V O L U M E 22, NO. 9, S E P T E M B E R 1 9 5 0
1177 minimized. T o determine the possible extent of such interference, sulfur dioxide was metered into the mixing tube, and the scrubber sampled the air-sulfur dioxide mixture for 15-minute periods using distilled water as the solvent. The sulfur dioxide content of the resulting solution was determined by oxidizing the sulfite to sulfate, which was then determined turbidimetrically. The results are presented in Figure 5. The over-all efficiency varied from 1 to 9% as the sulfur dioxide concentradon was decreased from about 7 to 0.6 mg. per cubic meter. Probably high efficiencies for sulfur dioxide, if desired, could be obtained by using sodium hydroxide solutions in the scrubber. ACKNOWLEDGMENT
0
I
2070 at a concentration of 0.2 mg. per cubic meter of air. However, the recoveries averaged about 100% when 0.1 %, ’ sulfuric mid was used. The results are plotted in Figure 4. The Venturi scrubber would be expected to be inefficient for volatile substances, such as ammonia, unless some substance is present in the scrubber solution to convert these materials to a nonvolatile form. For a very small increment of time at the beginning of the sampling period the scrubber is efficient. During this time the scrubber solution consists essentially of pure water. -4s the scrubber operates, however, the solution rapidly absorbs the gas, until in a short time equilibrium conditions exist and the vapor pressure of the volatile substance in solution equals the partial pressure of this substance in the air. Obviously, gas collection by the scrubber from that time on would be zero. Thus the over-all efficiency of the scrubber will decrease with increasing lengths of runs. The decreasing efficiency with increasing con(bentration must result from deviations from Henry’s law. Sulfur Dioxide. Finally, because sulfur dioxide is often found along with sulfur trioxide in the atmosphere, the efficiency of the w u b b e r for collecting this gas was studied. It was expected that when pure water was used in the scrubber the efficiency for sulfur dioxide collection would be low, as was the cp,se in ammonia sampling. This would be an advantage when using the scrubber to collect sulfuric acid, inasmuch as interference from the dioxide, which in dilute solution oxidizes rapidly to sulfuric acid, would be
Acknowledgment is due the Smoke and Fumes Committee of the Western Oil and Gas Association, whose financial support and encouragement for the study of smog in Los Angeles made this work possible. LITERATURE CITED
(1) Am. Pub. Health .4ssoc., “Ventilation and atmospheric poliu-. tion. 11. Report of Subcommittee on Chemical Methods in
.4ir Analysis. Sampling and Sampling Devices,” Am. Pub. Health Assoc. Year Book, pp. 92-7, 1939-40. (2) Anthony, A. W., Jr., “Two Methods of Wet Scrubbing of Gasea for Reduction of Atmospheric Pollution,” Proc. Smoke Prevention Assoc. .4m., 26 pp., 1948. (3) Jacobs, M. B., “Analytical Chemistry of Industrial Poisons, Hazards, and Solvents, pp. 289-90, New York, Interscience Pub., Inc., 1941. ( 4 ) Johnstone, H. F., and Roberts, H. M., Ind. Eng. C h m . , 41. 241723 (1949).
Jones, W. P., Ibid., 41, 2424-7 (1949). (6) Magill, P. L., Am. Znd. Hug. Assoc. Q w r t . , 11, 55-64 (March (5)
1950). (7) (8) (9) (10)
(11) (12)
Magill, P. L., Proceedings of First National Air Pollution Symposium, Stanford Research Institute, Pasadena, Calif., Nov. 10 and 1 1 , 1 9 4 9 , p. 61. Moskowite, Samuel, Siegel, Jac, and Burke, W. J., N. Y . State Dept. Labor, I d . Bull., 19, 33-5 (1940). Sheen, R. T., Kahler, H. L., and Ross, E. M., IND.ENG.CHEW, ANAL.ED.,7, 262-5 (1935). Silverman, Leslie, Proceedings of First National Air’ Pollution Symposium, Stanford Research Institute, Pasadena, Calif., Nov. 10, and 11, 1949, p. 55. “Stanford Research Institute, “Smog Problem in Los -4ngeles County,” Second Interim Report, 1949. Western Precipitation Co., Los .4ngelc.s. Tech. Bull. 3-C: pp. 3-4.
