ANALYTICAL CHEMISTRY
642
react quite readily, and it may be determinable by this method. It is possible that some side reaction along with the oxidation may have been consuming aldehyde. ACKNOWLEDGMENT
Acknowledgment is made to Dale Eichlin and Richard C. Stahl who tied up the loose ends of the procedure after the departure of Rlrs. Segal from our laboratory. Acknowledgment is also made to the early work done by J. G. Hanna in an attempt to use silver oxide oxidation as a means of determining aldehydes. LITERATURE CITED (1)
(2) Bailey, H. C., and Knox, J. H., J . Chem. Soc., 1951,2741. (3) Bryant, W. M. D., and Smith, D. bl., J . Am. Chem. Soc., 57, 57-61 (1935). (4) Chelintzev, V. V., and Xikitin, E. K., J Gen. Chem. (U.S.S.R ), 3, 319-28 (1933). ( 5 ) Iddles. H. 4.,and Jackson, C. E., IND. ESG.CHEW,ANAL.ED , 6 , 454-6 (1934). (6) Marasco, M., Ind. Eng. Chem., 18, 701-2 (1926). (7) Messinger, J., Ber. deut. chem. Ges , 21, 3366-72 (1888). (8) Mitchell, J., Jr., and Smith, D hI.,ANAL.CHEM.,22, 746-50 (1950). (9) Ponndorf, W., Ber. deut. chem. Ges., 64B, 1913-23 (1937). (10) Siggia, Sidney, and hlaxcy, TT’illiam, ISD. ENG.CHEM.,A x . 4 ~ . ED.,19, 1023 (1947).
Alekse’ev, S. V., and Zvyagina, S. I., Sintet. Kauchuk, 5, 19-26 (1936).
RECEIVED for review September 29, 1952. Accepted November 1 2 , 1952.
Determination of Chlorate and Perchlorate in the Presence of One Another G . P. HAIGHT, JR.’ Department of C h e m i s t r y , T h e George Washington C‘niversity, Washington 6, D . C . LTHOUGH
the reduction of chlorate to chloride is easy and
A quantitative in the presence of a variety of reducing agents,
the reduction of perchlorate is limited to a few special agents ( 1 ) whose action is generally slow, and thus far only one procedure for the determination of perchlorate by wet reduction is availableLe., reduction by titaniuni(II1) sulfate in an atmosphere of hydrogen (7). The discovery of the strong catalytic activity of molybdenum toward polarographic reduction of perchlorate in sulfuric acid solution ( d , 4 ) has been applied to the analysis for molybdenum by the author (b), and it was deemed desirable to see if the effect could be turned to analysis of perchlorates and chlorates. This has been done with marked success using stannous ion and/or zinc amalgam for reduction. MATERIALS
Amalgamated zinc was prepared as for a Jones reductor. ilkfree stannous solutions were prepared by dissolving stannous chloride or stannous sulfate in 1 AI sulfuric acid. Oxygen-free nitrogen was bubbled through the solutions for 20 minutes. These solutions were standardized daily with standard potassium permanganate solutions, prepared and standardized by the conventional procedure. PROCEDURES AND RESULTS
.
Haight and Sager (3) have shown that tetravalent molybdenum is probably the catalytic agent in a study of the kinetics of the reduction of perchlorate with Sn(I1). Reduction of molybdenum to the trivalent state stops the reaction with the result that very few reducing agents are effective. So far the only known agents which are able to bring about this catalytic reduction are stannous ion, amalgamated zinc, and mercury cathodes. Chromium(I1) and vanadium(I1) have proved unsuitable for this system because they reduce molybdenum to the trivalent form. Results using Sn(I1) and amalgamated zinc will be discussed for chlorate and perchlorate. Reduction with Zinc Amalgam. If the zinc is of high purity and the amalgam carefully made, it is possible to work with 4 to 6 M sulfuric acid. Since the reactions in question are not fast and are speeded up by hydrogen ion, i t is advantageous to work in as strong acid as possible. Perchlorate is not reduced in the absence of molybdenum while chlorate is reduced to chloride either in the presence or absence of molybdenum, though much faster if the molybdenum is there. If only chlorate is present, 1 Present address, Chemistry Lawrence, Kan.
