Wired Chemist

1. Experimental Procedure

   Caution: Inhalation of dust from sulfur, calcium carbonate, and calcium sulfate should be avoided. Due to the highly corrosive nature of potassium hydroxide, it should be handled carefully and with caution. Contact of all chemicals with eyes and other mucous membranes should be avoided. Therefore, appropriate eye protection should be worn at all times, and it is recommended that all work be done in a fume hood.

   Construction of side-arm test tube apparatus:

   • Obtain three 6-inch side arm test tubes, two 6-inch pieces of Tygon tubing, three pieces of glass tubing (0.6 cm external diameter; one 3-inch piece, and two 6-inch pieces) and three No. 2 rubber stoppers with bored holes. Carefully insert each piece of glass tubing into the bored hole of a stopper so that about 1 inch of one end of the tubing protrudes through the stopper. Plug the top of each side-arm test tube with a stopper / glass tubing assembly. The side-arm test tube fitted with the stopper containing the 3-inch piece of glass tubing will always contain the sample of sulfur to be burned, and for sake of clarity, will hereby be referred to as test tube #1. The test tube containing the scrubbing reagent is designated as test tube #2, and the test tube containing the distilled water as test tube #3. Connect the side arm of test tube #3 to an aspirator. For Part 1 of the experiment, connect test tube #1 to test tube #3 by using a piece of Tygon tubing that connects the glass tubing of test tube #1’s stopper to the glass tubing of test tube #3’s stopper (set test tube #2 aside). For Part 2, disconnect test tube #1 from test tube #3, and reconnect test tube #1 to test tube #2 by using a piece of Tygon tubing. Then connect the side arm of test tube #2 to the glass tubing of test tube #3.

   Part One. The increase in rainwater acidity brought about by SO₂(g)

   • Place approximately 0.3 g of sulfur into test tube #1, and approximately 9 mL of distilled water into test tube #3 (connected to the aspirator). If needed, adjust the position of the glass tubing in the stopper of test tube #3 so that the end of the glass tubing inside of the test tube is immersed in the water. Using a piece of pH paper, test the initial pH of the distilled water and record your observation. Turn on the aspirator to maximum suction (evidenced by the production of vigorous bubbles in the distilled water), and heat the sulfur in test tube #1 using direct heat from a Bunsen burner flame for approximately 1-2 minutes. Turn off the aspirator, and use another piece of pH paper to test the new acidity of the distilled water. Record your observation. (Note: If a noticeable increase in acidity (i.e. a lower pH) is not observed, ask your lab instructor for assistance, and re-run the experiment before moving on to Part Two.)

   Part Two. Exploration of four reagents as potential SO₂ scrubbers

   • Make the necessary modifications to the test tube set up by following the directions given in "Construction of side-arm test tube apparatus" for Part Two.

     Interaction of SO₂(g) with calcium carbonate (CaCO₃)

     • As in Part 1, place approximately 0.3 g of sulfur into test tube #1 and about 9 mL of distilled water into test tube #3 (once again, be sure that the end of the glass tubing inside test tube #3 is submerged in the water). Note the initial pH of the distilled water using a piece of pH paper. In test tube #2, place enough ground calcite (CaCO3) so that about 1/2 inch of the glass tubing extends down into the sample. Turn on the aspirator to maximum suction, and heat the sulfur in test tube #1 using direct heat from a Bunsen burner flame for approximately 3-4 minutes. Turn off the aspirator, and re-test the acidity of the distilled water. Record your observation. Next, replace the ground calcite in test tube #2 with an aqueous calcium carbonate slurry (instructions follow), and re-run the experiment exactly as performed previously, making sure to replace both sulfur and water with fresh samples before doing so. [To prepare the calcium carbonate slurry, place about 0.3 g of calcium carbonate in the test tube, add 9 mL of distilled water, and stir vigorously with a glass stirring rod until a cloudy solution is obtained (do not expect the calcium carbonate to dissolve).] Lastly, observe the effect that constant stirring of the aqueous calcium carbonate slurry has on the results by making the following modifications to the procedure: Place a magnetic spin vain in a fresh calcium carbonate / water mixture, and place a stirrer / hot plate directly under this test tube. Turn on the stirring mechanism to medium-high power. Run the experiment as carried out previously, making sure to test and record the pH of the distilled water both before and after the reaction.

     Interaction of SO₂(g) with calcium sulfate (CaSO₄)

     • In test tube #2, place enough drierite (calcium sulfate) so that about 1/2 inch of the glass tubing is surrounded by particles. Place a new sample of sulfur into test tube #1, and replace the distilled water in test tube #3 with a fresh 9 mL-portion. Run the experiment as carried out previously, making sure to test the pH of the distilled water in test tube #3 both initially and finally. Perform this reaction a second time using a calcium sulfate slurry in place of the drieritem (prepare the slurry in the same manner as was used for the calcium carbonate, using about 0.5 g CaSO₄ powder and 9 mL of distilled water).

     Interaction of SO₂(g) with sodium carbonate (Na₂CO₃)

     • Place approximately 10 mL of a 1M Na₂CO₃ solution in test tube #2. If necessary, adjust the position of the glass tubing in the stopper so that the end of the tubing is immersed in the solution. Replace the sulfur and distilled water with fresh samples, and run the experiment to observe the scrubbing ability of the sodium carbonate.

     Interaction of SO₂(g) with potassium hydroxide (KOH)

     • (Due to the corrosive nature of potassium hydroxide, it should be handled carefully. Direct contact with skin and eyes should be avoided.) Place enough KOH pellets in test tube #2 so that about 1/2 inch of the glass tubing is surrounded by sample. Replace the sulfur and distilled water with fresh samples, and run the experiment to observe the scrubbing ability of potassium hydroxide. Perform the reaction a second time using approximately 10 mL of a 1M KOH solution in place of the KOH pellets.

2. Background Information

   As concern about the harmful effects caused by acid rain has grown over the past years, various measures have been taken to reduce its production. One of the primary pollutants responsible for the formation of acidic rainwater is sulfur dioxide gas (SO₂) released into the atmosphere from coal-burning industrial factories and power plants. One method used to decrease the amount of sulfur dioxide being emitted is a process called Flue Gas Desulfurization, or "scrubbing" (1). This abatement process involves allowing the SO₂(g) to interact with a base such as limestone (CaCO₃) before it leaves the gas stacks of the factory. A slurry of limestone and water is sprayed down onto the coal combustion gases as they pass upwards through the stacks, thus converting the sulfur dioxide gas into calcium sulfite (1):

   SO₂(g) + CaCO₃(s) → CaSO₃(s) + CO₂(g)

   Other bases such as sodium hydroxide (NaOH) and potassium hydroxide (KOH) which are also potentially effective SO₂ scrubbers are not usually used on the industrial level because they are cost-prohibitive in large quantities (1) and substantially corrosive.

(1) Bunce, Nigel. Environmental Chemistry, 2nd Edition. Wuerz Publishing: Winnepeg, Canada, 1994.