938 A N A L Y T I C A L C H E M I S T R Y

from 20 to 40 ml. The average deviation from the theoreti- cal weight was 0.1% for precipitates that were washed with hydrochloric acid solution.

The factor 2.890 converts weight of thallous sulfate to tetra- phenylarsonium chlorothallate. The weight of thallous perrhen- ate is converted to the equimolecular mixture of tetraphenyl- arsonium perrhenate and tetraphenylarsonium chlorothallate by the factor 2.998. Thallium was not separated from rhenium but vias simultaneously precipitated with tetraphenylarsonium chloride.

SUMMARY

Thallium may be quantitatively determined by precipitation from hydrochloric acid solutions containing the element in the trivalent state. The oxidation of the thallous ion may be ac- complished with hydrogen peroxide or any other effective oxi- dizing agent which introduces no interfering substance. The

precipitating reagent is a water solution of tetraphenylarsonium chloride. The amount of excess of this reagent is not critical. The precipitate obtained should be washed with hydrochloric acid solution to prevent hydrolysis, and dried in an oven at 110 c.

LITERATURE CITED

(1) ChrBtien and Longi, Bull. SOC. chim., 11, 241 (1944). (2) Feigl, Qualitative Analysis by Spot Tests, 2nd ed., p . 93, New

York, Kordemann Publishing Co., 1939. (3) Hawley, J . Am. Chem. SOC., 29, 300 (1907). (4) Meyer, Z . anorg. Chem.. 24, 321 (1900). (5) hloser and Brukl, Monatsh, 47, 709 (1926). (6) Scot t , Standard Met,hods of Chemical Analysis, 5 th ed., p.

(7) Willard and Smith, IND. ENG. CHEM., ANAL. ED., 11, 305 (1939). RECEIVED February 5 , 1948. Contribution 52 from the Chemistry Depart- ment, University of Tennessee, Knoxville.

942, N e w York, D. Van Nostrand Co., 1939.

Determination of Alkyl Sulfides and Disulfides SIDNEY SIGGIA AND R. L. EDSBERG, General Aniline & F i l m Corporation, Easton, Pa.

A procedure for determining alkyl disulfides, which was found to work equally w-ell for alkyl sulfides, involves oxidation with bromine. Alkyl sulfides and di- sulfides can be determined in the presence of each other by the application of a second method. Samples containing less than 10 mole % thiol can also be de- termined. The procedure is precise to about *0.3% in the best cases and goes as low as *l.Oqc in samples where the end point is rather slow.

OLTHOFF et al. (4) devised a method for determining alkyl K disulfides which involves reduction to the corresponding thiols (mercaptans) and amperometric titration of the thiols u ith silver nitrate. This method has the disadvantage of being reproducible to only about 2% in the optimum range of sample size and of yielding results that are about 5% low.

A procedure yielding a higher precision and accuracy was sought, and the following oxidation method was devised. The method described involves oxidation of the disulfide with bromine to the formation of the corresponding sulfonyl bromide.

(1)

The oxidation is brought about by adding standard bromate- bromide solution to an acid solution of the disulfide. As soon as the bromate-bromide strikes the acid solution, the bromine is liberated and is consumed by the disulfide. The end point is taken as the appearance of the first permanent bromine colora- tion. The usual method of adding excess bromate-bromide and determining the excess reagent iodometrically cannot be used in determining disulfides because of substitution reactions which consume bromine and cause high results. The substitution reactions do not interfere as long as there is no excess bromine present.

This same procedure was found to be applicable to the deter- mination of alkyl sulfides. The reaction is the oxidation of the sulfide to the sulfoxide.

RSSR + 5Brz + 4H20 + 2RS02Br + 8HBr

RzS + Brz + R2S.Brz -+ RzSO + 2 HBr (2) Hz0

In this case, as with the disulfides, using an excess bromate- bromide and determining the excess cause high results. In the case of the sulfides, the trouble is not only substitution but further oxidation of the sulfoxide to the sulfone. Neither of these reactions interferes as long as excess bromate-bromide is avoided, A similar procedure was used by Sampey, Slagle, and Reid (5), who titrated the sample, in benzene solution, with a solution of

bromine in water. The main disadvantage of this method lies in the fact that bromine water is not a very stable standard solu- tion. Bromine vapors are continually lost from the solution.

