Microscale Gas Chemistry:
General Safety Precautions.
Always wear safety glasses. Gases in syringes may be under pressure and could spray liquid chemicals. Follow the instructions and only use the quantities suggested. Hydrogen sulfide is extremely toxic. These experiments should not be conducted by individuals who have little experience with the techniques of gas syringe manipulation. Although the syringe method minimizes the risk of accidental exposure to the gases generated, as a precaution the gas-generation and gas-washing steps should be performed in a working fume hood or outdoors. Hydrogen sulfide is also a flammable gas.
hood or work outdoor.
The gas-generation and gas-washing steps should be carried out inside a working fume hood or outdoors.
Hydrogen sulfide is extremely toxic. H2S(g) has the familiar smell of rotten eggs. Its odor can be detected at 2 ppb! Low level exposure can cause headache, dizziness and nausea. Inhaling higher concentrations of the gas can cause collapse, coma, and death from respiratory failure. The odor of H2S does not increase in proportion with its concentration, so higher concentrations of the gas do not smell worse than low levels.
Because hydrogen sulfide is extremely toxic, these experiments should only be conducted by those individuals who are experienced with the techniques of gas syringe manipulation.
We recommend lubricating the black rubber diaphragm of the plunger with silicone spray (available from hardware stores) or medium-grade silicone oil (Educational Innovations, $5.95 Part #GAS-150; Fisher Catalog Number S159-500; $34/500 mL.)
Equipment. (This equipment can be ordered from a variety of vendors including Educational Innovations, Flinn Scientific (US sales only), Micro Mole, and Fisher Scientific. Part numbers and links to their websites are provided.)
- 60-mL plastic syringes with a LuerLOK fitting
- Latex LuerLOK syringe cap fittings
- Plastic vial caps that fit within the barrel of the syringe
- Small plastic weighing boat
- Clear plastic beverage cup, 9 oz (260 mL)
- 250 mL flask with suitable stopper
- 3-cm piece of latex tubing
Preparation of Neutralization Solution.
4 g NaOH Contact the author for chemicals needed
universal indicator solution, 5-mL concentrated ammonium hydroxide (only the fumes will be used)
100 mL of 1 M NaOH (4 g NaOH in H2O to make 100 mL) in a
250 mL flask.
Keep the flask stoppered when not in use. Label the flask ‘1 M
for neutralization.’ This solution will be used to neutralized
excess reagents in the experiments.
of Hydrogen Sulfide.
The hydrogen sulfide (H2S) gas samples used in these experiments are generated by Method A. It may be practical to simultaneously generate three syringefuls of H2S(g) which are needed to perform all six of these experiments. Experiments 1, 2 and 3 collectively require one syringeful of H2S(g), Experiment 4 requires one syringeful, and Experiments 5 and 6 collectively require one syringeful of H2S(g). After gas generation has stopped, pull the plunger further outward an additional 5 mL in order to create a slightly reduced pressure inside the syringe. While working inside the fume hood, remove the latex syringe cap while it is directed upwards. Rotate the syringe 180o in order to discharge the reaction mixture into the beaker containing the NaOH Neutralization Solution. Immediately recap the syringe.
Transfer (instead of washing)
The gas-filled syringe is not "washed" in order to remove traces of unwanted chemicals from the inside surfaces of the syringe before the gases can be used in experiments. Another simple technique is used to accomplish the same objective. Using a 3-cm piece of latex tubing, connect the H2S-filled syringe to a clean dry syringe. Hold the two syringes in a vertical position with the clean, dry syringe on top (Figure 1). Transfer the hydrogen sulfide to the clean dry syringe by simultaneously pushing and pulling on the two plungers in 10-mL increments. Do not transfer any of the liquid reagent. After transfer is complete, pull the plungers outward by 3- 5 mL to assure reduced pressure in the syringes. Remove the connector tubing and cap the syringes. Dip the conncector tubing into the Neutralization solution in order to prevent odor.
Unwanted samples of hydrogen sulfide should be destroyed. This is accomplished most efficiently by suctioning some of the 6 M NaOH into the syringe as described in Experiment 3. Glassware and syringes should be washed inside the hood before they are removed.
Indicator/pH 8 Solution.
Experiments 2 and 4 require a slightly basic universal indicator solution. Prepare a solution by mixing 50 mL distilled water plus 5 mL universal indicator solution. Raise the pH to 8 by bubbling through the solution a pipetful of gaseous ammonia taken from the vapors above a solution of concentrated ammonium hydroxide solution.
Experiment 1. H2S
is Slowly Oxidized.
2 H2S(aq) + O2(aq) 2 S(s) + 2 H2O(l)
Place 10 mL distilled
in a 18 x 150 mm test tube (capacity 30 mL). Stopper the test
with a rubber stopper. Prepare a syringeful of H2S as described
and transfer to a clean, dry syringe as described
Before removing the syringe cap, pull the plunger outward by 5 mL, thus
creating slightly reduced pressure within the syringe. Replace
syringe cap with a 15-cm length of latex tubing and bubble 10 mL of the
gas below the surface of the water in the test tube. Remove the
assembly and pull about 5 mL air into the syringe to remove most of the
H2S from the latex tubing. Replace the tubing with the syringe
and set the syringe aside for use in Experiments 2 and 3. Stopper
the test tube and shake the test tube vigorously to dissolve some of
H2S(g). Set the stoppered test tube aside
observe it over the next several hours. After 24 hours most of
H2S will have been destroyed by oxidation.
the resulting solution by adding it to the Neutralization Solution.
Experiment 2. H2S
is a Weak Acid.
