• The?Ka?of a weak acid may be determined by finding the pH at the half equivalence point
• At the half equivalence point?Ka?= [H+]

Key steps in the procedure

• A pH probe or meter is calibrated before use
• 25 cm3?of 0.1 mol dm-3?ethanoic acid is pipetted into a conical flask and a few drops of phenolphthalein indicator are added
• A burette is filled with?0.1 mol dm-3?sodium hydroxide solution
• The contents of the conical flask are titrated against the sodium hydroxide until the indicator just turns pink
• A further pipette containing 25 cm3?of the acid is then added to the flask and the pH is measured

Specimen Results

• pH at half equivalence point = 4.75

Analysis

• After the addition of the second portion of acid, the solution is effectively a half-neutralised sample of acid
• This is the half-equivalence point at which the?Ka?= [H+]
• By measuring the pH at that point you can convert it to [H+] and hence find the?Ka?of the acid

pH = 4.75

[H+] = 10-pH?= 1.8 x 10-5?mol dm-3

∴ ?Ka?= 1.8 x 10-5?mol dm-3

• You can also say that the pKa?= pH at the half-equivalence point

The pH at the half-equivalence point in a weak acid titration gives the pKa of the weak acid