Mixtures of alkyl disulfides and sulfides can be determined by first applying the bromine oxidation, which yields both the disulfide and the sulfide. Then, by using the method of Kolt- hoff el al. (4 ) the disulfide alone can be determined; the sulfide is then obtained by difference. The extent of the application of this method is limited by the relatively low precision and ac- curacy of the reduction method, and by the fact that the final result depends on a subtraction and that the disulfide oxidation involves 5 moles of bromine while the sulfide oxidation involves 1 mole. The best results are obtained on samples low in di- sulfide and high in sulfide; here, the error in the disulfide result does not figure too prominently in the final results.

Thiols are also oxidized by bromine and act as interferences if they are not determined separately and the results corrected for their presence.

2RSH -+ Br2 RSSR 5BrZ + 2RS02Br HzO

This procedure can be used to determine thiols. The precision is high (*0.5%), but the accuracy is low. When pure ethane- thiol was run by this method, the results were very precise but were 3% low. 1-Pentanethiol results were 83 low. However, if the sulfide or disulfide contains thiol equivalent to 10% or less of the total titration, the absolute error in the first result is not significant. For instance, if the sample contains ethanethiol equivalent t o 10% of the total titration, then the error in the .total analysis is only 0.3Ye. In the case of 1-pentanethiol, the error would be 0.87e. The error introduced by the thiol increases as the amount of thiol in the sample increases.

Samples of disulfide or sulfide containing thiols are determined by the bromination procedure described below, which yields both the thiol and the disulfide or sulfide; the thiol alone can be

V O L U M E 20, NO. 10, O C T O B E R 1948 939

50 ml. of bromate-bromide solution in a 250-ml. iodine flask. To the solution are added 6 ml. of 6 'V sulfuric acid; no bromine vapors are allowed to escape. The flask is chilled, and 10 ml. of the 2OY0 potassium iodide are added via the well around the glass stopper of the flask. Precautions must be taken to prevent escape of bromine. The flask is shaken to account for the bromine in the atmosphere of the flask. The liberated iodine is then titrated with 0.1 N thiosulfate, using starch indicator.

Glacial acetic acid, C.P. concentrated hydrochloric or sulfuric acid.

PROCEDURE

A sample containing about 0.001 to 0.002 mole of alkyl sulfide or about 0.0003 mole of alkyl disulfide is weighed into a 250-ml. Erlenmeyer flask and dissolved in 40 ml. of glacial acetic acid. To this solution are added about 10 ml. of water (less water can be added if the sample comes out of solution). In the case of alkyl sulfides, 3 ml. of concentrated hydrochloric acid are sufficient to achieve complete reaction (see Table I). In the case of the alkyl disulfides, 3 ml. of concentrated hydrochloric acid are usually in- sufficient and more acid is generally required; 25 ml. of either con- centrated hydrochloric acid or concentrated sulfuric acid nTill cause the reaction to proceed completelv (see Table 11). After the acid has been added, the contents of the flask are titrated with standard 0.1 N bromate-bromide solution until the bromine color persists. The end point can be noted within 2 drops of reagent. A blank should be run on the acetic acid-water mixture used as solvent to correct for the excess bromine needed for the end point. This blank is subtracted from the titration; the blank is small, but

Table I. Sulfides Concd.

.kcid Used, hll. %

a-Butyl sulfide in 50 ml. of 80% HAC-20% Hz0 3 HC1 99.5 13 HC1 99 .o 25 HzSOr 99.5

Benzyl sulfidea in 50 ml. of 80% HAc-20% HzO 3 HC1 99.5 100.3 100.1

3 HCl 98.3 99.0 9 9 . 1 98.3

Ethyl sulfide in 100 ml. of 80% HAc-20% HzO

5 Sulfur analysis was run on this sample: ?Z S found = 15.20, % S calcd. = 14.95.

Table 11. Disulfides Concd.

