Instead of saying that a corner block is one thousandth of the
complete stack, or F = 0.001, we could say that relative to the
stack as a whole, a single corner block represents pF = 3.
This is the basis of the pH scale, with ions in solution replacing
blocks in a stack. The pH of a solution is defined as the negative
logarithm to base 10 of the hydrogen ion concentration:
In the same way, pOH is the negative logarithm of the hydroxide
ion concentration, pOH = -log10 [OH-], and
pK is the negative logarithm of any equilibrium constant, pK = -log10
K. The table on page 7 gives the value of pKa as well
We can write the water dissociation equilibrium expression as Kw
= [H+] [OH-] = 10-14; then pKw
= pH + pOH = 14.00. The product of the concentrations of H+
and OH- ions is a constant, 10-14, and the
sum of pH and pOH is another constant, 14.
Pure water has a pH of 7.00, acids have pH values less than 7,
and bases have pH values above 7. The pH of several commonly encountered
liquids are shown below.