R E C E I V E DMarch 21; 1950.
Spectrophotometric Determination of Phosphorus in Organic Phosphates W. R. SIMMONS’ A N D J. H. ROBERTSON, University of Tennessee, Knoxuille, Tenn. The molybdivanadophosphoric acid procedure for the determination of orthophosphate is adapted to the analysis of aliphatic phosphates after hydriodic acid conversion and to both aliphatic and aromatic phosphates after conversion by catalytic oxidation.
I
N A previous paper (6) the authors reported that hydriodic acid conversion of aliphatic phosphates yields colorless solutions, whereas similar treatment of aromatic phosphates gives solutions of varying color from pale green to pale orange; and that conversion of both aliphatic and aromatic phosphates by sulfuric-nitric-perchloric acid oxidation in the presence of a molybdenum catalyst produces slightly greenish colored solu1 Present address, Experiment Station, Hercules Powder Company, Wilmington,’Del.
tions. .4lthough the solutions obtained by hydriodic acid conversion of aliphatics appear to be colorless, when viewed through a long column they show a faint yellowish cast. B e c u s e of the importance of the molybdivanadophosphoric acid colorimetric method of determining orthophosphate, reported by M i e son (6) and modified by Kitson and Mellon ( 4 ) , i t appeared desirable to establish the degree of applicability of the method to organic phosphates decomposed by hydriodic acid or by catalytic oxidation. The present paper describes this study.
ANALYTICAL CHEMISTRY
1178 The residual color after hydriodic acid conversion of aromatic phosphates is stable, requiring vigorous oxidation for complete removal. The addition of small amounts of ammonium persulfate, bromine water, perchloric acid, hydrogen peroxide, or potassium permanganate to the boiling solution has little effect in reducing the color. Evaporation to fumes with perchloric acid destroys the color in the case of tri-p-cresyl and tri-+cresyl phosphates, but the meta derivative requires evaporation to dryness. From the outset, therefore, it appeared doubtful whether the molybdivanadophosphoric acid method would fmd convenient adaptation to hydriodic acid conversion in the analysis of aromatic phosphates. The residual color after conversion of organic phosphates by catalytic oxidation is due to the presence of molybdenum which is found not to interfere in the procedure herein described for the determination of orthophosphates. Gibbs (9) reported that molybdiphosphoric acids form readily when soluble phoaphates and molybdates are brought into solution together. The green color present after catalytic oxidation is attributed to some heteropoly acid of phosphate and molybdate, the exact nature of which is dependent upon the pH and the relative concentrations of phosphate and molybdate (8). Kitsoil and Mellon ( 4 ) found that unless too much ammonium ion is prevent the addition of ammonium vanadate solution readily converh molybdiphosphoric acids to the heteropoly molybdivanadophosphoric acid. This observation was confirmed in the present investigation. The necessity for adding vanadate M o r e molybdate for the complete conversion of phosphate to molyhdivanadophosphoric acid exists only under conditions favorable for the formation of ammonium molybdiphosphate which is not readily converted to the desired complex upon the addition of vanadate. Bolin and Stambery (1)reported that the presence of a small amount of molybdenum does not interfere in the analysis of various agricultural materials by the molybdivanadophosphoric acid procedure using a Cenco photelometer with a 420 mp filter. In the present study measurements were made 011 a Beckman Model DU spectrophotometer a t 460 mp, the wave length used by Kitson and Mellon ( 4 ) . The acidities of the solutions in the present study were ztppmximately 0.6 S. SOLUTIONS
