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.

Use a fume hood or work outdoor.
    The gas-generation and gas-washing steps should be carried out inside a working fume hood or outdoors.

Toxicity.
    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.

Suitability.
    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.

Syringe Lubrication.
    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

4 g NaOH

Contact the author for chemicals needed

universal indicator solution, 5-mL

concentrated ammonium hydroxide (only the fumes will be used)

    Prepare 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 NaOH for neutralization.’  This solution will be used to neutralized excess excess reagents in the experiments.
 

Preparation 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 180^o in order to discharge the reaction mixture into the beaker containing the NaOH Neutralization Solution.  Immediately recap the syringe.
 
 

Syringe-to-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.

Figure 1

Disposal.
    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.

Universal 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.
Equipment:

• 18 x 150 mm test tube with suitable stopper
• 15-cm length of latex tubing

• H2S(g), 15-mL

Place 10 mL distilled water in a 18 x 150 mm test tube (capacity 30 mL).  Stopper the test tube with a rubber stopper.  Prepare a syringeful of H2S as described above and transfer to a clean, dry syringe as described above.    Before removing the syringe cap, pull the plunger outward by 5 mL, thus creating slightly reduced pressure within the syringe.  Replace the 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 syringe/tubing 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 cap 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 the H2S(g).  Set the stoppered test tube aside and observe it over the next several hours.  After 24 hours most of the H2S will have been destroyed by oxidation.  Discard the resulting solution by adding it to the Neutralization Solution.
 

Experiment 2.  H2S is a Weak Acid.
Equipment:

• Plastic cup, 9-ounce, 250-mL

• H2S(g), > 30-mL
• 20 mL of the universal indicator/pH 8 solution (Instructions given above)

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).
Equipment:

• 250-mL beaker
• Plastic cup or beaker, 250 mL (9-ounce)

• H2S(g), 60-mL
• 25 mL 6 M NaOH (6 g NaOH(s))

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 performed in a plastic cup, however.  Pour the 25 mL 6 M NaOH into a plastic cup or beaker.  Use the H2S(g) that remains from Experiments 1 and 2 or prepare a fresh syringeful of H2S as described above.  It is unnecessary to transfer the gas to a clean syringe for this experiment.  Pull the plunger back by 5-mL in order to create a reduced pressure in the H2S-filled syringe, then remove the latex syringe cap and draw a 5 milliliters of NaOH(aq) into the syringe.  Hydrogen sulfide reacts instantaneously with the NaOH(aq).  The plunger may move rapidly inward and/or the NaOH solution will be drawn rapidly into the syringe.  The reaction is so rapid, it could be surprising.  The cup is used because its walls will contain any splashed NaOH(aq).  Discard the resulting solution by adding it to the Neutralization Solution.
 
 

Experiment 4.  H2S Burns in Oxygen with a Blue Flame. Equipment: 15 cm piece of latex tubing  glass Pasteur pipet 500-mL flask with suitable stopper birthday candle supported in a one-holed rubber stopper  matches or lighter Chemicals: H2S(g), 60-mL 25 mL 6% H2O2(aq)  0.10 g MnO2(s) 25 mL of the universal indicator/pH 8 solution (Instructions given above)      Fit a 15 cm piece of latex tubing into the end of a glass Pasteur pipet as shown in Figure 2.  It should make a snug fit.  Enrich a 500-mL flask with O2(g) by decomposing 25 mL 6% H2O2(aq) with 0.10 g MnO2(s) inside the flask.  (Do not drain the reagents from the flask.)  Place an inverted beaker over the flask's neck to minimize O2 loss.  Prepare a syringeful of H2S as described above.  Equip a one-holed rubber stopper with a birthday candle.  Set the candle a safe distance away from the syringe and light the candle.

Figure 2 Figure 3

     The general arrangement of the experimental apparatus is shown in Figure 3.  Fit the syringe with the pipet/tubing assembly.  Two people are needed to complete this procedure.  Move the tip of the pipet into the vicinity of the candle flame.  Slowly discharge the H2S into the candle flame at a rate suitable to ignite the gas and maintain a flame.  Move the burning pipet into the O2-filled flask.  The H2S will burn with a hotter flame and the characteristic blue flame will be evident.  An audible "roar" will be heard coming from the mouth of the flask.  Important!  Never withdraw the plunger while the pipet is lit!  H2S forms explosive mixtures with air.

   The combustion of H2S(g) in O2(g) produces SO2(g) as follows:

Sulfur dioxide is an acidic oxide.  In order to test for the presence of SO2, withdraw 40 - 50 mL of the gas mixture (of SO2 and O2) from the flask with a clean syringe equipped with 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 experiments 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.

 

Experiment 5.  Precipitation Reactions.
Equipment:

• 12-well plate shown
• six test tubes, 18 x 150 mm, with suitable stoppers
• six plastic disposable pipets
• gallon (4-L) sealable food storage bag
• filter funnel and filter paper (optional — See Bi⁺³ solution below)

• H2S(g), 60-mL
• Cd⁺²: 0.1 g Cd(NO3)2^.4H2O in 5 mL H2O
• Cu⁺²: 0.1 g CuSO4^.5 H2O in 5 mL H2O
• Pb⁺²: 0.1 g Pb(NO3)2 in 5 mL H2O
• Bi⁺³: 0.1 g Bi(NO3)3^.5H2O in 20 mL H2O; stir for 30 minutes and filter or allow to settle overnight; you will use the clear aqueous portion
• 10 mL 6 M NaOH(aq): 2.4 g NaOH
• 10 mL 30% H2O2(aq) (only necessary if Experiment 6 is being performed)

Transfer 3 mL of the appropriate metal ion solution to each well of a 12-well plate shown 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, only 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.

Figure 4

Allow the reactions to proceed for at least 5 minutes before going on to Experiment 6.  Do NOT open the plastic bag.  The two wells of NaOH(aq) will absorb the excess H2S(g) overnight.  Clean-up: No clean-up is necessary 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 Experiment 6.
 

Experiment 6.  Reoxidation 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:

Within 2 - 3 hours the dark color of bismuth sulfide will be replaced with white, insoluble bismuth sulfate:

Readers may be interested to know that skunk scent contains trans-2-butene-1-thiol, CH3CHCHCH2SH, (or “TBT”) which is somewhat related to H2S in that both molecules possess the thiol group, SH.  As with the oxidations studied in Experiment 56, TBT can be oxidized to the odorless sulfonic acid trans-CH3CHCHCH2SO3H.  This chemistry is used in a “home skunk remedy” for treating pets who have been sprayed by skunks. (“Skunk Non-scents,” Nancy Touchette, Chem Matters, page 7, October, 1996.)

Photograph of reaction after reoxidation (second row) of metal sulfides.

Clean-up:  Allow the bag to stand overnight.  The NaOH(aq) will react with excess H2S(g). Without opening the plastic bag, suction a few mL of the NaOH(aq) into the syringe to remove traces of H2S(g).  Wear gloves to avoid contact with unreacted H2O2(aq).  It is now safe to open the bag indoors.  Remove the contents carefully and discard the bag and pipet in the trash.  Discard metal ions according to local regulations.  Wash the syringe contents (NaSH(aq)) down the drain with plenty of water.
 
 

    At the 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 necessary to use a 3-cm diameter brush to clean the inside of the barrel.  Rinse all parts with distilled water.  Be careful with the small parts because they can easily be lost down the drain. Important: Store plunger out of barrel.