(1) 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.

(2) 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

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.

b. **pH Scale**. This transformation results in a range of pH numbers from 0 to 14,

which is called the pH scale.

(1) 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.

(2) 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:

(3) 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.

MD0803

2-20

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