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pH
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 (see Chemical Analysis).
Universal Indicator
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.
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