The basicity of a solution can be described by its concentration of OH^-. One way to express this concentration is in pOH units, which stands for the Power of the OH ion concentration. pOH is defined as

pOH = -log₁₀[OH^-]

By the definition of pOH, more basic solutions will have a lower pOH. If the pOH of a solution is below 7, it is an base, if it is above 7, it is acidic. Why 7? Because water autoionizes: the concentration of OH^- in pure water is 1.0*10^-7 M, and thus the pOH of pure water is -log(10^-7) = 7.

Example 1: Blood is slightly basic, with a [OH^-] of about 3.15*10^-7M. What is the pOH of blood?

Solution: The pOH is the -log₁₀ of the hydroxide ion concentration, so

pOH = -log₁₀(3.15*10^-7) = 6.5

Example 2: The pOH of household vinegar is about 11.0. What is the hydroxide ion concentration of vingear?

Solution: Set up the equation as usual, then take the inverse log (10^x on your calculator) of both sides to eliminate the log

pOH = -log₁₀([OH^-])

11.0 = -log₁₀([OH^-])

-11.0 = log₁₀([OH^-])

10^-11 = 10^{log₁₀([OH-])} = [H⁺]

[OH^-] = 10^-11 M

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