Microscale Gas Chemistry:

Experiments with Sulfur Dioxide

 Link to SO2 data page including physical properties.

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.

Reducing 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.

    Because 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.

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 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.)

  • 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)
  •     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:

    NaHSO3(s) + HCl(aq)  SO2(g) + NaCl(aq) + H2O(l)

    Preparation 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.

    Preparation 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.

    Preparation of Sulfur Dioxide in the Microwave Oven.
         Samples of SO2(g) also can be prepared conveniently in a microwave oven.  (See details.)
    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 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.

    Figure 1

        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.

    Universal 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.

    Experiments with Sulfur Dioxide.

    Experiment 1. Sulfur Dioxide Reacts with Water. 
    • plastic cup or beaker, 250-mL (9 ounce)
    • SO2(g), 20 - 30-mL
    • Universal Indicator/pH 8 Solution (See: Instructions given above)
          Transfer 50mL Universal Indicator/pH 8 Solution to a plastic cup or beaker.  Generate SO2 as described above.  Remove the syringe cap and attach a 15 cm length of latex tubing to the syringe.  Slowly dispense 10-mL of the SO2 near the surface of the water (Figure 2) and notice the production of an acidic solution at the surface.    The reaction is:

    SO2(g)  SO2(aq)

    Figure 2

        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.

    Chemicals:     Add 50 mL of distilled water to a beaker.  Add 50 mL 3 M NaOH to another beaker.  Generate two syringefuls of SO2.  Replace the syringe caps from a SO2-filled syringes with separate 15-cm lengths of latex tubing.  Hold the syringe by the barrel and not by the plunger for this next part!  Draw 2-3 mL of the solution into the syringe and then pinch the tubing closed with your fingers.  The plunger will rapidly be pulled inward as the SO2 reacts.  (The action is quite fast and may be surprising to some.)

    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.

    Chemicals:     Prepare a very dilute aqueous solution of potassium permanganate.  Prepare a syringeful of SO2(g) as described above and transfer to a clean, dry syringe.  Pour 5-mL of the permanganate solution into the weighting boat and draw the solution into the SO2-filled syringe.  Cap with the syringe cap and shake the solution vigorously.  The pink color of permanganate will disappear.  The stoichiometry for the oxidation by permanganate under neutral or slightly acidic conditions is:

    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.

    Chemicals:     Prepare a dilute aqueous solution of bromine if not already available.  Prepare a syringeful of SO2(g) and transfer to a clean, dry syringe.  Pour  5-mL of the bromine solution into a weighing boat and then draw the solution into the syringe. Install the syringe cap and shake the solution vigorously.  The red color of bromine will slowly disappear.  The stoichiometry for the oxidation by Br2(aq) under neutral or slightly acidic conditions is:

    SO2(g) + Br2(aq) + 2 H2O(l)  2 Br-(aq) + SO4-2(aq) + 4 H+(aq)

    Experiment 5. Sulfur Dioxide Reacts with Dichromate.

    Chemicals:     Prepare a dilute aqueous solution of potassium dichromate if not already available.  The dichromate ion exists in equilibrium with chromate ion; the later becomes predominant as the solution is diluted.  In both cases, Cr is in the +6 oxidation state.  Prepare a syringeful of SO2(g) as described above and transfer to a clean, dry syringe.  Pour 5-mL of the chromate/dichromate solution into a weighing boat and then draw the SO2-filled syringe into the syringe.  Cap with the syringe cap and shake the solution vigorously.  The orange color of Cr+6(aq) will turn green, indicative of Cr+3(aq).  The stoichiometry for the reaction under neutral or slightly acidic conditions is:

    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)

    Experiment 6. Reaction Between H2S and SO2 Yields Elemental Sulfur. 
    • large test tube, 25 x 250 mm with suitable stopper
    • ring stand and clamp
    • 15-cm length of latex tubing, two pieces
    • tape (electricians tape)
    Chemicals:      Elemental sulfur is produced from hydrogen sulfide gas obtained from gas wells.  In the first step, some of the H2S is burned to produce SO2.  The SO2 is then reacted with more H2S to produce elemental sulfur.  The two steps are given as follows:

    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)

    Figure 3

    Yellow colloidal sulfur appears
    under the surface of the water.

