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pH as a Scale of Acidity


The term pH denotes hydronium ion concentration in a solution. Water undergoes slight ionization to give hydronium ion (acidic character), and hydroxyl ion (basic character) - water is amphoteric.

H2O(l)    reversible reaction arrow H3O+(aq) + OH-(aq)

Therefore, a solution would either be acidic or basic depending on which ion is more in concentration, i.e. H3O+ or OH-. For pure water, the concentration of both ions are the same, and each is found at 25oC to be 10-7 mol/dm3. The equilibrium constant for the autoionization of water above is

Kw = [H3O+] [OH-]

Where Kw is the ionic product of water, [H3O+] is the concentration of hydronium ions and [OH-] is the concentration of hydroxyl ions. For pure water:

[H3O+] = [OH-] =10-7 mol/dm3
Therefore, Kw = 10-7 x 10-7 = 10-14 mol2 dm-6
l.e. Kw = 10-14 mol2 dm-6 at 25oC

lf however the concentration of H3O+ is more than 10-7 mol dm-3, the concentration of OH- will be less than 10-7 mol dm3, and the solution will be acidic.

In the same sense, the higher the concentration of OH- than 10-7 mol dm3, the lower the conc. of H3O+ than 10-7 mol dm-3, and the solution will be basic.

When the conc. of both are exactly equal, the solution will be neutral.

The dissolution of substances in water may increase or reduce the concentrations of any of H3O+ or OH-. lf [H3O+] is increased more than [OH-], the solution is acidic. The solution is basic if [OH-] is greater.

The pH of a solution is defined as the negative logarithm to base 10 of the hydronium ion concentration.

l.e. pH = - log [H3O+]

Note: the lower the numerical value of pH, the higher the concentration of H3O+ and the lower the conc. of OH- and vice versa.

It is also possible to use hydroxyl ion concentration (i.e. pOH) to indicate alkalinity.

l.e. pOH = - log [OH-]

Since [H3O+] [OH-] = 10-14

pH + pOH = 14

The pH scale

ph scale

pH Calculations

1. Calculate the pH of a solution, which has hydronium ion concentration of 6.0 x10-5 M

pH = -log [6.0 x10-5]
pH = - [log 6.0 + log10-5]
pH = - [0.78 - 5] = 4.22

(2). Calculate the hydronium ion concentration of a solution which has a pH of 11.73

pH = - log [H3O+]
11.73 = - log [H3O+]
log [H3O+] = - 11.73
[H3O+] = antilog of - 11.73

To obtain the antilog of - 11.73 from the anti-logarithm table, - 11.73 must be expanded into a positive and a negative number. The negative part should be an whole number, while the positive number a fraction.

To obtain these two numbers, break -11.75 into -11 and -0.73. Then, you can subtract 1 from -11 and balance up by adding 1 to -0.73.

i.e. -11 - 1 = -12 and -0.73 + 1 = 0.27.
Therefore, -11.73 = - 12 + 0.27
From the antilog table, antilog of -12 is 10-12, antilog of 0.27 is 1.9.
Therefore [H3O+] = 1.9 x10-12 M





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