PH (HYDROGEN ION CONCENTRATION)

(1) A convenient method of expressing the acidity or basicity of a solution in terms of the logarithm of the reciprocal (or negative logarithm) of the hydrogen ion concentration. The pH scale runs from 0 to 14; a pH value of 7.0 indicates a neutral solution. Values above 7.0 pH indicate basicity (basic solutions); those below 7.0 pH indicate acidity (acidic solutions). Natural waters usually have a pH between 6.5 and 8.5. Because the units are derived from common logarithms, a difference of one pH unit indicates a tenfold (101) difference in acidity; similarly, a difference of two units indicates a hundredfold (102) difference in acidity. The term originally derived from "potential of hydrogen," or hydrogen power. (2) A term indicating the hydrogen ion concentration of a solution, i.e., a measure of the solution's acidity. The term (from French, pouvoir hydrogène, or literally, "hydrogen power") is defined as the negative logarithm of the concentration of H+ ions (protons): 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 Acid and Base), 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 0.107 moles/liter. Consequently, the pH of pure water is -log (0.107), which equals log 107, or 7. If an acid is added to water, however, an excess of H3O+ ions is formed; their concentration can range between 0.106 and 0.10 moles/liter, depending on the strength and amount of the acid. Therefore, acid solutions have a pH ranging from 6 (for a weak acid) to 1 (for a strong acid). Inversely, a basic solution has a low concentration of H3O+ ions and an excess of OH- ions, and the pH ranges from 8 (for a weak base) to 14 (for a strong base).