Calculating pH
The pH at various points in the titration curve can be calculated. This is useful because, for example, when detecting the equivalence point by an indicator, it is necessary to know the pH of the equivalence solution before chosing the indicator.
Titration of monoprotic weak acids with OH –
HA + OH –
A– + H2O
The known Ka of the acid and the corresponding expression is used to do these calculations, making appropriate substitutions for [HA] and [A –].
Titration of monoprotic weak acids with OH –
HA + OH –
A– + H2O| Ka = | [A – ][H3O+] |
| [HA] |
The known Ka of the acid and the corresponding expression is used to do these calculations, making appropriate substitutions for [HA] and [A –].
In the buffer region before equivalence, c(HA) = [HA] and c(A –) = [A –].
At the equivalence point the major species that is "pH active" is the weak base A –.
As the extent of reaction of a weak base with water is low, [A –] = c(A –).
To the extent that the weak base does react with water, HA and OH– are formed in equal amounts.
Thus [HA] = [OH –] = Kw/[H3O+].
At the equivalence point the major species that is "pH active" is the weak base A –.
As the extent of reaction of a weak base with water is low, [A –] = c(A –).
To the extent that the weak base does react with water, HA and OH– are formed in equal amounts.
Thus [HA] = [OH –] = Kw/[H3O+].