relating to the oxidation of 1,a-propanediol and 1,2-propanediol with vanadium(V) in a medium initially 9.98' in sulfuric acid. Under these conditions each formula weight of 1,3-propanediol requires 10 equivalents of vanadate. An equation which accounts for this finding involves the production of formic acid: HOCHzCHzCHzOH 10 VOz+ 10 H + + 3 HCOOH 10 VO++ 6 HzO
+
+
+ +
The intermediate formation of malonic acid does not appear to be involved in this process; on the basis of experiments with the latter substance, a vanadate consumption in excess of 15 equivalents per formula weight of 1,3-propanediol would have been expected, had its production occurred in the course of this reaction. The effect of the relative position of the hydroxyl groups is demonstrated by comparing these data with those for the oxidation of 1,2-propanediol under identical conditions. Were this reaction to take the expected course, with the production of equimolar quantities of formic and acetic acids, 6 equivalents of oxidant mould be required. The extent of oxidation found experimentally is considerably in excess of this figure; no simple stoichiometry can be proposed to explain these data. As seen from Table VII, the oxidation of glycerol in a medium 9.9F in sulfuric acid involves the consumption of vanadium(T7) significantly in excess of that
required for oxidation to three forniula weights of formic xcid. Further study has shown that Ihis difficulty can be eliminated by a suitable choice of reaction conditions, a i d that glycerol may be successfully analyzed through the use of this oxidizi3g agent (IO). Stability of Vanndium(V) Solutions. I n connection with these studies, considerable infor .nation mas accumulated as to the stability of acidic vanadate solutions on normal storage in clear, soft g l : ~bottles for extended periods 01' time; representative data are & e n in Table VIII. The normality viilues represent the average of several ;itrations; reproducibility is indicated 1)ythe average deviation occurring between results in each standardization. Given also is the percentage change in concentration since the initial standardization. These solutions retained essentially the same titer throughout test periods which ranged from 1 month to 1 year. No consistent trenS of increase or decrease in normality can be noted. The differences observed between values in any set more likely ieflect uncertainties in concentration of the solutions used for standardization 1 ather than a tendency for the van idium(V) solutions to change in normality. These exp1orator;r investigations indicate that the use of vanadium(V) solutions for the vo umetric analysis of organic compounds s feasible in several instances. No specilicity of action can
be detected from the data presented; this drawback is common to many oxidizing agents in current use. Oxidative procedures with vanadiuni(V) solutions are simple, however, and close control of the variables of temperature and acid concentration does not appear to be critical. Finally, the ease of preparation of the reagent and its excellent stability warrant further consideration of its applicability in the analysis of organic compounds. LITERATURE CITED
(1) Bonner, it'. A., J. Am. Chem. SOC. 70,3508 (1948). (2) Calcott, W.S., English, F. L., Domning, F. B., Ind. Eng. Chent. 16, 27 (1924 ).
(3j-i!?oiette, Andrk, Gaudefroy, Ghisdain, Compt. rend. 237, 1523 (1953); Bull. SOC. Ehim. France 21, 956 (1954). (4)Raq, G. G., Rao, V., Narasimha Sastri, M., Current Sci. (India) 18, 381 (1949). (5) Rao, G. G., Sankegowda, H., Ibid., 21,188(1952). (6) Ibid., p. 189. (7)Singh, Balrvant, Singh, Ranjit, AnaE. Chim. Acta 10, 408 (1954); 11, 412 (1954). \----
(8) Singh, Balwant, Singh, Suyjit, Ibid., 13,405 1955). (9) Tsuba i, Isamu, N i p p o n Kagaku Zassi 66, 10 (1945); h'ippon Kagakzi Zasshi 71,454 (1950). (10)West, D. M.,Skoog, D. A., ANAL. CHWI.31,586 (1959). (11) West, D. M.,Skoog, D. A., Anal. Chhim. Acta 12,301(1955).
i,
RECEIVEDfor review July 18, 1958. Accepted November 24,1958.
