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28 November 2003

Make Your Own Chemicals: Ammonium Sulfide

by Christian Thorsten

WARNING: Among other hazards, this procedure involves generation of hydrogen sulfide-- a very poisonous gas. If you choose to attempt this or any of the experiments or procedures described on this site, you do so entirely at your own risk.

Ammonium sulfide is a very useful reagent in mineral analysis. It's easy to prepare in any modestly-equipped laboratory, though there's the 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 (Erlenmeyer Flask) or reagent bottle; powdered sulfur; paraffin wax shavings; alcohol lamp or candle.

Procedure:
Place an equal volume of paraffin shavings and sulfur powder into the test tube. Fit the test tube with a one-hole stopper through which a gas delivery tube has been pushed with thickly-gloved hands*. Hold this test tube / stopper / gas delivery tube assembly with asupport stand and clamp. Into the conical flask or reagent bottle, pour ammonia solution until the vessel is 1/2 to 2/3 full. Place the free end of the gas delivery tube into this solution.

The rest of the procedure must be done outside or in a fume hood! Hydrogen sulfide is poisonous and also tends to "saturate" the sense of smell; a lethal dose will smell no more unpleasant than a merely annoying concentration**.

Gently heat the test tube containing the paraffin and sulfur. These will melt and eventually turn brown; H₂S gas will begin to bubble out of this mixture and into the ammonia solution at the other end. Keep bubbling the hydrogen sulfide gas through the ammonia solution for about 15 minutes. More time (that is, more H₂S) is of course required to saturate a larger volume of ammonia solution. Do not heat the contents of the test tube too strongly, however. The molten paraffin-sulfur mixture could boil over, not to mention that it's flammable.

Withdraw the gas delivery tube from the ammonia solution before you stop heating the test tube. This will prevent liquid from drawing up into the hot test tube and shattering it.
When finished generating H₂S, let everything cool and fill the remainder of the conical flask or reagent bottle with ammonia solution. Swirl gently. Stopper the container tightly and store in a cool, dark place. You should regard this reagent as a poison and treat it with appropriate caution. Do not get it on your skin; while it's often labeled "(NH₄)₂S", the reagent inevitably contains some NH₄HS, which is actually more dangerous than hydrogen sulfide via skin absorption and can also be lethal (see Merck Index entry for "ammonium bisulfide"). Wear vinyl or polyethylene gloves when handling ammonium sulfide reagent, and work in a well-ventilated area. Do not inhale the fumes- 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).

Notes:

* 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 to lessen friction.

** 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.

Works Cited:

Smith, Orsino C. Identification and Qualitative Analysis of Minerals. Princeton, NJ: D. Van Nostrand Co., Inc., 1946.

Windholz, Martha, ed. The Merck Index. Rahway, NJ: Merck & Co., Inc., 10th Edition, 1983.