Department,
University of
Kansas,
it may be reduced quantitatively in a Jones reductor to chloride in the presence of molybdate and the chloride determined conventionally with silver nitrate solution after reoxidation of the molybdenum(II1) formed with permanganate. PROCEDXRES FOR CHLORATE . W D / O R PERCHLORATE. In analyzing for chlorate alone, about 1 millimole of chlorate is dissolved in 25 to 50 ml. of 4 M sulfuric acid, and 5 to 10 ml. of 0.1 M sodium molybdate solution are added. Reduction is carried out either b y conventional method in a Jones reductor or by shaking in contact with zinc amalgam (enough to cover the bottom of a 250-ml. Erlenmeyer flask) until the molybdenum is in the green trivalent state. The solution is transferred to a flask for titration and enough potassium permanganate solution is added to oxidize the trivalent molybdenum to colorless molybdate. If too much potassium permanganate is added, it may be removed with a little hydrogen peroxide solution. An excess of standard silver nitrate solution is added, and the solution is titrated with standard potassium thiocyanate solution to the red ferric thiocyanate end point. Chloride may also be determined gravimetrically. A separate chloride determination may be done directly on the initial sample if that ion is originally present. In analyzing for chlorate in the presence of perchlorate, the addition of molybdate in the above procedure is omitted. The sulfuric acid solution must be boiled in the presence of zinc amalgam for 15 or 20 minutes and then transferred to a titrating flask for volumetric chloride determination. No treatment with permanganate is necessary. Chloride remaining in the solution represents chlorate plus any chloride originally present, and may be determined by titration or gravimetrically. In determining perchlorate plus chlorate, or perchlorate alone, molybdate is added, but the Jones reductor may not be used. The solution must be boiled in contact nith amalgamated zinc for 15 to 30 minutes, or until the solution is green. This serves as a good indicator for completion of the reaction. Studies have shown that molybdenum(V1) is not reduced completely even to the pentavalent state as long as perchlorate is present. Khen the perchlorate is all reduced, green molybdenum(II1) may be formed. The odor of elementary chlorine was detected in evolved gases, and results were always about 1 to 5% low. Refluxing or use of a chlorine trap might enable one to catch and determine escaping chlorine. Otherwise a small correction factor may be applied. Analysis of reduced solutions for chloride is the same. Results must be corrected for chloride originally present and for chlorate originally present as determined on separate samples.
Reduction with Stannous Ion. The reduction of chlorate with stannous ion is well known, but, in the absence of molvbdate, perchlorate is not reduced by this agent except in hot concentrated hydrochloric acid (6). I n the presence of molybdate, both chlorate and perchlorate are reduced to chloride, the former instantaneously, the latter only slowly a t ordinary temperatures. The reduction of chlorate in the absence of molybdate is slower, but analytically feasible. .4nalyses are made by addition of excess stannous ion as chloride or sulfate to the sample in 6 M
643
V O L U M E 2 5 , N O . 4, A P R I L 1 9 5 3 sulfuric acid, heating under nitrogen, and back-titrating the excess stannous with standard potassium permanganate solution. Molybdenum is reoxidized to its initial hexavalent form which is colorless, this change warning the analyst of the approaching end point. Chloride added with stannous ion did not cause end point errors with potassium permanganate in the solutions studied.
PROCEDURES FOR CHLORLTE AND/OR PERCHLORITE. For determining chlorate alone, the sample is dissolved in 25 to 50 ml. of 6 .If sulfuric acid, and 5 to 10 ml. of 0.1 iM molybdate and a twofold excess of standard stannous chloride or sulfate solution are added. The excess stannous ion is titrated immediately with standard potassium permanganate solution. For determining chlorate in the presence of perchlorate, the addition of molybdate is omitted. The solution is boiled 1 to 5 minutes under a nitrogen atmosphere, cooled under nitrogen, and then titrated with standard potassium permanganate. ('hloride should be low or absent entirely because stannous chloride reduces perchlorate in acid solution a t temperatures above 50" C. (6). Therefore reduction with zinc is preferable for chlorate in the presence of perchlorate. In analyzing for perchlorate alone or for perchlorate plus chlorate, the sample is treated as is the sample for determining chlorate alone, but it is boiled under nitrogen for 15 to 30 minutes, followed by cooling under nitrogen and titration with standard permanganate. The length of treatment must be found by practice. The author found 0.01 A' perchlorate solutions to require about 30 to 40 minutes. More dilute solutions would require more time. The fact that this reaction is not fast suggests that veri low perchlorate concentrations would be undesirable because of the long time requirement for each determination. This is a general difficulty Rith perchlorate reductions and does not seem to have been eliminated here. Results for perchlorate must be corrected for chlorate found independ~ntly. No correction for chloride need be applied in this case. Results are given in Tables I and 11. Interferences. The catalytic activity of n~olybdenun~ toward the reduction of nitrate has been observed polarographically by the author and reported by Johnson and Robinson ( b ) . The :tuthor is studying the cats13 tic reductions of nitrate and nitro organic compounds all of which \ \ o d d interfere \\ith the method using stannous ion but might not in the case of zinc unIt's's nitrogen ouychloride should be formed and escape Eie-
Table I.