Analysis of Dilute Aqueous (Glycerol Solutions with Qui nq uevalent Van adiLIm D. M. WEST'and D. A. SKOOG Deparfmenf of Chemistry and Chemical Engineering, Sfanford Uniiersify, Sfanford, Calif.
b A method for the analysis of glycerol is based on its oxidation to formic acid by acidic solutions of vanadium(V). The reaction is carried out in the presence of an excess of the oxidizing agent, such excess being determined by back-titration with a ferrous iron solution. Within the range of applicability, results accord well with those obtained by periodate, cerate, and dichromate oxid a tion.
G
may be quantitatively analyzed in several mays, the method chosen depending in part upon the nature of the sample. When a small quantity of water is the prinLYCEROL
586
ANALYTICAL CHEMISTRY
cipal impurity, as in refined glycerol, the assay is commoIily carried out by a specific gravity mlwnrement (4) or Karl Fischer titratior: (6). Crude glycerol samples and dilnte aqueous solutions are more readit7 analyzed by oxidative methods employing dichromate ( I ) , quadrivalent cer um (7), periodate (S), iodate (8), or permanganate (6). I n addition, acetylation (9), the reaction of glycerol with hydriodic acid ( I @ , and the formation of sodium cupri-glycerol compltx (2) have been used. During an investig lion of the oxidation of organic compounds with standard solutions of qui Iquevalent vanadium (IO),conditions were found under
which glycerol is stoichiometrically oxidized according to the equation: HOCHzCHOHCHzOH 8 H + 4 BHCOOH
+ SVOz+ +
+ N O + ++ 5Hn0
This reaction has been critically studied to evaluate its usefulness as an analytical method. It is carried out in the presence of a n excess of the oxidizing agent, such excess being determined by back-titration with a ferrous animonium sulfate solution of known titer in the presence of N-phenylanthranilic acid as indicator. 1 Present address, Department of Chemistry, San Jose State College, San Jose 14, Calif.
9.9 F
, .
8.0
8
6.0
gk c 8
4.0
52 23
55
uz
21 0 0
40
20 REACTION
TIME.
60 MINUTES
Figure 1. Rate of oxidation of glycerol as function of initial sulfuric acid concentration REAGENTS AND SOLUTIONS
Aininonium nietavanadate, approximately 0.3N. About 33 grams of C.P. ammonium metavanadate are dissolved in 900 ml. of distilled water and 100 ml. of concentrated sulfuric acid. If the solution is turbid, i t is filtered through a fritted-glass crucible. Standardization may be carried out against hIohr’s salt of known purity or sodium oxalate (11).
Ferrous ammonium sulfate, approximately 0.12N. About 48 grams of ferrous ammonium sulfate hexahydrate are dissolved in 900 ml. of distilled water and 100 ml. of concentrated sulfuric acid. The normality of this solution may be determined by titration with primary standard potassium dichromate. A’-Phenylanthranilic acid, 0.066F. The indicator is prepared by dissolving 0.213 gram of Eastman Kodak Co. N phenylanthranilic acid in 30 ml. of 5% sodium carbonate; this is subsequently diluted to 150 ml. with distilled water. Three to 5 drops of the indicator solutions are used for each titration. Glycerol. Commercial refined glycerol was used without further purification. Assay of the stock glycerol was made by specific gravity measurement and Karl Fischer titration, good agreement being obtained between the two methods. Dilute aqueous solutions were prepared by diluting weighed quantities of the stock glycerol to known volumes with distilled mater. RECOMMENDED PROCEDURE
A 10-ml. aliquot of the standard vanadate solution, after dilution with 100 ml. of distilled water and 25 ml. of concentrated sulfuric acid, is titrated a t room temperature with the ferrous ammonium sulfate solution to the change of N-phenylanthranilic acid from violet to green. Another 10-ml. aliquot of vanadate is added to a 10-ml. sample of dilute glycerol solution in a 250.ml. Erlenmeyer flask. The resulting solution is heated for 1 hour in a boiling water bath, after which it is diluted with 100 ml. of distilled water and cooled
