Alfred R. Armstrong
College of William and Mary Williamsburg, Virginia
Precipitation of the Hydrogen Sulfide Group of 1011using Thioacetamide
In the current texts in qualitative analysis there is considerable variation in the procedures recommended for precipitation of the hydrogen sulfide group using thioacetamide as the source of hydrogen sulfide. Hogness and Johnson' substitute thioacetamide for H B gas without change in the concentration of the H + ion. Barber and TaylorZ begin the precipitation at the classical H + ion concentration of 0.2-0.3M, and then dilute the solution to 0.1M H + ion concentration for complete precipitation of PbS, CdS, and SnS. Kinga uses essentially the same procedure as Barber and Taylor. Nordmanq4 after an initial precipitation at 0.3M H + ion, dilutes to about 0.15MH+ ion concentration. Moeller5 recommends precipitation of the group from a solution initially 2M in HC1 and states that hydrolysis of the thioacetamide t o acetate reduces the acidity of the solution to the customary concentration of 0.3M H + ion. Swift and Butler6 find that the hydrolysis of thioacetamide t o hydrogen sulfide and acetamide is first order with respect to H + ion concentration and to thioacetamide concentration. Further, the acetamide is hydrolyzed to acetate so slowly that within the time required for the precipitation and separation of the hydrogen sulfide group the use of thioacetamide does not cause difficultyin the adjustment or control of the pH. With respect to the practice of beginning the precipitation in a small volume of solution of high acidity and then diluting the solution to obtain quantitative precipitation of the lead and cadmium these authors state: A similar dilution of the thioacetamide solution would leave a solution which is not saturated with hydrogen sulfide and from
which, unless care is observed in any subsequent heating, hydrogen sulfide can be expelled more rapidly than it is produced by hydrolysis. Thus lead sulfide precipitates produced by such a dilution with cold water have dissolved when the solution was subsequently heated to boiling even though an excess of thioacetamide was present. Finally, in an attempt to adapt thiaacetamide to the precipitation of the hydrogen sulfide group of elements, the precipitation of lead was found to be incomplete
' H o o ~ ~ sT.s ,R., AND JOHNSON, W. C., "Qualitative Analysis and Chemical Equilibrium," 4th ed., Henry Holt and Company, New York, 1954, p. 419. BARBER, H. H., A N D TAYLOR, T. I., "Semimicro Qualitative Analysis," Revised ed., Harper and Brothers, New Yark, 1953, p. 173. KING,E. J., "Quditsltive Analysis and Electrolytic Solutions," Harcaurt, Brace and Company, New York, 1959, pp. 421-23. ' NORDMANN, .I."Qualitative , Testing and Inorganic Chemistry," John Wiley & Sons, Inc., New York, 1957, pp. 268-69. MOELLER, T., "Qualitative Analysis," MeGraw-Hill Book Co., Inc., New York, 1958,pp. 450-51. E. A,, Anal. C h m . , 28, 14643 SWIFT,E. H., AND BUTLER, (1956).
under conditions which had been satisfactory with gaseous hydrogen aulfide.
The escape of hydrogen sulfide from the hot solution appears t o be a major difficulty in obtaining complete precipitation of the hydrogen sulfide group. At the temperature required for rapid hydrolysis of thioacetamide the H,S gas is lost nearly as fast as it is formed; hence, the concentration never gets high enough for complete precipitation of the sulfides of lead and cadmium. The loss of hydrogen sulfide gas during the hydrolysis of the thioacetamide can be prevented by capping the reaction vessel with a balloon. After the hydrolysis is complete the trapped hydrogen sulfide can he redissolved by cooling and agitating the capped reaction vessel. The larger amount of H2Sretained in the solution during hydrolysis, together with that redissolved from the balloon, gives complete precipitation of the hydrogen sulfide group from a solution 0.3M in H + ion. For precipitation on a semimicro scale a 13- X 100mm test tube with a volume of ahout 10 ml is a satisfactory reaction vessel. The penny balloons from the toy counter of a variety store fit snugly over the lip. If the test tube is half full of liquid there is only 5 ml of air in the system. If 1millimole of thioacetamide is present, its hydrolysis gives 20 ml of HIS gas. Thus the gas in the capped system is rich in H2S and the solution can be saturated with the gas by cooling and agitating the tube. Thioacetamide is much more readily oxidized than is hydrogen sulfide gas. Hydrogen peroxide rapidly oxidizes thioacetamide to free sulfur. Roughly, one drop of 3% HzOz solution oxidizes one drop of 1M thioacetamide. Hence, if hydrogen peroxide is used to oxidize Sn(I1) to Sn(IV) prior to precipitation, the quantity added should he kept to a minimum. On the positive side, thioacetamide reduces As(V) to As(II1) and As83 is precipitated rapidly. When thioacetamide is used as the source of H2S the arsenic is precipitated without using NHJ as a reducing agent. Under the conditions usually encountered in student unknowns the following procedure gives complete precipitation of the hydrogen sulfide group on a single. treatment with thioacetamide. A.n unusually high concentration of chloride ion can prevent the precipitation of cadmium. If a large amount of oxidizing agent is present the thioacetamide may be destroyed. Adjust the volume of the unknown solution to 2 to 3 ml. Neutralize the solution, make it just acidic with HC1, and then add 5 drops of 6M HC1 in excess. Add two drops of 3% HzOzsolution and heat the solution t o boiling to oxidize any Sn(I1) to Sn(1V). Add 1 ml of 1 M thioacetamide solution and dilute to 5 ml. Transfer the solution to 13- X 100-mm test tube. Volume
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Cap the test tuhe with a deflated balloon. Heat the tube in a bath of boiling water for 10 min. (Slightly less air is trapped in the system if the tube is heated for about a minute, capped, and then returned to the boiling water for ten minutes.) Without removing the balloon, cool the test tube under the tap or in a beaker of cold water. For about a minute, agitate the cold test tuhe as vigorously as possible without getting the solution up into the balloon. Remove the balloon, centrifuge, and decant the supernatant liquid into a second 13- X 100-mm test tube. Test the acidity of
414 / Journal of Chemical Education
the solution with methyl violet paper, and, if necessary, adjust the H + ion concentration to the original value of 0.3M. To test for complete precipitation, add 5 drops of 1M thioacetamide, cap the test tube with a balloon, and heat it for 10 min. in a bath of boiling water. Cool and agitate the tuhe to redissolve trapped HrS. If any additional precipitate is formed, pour the suspension into the test tube containing the previously precipitated sulfides and centrifuge again. Continue t,o test for completeness of precipitation until no further precipitation occurs.
