PH Theory

pH, term indicating the hydrogen ion (positively charged hydrogen atom) concentration of a solution, a measure of the solution’s acidity. Hydrogen ions are usually represented by the symbol H+. The term (from French pouvoir hydrogène, “hydrogen power”) is defined as the negative logarithm of the concentration of H+ ions: pH = -log10[H+], where [H+] is the concentration of H+ ions in moles per liter (see Mole). Because H+ ions associate with water molecules to form hydronium (H3O+) ions (see Acids and Bases), pH also is often expressed in terms of the concentration of hydronium ions.

In pure water at 22° C (72° F), H3O+ and hydroxyl (OH-) ions exist in equal quantities; the concentration of each is 1 x 10-7 moles/liter, creating a neutral solution. Consequently, the pH of pure water is –log (1 x 10-7), which equals log (1 x 107), or 7. If acid is added to water, however, an excess of H3O+ ions is formed: H+ (acid) plus H2O (water) yields H3O+ (hydronium ions). When the concentration of H3O+ exceeds the concentration of OH-, the solution becomes acidic. In an acidic solution, the concentration of hydronium (H3O+) ions can range from 1 to 1 x 10-7 moles/liter (but not including 1 x 10-7), depending on the strength and amount of the acid. Therefore, acid solutions have a pH ranging from 0 up to, but not including, 7. Acids with lower numbers are stronger. Inversely, when the concentration of OH- exceeds the concentration of H3O+, the solution becomes basic. In a basic solution, the concentration of hydroxyl (OH-) ions can range from 1 to 1 x 10-7 moles/liter (but not including 1 x 10-7). This corresponds to a concentration of hydronium ions that ranges from 1 x 10-14 to (but not including) 1 x 10-7 moles/liter. Therefore, basic solutions can have a pH ranging from 14 down to, but not including, 7. Bases with higher numbers are stronger.

The pH of a solution can be measured by titration, which consists of the neutralization of the acid (or base) by a measured quantity of base (or acid) of known concentration, in the presence of an indicator (a compound the color of which depends on the pH). The pH of a solution can also be determined directly by measuring the electric potential arising at special electrodes immersed in the solution

Chemical indicators change color in response to a solution’s pH. Most chemicals used as indicators respond only to a narrow pH range. A universal indicator, however, is sensitive to a large pH range because it uses several indicator chemicals. This illustration shows the color range of a typical universal indicator with the corresponding pH scale.