Page 22 - Pharmaceutical analytical chemistry |

Scientists frequently express the concentration of a species in terms of its
p-function or p-value. The p-value is the negative logarithm (to base 10)
of the molar concentration of a species. Therefore, for a species X

pX = - log [X]
p-Values offer the advantage of allowing concentrations that vary over
ten or more orders of magnitude to be expressed in terms of small
positive numbers.

The ion-product constant for water

Aqueous solutions contain small amounts of hydronium and hydroxide
ions as a consequence of the dissociation reaction

2H2O ↔ H3O+ + OH-
An equilibrium constant for this reaction can be formulated as follows

The concentration of water in dilute aqueous solutions is enormous,
however, when compared with the concentration of hydrogen and
hydroxide ions. As a consequence, [H2O] in the above equation can be
taken as constant. So, K [H2O]2 = Kw = [H3O+] [OH-] where the new
constant Kw is given a special name, the ion-product constant for water.
At 25ₒC, the ion-product constant for water is 10-14. It allows us to
calculate the hydronium and hydroxide ion concentrations of aqueous
solutions.
In pure water, the concentrations of these two species are equal, since
there are no other sources of H+ or OH- except H2O dissociation.
Thus, [H+] = [OH-] = 10-7 M
Using the p-value:

pKw= pH + pOH = 14

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