FriendlyFinger - 27-6-2006 at 23:43
The other day I tried to titrate suspected 6% MeNH2 in MeOH, so I prepared 25 ml HCl containing 0.05 HCl mole with 3 drops of phenolphthalien and
in the burette was the MeOH which I thought contained about 0.1 but found it hard to get a sharp end point. The vague result was approx 3% MeNH2.
What did I do wrong?
pantone159 - 28-6-2006 at 09:59
Phenolphthalein is not a good indicator when titrating weak base (e.g. MeNH2) with strong acid (HCl).
When titrating a weak base, you want an indicator that changes (if I did my calculations right!) at
pH = 0.5 * pKa, where pKa is the constant for MeNH3+, and pKa = 14 - pKb.
Since MeNH2 has pKb = 3.36, you want an indicator that changes around pH 5.32.
Methyl red qualifies, and quinine probably will work as well (fluorescence changes around this pH value).