Indicators and Carbonic Acid

This one is just plain fun. Fill two 1-L graduated cylinders with 900 mL of water plus 10 mL of 1 M NH3 and place a magnetic stirring bar in each. Then put 2 mL of phenolphthalein indicator solution in one and 2 mL of bromothymol blue indicator solution in the other. Both indicators will have the color of their basic species. 

Drop a few chunks of Dry Ice (solid CO2) into each cylinder. As CO2 bubbles through each cylinder, the solutions become acidic. First, pink phenolphthalein color 

Add about 20 mL of 6 M HCl to the bottom of each cylinder through Tygon tubing attached to a funnel. Then stir for a few seconds on a magnetic stirrer. Explain what happens. The sequence of events is shown in Color Plate 4. 

CO2 in water makes carbonic acid, which has two acidic protons  

The value ATai = 4.46 X 10 applies to the equation all dissolved CO2 HCO7 + H  

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