pH and pOH

pH is a measure of hydrogen ion concentration [H⁺] or hydroxonium ion concentration [H₃O⁺] in a solution. It is a measure of the strength of the acid or base character of a substance.

pOH is a measure of hydroxyl ion concentration [OH^–]

Ionic Product of Water (K_w):

H₂O_(l) \( \rightleftharpoons \) H⁺_(aq) + OH^–_(aq)

2H₂O_(l) \( \rightleftharpoons \) H₃O⁺_(aq) + OH^–_(aq)

The concentration of the two ions in water H⁺ & OH^– is 10^-7

[H⁺] is the concentration of hydrogen ion

[OH^–] is the concentration of hydroxyl ion.

The product is

[H⁺] [OH^–] = 10^-7 × 10^-7 = 10^-14

K_w is the ionic product of water or dissociation constant of water at 25ºC.

When [H⁺] = [OH^–], the liquid is neutral

[H⁺] = [OH^–] = 10^-7 mol/dm³

The ionic product, K_w = [H⁺] [OH^–] = 10^-14 enables pOH to record alkalinity as well as acidity.

Calculation of pH and pOH:

pH is defined as the negative Logarithm to base 10 of hydrogen ion concentration [H⁺] expressed in moles per dm³.

pH = -Log₁₀[H⁺], pH = – log₁₀ [H₃O⁺]