SSC 601: Advanced Soil Chemistry

Ion product of water

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, Ka, of water at 250C is 1.8 x 10-16 .



Capture 2

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, Kw, is given by,


Kw = [H+] x [OH-]                                  (2)

Kw = 9.99162 x 10-15                              (3)


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


Kw = [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.

(Next Lesson) General concept of ion exchange in soils
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Prof. Dr. GKM Mustafizur Rahman
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