diluted to the mark, shaken, and allowed to stand for 10 minutes. The photoelectric spectrophotometer was adjusted to read 100% transmittance for a blank containin 5 ml. of each of solutions A, C , D, and E, diluted to 50 ml. %he results were plotted on semilogarithmic paper as per cent transmittance against concentration of phosphorus in milligrams per 50 ml. The straight line provided a standard graph for reference of all analyses made on this instrument with the same .reagents a t the same dilution. A second series of ten standards was similarly prepared, using twice the volume of all reagents with the exception of solution A, of which only 5 ml. was used, and diluting to the mark in 100-ml. volumetric flasks. These standards were for use in making measurements on those phosphate solutions which were diluted to 1W ml. to keep the concentration of phcsphorus less than 1.0 mg. per 50 ml., the highest concentration a t which measurements were to be made. For both series of dilutions the slope was found to be -0.4989. The weight of phosphorus in 50 ml. of solution is given by the expression Mg. of P in 50 ml. =
2
- log T m
0.4989 where 2' is the per cent transmittance. Preparation of Standard Graphs for Catalytic Oxidation. Two graphs were prepared as described above, except that solution B w&s substituted for solution A. The slopes of these graphs were the same and identical with those for hydriodic acid conversion within the limits of experimental error. Measurement of Hydriodic Acid Conversion Samples. Fivemilliliter aliquots of samples of organic phosphates decomposed by hydriodic acid conversion were transferred from their 250-ml. volumetric flasks to those used for color development. Aliquoh containing less than 1.0 mg. of phosphorus were transferred to 50-ml. volumetric flasks and aliquots containing from 1.0 to 2.0 mg. of phosphorus were transferred to 100-ml. volumetric flasks. For the 50-ml. dilutions, 5 ml. each of solutions C , D, and E were added in the order named, and the solution was made up to the mark and allowed to stand 10 minutes. For the blank, 5 ml. each of the solutions A , C, D, and E were similarly mixed, diluted, and allowed to stand 10 minutes. The per cent transmittance was measured and the concentration of phosphorus was calculated from Equation 1. For the 1OO-ml. dilutions, 10 ml. each of solutions C, D, and E were added, and the final solution was made up to the mark and allowed to stand 10 minutes. For the blank, 5 ml. of solution A and 10 ml. each of solutions C, D, and E mere similarly diluted. The per cent transmittance was measured and the concentration of phosphorus was calculated. The percentage of phosphorus pentoxide in the unknown samples was cdeulated as follows: Dilution volume mg. of P in 50 ml. X 11.456 50 ml. (2) % "05 = grams of unknown sample
A. Blank for Hydriodic Acid Conversion. Thirty milliliters of reagent grade hydriodic acid, specific gravity 1.7, 55 and 58%, mg. of P in 50 ml. X 22.912 were diluted to 100 ml.,,decomposed by the cautious addition of (3) ml' % "05 = grams of unknown sample 30 ml. of concentrated nitric acid, diluted somewhat, boiled down to 100 ml. to remove all iodine, transferred to a 250-ml. volumetric flask, and diluted to the mark. B. Blank for Catalytic Oxidation. A mixture of 5 ml. of catalytic oxidation reagent (6), 10 ml. Table I. Comparison of M e t h o d s f o r D e t e r m i n a t i o n of P h o s p h o r u s of concentrated nitric acid, and 2 ml. of conceni n Organic P h o s p h a t e s trated perchloric acid was evaporated to fumes of ~ h ~ Hydriodic ~ ~ Acid~Conversion i ~ ~Catalytic l Oxidation perchloric acid, transferred to a 250-ml. voluPtOr AlkaliSpectroAlkaliSpectrometric flask, and diluted to the mark. Equivalents, metric photometric metric photometric C. Nitric Acid, 1 to 2. Compound or Mixture % titration procedure titration procedure D. Ammonium Vanadate, 0.25%. T w o and PzOr Found, % ~._ oiie half grams of ammonium vanadate were dis56.79 55.7 56.51 55.7 Hexaethyl tetra 56.1 solved in 500 ml. of warm water, the solution was 55.53 56.8 55.13 54.7 Hexaethyl tetra 56.1 55.87 56.4 56.01 55.0 Monoethyl 56.3 cooled, 20 ml. of concentrated nitric acid were 57.66 57.2 57.38 56.2 Monoethyl 56.3 added, and the mixture was diluted to 1 liter. 