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pH is a logarithmic scale of active hydrogen ion concentration in a aqueous solution. In pure water, the concentration of hydrogen ions is 10-7M. The pH scale converts this to a convenient positive number.
pH is represented in a scale between 0-14 in aqueous solutions. A solution with a pH less than 7 is considered acidic, whereas a solution with a pH greater than 7 is considered alkaline. A solution with the pH of 7 is considered neutral.
Pure water undergoes autoprotolysis to yield equal numbers of hydrogen and hydroxide ions.
The equilibrium constant at 25oC is:
pOH is the negative logarithm of the active hydroxide concentration. The equilbrium expression can be rewritten as:
This equation applies for all aqueous systems. Very strong acids and bases can have pH's <0 and >14 respectively.
Although pH is defined for aqueous solutions, measurements can be made in non-aqueous solutions provided the solvent is sufficiently conductive, or a conductive solvent is added to a non-conductive sample.
pH is probably the most common of all routine measurements with numerous applications in laboratories, industries of all kinds, and the environment. The most common mode of measurement is using either a combined glass pH electrode, or a pH electrode coupled with a separate reference. The voltage developed has very low power, and hence measurement requires a special, high impedance meter.
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