Thus, a 0.01 M H2S solution will have a pH = 4.5. If necessary, prepare a syringeful of H2S as described above. Wash the gas. Before removing the syringe cap, pull the plunger outward by 5 mL, thus creating slightly reduced pressure within the syringe. Remove the syringe cap, suction 20 mL of the universal indicator/pH 8 solution into the H2S-filled syringe, replace the latex cap and shake to mix the reagents. The pH of the solution will drop from 8 to 4 as the H2S dissolves in the solution. Discard the resulting solution by adding it to the Neutralization Solution. Keep the syringe capped when not in use. Excess H2S(g) can be used in Experiment 3.
H2S(aq) + H2O(l) H3O+(aq) + HS-(aq) Ka = 1 x 10-7
H2O(l) + H2O(l) H3O+(aq) + OH-(aq) Ka = 1 x 10-14
Experiment 3. Reaction
Between H2S(g) and NaOH(aq).
H2S(g) + NaOH(aq) NaHS(aq) + H2O(l)
Prepare the 6 M NaOH in a
glass beaker — too much heat is generated to do this part in a plastic
cup. The reaction with H2S can be
in a plastic cup, however. Pour the 25 mL 6 M NaOH into a plastic
cup or beaker. Use the H2S(g) that remains
Experiments 1 and 2 or prepare a fresh syringeful of H2S
as described above. It is unnecessary to transfer the gas to a
syringe for this experiment. Pull the plunger back by 5-mL in
to create a reduced pressure in the H2S-filled
then remove the latex syringe cap and draw a 5 milliliters of NaOH(aq)
into the syringe. Hydrogen sulfide reacts instantaneously with
NaOH(aq). The plunger may move rapidly inward and/or the NaOH
will be drawn rapidly into the syringe. The reaction is so rapid,
it could be surprising. The cup is used because its walls will
any splashed NaOH(aq). Discard the resulting solution by adding
to the Neutralization Solution.
Burns in Oxygen with a Blue Flame.
The combustion of H2S(g) in O2(g) produces SO2(g) as follows:
2 H2S(g) + 3 O2(g) 2 SO2(g) + 2 H2O(g) DH = -1036 kJ
Sulfur dioxide is an
oxide. In order to test for the presence of SO2,
withdraw 40 - 50 mL of the gas mixture (of SO2
O2) from the flask with a clean syringe equipped
a length of latex tubing. Pour 25 mL of a slightly basic aqueous
solution of universal indicator into the SO2/O2-filled
flask and it will turn to its acidic color. Excess
H2S(g) in the syringe can be used in other
or should be destroyed as described above in the Disposal section.
Hydrogen sulfide burns in an atmosphere enriched with oxygen. An audible 'roar' can be heard!
Using universal indicator (green in beaker) to test the acidic nature of the gas produced.
Transfer 3 mL
of the appropriate metal ion solution to each well of a 12-well plate
in Figure 4. Transfer 5 mL of 6 M NaOH to each of two wells
(labeled “OH-”) and 5 mL of 30% H2O2(aq)
to each of two wells. (If Experiment 6 is not being performed,
one well of each metal ion is required and the wells of H2O2(aq)
are not required.)
Prepare a syringeful of H2S(g) and wash the gas. Cap the syringe with a latex cap. Place the filled well plate along with the H2S-filled syringe and a plastic disposable pipet into a large plastic bag. Zip the bag shut with a pocket of trapped air to support the bag above the well plate.
Cd+2(aq) + H2S(aq) + 2 H2O(l) CdS(s) + 2 H3O+(aq)
Allow the reactions to
for at least 5 minutes before going on to Experiment 6. Do NOT
the plastic bag. The two wells of NaOH(aq) will absorb the excess
H2S(g) overnight. Clean-up: No clean-up is
at this point if Experiment 6 is being performed. If Experiment 6
is not being performed, follow the Clean-up procedure at the end of
of Metal Sulfides.
Without opening the plastic bag, use the plastic disposable pipet to transfer at least 3 mL of H2O2(aq) to one of each pair of wells for each metal sulfide. Within a few minutes, bubbles will appear in the well containing CuS. Within 40 minutes the solutions containing CdS and PbS will have returned to clear. In both cases, the sulfide anion has been oxidized to the sulfate ion as per:
CdS(s) + 4 H2O2(aq) Cd+2(aq) + SO4-2(aq) + 4 H2O(l)
Within 2 - 3 hours the dark color of bismuth sulfide will be replaced with white, insoluble bismuth sulfate:
Bi2S3(s) + 12 H2O2(aq) Bi2(SO4)3(s) + 12 H2O(l)
|Readers may be
to know that skunk scent contains trans-2-butene-1-thiol, CH3CHCHCH2SH,
(or “TBT”) which is somewhat related to H2S in
both molecules possess the thiol group, SH. As with the
studied in Experiment 56, TBT can be oxidized to the odorless sulfonic
This chemistry is used in a “home skunk remedy” for treating pets who
been sprayed by skunks. (“Skunk Non-scents,” Nancy Touchette, Chem
page 7, October, 1996.)
Photograph of reaction after reoxidation (second row) of metal sulfides.
end of the experiments, wipe excess lubricant off of rubber diaphragm.
Clean all syringe parts (including the diaphragm), caps and tubing with
soap and water. Use plenty of soap to remove oil from the rubber
seal. This extends the life of the plunger. It may be
to use a 3-cm diameter brush to clean the inside of the barrel.
all parts with distilled water. Be careful with the small parts
they can easily be lost down the drain. Important: Store plunger out of
in Chem13 News in May, 1997. The authors of the original
From Department of Chemistry, Creighton University, Omaha, Nebraska 68178 USA
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(This page last updated 30 October 2008)