Add two to three drops of phenolphthalein indicator solution. Stir. (If available, a magnetic stirrer will be helpful.) Test the conductivity. (It is best to use a conductivity tester with a 60-W light bulb for this and to leave the electrodes immersed in the liquid.) Slowly add Ba(OH)2 solution from the other buret with stirring. Note the conductivity as the base solution is added. Stop the Ba(OH)2 addition as close as possible to the neutrality point. The light bulb should glow more dimly until it completely stops glowing. This is the neutrality point. The phenolphthalein color should become visible at the same point.
After students have time to observe and record their observations, add additional Ba(OH)2 solution. The light bulb should begin to glow increasingly brightly and the phenolphthalein color should be come more pronounced. Ask students to propose explanations for the behavior of the conductivity light bulb.
Demonstration 5: pH and Environmental Gases
Dissolved gases often produce acidic or basic solutions.
5 Flasks, conical or round, 250-mL
Universal indicator solution
Gas source for CO2, NH3, and SO2
Dry Ice, if available
Dry Ice can cause severe frost bite and should be handled with gloves or tongs. Other gas sources need to be handled carefully. Use the fume hood. Goggles should be worn.
Add 100 mL of distilled water to each flask. Add universal indicator solution to produce an intense color. Bubble each individual gas, one at a time, into the distilled water in the separate flasks. Add a piece of Dry Ice to the fourth flask. The fifth flask can be left with distilled water as a control. Have students observe and interpret the results.
Demonstration 6: pH Measurement
pH can be measured in several convenient ways.
Narrow and broad range pH paper
pH pen or pH meter
Solutions with various pH values
Become familiar with the operating instructions for the pH measuring device.
If a pH meter is available and used, show students not only how to use the meter but also how to calibrate the instrument at a pH close to the value to be measured. Adapt this demonstration to your needs and available pH measuring devices. Indicators may also be used to supplement the pH paper or meter observations.
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