        In this experiment we will demonstrate Step 2 of the sequence.  This experiment should not be attempted by those inexperienced with gas generation and manipulation using syringes.  Produce 30-mL of SO2 as described above and 30-mL H2S (reference given above).  Fill a large test tube with water and then pour the water out.  Moisture catalyzes the reaction and MUST be present.  Position the test tube vertically with a ring stand and clamp.  Tape the end of two lengths of latex tubing together near the open end.  Connect a length of latex tube to each of the two gas-filled syringes.  Place the open ends of the latex tubes near the bottom of  the test tube as shown in Figure 3. Simultaneously transfer 10-mL incremental amounts of SO2 and H2S to the test tube.  Although the stoichiometry calls for 2-mL H2S for every 1-mL SO2, it is best to keep the H2S as the limiting reagent. Soon after the gases come in contact, canary yellow sulfur will completely line the inside of the test tube.

        Slowly 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.
    • Clean syringes
    • 3-cm length latex tubing
    • flowers (impatens, African violets, red roses work well)
    • radish
    • fruit drinks (optional)
    • SO2(g), 60-mL (multiple syringes needed)
       Radishes and certain flowers including purple African violets and red roses are rapidly discolored by exposure to SO2.  Generate a syringeful of SO2 as described above and set it aside.  Place flowers or a radish in the syringe body of a clean syringe.  Install the plunger as far as possible, but not to crush the flowers (as shown in Figure 4).  With the short connector tubing, transfer 20 ? 30 mL SO2(g) to the syringe with the flowers or radish.  As usual, it may be necessary to assist the movement of the plunger outward as the gas is being pushed inward.  After 20 ? 30 mL of gas has been transferred, pull back 5-mL on the plunger of the syringe containing the flower/radish to create a reduced pressure.  Remove the latex connector tubing and cap both syringes.  Observe the experiment over the next several minutes and again after one hour and again after overnight.  The flower color change takes place within seconds.  The radish becomes noticeably lighter within a few minutes and white overnight.  Some fruit drinks also are affected by exposure to SO2.

    Figure 4


    Flower (impatens) leaf
    before exposure to sulfur dioxide

    Flower quickly fades after exposure to sulfur dioxide
    Normal radish (left) compared to one exposed to sulfur dioxide for several hours (in syringe)

    Experiment 8. Acid Rain Microchemistry.
    • 24-well plate
    • gallon-sized (4-L) sealable plastic food storage bag
    • SO2(g), 60-mL (multiple syringes needed)
    • Universal indicator/pH 8 solution, 150-mL
         Coal combustion produces sulfur dioxide which acts to produce acid rain.  In this experiment, a 24-well plate is used to create a series of lakes, six of which are buffered.  The 24-well plate is enclosed in a plastic bag to create an ecosystem.  The layout of a typical ecosystem is shown in Figure 5.  The "B" marks indicate the six lakes that will be buffered. Fill all 18 of the unlabeled wells with this solution.  If this experiment is to be used as a classroom demonstration using the overhead projector, fill the wells so they are slightly overfilled as shown in Figure 6.  Use a pipet to add the final drops to each well. To the remaining Universal Indicator solution, dissolve 0.1 g of sodium bicarbonate, NaHCO3.  Fill the remaining six lakes with this solution.  Place a 6-cm length of a plastic pipet stem between the four middle wells in order to prop up the plastic bag above the surface of the filled wells.  Next, slip the filled well plate into a plastic bag as shown in Figure 7.  Generate 60 mL SO2 as described above.  If a smaller bag is used, pierce a small hole through the bag with a sharp pencil and work the latex tubing through the hole as shown in Figure 7a. (Moistening the tubing with alcohol helps to facilitate this process.)  Zip the bag shut. If a larger bag is used, the syringe can be placed inside as shown in Figure 7b.  Once inside, remove the latex cap from the syringe by manipulating the syringe through the plastic bag.  Place the assembly on the overhead projector.  Discharge the gas into the bag.  As the gas drifts across the "landscape," the unbuffered lakes will become acidic.  The buffered lakes will eventually become acidified as well.  The entire acidification process takes 1-2 minutes for the unbuffered “lakes” and over 5 minutes for the buffered ones. 

    Figure 5

    Figure 6
    Side views of an underfilled well (left) 
    and a properly filled well (right)

    Figure 7a

    Figure 7b

          Allow the bag to stand overnight.  All of the sulfur dioxide will dissolve in the water.

        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:

    SO2(g)  SO2(aq)

    SO3(g) + H2O(l)  H2SO4(aq)

    The 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.

    Clean-up and Storage.

      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.

    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, 
    Michael Anderson, co-PI
    Joseph Nguyen, chemistry major, Creighton graduate, May, 2000, now working in Residence Life at Idaho State

    Joseph Lannan, Blair High School, Blair, NE

    Return to previous screen

    Go to Mattson Home Page

    (This page last updated 29 January 2002)