the hydrogen ion, aH + , is being considered. The approximate equivalence to molarity, [ H+ ] can be presumed only in very dilute solutions ( ionic strength <0.1 ).
A logarithmic scale is convenient for expressing a wide range of ionic activities. Equation 1 in logarithmic form and corrected to reflect activity is :
( -log10 aH+ ) + ( -log10 aOH- ) = 14
or
pH + pOH = pkw
where :
pH+ = log10 aH + and
pOH = log10 aOH-
Equation 2 states that as pH increases pOH decreases correspondingly and vice versa because pKw is constant for a given temperature. At 25๐C, pH 7.0 is neutral, the activities of the hydrogen and hydroxyl ions are equal, and each corresponds to an approximate activity of
10-7 moles/L. The neutral point is temperature-dependent and is pH 7.5 at 0๐C and pH 6.5 at 60๐C.
The pH value of a highly dilute solution is approximately the same as the negative common logarithm of the hydrogen ion concentration. Natural waters usually have pH values in the range of 4 to 9 , and most are slightly basic because of the presence of bicarbonates and carbonates of the alkali and alkaline earth metals.
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