|
|
|
|
|
|
|
|
|
Household "ammonia" is a solution of approximately 5% NH3 in water. Some cleaning concentrates are as high as 10% in NH3. Dissolved ammonia gives a solution of ammonium hydroxide, which ionizes partially into NH4+ ions and OH- ions. |
| Ammonia solution itself will precipitate certain
metal cations as the hydroxides and has its own place in qualitative analysis. |
|
Ammonium Sulfide - Lab Notes
WARNING! Among other hazards, this procedure involves generation of hydrogen sulfide, a highly toxic 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. Please review the Terms of Use for this web site.
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.
A, the test tube or bottle which contains your H2S generating mixture, is fitted with a one-hole stopper and glass tube. The tube leads into a flask B fitted with a two-hole stopper. The second tube leads out of B into C, a small reagent bottle or flask containing ammonium hydroxide (aqueous ammonia).
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 ("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.Copyright
Legal Notice: In order to use this website or any information contained herein, you must read and agree to the Terms of Use. Please be advised that malicious or negligent use of toxic materials can invite criminal prosecution.
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 or a small amount of glycerol to lessen friction.
Back to article
** 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.
Back to article
Works Cited:
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.
Back to the Experiments
CR Scientific Catalog
Main Page