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Unit 2Elements of Chemical Change

2-9. pH AND ACIDITY

In discussing acids, bases, and salts, we often refer to a solution or compound being acidic, neutral, or basic in a qualitative manner. This concept is useful to us in a general sense, but would be of much greater value if we could speak in quantitative terms. It would be valuable if we could answer the question of how acidic one solution is in relation to another solution.

pH. The solution to this problem is not as difficult as it may seem. Acids donate protons (hydrogen ions, H+) in solution. Thus, the acidity of a solution must be related to this property.

In fact, the acidity of a solution is the concentration of hydrogen ions in that solution. Since we can calculate the hydrogen ion concentration, as you will learn later, we can now determine a numerical value of the acidity of a solution. The concentrations of hydrogen ions in both acidic and basic solutions are generally very small. A strong solution of HCl, for example, may contain only 0.01 mole of hydrogen ions per liter of solution. A solution of NaOH may have as little as 0.00000000001 mole of hydrogen ion per liter of solution.

To simplify the expression of such terms, chemists have transformed the concentration values into numbers, called pH numbers, which are easier to utilize. This is done according to the following equation:

pH = -log[H+]

The abbreviation log stands for logarithm. (For example, log 1 = 0, log 0.1 = -1, log 0.01 = -2, log 0.001 = -3, log 0.0001 = -4.) The expression [H+] here is the concentration of hydrogen ions in moles per liter. If we consider the two previous examples, you can see how this transformation aids us. The pH of the HCl solution would be -(-2.0) = 2.0; the pH of the NaOH solution would be -(-11.0) = 11.0. These numbers, 2 and 11, are certainly easier to work with than 0.01 and 0.00000000001.

pH Scale. This transformation results in a range of pH numbers from 0 to 14, which is called the pH scale.

The limits of the scale are related to the dissociation; how they are arrived at is beyond our scope. Further information on this relationship can be found in an inorganic chemistry textbook.

While you will not need to calculate a pH value, you will need to interpret what a pH value means at times. To learn this function, examine the following pH scale:

A pH value less than 7.0 means the solution is acidic; the lower the number, the more acidic. A solution with a pH of 2.0 is more acidic than one with a pH of 4.0. Any pH value greater than 7.0 means the solution is basic with larger numbers indicating solutions that are more basic. The only value on the scale that indicates a neutral solution is 7.0. The pH values for some common pharmaceutical products are given below.

Measurement of pH. There are three common methods for measuring pH, which you may encounter in medicine.

• Litmus paper. Litmus paper is a paper coated with a dye, which is red in an acid pH or blue in a basic pH. It will only indicate whether a solution is acidic or basic; it will not give an actual pH value.
• pH paper. pH paper works on the same principle as litmus paper but uses several different dyes. By comparing paper color with a chart, the pH of a solution can be determined within one pH unit. If a closer measurement is needed, special narrow-range papers can be used to determine the pH within 0.1-pH unit.
• pH meter. The most accurate tool for pH measurement is the pH meter. This makes use of an electrode dipped into solution and is accurate to about 0.01-pH unit, depending on the particular machine.