The applet below draws the graph of pH against volume of titrant added for the titration of strong acid by a strong base. Specifically:
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Press the run button to draw the pH curve for the strong acid/strong base titration.
Note that for this titration:
- the equivalence point is at exactly pH 7.0
The shape of the graph is explained as follows:
Before the equivalence point:
Added OH-(aq) present in the NaOH(aq) titratant is neutralised by the H3O+(aq) in the analyte solution to produce water:
OH-(aq) + H3O+(aq) --> 2H2O(l) Na+ (from the NaOH solution) and Cl- (from the HCl solution) are spectator ions in the process. The overall result is that the added base is neutralised, acid is consumed and a solution of the salt sodium chloride, NaCl(aq), is produced.
The resulting pH is simply given by [H3O+], the concentration of hydronium ions remaining:
pH = -log10[H+] Where [H+] is equivalent to [H3O+].
At the equivalence point:
All the acid has just been neutralised by the base to give a solution of sodium chloride in water only. The only source of hydronium ions is through the dissociation of water:
2H2O(l) H3O+(aq) + OH-(aq) given that:
[H3O+] x [OH-] = 1 x10-14 mol2/L2
and that:
[H3O+] = [OH-] = 1 x 10-7 mol/L
using:
pH = -log10[H+] = 7
After the equivalence point:
The pH of the solution is determined by the equilibrium for the dissociation of water with the concentration of H3O+(aq) being progressively decreased as the increasing concentration of OH-(aq) shifts the equilibrium to the left of the equation below:
2H2O(l) H3O+(aq) + OH-(aq)