WARNING! Among other hazards, this procedure involves generation of hydrogen sulfide, a highly toxic gas. H2S can kill you.
Ammonium sulfide is a very useful reagent in mineral analysis. It's easy to prepare in the modestly-equipped laboratory, though there's the significant hazard of hydrogen sulfide during its production. Once made, however, a bottle of the reagent will last quite a long time.
Supplies: ammonia solution (ammonium hydroxide); test tube; support stand and clamp; one-hole stopper; gas delivery tube; conical flask or reagent bottle; source of H2S; alcohol lamp.
Procedure: Into a test tube or gas generating bottle, place your source of H2S. The test tube of generating bottle is then fitted with a one-hole stopper through which a gas delivery tube has been fitted with thickly-gloved hands*. This test tube / stopper / gas delivery tube assembly is held with a support stand and clamp.
Ammonia solution is poured into the conical flask or reagent bottle until the vessel is 1/2 to 2/3 full. The free end of the gas delivery tube is then placed into this solution. For safety you should create a "trap" in the line as pictured below. This will help protect from suckback, which could shatter the test tube (if your H2S generating method involves heat).
The apparatus shown above must be secured with appropriate clamps and stands.
The rest of the procedure must be done outside or in a fume hood! Hydrogen sulfide is highly toxic and also tends to "saturate" the sense of smell; a lethal dose will smell no more unpleasant than a merely annoying concentration**.
If using a method of H2S generation that requires heat, the test tube is heated cautiously over an alcohol burner flame. H2S gas will begin to bubble out of this mixture and into the ammonia solution at the other end. The hydrogen sulfide gas is allowed to keep bubbling through the ammonia solution for about 15 minutes. A longer period of H2S bubbling is of course required to saturate a larger volume of ammonia solution. The contents of the test tube must not be heated too strongly, however. Hydrogen sulfide is highly flammable.
The gas delivery tube must be withdrawn from the ammonia solution before one stops heating the test tube, especially if no "trap" is present. Otherwise, liquid will draw up into the hot test tube and shatter it.
When generation of H2S has reached a satisfactory level, the heat is withdrawn and everything is allowed to cool. The remainder of the conical flask or reagent bottle is filled with ammonia solution, stoppered, and swirled gently. For storage, the container must be stoppered tightly and kept in a cool, dark place having adequate ventilation. One must regard this reagent as a poison and treat it with appropriate caution. It must not contact the skin; while it's often labeled "(NH4)2S", the reagent inevitably contains quite a bit of NH4HS, which is actually more dangerous than hydrogen sulfide via skin absorption and can also be lethal (see Merck Index entry for "ammonium bisulfide"). Vinyl or polyethylene gloves-- preferably two layers (i.e., "double bagged")-- are necessary when handling ammonium sulfide reagent, and all work with it must occur in a well-ventilated area. The fumes are toxic and mustn't be inhaled; they are a mixture of hydrogen sulfide and ammonia.
The ammonium sulfide solution, prepared as outlined above, can be used to precipitate certain metal cations as sulfides (some, such as iron, precipitate as hydroxides if the pH is adjusted toward alkalinity). For detailed information on sulfide precipitations, see Orsino Smith's procedures P-8 and P-14 (1946). Smith's text also mentions several other uses for the ammonium sulfide reagent in spot tests and blowpipe analysis (1946).
Adding sulfur will cause the ammonium sulfide solution to turn yellow-orange as ammonium polysulfide [(NH4)2Sx] is formed. The polysulfide is also used in mineral analyses, though not as often as the regular sulfide.
Sometimes ammonium sulfide solution will turn yellow-orange even when no sulfur has been added. This happens via oxidation by atmospheric oxygen; the result is also polysulfide.
* When pushing glass tubing through a hole in a stopper, thick leather gloves should be worn. The tubing should also be wet with some water or a small amount of glycerol to lessen friction.
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** Some have suggested that H2S actually becomes "odorless" to the sense of smell at high concentrations. This much H2S would probably be lethal, so it's tough to verify if the statement is true or not.
To test if hydrogen sulfide gas is being generated, do NOT smell it; instead, use a piece of filter paper moistened with a drop of lead acetate or lead nitrate solution. The paper will turn dark brown or black in the presence of H2S.
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Smith, Orsino C. Identification and Qualitative Analysis of Minerals. Princeton, NJ: D. Van Nostrand Co., Inc., 1946.
The Merck Index. Rahway, NJ: Merck & Co., Inc., 10th Edition, 1983.
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