pH: It’s a gas

Acid rain has been written about in the scientific and popular press for some time. This interest and exposure has led to the introduction of acid rain topics into the school science curriculum. A consideration of acid rain necessarily involves introducing students to the pH scale. The pH scale is used as a measure of the concentration of hydrogen ion (H+ ) in solution. The problem is that the pH scale is an inverse power of 10 scale; it is defined as minus the logarithm of the hydrogen ion concentration.

For the beginning and even advanced students there is nothing intuitive about the relation between hydrogen ion concentration and the pH scale. One activity that attempts to convey this relationship is the serial dye dilution demonstration. While this a good demonstration of the power of 10n scale, it does not convey the sense of chemical activity of hydrogen ions as a function of pH.

What I propose is that the reaction of hydrogen ion with the hydrogen carbonate (bicarbonate) ion (HC03 _) to generate carbon dioxide (C02 ) can be used in conjunction with the dye demonstration to give a more meaningful illustration of the pH scale. In equation form the reaction between hydrogen ion and hydrogen carbonate ion appears as:

H+ (aq) + HCO3--(aq) ----> H2O + C02 (q) (1)

By this equation it is seen that the relationship between hydrogen ion and carbon dioxide gas is a direct one. The carbon dioxide gas produced can be captured in some visible way, such as a balloon, thereby providing a clear chemical activity link to hydrogen ion concentration and, thereby, pH.

Following is the design of such a demonstration. To capture the carbon dioxide gas 10-centimeter pear shaped balloons were used. Balloons of this shape are approximately spherical when inflated and can easily contain 3.5-4.0 liters of gas. As a reaction vessel, 15-centimeter test tubes with an opening of 3 centimeters (including lip) were used. The balloons must fit snugly over the test tubes to prevent gas leakage.

Stoichiometeric calculation using equation 1 and the ideal gas law show that to produce 2.5 liters of carbon dioxide gas require about 0.1 moles of hydrogen ion and 8.4 grams of sodium hydrogen carbonate (NaHC03). Given the size of the reaction vessels a pH of -1 was chosen as the starting pH. This pH corresponds to 10 molar hydrogen ion. The source of hydrogen ion was the strong acid hydrochloric acid. If larger reaction vessels are used the starting pH could be 0 (1 molar hydrogen ion).

To deliver 0.1 moles of hydrogen ion would require 100 ml of the 0 pH solution whereas it would require 10 ml of the –1 pH solution. The required amount of sodium hydrogen carbonate was weighted out (for readers who do not have a balance, 8.4 grams is about 1 1/2 level teaspoons of sodium hydrogen carbonate) and loaded into each of five balloons.

The balloons loaded with sodium hydrogen carbonate are carefully fitted over test tubes containing 10ml of either –1, 0, +1, +2, or +3 pH solutions. Each contains 10 times less hydrogen ion and, as a consequence, will produce 10 times less carbon dioxide gas when the 8.4 grams of sodium carbonate are added. The contents of the balloons are emptied into the test tubes. Carbon dioxide generation is vigorous which makes the balloons inflate rapidly. The test tubes may need to be shaken a couple of times to ensure complete reaction. The loss of acid chemical activity, due to the decreasing amounts of hydrogen ion at successively higher pHs, is quite evident from the size of the balloons.

For more advanced classes, and to add a little chemical mystery to this demonstration, vinegar can be used as a source of hydrogen ion. Vinegar is about 4% acetic acid that makes it a bout 0.7 molar acetic acid. F acetic acid was a strong acid it would have a pH of 0.2, its actual pH is between 2 and 3. This is the case since acetic acid is a weak acid. A solution of a strong acid whose pH is 2 or greater produces little carbon dioxide gas as the above demonstration will clearly show.

By first measuring the pH of vinegar then adding the balloon load of sodium hydrogen carbonate produces quite a paradox. The reaction with vinegar is vigorous and the balloon inflates to a size indicating a solution of 0-1 pH, based on the demonstration. This apparent paradox can lead to a discussion of Le Chatelier’s Principle and weak and strong acids.

Questions? Comments??
John Pellerin <>