pH represents hydrogen ion concentration in a logarithmic scale. However, in solutions with relatively large ionic strength (large concentration of ions) the total amount of hydrogen ions are not entirely effective in affecting pH. Some of these ions will be occupied in ion associations. Therefore, activity, a, is used to represent the effective concentration. The following equations summarize how to make pH calculations using activity:
Where, aH+
is the activity, γH+
is the activity coefficient, and CH+
is the molar concentration of the hydrogen ion.
Example: calculate the pH of a 0.05 M solution of HCl assuming activity coefficient of hydrogen ion equal to 0.84.
If the calculation is made without activity considerations then
pH = -log 0.05 = 1.30
If activity is considered then
pH = -log (0.83 × 0.05) = 1.38
Example: calculate the pH of a 0.05 M solution of HCl assuming activity coefficient of hydrogen ion equal to 0.84.
If the calculation is made without activity considerations then
pH = -log 0.05 = 1.30
If activity is considered then
pH = -log (0.83 × 0.05) = 1.38
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