49.45 48.4 49.13 48.9 48.6 Tetraethylb yrobase E. Ammonium Molybdate, 5%. Fifty grams 17.74 17.4 17.84 17.5 Tributoxy etxyl 17.8 38.18 37.9 38.03 37.6 Triethyl 38.9 of ammomum molybdate were dissolved in water 39.37 37.9 38.69 37.8 Triethyl '38.9 a d diluted to 1 liter. Mean for aliphatios
PROCEDURE
Preparation of Standard Graphs for Hydriodic Acid Conversion. A 0.4393-gram sample of twicerecrystallized potassium dihydrogen phos hate was dissolved in distilled water and dilutefto 1 liter in a volumetric flask. This solution contained exactly0.1 mg. of phosphorusper ml. Ten aliquots ranging from 1 to 10 ml. were measured from a IO-ml. buret iuto 50-ml. volumetric flasks. To each aliquot 5 ml. of each of solutions A, C, D, and E were successively added, and the mixtures were
46.16
45.39 43.3 21.3 21.1 19.5 18.6 19.0 17.3 11.7 18.36
46.32 44.17 35.82 37.89 19.68 21.66 19.62 17.64 12.98 23.61
45.96 44.8 21.8 20.7 18.8 19.9 20.0 18.7 13.1 19.00
46.11 43.70 21.19 21.20 19.18 19.40 19.36 18.71 12.77 18.81
21.8 21.8 19.2 19.2 19.2 19.2 12.8 Mean for aromatics 19.03 0 Upon basis of 100% pur.ity. b Technical tetraethyl yrophosphate. C 50% technical tetraetgyl pyrophosphate with organic solvent and emulsifier. d Recrystallized twice from methanol and water. 6 Isomer not specified, but identified through derivatives as chiefly met8 isomer.
1179
V O L U M E 2 2 , NO. 9, S E P T E M B E R 1 9 5 0 Measurement of Catalytic Oxidation Samples. lle+surement on 5-ml. aliquots of samples decomposed by catalytic oxidation were made descrihed above, except that solution R was substituted for solution .\ in the blanks.
Catalytic Oxidation. Values obtained by the molvhdivanadophosphoric acid method agree well with those obtained by the alkalimetric titration procedure. The standard deviation for aliphatics was 0.28%and for aromatics 0.09%.
DISCUSSION
I u Table I each value for the per cent phosphorus pentoxide found represents tht, mean of measurements on duplicate aliquots from each of two to eight samples of the organic phosphate. The data for alkalimetric titration of aliquots of the same solutions analyzed here and reported prwiously ( 6 )for both aliphatics and aromatics are included for comparison. Hydriodic Acid Conversion. For aliphatic phosphates, results by the molybdivanadophosphoric acid method are slightly higher than by the alkalimetric titration procedure. For the aromatics they are somewhat .high, in general, and in the case of triphcnyl phosphate and tri-ni-crwyl phosphate the results are EO high as to make the method roinpletely unreliable when applied to them. These results itre :Lttributed to the presence of residual colored organic matter. The time required for the destruction of this color by evaporation Jvith oxidizing agents makes the method of hydriodic acid.conversion of aromat,ics less desirable than catdytic oxidation for the spectrophotometric determination. The standard deviation for the aliphatics \vas 0.3870,and the results, while higher than I)y alkalimetric titration, are in agreement with those similitrly obtained after conversioii by catalytic oxidation. Comparison of rtssults recorde:l in Ta1)le I suggests that the results obtained l)y alkalimetric titration after hydriodic acid conversion may be slightly low and those by the spectrophotometric procrtlure :&ftcrsimilar conversion slightly high.