Reduction with Zinc Amalgam
AloO,-Added, Milhole 0.0 0.0
0.0 0.0 0.0 0.5 0.5 0.5 0.5
C l o t - Bdded, Clod- Added, hlillimoles hlillimoles 1 1 2 5 5 1 2 5 5
008 008 016 020 020 008 016 020 020
7 286 3 643 3 643 3 643 7.286 7.286 3 643 3 643 7 286
Table 11. Time Boiled, 3Iinutes 0 0 0 1 30 30
1100,- .4dded, Millimoles 1 0
C1- Found hlillimoles' 1.007 1.013 2.018 5.023 5.028 8.12 5,621 8.58 12.17
Error in Cl-. Millimole -0.001 005 +0.002 +O. 003 f O 008 -0.18 -0,038 -0.083 -0 07
to
Reduction with S n + + c103Bdded, hleq. 0 4950 0 4950 0 4950 0 4950 Sone Sone None Xone None None None 0 4950
nients such as chromium and vanadium, which in their lowePt positive valence states reduce molybdenum to thc trivalent state and prevent catalysis, should not interfere with the stannous reduction if they are originally present in their higher valence states. Experiments show they do interfere with zinc reductions Zinc reduces them to lower valences which prevents reoxidatinn of trivalent molybdenum and destroys its catalytic activity. DISCUSSION
T h e better procedure for analysis of chlorate in the preseiicp of perchlorate seems to be the reduction of chlorate in strong sulfuric acid with zinc amalgam. Complications arise if stannous ion is used, especially if chloride is present in the mixture to be analyzed or if the amount of chlorate is large compared to perchlorate. Chloride will enable stannous ion to reduce pcrchlorate a t the high temperature required for complete reaction. I n addition the danger of air oxidation of the stannous ion is eliminated as a source of error if zinc is used. However, if perchlorate is absent, the catalytic reduction of chlorate \! ith stannous ion and molybdate is very rapid and convenient. The reaction is instantaneous in 4 Jf sulfuric acid and a t room tcniperature. If the excess stannous ion is titrated inimediatcl\- no precautions for excluding air are necessarv, and the difficulties of using strong sulfuric acid in a Jones reductor are avoided. Both procedures for perchlorate suffer from the difficultj- that the reactions proceed a t measurable rates of speed which means that the detection of small amounts of perchlorate (0.2 niillimoles per liter of reaction mixture or less) would be very difficult. Whether all the loss during zinc amalgam reductions was due to evolution of chlorine or whether a part was due to incomplete reaction was not determined. The stannous reduction of perchlorate involves no loss of chlorine if stannous ion is present in moderate excess. The slowness of the reaction near the end is a handicap, but the large number of equivalents involved in changing perchlorate to chloride ion makes the use of a soluble reducing agent attractive. These reactions all proceed faster in stronger sulfuric acid but if the concentration of that acid becomes too high, it becomes a strong oxidant and is subject to attack by stannous ion or by amalgamated zinc. Most of this work was done in 4 to 5 X sulfuric acid, and that range is recommended for all procedures. Up to 7.0 'If may be used mith caution. Lower concentrations tr-odd require more time for reactions. The minimum error with zinc reductions and the maximum total error with stannous reductions appears to be 0.04 millimole of perchlorate per 100 ml. of reacting solution. This error is probably absolute in character, not relative to size of sample, and would require perchlorate concentrations in large excess of 4.0 millimolar to reduce relative error. Chlorate determinations are apparently as precise as volumetric and gravimetric apparatus allow. A similar differential method for nitrite and nitrate using stannous ion appears possible and is under investigation.
(304-
SnT-rstd. Error in Ci-, HzSO4, M Neq. Millimole 2.25 0.4968 TO 0003 2.25 1 0 0,4944 -0 0001 2.25 0 0 0.4938 -0 0002 1 0 2.25 0 48440 -0 0001 1 0 4.5 23.05 -0 05 1 0 4.5 23.04 -0 05 15 4.5 2 0 23.10 -0 04 20 3.6 1 0 11.58 -0 01 7.2b 20 1 0 11.67 +o 001 0 1 0 3.6 10.4; -0 1 5 20 3.6 0 0 0 0 0 0 20 4 5 1.0 23.67 -0 018C " SnSOl must be used here. Clod- is reduced by SnClz a t temperatures above 5o0 C . b This would appear t o br a n ripper Ilmit. A blank on slightly higher concentration of HzS0, exploded ( M O O & - -was present) with evolution of SO, indicating. perhaps, a n autoratalvtic reduction. .4dded, Meq. A-one Sone None 33.32 23.32 23.32 23.32 11.66 11.66 11.66 11.66 23.32
LITERATURE CITED
(1) Bredig, G., and hlichel, J., 2. p h y s i k . Chem., 100,
124 (1922). (2) Haight, G. P., ANAL.CHEW,23, 1505 (1951). (3) Haiaht. G. P.. and Sager. W. F.. J . Am. Chem. Sic.. 74. 6056 (1952,. (4) Holt,je,'R.[and Geyer,'R., 2. anorg. Chem., 246, 265 11941). (5) john son,^ -M. G., and Robinson, R. J., A s . 4 ~ . CHEM.,24, 366 (1952). (6) Rivest, R., private communication from the University of California. (7) Rothmund, V., Z. anorg. Chem., 62, 108 (1909). RECEIVED for review March 19, 1952. Accepted Xovember 17, 1952. Presented before t h e Pittsburgh Conference o n -4nalytical Chemistry and .4pplied Spectroscopy, March, 1952.
