DECEMBER, 1948
675
.
.
PREPARATION OF AMMONIUM MONOSULFIDE FROM AMMONIUM POLYSULFIDE *
COLORLESS AMMONIUM sulfide is desired in several procedures of inorganic qualitative analysis. For its preparation, Middleton and Willard' direct that ice cold 3 N aqueous ammonia be saturated with hydrogen sulfide, kept in completely filled bottles sealed with paraffin, treate,d just before use with an equal volume of 3 N ammonia, and discarded a t the end of the laboratory period. No reference has been found in Chemical Abstracts to any method of keeping ammonium sulfide colorless or of decoloring a solution which has become yellow by oxidation. Comparison of standard electrode potentials shows that active metals would reduce free sulfur or the polysulfide ion to sulfide, but the introduction of such a metal MIDDLEION, A. R.,AND J. W. WILURD, '%mimicro Qmlitative Analysis," Prentice-Hall, Inc., 1942, p. 432.
CARL E. OTTO University of Maine, Orono, Maine
would usually ruin the solution for use as a qualitative reagent. However,if the metal ion wereprecipitated, the solution~mightbe usable. Chromium and aluminum seem active enough to reduce sulfur to sulfide or to hecome oxidized in preference to sulfide. In the basic solution they would be precipitated as the hydrous oxides. The concentration of sulfide ion would then remain unchanged upon exposure of the solution to air. Other metals, such as nickel or zinc, could be precipitated as sulfides, and, since the sdfides of the inactive metals are quite insoluble, the concentration of the metal ion is lowered sufficiently to permit many, if not all, of them to reduce sulfur to sulfide. Thus the equilibrium ratio, IS-]/[Sa-1, should he 1.3 X 108 with mercury and 4.0 X 108 with copper. This type of reaction, while keeping the solution colorless, would result in
676
lowered concentration of sulfide ion as oxygen is absorbed from the air. Experimental work consisted of placing a metal in a 3 N solution of (NH&S which had become colored, obsewing the result and then making qualitative tests on the solutions that became colorless. The results are listed below. Aluminum wire As received Acid washed Sand-papered Amalgamated
No decolorizing No decolorizing No deeolarieing Deeoloriaed in about one hour. By next day the entire solution had become a gel of aluminum hydroxide Deealorised in one to two days. Zinc (mossy) White ~ .r e c.i ~ i t a tine bottom of bottle and coating the metal Devarda's Alloy (Al 45, Decolorized in two to three days. Black precipitate on surface of Zn 5, Cu 50) metal. White precipitate suspended throughout solution Chromium No decolorizing Granulated Amalgamated No decolorizing Nickel (grmulated) Solution gradually became very dark brown. After weeks of standing clumps of precipitate could be seen, but they remained suspended throughout the solution Decolorized in about 30 min. Black Copper wire coating on the wire Black coating on the surfdce of the Mercury mercury, but the solution never became colorless
It is probable that aluminum and chromium are protected by a tightly adhering coating of oxide which p r e vents further action unless the metal is amalgamated, and then the aluminum reacts too readily. The greater activity of copper over merculy, although theoretically about the same equilibrium concentrations should result, is probably due to the greater area of contact furnished by the wire form of the coppgr. The solutions decolorized by zinc and by copper were tested further. Each of these when acidified with hydrochloric acid evolved hydrogen sulfide and left a solution that was as clear as water. Other portions were twice evaporated just to dryness with nitric acid and then taken up with water acidified with nitric acid. The solution decolorized by zinc was tested for zinc with dithizone,2 but no red color was formed. It is statedin the reference that 0.025 y zinc can be detected by this method. The solution decolorized by copper was tested for copper with ferric thiocyanate and sodium t h i o ~ d f a t e . The ~ result of this test indicated the presence of copper. Decolorization occurred in 30 seconds as compared to three minutes for the blank. Another portion of this solution was then tested with benzoin xim me.^ A negative result was obtained. The reference
'
FEIGL,F., AND J. W. MATTHEWS, "Qualitative Analysis hg Spot Tests," Nordemann Publishing Co., Inc., 1937, p. 108. Ibid., p. 39. 'Ibid., p. 40.
JOURNAL OF CHEMICAL EDUCATION
states that 0.02 y copper can be detected by the first method and 0.1 y by the second. It appears, therefore, that the copper ion in solution was low enough in concentration not to interfere in many qualitative tests. There was less than 0.1 y in the drop taken for test, and may be less than 0.02 y if some unknowh substance catalyzed the ferric thiocyanatethiosulfate reaction. Strips of filter paper moistened with lead acetate solution turned brown when held a few seconds above the unstoppered bottles of solution decolorized by zinc or copper. In fact, the odor of hydrogen sulfide was very apparent. Solutions of manganous nitrate (the common metal requiring the highest concentration of sulfide to form a precipitate) containing 10 mg., 1mg., and 0.1 mg. of the metal per ml. were prepared and separately tested with the solutions decolored by zinc or copper and with the original yellow polysulfide solution. In all cases a flesh-colored precipitate was obtained when the manganese concentration was 10 mg./ml. or 1 mg./ ml. and no precipitate when it was 0.1 mg./ml. These tests showed that the concentration of sulfide ion was not materially decreased even though the bottles had been opened several times and allowed to stand only partially filled for several weeks during the course of the experiments. The air introduced when the bottle was momentarily opened wodd tinge nith yellow the thin film of solution on the glass wall above the liquid level, but this disappeared on standing or shakmg. This observation indicated that oxidation of the sulfide ion would continue to occur upon exposure to air. No exact measurements of the decrease in sulfide ion. concentration with time were made because this decrease would depend upon the tightness of closure of the bottle and frequency of opening. The above tests with manganese, however, show that the solution remains usable for weeks. Instead of discarding the solution each day to avoid interference by polysulfide ion, it could be kept until it failed to respond to some selected qualitative test. Of course, one that is not too sensitive should be chosen. The fact that metallic copper does not react with monosuliide ion, or reacts extremely slowly, was shown by thrusting a clean wire into the solution that had been decolorized with copper. This remained clean for hours and by the folloGing day had only a slight brownish tinge that remained apparently unchanged for a t least two weeks. -This slight coloration might have been due to polysulfide formed by oxygen, which entered when the bottle was opened to introduce the clean wire. When placed in some of the original yellow polysulfide solution, a clean copper wire was black in ten seconds. In conclusion, it has been shown that yellow ammonium sulfide can be decolorized by metallic zinc or copper to yield a colorless solution useful in qualitative analysis. These metals will not preserve a sdlfide solution with concentration absolutely unchanged, but will, by precipitation of a metal sulfide, prevent the accumulation of polysulfide ions.
