pH

A measure of the hydrogen ion concentration, [H⁺], and hence of the acidity or alkalinity of an aqueous solution. It is the logarithm of the reciprocal of [H⁺],  i.e.  log₁₀(1/[H⁺]).  The concentration of H⁺ in a litre of pure water (i.e. a neutral solution) is 10^-7 moles, giving a pH value of 7. One integer on the pH scale indicates a tenfold difference in H⁺ concentration. The lower the pH value, the higher the concentration of H⁺ and vice versa.