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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 wid 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 AND 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
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A N h,L Y T I C A L C H E M I S T R Y
chromium plus vanadium. Vanadium is then selectively oxidized with permanganate ion according to the method of Ducret (3)and titrated amperometrically with ferrous solution. Chromium is obtained by difference. The method haa been applied successfully to steel, crude oil, asphalt, and oil residuum. APPARATUS
The apparatus used for amperometric titration was similar to ' that described by Laitinen, Jennings, and Parks (7), and consisted of a Fisher Elecdropode (Fisher Scientific Company, Pittsburgh, Pa.) equipped with a saturated calomel reference electrode, salt bridge, constantrspeed stirring motor, and rotating platinum indicator electrode. REAGENTS
Perchloric acid, approximately 60 to 72%. Potassium permanganate solution, approximately 0.1 M . Ferrous Ammonium Sulfate Solutions. To prepare 0.1 N ferrous solution, dissolve 39 grams of reagent grade ferrous ammonium sulfate hexahydrate in 1 liter of 1 N sulfuric acid. PrepareO.O1 Nand 0.001 Nsolutions by dilution of the 0.1 N solution. Standardize these solutions immediately before use by amperometric titration with standard potassium dichromate solution of suitable strength. Sodium azide, 4% aqueous solution. PROCEDURE
To the solution of chromium and/or vanadium contained in a 50-ml. borosilicate glass Erlenmeyer flask, add 2 to 3 ml. of
Table I. Optimum Conditions for Amperometric Titration of Chromium and Vanadium Chromium plus Vanadium Content. Me. Titratiqg Conditions Volume of solution, ml. Galvanometer sensitivity, microamperes per mm. Normality of ferrous solution
1.0-0.1 40
0.1-0.01. 30
0.50
0.20 0.01
0.1
