SSC 601: Advanced Soil Chemistry

The molecular weighs of water is 18.015 and 1L of pure water at 25⁰C weighs about 1000g. Therefore, the molar concentration of water is 1000/18.015 = 55.509 mol L^-1. One molecule of water in every 10⁷ molecule dissociate into H⁺ and OH^- ions. However, this slight dissociation of water is very essential for many of chemical processes and reactions in water.

H₂O = H⁺ + OH^-

The dissociation constant, K_a, of water at 25⁰C is 1.8 x 10^-16 .

Since concentration of H₂O is 55.509 M and [H⁺] is negligible,

so, [H₂O] - [H⁺] → H₂O

[H⁺] x [OH^-] = 1.8 x 10^-16 x [H₂O]

[H⁺] x [OH^-] = 1.8 x 10^-16 x 55.509

[H⁺] x [OH^-] = 9.99162 x 10^-15

Ion product of water, K_w, is given by,

K_w = [H⁺] x [OH^-]                                  (2)

K_w = 9.99162 x 10^-15                              (3)

Since, concentration of H⁺ and OH^- in water are equal, therefore, equation (3) can be written as,

K_w = [H⁺] x [OH^-] = [H⁺]²                        (4)

From equation (3) and (4), we can get,

[H⁺]² = 9.99162 x 10^-15 = 10^-14                     (5)

Taking negative log on both the sides, equation (5), becomes,

2pH = 14.0

pH = 7.0

The pH of pure water is 7.0 under control conditions, but under ordinary laboratory conditions, pH of distilled water is acidic (pH ≈ 5.0) because of removal of basic cations during distillation and absorption of CO₂ from atmosphere.

Remember that a meq of any cation is that amount of cation required to replace 1 meq of another cation.