to room temperature. Approximately 25 ml. of concentrated sulfuric acid are introduced. The excess vanadate is then titrated with the ferrous ammonium sulfate solution, at room temperature, AT-phenylanthranilic acid being used as indicator. For satisfactory results, the volume of ferrous ammonium sulfate solution used in the back-titration must be greater than 10% and less than 50% of that required to titrate the vanadate aliquot alone. STOICHIOMETRY
The influence of the time of heating on the reaction is shown in Figure 1, where the extent of oxidation for selected reaction conditions is plotted against the heating period. For initial sulfuric acid concentrations ranging up to 4.3F an essentially stoichiometric reaction is to be expected; the time required for this is nearly independent of the acidity within this region. At high initial acid concentrations the reaction is very rapid, but no longer stoichiometric. I n no experiment was provision made for refluxing of the reaction mixtures and there was a pronounced decrease in volume in the course of heating. Under conditions of reflux, the reaction was prohibitively slowed. Effect of Acid Concentration. Ordinarily, these oxidations were carried out in the presence of sulfuric acid; its effect upon the course of the reaction is shown in Figure 2, where the average extent of vanadate consumption over several arbitrarily selected heating times is plotted against the initial sulfuric acid concentration of the reaction mixture. Oxidation of dilute glycerol solutions with vanadate proceeds very rapidly under conditions involving high concentrations of sulfuric acid, but with the consumption of oxidant significantly in excess of that required for the production of
three formula weights of formic acid per formula weight of glycerol. It is possible to account for the vanadate in excess of that required for an &electron change as being due to a secondary oxidation of formic acid. There appears to be Iittle tendency for the reaction to proceed rapidly to the ultimate production of carbon dioxide and water, even undc r very strenuous conditions. This is due not only to the inherently slow reaction between quinquevalent vanadium and formic acid, but also to the lcss, through evaporation or dehydration, of appreciable quantities of the laher substance during heating. I n less acidic media, the vanadate consumption closely approximates that required for the stoichiometric oxidation of glycero to formic acid; concurrently with this, however, the time required for the reaction is markedly increased. I n the :ibsence of acid, reaction does not occur to any observable extent. Limited investigations in which phosphoric and perchloric acids were substituted for sulfuric acid indicated that there was no advantage in the use of either. The use of phosphoric acid was difficult; the vanadate solution tended to throw down a yellow precipitate, presumably a heteropoly acid. Oxidation of glycerol with this solution was marki:dly slower than with a sulfuric acid solution of similar strength. The same mas true n-ith the vanadate solution prepared with perchloric acid; it is posr,ible that the slower rate of reaction n this instance was due to the evapclrntion of appreciable quantities of pel chloric acid during heating. Inffuence of Excess Vanadate. A series of dilute glycerol solutions, representing approximately a tenfold range of concentration, was prepared by diluting weighed quantities of rcfined glycerol t o exactly 1 liter in volumetric flasks. The percentage composition of the stock material had previously been determined by specific gravity measurement and also by Karl Fischer titration of the water present. Each 3olution was repeatedly analyzed, using a 0.3N vanadate solution in a medium initially 0.9F in sulfuric acid and heating for 50 minutes in a boiling mater bath. I n adciition, the glycerol content of each solution was determined in replicate b q periodate and ceriuni(1V) oxidatioi s. Table I summarizes the results obtained I)y these several methods of analysis; shown also are data with respect to several other dilute glycerol solutions whose concentrations were determir ed by a method using potassium dichromate. These data show that thi: vanadate oxidation yielded results comparable with the other methods when the glycerol concentration wati held between 17 and 31 ing. VOL. 31, NO. 4, APRIL 1959
