JOURNAL OF CHEMICAL EDUCATION
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THIOACETAMIDE AS A SULFIDE PRECIPITANT IN QUALITATIVE AND QUANTITATIVE ANALYSIS HARRY E. GUNNING Illinois Institute of Technology, Chicago, Illinois
INSPITE of its dangerously high toxicity and objection- and the aqueous solutions are stable a t room temperaable odor, hydrogen sulfide has long remained an im- ture, the solution begins to hydrolyze rapidly a t temportant reagent in most qualitative analysis schemes. peratures above 80°C. The hydrolysis, which occurs In quantitative analysis the use of hydrogen sulfide has presumably through the thioenol form, CH3-C=NR, /I been considerably restricted, largely because of its S-H tendency to form colloidal precipitates. During the past five years, thioacetamide has been yields first acetamide and hydrogen sulfide, and finally progressively replacing hydrogen sulfide in analytical further hydrolysis of the acetamide forms ammonium laboratories. The reasons for this are not far to seek. acetate. Thioacetamide is a superior sulfide precipitant to hyS 0 drogen sulfide; furthermore, it possesses none of the cH,LivHa H1o + cHrII-iw H,s physiological disadvantages of the gaseous reagent. It is the purpose of this paper to discuss the use of thio0 acetamide as a sulfide precipitant in both qualitative cHrd-ivHa H1o cH$and quantitative analysis. S Thioacetamide first achieved prominence as a fungiII Thioacetamide, CH3-CNHa, is a white, water- cide in surface-decay control of citrus fruits ( 1 ) . Its soluble solid, m. p. 115-16°C. While both the solid use as a substitute for hydrogen sulfide in qualitative
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analysis was first suggested in this country by Barber and Grzeskowiak (8, 5).
quantitatively as sulfide with thioacetamide, from acidic solutions. Cadmium (12), as the sulfide, can be brought down THIOACETAMIDE IN QUALITATIVE ANALYSIS from either acidic or ammoniacal solutions with thioI n the use of thioacetamide as a substitute for hy- acetamide. drogen sulfide in precipitating Group I1 and Group I11 Lead (IS) is first precipitated as PbS, which is dried sulfides, no basic change in analytical procedure is re- a t 110°C. Best results are obtained if the sulfide is quired. I n a typical case, to the filtrate from Group oxidized to sulfate before weighing. I, made approximately 2 N in acid, a slight excess of a Tin (14) must first be oxidized to Sn (IV) before pretwo t o five per cent solution.of aqueous thioacetamide cipitation in acid solution as SnSz with thioacetamide. is added. The mixture is heated for five minutes on a The authors recommend the addition of a smaU amount water bath, at which time the pH is lowered to 0.5, and of mercuric chloride after precipitation. the sulfides of As, Sb, Sn, Hg, Cu, Bi, Pb, and Cu come I n the determination of mercury (16),the sulfide can down. An excellent description of a complete analyt- be precipitated from acid solution in a weighable form. ical scheme employing thioacetamide is given by Blo- However, high values are obtained, although these can mendal and Veerkamp (4). A textbook suitable for a be made quantitative by correction with a n empirical one-semester introductory course in semimicro quali- factor. tative analysis has recently been published (6). I n an excellent review paper Flaschka has summarized Experience has shown that a major analytical ad- the results of his studies on the use of thioacetamide in vantage of thioacetamide over hydrogen sulfide is that quantitative analysis. Data are given on the precipof the superior coagulative properties of the precipi- itation of the sulfides of silver, thallium, mercury, coptates formed. This is particularly important a t low per, bismuth, lead, arsenic, antimony, tin, molybdenum, metal-ion concentration where thioacetamide brings zinc, cadmium, cobalt, nickel, manganese, vanadium, down a precipitate which can be easily filtered or cen- and selenium. trifuged, whereas, under similar conditions, the precipitate with hydrogen sulfide is usually colloidal in nature. COST AND SAFETY ADVANTAGES The ability of thioaceta.mide to precipitate sulfides Experience with thioacetamide in freshman chemistry quantitatively, in separable form, has made it suitable laboratories under the author's supervision has shown not only for macro and semimicro qualitative analysis, that it is decidedly more economical than hydrogen sulhut also for microanalytical work. Detailed procedfide, despite the fact that thioacetamide is by no means ures for its use in microchemical work are given by a cheap chemical. This economy results largely from Flaschka (6). the fact that only a slight excess of thioacetamide is required. THIOACETAMIDE IN QUANTITATIVE ANALYSIS Since thioacetamide generates hydrogen sulfide in Research on the use of thioacetamide in quantitative ,silu, where it is immediately c
