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 pK_a as well as K_a.

We can write the water dissociation equilibrium expression as K_w = [H₊] [OH_-] = 10^-14; then pK_w = 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.