SUMMARY
Analyses of organic phosphates, decomposed by hydriodic w i d and by catalytic oxidation, were completed by the molybdivanadophosphoric acid spectrophotometric procedure, and compared with values obtained for aliquots of the same solutions by the molybdiphosphate-alkalimetrictitration procedure. Results obtained by the spectrophotometric procedure compare fitvorably with results obtained by the titration procedure for aliphatic phosphates converted by hydriodic acid and for both aliphatic and aromatic phosphates converted by catalytic oxidation. The results obtained by the spectrophotometric procedurr for aromatics after conversion by hydriodic acid are unreliable. LITERATURE CITED
( I ) Holin. D. W., and Stamhery, 0. E., IND.ENG.CHEM.,ANAL.ED., 16, 345 (1944). (2) Emeleus, H. J., and Anderson, J. S., “Modern Aspects of Iriorganic Chemistry,”p. 183, New I’ork, D. Van Nostrand C*o., 1948.
(3) Gibbs, M .D.. Am. CheTn. J . , 3, 317 (1881). ( 4 ) Kitson, R. E., and Mellon, M. G., IND.EX. CHEM.,ANAL.ED., 16, 379 (1944). (5) Misson, G., C h e m - Z l g . , 32, 633 (1908). (6) Simmons, W. R.. and Robertson. J. H., ANAL.C,HEM.,22, 294 (1950). RECEIVED April 17, 1950. .Contribution 84 from the Department of Chenristry, University of Tennessee.
Determination of Small Amounts of Chromium and Vanadium by Amperometric Titration THOMfiS D. 1’.4KKS1 A N D ELIGIO J . AGAZZI, Shell Development C o m p a n y , hmeryville, Calif. An amperometric method for the rapid determination of chromium and vanadium is described. Chromium and vanadium are oxidized to chromate ion and vanadate ion by heating with perchloric acid and treatment with permanganate. Chromate and vanadate ions are titrated amperometrically with ferrous solution at the rotating platinum electrode. Reduced vanadium is selectively reoxidized to vanadate ion with permanganate ion and again titrated amperometrically. Chromium is obtained by difference. The method has been successfully applied to steel, crude oil, oil residuum, and asphalt samples. .4s little as 5 micrograms of chromiuni or vanadium can he determined.
K
OLTHOFF and &fay ( 5 ) havcl shown that chrom:tte ion in very dilute acid solution can be accurately determined by amperometric titration with ferrous ion using a rotating platinum indicator electrode. Ducret (3) determined chromatc and vanadate ions in mixtures by titration with ferrous ion using sulfonated diphenylamine as indicator; he first determined chromate plus vanadate ions by rc,duction with ferrous ion, and then determined vanadium alone after selective reoxidation with permanganate ion. This mcthod fails when dealing with small amounts of these ibns becausc of t h r l u g e indicator correction (5). The chemistry and electrodcreactioiis involved in thc, amperometric titration of chromate ion have twen amply discussed by Kolthoff and May ( 5 ) . In these l:thoratories, their method was found to be applicable also to thcx drtcrmination of small amounts of vanadium by titration of v:tnadatc ion with ferrous ion. The I
Present address, Stanford Research Institute, Stanford, Calif.
principltl is the same, except that the reduction of vanadate ion to vanadyl ion involves the addition of only one electron. Using 0.001 A’ ferrous solution, as little as 5 picrograms of vanadium can he readily titrated. Oxidation of chromium and vanadium to chromate and vanadate ions may be accomplished by a number of oxidizing agents, such as bromate ion (6, I 2 ) , persulfate ion ( I S ) , permanganate ion ( I S ) , or perchloric acid (10,11, f3). Of these reagents, perchloric acid was selected because of the simple and rapid manner in which the oxidation can be performed. However, some reduction of the chromate ion occur3 by virtue of the hydrogen peroxide formed by decomposition of the perchloric acid ( I O , I S ) . Rapid cooling and dilution were found to be inadequate as a means of avoiding this reduction. Complete oxidation of the mixture was achieved by treating with permanganate ion after c.)oling and diluting. I n the method described, chromate and vanadate ions are titrated amperornetrically with ferrous solution to measure the
