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.
Use a fume
hood if available.
The gas-generation and gas-washing steps should be carried out inside a working fume hood if possible.
syringe pressure to prevent unintentional gas discharges.
The gases inside the syringe are possibly at a pressure slightly higher than the external room pressure. If the syringe cap is removed under these circumstances, an unintentional discharge of the gas in the syringe will occur. To prevent this, pull the plunger back by 5-mL or so before removing the syringe cap. Then some air will rush into the syringe when the cap is removed rather than discharging some gas into the room. The presence of small amounts of air does not affect the experiments described in this chapter.
Sulfur dioxide has an irritating odor and is a poisonous gas. Care must be taken when handling SO2(g). Exposure to concentrations as low as 8 ppm will produce coughing. If you start coughing due to SO2 inhalation, leave the laboratory to seek fresh air. Deadly concentrations for rats start at 1000 ppm. To put these concentrations in perspective, if 8 mL SO2 were dispersed evenly into a volume of 1 m3, the concentration of SO2 would be 8 ppm. Exercise caution when working with poisonous gases and vacate areas that are contaminated with unintentional discharges of gas.
SO2 is extremely water soluble, wash out syringes
with plenty of water to minimize the amount of the gas that dissipates
into the room.
All of these experiments are suited for use as classroom demonstrations. These experiments are not advised for use as laboratory experiments conducted by high school students due to the toxicity of SO2. The experiments are all suitable for university-level students.
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 or 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)
This quantity of sodium bisulfate will produce approximately 50 - 55 mL of SO2. The production of SO2 is relatively slow and it typically takes over a minute to fill a syringe. The reaction is:
4 g NaOH 1.7-g sodium bisulfite, NaHSO3(s) 5 mL 6 M HCl(aq) 5 mL universal indicator solution concentrated ammonium hydroxide (only the fumes will be used)
NaHSO3(s) + HCl(aq) SO2(g) + NaCl(aq) + H2O(l)
of Neutralization Solution.
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.
of Sulfur Dioxide.
The SO2 gas samples used in these experiments are generated by Method A. This quantity of NaHSO3 used (1.7 g) requires the use of a larger vial cap. (If this amount causes the vial cap to sink, start by floating an empty vial cap on a syringe filled with water. Then add NaHSO3 until the cap is nearly ready to sink. Lower the vial by floatation in the usual way.) If 1.7-g NaHSO3 is used, about 55 mL SO2 will be generated. If the plunger does not move easily in the barrel, gently pull the plunger outward every 10 seconds or so in order to accommodate the gas produced. The SO2(g) will effervesce from the solution. Stop the gas generation after the syringe is full by removing the latex syringe cap while it is directed upwards. Rotate the syringe 180o in order to discharge the reaction mixture into a container of water and then recap the syringe. Fit the latex syringe cap over the LuerLOK fitting. Care must be taken to stop the gas generation after the syringe is full. This is done by removing the latex syringe cap while it is directed upwards. Rotate the syringe 180o in order to discharge the reaction mixture and then recap the syringe.
of Sulfur Dioxide in the Microwave Oven.
Samples of SO2(g) also can be prepared conveniently in a microwave oven. (See details.)
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 SO2-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 sulfur dioxide 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 sulfur dioxide should be destroyed. This is accomplished most efficiently by suctioning some of the Neutralization Solution into the syringe. Glassware and syringes should be washed inside the hood before they are removed.
Indicator/pH 8 Solution.
Several of the experiments require a slightly basic universal indicator solution. Prepare a solution by mixing 200 mL distilled water plus 20 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.
1. Sulfur Dioxide Reacts with Water.
It may be necessary to dispense an additional 10 or 20-mL of the SO2 to achieve the desired effect.
The ammonia present reacts with sulfurous acid to produce aqueous ammonium bisulfite:
SO2(aq) + H2O + NH3(aq) NH4HSO3(aq)
Experiment 2. Sulfur
Dioxide Reacts More Quickly with NaOH(aq) than with Water.
SO2(g) + NaOH(aq) NaHSO3(aq)
Syringe-syringe transfer is simple with a latex tube.
This technique is used for many experiments throughout this
series including the next three experiments.
Experiment 3. Sulfur Dioxide
Reacts with Permanganate.
5 SO2(g) + 2 MnO4-(aq)+ 2 H2O(l) 2 Mn+2(aq) + 5 SO4-2(aq) + 4 H+(aq)
Cleaning-up Stained Syringes:
Brown stains left from permanganate solutions can be removed from syringes
with 1 M HCl(aq)
Experiment 4. Sulfur Dioxide
Reacts with Aqueous Bromine.
SO2(g) + Br2(aq) + 2 H2O(l) 2 Br-(aq) + SO4-2(aq) + 4 H+(aq)
Experiment 5. Sulfur Dioxide
Reacts with Dichromate.
3 SO2(g) + 2 CrO4-2(aq)+ 4 H+(aq) 3 SO4-2(aq) + 2 Cr+3(aq) + 2 H2O(l)
3 SO2(g) + Cr2O7-2(aq)+ 2 H+(aq) 3 SO4-2(aq) + 2 Cr+3(aq) + H2O(l)
6. Reaction Between H2S and SO2
Yields Elemental Sulfur.
Step 1. Combustion of H2S:
2 H2S(g) + 3 O2(g) 2 SO2(g) + 2 H2O(g)
Step 2. Redox Combination:
16 H2S(g) + 8 SO2(g) 3 S8(s) + 16 H2O(l)
add Neutralization solution to the test tube until it is 1/3-full.
Rest an oversized stopper (suitably sized stopper placed upside down) over
the test tube opening in order to minimize gas dispersion.
|Experiment 7. Sulfur Dioxide
Discolors Many Natural Colors.
Flower quickly fades after exposure to sulfur dioxide
|Experiment 8. Acid Rain
Sulfur dioxide is produced when coal, which by nature contains varying amounts of sulfur compounds, is combusted. The sulfur leaves the smoke stack in the form of SO2(g). In the atmosphere, sulfur dioxide is usually converted to sulfur trioxide which is far more "water soluble" than sulfur dioxide. Actually both SO2 and SO3 are acid anhydrides and react with water to produce aqueous sulfur dioxide and sulfuric acids, respectively:
SO3(g) + H2O(l) H2SO4(aq)
falls to earth with rain. In this acid rain demonstration, SO2(g)
and aqueous sulfur dioxide are acidifying the "lakes" while in our environment,
SO3(g) and H2SO4(aq)
are the normal culprits.
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
|This article first appeared
in Chem13 News in September, 1997. The authors of the original
Chem13 article are:
From Department of Chemistry, Creighton University, Omaha, Nebraska 68178 USA
Bruce Mattson, faculty member, principal investigator,
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(This page last updated 29 January 2002)