Acid Csed, hI1. R

Ethyl disulfide in 50 ml. of 80% HAc- 20% Hz0

Butyl disulfide In 50 ml. of 80y0 H.Ic- 20% HzO

Butyl disulfide in 25 inl. of 80% HAc-

Phenyl disulfide0 in 50 mi. of 80% 20% Ha0

HAC-20C-i HzO

1-Cystine" in 50 nil. of water

3 HC1 i 1 . 3 10 HCl 86 4 20 HzSOd 95 6 25 HzSOi 95 3 30 HzS04 95.3

3 HC1 5 HC1

10 HC1 2.5 HC1

5 6 .V HiSOi 5 12 N HgS04

10 12 %SO4 10 HzSOr 20 HzSOa 25 H2304 30 H~SOI

87.2 88.7 95.8 96.8 i 9 . 6 83.0 86.9 97.8 98.8 98.8 98.8

it is still significant. For optimum results the disulfide samples should be slightly

warm during the titration (30" to 50" C.). If the solution is a t room temperature, the end point is a little slow, and a slight error can be incurred. The sulfide samples can be titrated a t room

10 H~SOI 98 8 temperature.

94 5 DISCUSSION 95 0 94 3

25 HCl 94 0 20 &SO4 Consumption of Brz

too slow for good end point

3 HCl

The analyses require very little time and a minimum of ap- paratus. The end point is not of the sharpest type, but is easily discernible; by using samples that yield a 30-ml. titration, the

3 HC1 100 0 titration can be reproduced to 0.5% or better. The blank which accounts for the amount of excess bromine needed to see the end 99 6 25 HC1 99 6

20 HzSOa 99 6 point is about 0.25 ml. a Sulfur analyses mere run on these samples. Phenyl disdfide, % S found = 27.94; 1-Cystine, 70 S found = 26.87; 70 S calcd. = 26.6.

% S calcd. = 29.4. BIBLIOGRAPHY

(1) Gilman, H., "Organic Chemistry," Vol. I, p. 889, New York, John TViley & Sons, 1943.

determined by the amperonietric titration method of Kolthoff and Harris (3). (1946).

ti! $ ~ ; ; l ~ ~ ' ~ ~ ~ , ~ ~ ; i a , IsD. ENG, CHEM., AsaL, ED,, 18, 161-2 (4) Kolthoff, May, Morgan, Laitinen, and O'Brien, Ibid., 18, 442-4

REAGENTS (1946). (5) Sampey, J. R., Slagle, K . H., and Reid, E. E., J . Am. Chem. Soc.,

0.1 S Bromate-Bromide. iifter 2.78 grams of dry potassium bromate and 10 grams of ootassium bromide are dissolved in

54, 3401 (1932).

water and diluted to 1 liter, the solution is standardized by putting RECEIVED February 2, 1948.

Determination of Moisture in lecithin and Crude Soybean Oils

K. M. BROBST, A. E. Staley Manufacturing Company, Decatur, i l l .

HE determination of moisture in commercial lecithin pre- T sents many problems. Thermal drying and toluene dis- tillation methods are not reliable, although the latter method is in general use a t present (4) . The specific disadvantages of the toluene distillation method when applied to lecithin are excessive foaming in the boiling flask and poor phase separation in the water trap. Results thus obtained may vary as much as 2070 (6 ) *

As lecithin is a material of relatively low water content, it was

thought desirable to investigate the Karl Fischer method. This paper presents an accurate and rapid method for the determina- tion of water in lecithin. The procedure described employs the Karl Fischer reagent with the determination of the end point by the dead stop method (18).

The Fischer reagent has been widely used for a variety of products (2-3> 5, 7-13', 15-29). The limitations of the method have been discussed by Mitchell et al. (14) and by Suter (16) . By careful attention to the end-point approach, and by elimina-