Acids and bases II

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3) pH, dissociation constants (Ka, Kb) and ionic product of water (Kw)

The pH of a solution is the negative logarithm to base ten of the molar concentration of hydrogen ion. Students should note that pH only tells you how acidic or basic a solution is, it does not tell you about the strength of the acid or base. A concentrated solution of a weak acid will have a low pH , but it does not mean that the acid is a strong acid

In order to measure the strength of the acid , chemists use the acid dissociation constant. The acid dissociation constant is an equilibrium constant. Basically it is the ratio of concentration of protons and conjugate base to the concentration of undissociated acid molecules. Since acid dissociation constants are equilibrium constants, they are not affected by concentration, unlike pH which is concentration dependent. The greater the Ka, the stronger the acid.


Dissociation of water
Water dissociates to a very small extent to form hydrogen ions and hydroxide ions. This is represented by the ionic product for water Kw, which is the [H+] X [OH-], 10e-14 for pure water at 25 degree celsius. Note that like acid dissociation constant, the ionic product for water is effectively an equilibrium constant, hence it is affected by temperature, i.e. Kw will vary with temperature.

4) Indicators for acid base titration
The most common indicators are litmus, methyl orange and phenolpthalein. Methyl orange is red at acidic pH, orange at pH 3 to 4 and yellow at pH 5 onwards. Litmus paper is red at acidic pH and blue at pH values of 7 and above. Phenolpthalein is colourless at at acidic pH (pH 0 to 7 ) and red at alkalike pH (pH 9 and above). Students shouldnote that not all indicators change colour at pH 7.

In a titration of strong acid against a strong alkali, at equivalence point when the acid is completely neutralized, the pH changes by a large extent from around pH 4 to 10. Thus both methyl orange and phenolpthalein can be used to indicate that equivalenc epoint has been reached.

In the titration of a strong acid with a weak base, the pH change is less drastic, from around pH 4 to 8, hence phenolpthalein is not a suitable indicator as it changes colour at around pH 8. Methy orange will be a suitable indicator.

In the titration of a weak acid with a strong base, the pH change at equivalence point is from around pH 7 to 11. Phenolpthalein will be a suitable indicator as it changes colour at around pH 8.
In the titration of a weak acid with weak base, the pH change at equivalence point is very small. hence there are no suitable indicators. You can probably detect the pH change using a pH meter.

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