Microanalvtical Determination of Sulfur J
A Modified Bomb Method JOSEPH F. ALICINO, Fordham University, New York, N. Y.
T
bustion 1.5 ml. of concentrated hydrochloric acid are necessary. After the bromine is added to the acidified solution in the flask, the solution is concentrated to about 5 to 7 ml. and transferred to a weighed filter-stick crucible with appropriate rinsings (6). The precipitate should be washed thoroughly with a total of about 20 t o 30 ml. of 1to 200 hydrochloric acid and water.
HE Elek-Hill microbomb method for the quantitative
determination of sulfur in organic compounds ( 1 ) requires the use of a Neubauer or Gooch type of crucible for the precipitate. The quantitative transfer of microquantities of barium sulfate from a glass dish to a crucible is a p t to be attended with some difficulty, owing to the tenacity with which the precipitate sticks to glass. It was thought that if this excellent method of combustion could be modified to permit its use in conjunction with the automatic filtration technique (6) a decided advantage would be gained. This change, herein described, is in the nature of a reduction in the amount of fusion mixture, so t h a t the resulting sulfate solution can be evaporated to a small volume and transferred to a filter-stick crucible. After considerable experimentation this reduction was established, but only after substituting potassium chlorate for the sugar-nitrate mixture used b y Elek. The use of potassium chlorate in this connection was also observed by another worker (3). The elimination of potassium nitrate reduces the well-known coprecipitation of nitrates, which has been quantitatively studied in a recent paper dealing with macroamounts ( 2 ) . The lowered concentration of salt solution from which the sulfate is precipitated is a n added advantage ( 4 ) .
Table I gives the results of some typical analyses b y the proposed method. The average sample weight v a s 6.1 mg.; the largest and smallest samples were 9.8 and 3.2 mg., respectively. The average amount of barium sulfate yeighed was 8.6 mg.; the largest and smallest precipitates weighed 23.1 and 3.6 mg., respectively. Since the over-all agreement of these 33 individual analyses with the calculated per cent of sulfur is better than 1 part per 1000 (assuming all the compounds to be pure), it is clear that the effect of coprecipitation is negligible. To check this further, all but one of the precipitates of barium sulfate corresponding to the analyses in Table I were washed again after the initial weighing, reignited, and weighed again (6). The average loss in weight was 0.031 mg., distributed as follows:
With a mixture of 0.06 gram of potassium chlorate and 0.35 gram of sodium peroxide as the charge, most compounds were quantitatively oxidized. Some substances, particularly sulfones, required treatment with 3 to 4 drops of elementary bromine after the combustion. The bromine treatment is now used as a matter of routine. For this treatment and also to facilitate subsequent transfer of the solution, a 125-1111. “iodine flask” (with a lip for pouring) has been found most convenient. The procedure is about the same as given by Elek except for the oxidizing mixture given above. The bomb must be heated in a roaring flame for approximately 60 seconds, and in so doing, its lead washer must be protected from excessive heat. This can be accomplished by placing the bomb with its holder in a hole cut out of an ordinary asbestos gad, mounted on a tripod, so that only the cup portion of the omb is in direct contact with the flame. To acidify the alkaline solution resulting from the com-
Since this average loss in weight is less than 4 parts per 1000 of the average weight of barium sulfate obtained in this series, i t would appear that coprecipitation does not seriously affect the results. The reduced amount of fusion mixture obviates the necessity of using reagents of special purity. The regular c. P. grade of sodium peroxide used for macroprocedures comes within the tolerance for this procedure. As Table I indicates, the accuracy of this method is comparable to the other existing determinations. This procedure also has the advantage of speed, especially in multiple runs.
Loss in Weight 1Vg
.
0 0.01 0.02
No., of Determinations
Loss in Weight
No: of Determinations
Me. 1 1 6
0.03 0.04 0.05
12 9 3
Summary The reduction of the fusion mixture for the determination of sulfur by the bomb method which, at the same time, permits the use of the filter-stick method for filtration is the basis for the proposed method. The sodium peroxide is reduced from 1.5 to 0.35 gram and 0.06 gram of potassium chlorate is substituted for the 0.30 gram of sugar-potassium nitrate mixture, giving a total of approximately one fifth of the quantity used in the original method.
TABLEI. TYPICAL AKALYSES Substance
Sulfur Found
% Cystine Thiourea Thiooarbanilide Sulfanilic acid Trithioformaldehyde Sodium j3-naphthoquinone sulfonate Diphenyl sulfone Diaminophenoxathiin Glutathione Sulfonal Ethyl-4-methylthiazole-5-acetate HBr
Methionine P-Br Benzylcysteine
S-Benzyl-AT-formyl cysteine Thiamine HCI Phenyl isothiocyanate (liquid)
26.74 26.68 42.01 42.07 13.97 14.20 18.50 18.57 69,58 69.81 12.18 12.39 14.53 14.66 14.06 14.00 10.60 10.53 27.95 28.09 2s.10 11.95 11.96 21.35 21.43 11.12 11.01 13.20 13.36 9.50 9.46 23.79 23.68
Sulfur Calcd. % 26.69 42.12 14.05 18.50 69.59
Acknowledgment
12.31
The author wishes to express his thanks to Francis W. Power, S.J., of this department for his interest in this work.
14.68 13.92 10.42
Literature Cited
28.09
(1) Elek, A., and Hill, D. W., J. Am. Chem. Soc., 55, 2550, 3479 (1933). (2) Fales, H. A., and Thompson, W. S., IND. ENQ.CHEM.,Anal. Ed., 11, 206 (1939). (3) Gottlieb, S., Columbia University, private communication. (4) Niederl, J. B., and Niederl, Victor, “Micromethods of Quantitative Organic Elementary Analysis”, p. 151, New York, John Wiley & Sons, 1938. (5) Pregl, “Quantitative Organic Microanalysis”, 3rd English ed. tr. by E. B. Daw, p. 112, Philadelphia, P. Blakiston’s Son & Co.. 1937. (6) Saschek, W., IND. ENQ.CHEM., Anal. Ed., 9, 491 (1937).
12.05 21.49 11.00 13.39 9.51 23.71
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