0
587
per 10 ml. of aliquot. Low results were obtained in tests on the glycerol solution whose concentration exceeded this range, owing to an insufficient excess of the oxidizing agent. Too great an excess of vanadate led to high results, due to an appreciable attack by that substance on the formic acid produced in the primary reaction. Analysis of the three solutions in Table I possessing the least glycerol content with B O.IN vanadate solution rather than 0.3N yielded average values of 9.27, 6.88, and 3.71 mg. per 10-ml. aliquot, in good agreement with the other methods of analysis. The excess of vanadate present in any situation thus exerts an appreciable influence upon the extent to which reaction occurs. Experiment has shown that there is a large decrease in volume, with concomitant increase in sulfuric acid concentration, when the reaction mixtures are heated in a boiling mater bath. It is also known that under conditions of reflux the reaction is slowed to a marked extent. The evaporation process, then, represents an important factor in the success of the method; therefore, it is necessary to observe the prescribed conditions as to the volume of the reaction mixture and also the reaction vessel employed. DISCUSSION
The results in Table I obtained by this procedure compare favorably with other methods of analysis. The one apparent limitation is that of the rather small range of glycerol concentrations over which the analysis is successful. Hon-ever, each procedure has its disadvantages. For example, the periodate procedure involves the use of a t least two solutions which require frequent standardization. The temperature a t which the oxidation is performed is somewhat critical, as is the reaction time. This latter variable is especially so in the dichromate oxidation of glycerol. From the standpoint of technique, the vanadate method is attractive. The reagent is stable over periods up to 1 year; while the stability of ferrous ammonium sulfate used for back-titrations is not great, only a volume ratio with respect to the vanadate solution is required, and this is readily deter-
588
0
ANALYTICAL CHEMISTRY
8.0
Figure 2. Influence of initial sulfuric acid concentration on extent of oxidation of glycerol for several heating periods
6.0
4.0
2.0
0 0
2.0 INITIAL
4.0
SULFURIC
ACID
6.0
8.0
CONCENTRATION.
10.0
FORMAL
Table 1. Comparison of Methods for Glycerol Analysis Glycerol Glycerol Found, Mg. Present, Mg.a ‘tanadate Periodate Cerium(1V) Dichromate 34.45 31.32 27.82 26.62 26.40 24.36 24.17 20.84 17.10 13.98 9.38 6.89 3.72
33.98 31.25 27.89
...
:
24 39 24.36 20.99 17.17 14.23 9.62 7.07 3.83
34.27 31.19 27.75 26.66
...
...
... 2 i : 32
... ...
...
26 1 48 26.19 24.02
24: i 2 ... 20.74 ... 17.07 *.. 13.93 ... 9.36 ... 6.88 ... 3.72 ... Based on specific giavity and Karl Fischer determination of stock glycerol.
not be as closely contrdled. With the exception of the periodate Oxidation, all of the oiidative methods are similarly limited by the presence of easily oxidized substances, which are also attacked by the se Jeral reagents. LITERATURE CITED
(1) Assoc. Offic. Agr. Ch:mists, “Methods of Analysis,” p. 480, !jth ed., Washington, D. C., 1940. (2) Bertram, S. H., Rritgers, R., Rec. trav. chim. 57, 681 (1338). (3) Fleury, P o d , Paris, Raoul, Compt. rend. 196, 1416 (1933).
24:09 20.90 17.08 13.93 9.44 6.91 3.77
Duke, F. R., IND. ENG. ED. 13, 558 (1941); ;er, R., Streit, J., Z. a m l . L.64, 13G (1924). .ley, A., BBlsing, Fr., Ber. 34, (1901).
‘est, D. M., Skoon, D. A., ANAL. ’ CHEAI. 3‘1,583 (1959). -‘ (11) West. D. M.. Skooa. D. A.. A n d ’ Chim. Acta 12, 301 (19%). (12) .%’isel, S.J Fanto, R., 2. h d w i r ~ s c h . Versuchsw. Deut.-Oesterr. 4, 190, 977 (1902); 5,729 (1902).
RECEIVEDfor review July 18, 1958. Accepted November 24, 1958.
