 +O. 04
VOL. 41,NO. 2, FEBRUARY 1969
359
Table 11. Effect of Varying Activity of lSsWAdded Activity added (cpm) Activity extracted (cpm) 23,300 978 46,600 1516 69,900 2578 93,200 2975 117,000 3597 140,000 4732 163,000 543 1 186,000 5845 210,000 6219 233 ,ooO 7303 7835 256, 000 8678 280, 000 Slope = 0.0297 count extracted/count added.
ous butanol-ether mixtures (3, ethyl acetoacetate (6),ethyl acetate (7), butyl acetate (8), 2-methyl-1-propanol (9), 3methyl-1-butanol (IO), 1-butanol in chloroform ( I I ) , and 1octanol(I2). All of these were investigated and it was found that the most complete extraction, combined with the smallest extraction of interferences, and the smallest volume of organic reagent needed was obtained with 2-octanone, which has a very low solubility in water. (This consideration is important because it is desirable to count the entire organic phase.) With this extractant, efficiency of extraction as measured with a2Pwas about 85 %’ in the ppb phosphate range, using 3 ml of 2-octanone per 100 ml of water sample. Thus, the distribution coefficient is of the order of 200 for tungstomolybdophosphoric acid. (5) C. Wadelin and M. Mellon, ANAL.CHEM., 25, 1668 (1953). (6) K. Z. Stoll, ibid.,11, 81 (1938). (7) J. Hure and T. Ortis, Bull. SOC.Chim. Fr., 1949,834. (8) T.Koto, et a/., TecRiio/.Repts. Tohoku Unia. 15 (l), 70 (1950). (9) C. Sideris, ANAL.CHEM., 14,762 (1942). (10)C. Rainbow, Nature, 157,268 (1946). (1 1) K. T. H. Farrer and S. J . Muir, A m . Chem. Zizst. J. and Proc., 11, 222 (1944). (12) F. L. Schaffer et al., ANAL.CHEM., 25, 343 (1953).
The pH was kept in the range 0.9-1.2 which was found to be optimum for the formation and extraction of molybdophosphoric acid into 2-octanone. The exact nature of the tungstomolybdophosphoric complex is uncertain. In the procedure, the molybdophosphoric acid is formed first and 185Wis then added. A study of the effect of the amount of tracer added on the activity of the extracted complex was made and the data are presented in Table 11. These data show that the amount of la5W activity in the final sample varies with the amount of lSsW tracer added in a continuous, linear manner. The slope of the line is 0.0297. Evidently, molybdenum atoms (or molybdate ions) are replaced by tungsten (or tungstate) in a continuous manner. Considering the various steps involved in the deterrnination, several parameters are critical, The concentrations of molybdate and tungsten must be uniform for all samples because the extent to which the displacement reaction occurs is dependent on these concentrations. Another consideration is the contamination of reagents with phosphate, which affects the results even more markedly if the amounts of reagents are vaned. The extraction step must be carried out with care, and volumes of aqueous and organic phases must be kept uniform because a difference of 0.1 ml of organic extractant will affect the extraction efficiency by almost 2%’. The length of time that the phases are allowed to separate before they are drawn off must be kept uniform for good results. The method was used successfully for phosphate concentrations ranging from 10 ppb to 100 ppm. Substances which normally interfere in phosphate determination are atoms which form heteropoly complexes with molybdate similar to molybdophosphoric acid. These are Ge, As, Si, and Nb and, with the exception of Ge, it was found that they interfered in this method to about the same extent as they do in the common spectrophotometric methods. NO interference from Ge was observed. RECEIVED for review July 11, 1968. Accepted November 21, 1968.
Spectrophotometric Determination of Primary Aromatic Amines with 9-Chloroacridine James T. Stewart, Terry D. Shaw, and Anthony B. R a y Department of Medicinal Chemistry, School of Pharmacy, The University of Georgia, Athens, Ga. 30601 THEREACTION of organic amines with 9-chloroacridine to give highly colored 9-aminoacridine hydrochlorides has been reported by Burckhalter et al. ( I , 2). There are, however, no reported examples of the quantitative determination of amines with 9-chloroacridine. In this paper we describe a new spectrophotometric method for determining small quantities of some primary aromatic amines with 9-chloroacridine. The sensitivity of the method applied to certain amines rivals that of the commonly used diazotization-coupling procedures. EXPERIMENTAL
Apparatus. Spectra and absorbance measurements were made with a Perkin-Elmer Spectrophotometer, Model 202, (1) J. H. Burckhalter et a / . ,U. S. Patent 2428355 (1947). (2) J. H. Burckhalter et a / . ,U. S. Patent 2419200(1947). 360
ANALYTICAL CHEMISTRY
and a Beckman Spectrophotometer, Model DU. Matched cells with a 1-cm optical path were used. Reagents and Chemicals. Aniline, p-nitroaniline, p-bromoaniline, 2-nitro-4-methoxyaniline, o-aminophenol, o-phenylenediamine, p-aminophenol hydrochloride, and 9-chloroacridine, all Eastman grade, along with p-methoxyaniline and p-aminophenol, both practical grade, were obtained from Eastman Kodak Co. 2,4-Dimethoxy-5-chloroanilinewas obtained from Pfister Chemical Works, Ridgefield, N. J. and p-aminophenylmercaptoacetic acid was obtained from Evans Chemetics, Inc., Waterloo, N. Y . All other chemicals used were the highest grade of the commercially available materials. Fresh solutions (10-6 mole/ml) were prepared daily by dissolving weighed amounts of the amines in ethanol. Solutions of 9-chloroacridine (10-6 mole/ml) were prepared immediately before use by dissolving weighed amounts in ethanol.
