The uequivalence point/u of a titration is when the acid and base have been mixed in exactly the correct proportions according to the equation.

For hydrochloric acid and sodium hydroxide:

(HCl_(aq) + NaOH_{(aq)} → NaCl_{(aq)} + H_2O_{(l)} )

Because the ratio is 1:1, the equivalence point will have been reached when 25 cmsup3/sup of 1 mol dmsup-3/sup HCl is added to 25 cmsup3/sup of 1 mol dmsup-3/sup NaOH.

bAdding strong acid into strong base/b

For example: adding hydrochloric acid to sodium hydroxide:

mtaimg/images/topics/20/20-7-1.png/mtaimg

The pH decreases very slightly until very near the equivalence point, when it decreases sharply. The equivalence point is at 7 because sodium chloride and water are neutral.

bAdding strong acid into weak base/b

For example: adding hydrochloric acid to ammonia solution:

mtaimg/images/topics/20/20-7-2.png/mtaimg

Initially, the pH falls rapidly as acid is being added. Soon, the pH decreases slowly as more acid is added, because a buffer solution has been set up, since there is a mixture of ammonia and ammonium chloride. The equivalence point is around 5 because ammonium chloride is slightly acidic. Past the equivalence point, there is an excess of strong acid, so the graph behaves like adding a strong acid to a strong base.

bAdding weak acid into strong base/b

For example: adding ethanoic acid to sodium hydroxide:

mtaimg/images/topics/20/20-7-3.png/mtaimg

Initially, there is an excess of strong base, so the graph behaves like adding a strong acid to a strong base. The equivalence point is around 9 because sodium ethanoate is slightly alkaline. Past the equivalence point, the pH decreases slowly as more acid is added, because a buffer solution has been set up, since there is a mixture of ethanoic acid and sodium ethanoate. Since ethanoic acid is a weak acid, the pH won't drop significantly.

bAdding weak acid into weak base/b

For example: adding ethanoic acid and ammonia solution:

mtaimg/images/topics/20/20-7-4.png/mtaimg

This is a combination of the graph of adding strong acid into weak base (before the equivalence point) and the graph of adding weak acid into strong base (past the equivalence point). Note that there is no rapid drop in pH here (no steep section).

iCredit to Chemguide for the above graphs/i

bCalculating Ksuba/sub from the pH at the half equivalence point/b

The half equivalence point is the point at which half the weak acid is neutralised. The Ksuba/sub of a weak acid can be calculated by finding the pH at the half equivalence point.

(K_a = \dfrac{[H^+]A^-}{HA} )

At the half equivalence point, HA = Asup-/sup, therefore:

(K_a = \dfrac{H⁺\cancel{A^-}}{\cancel{HA}} )

(K_a = H⁺ )