CHEMICAL REACTIONS

The titration is based on the oxidation of sulphur dioxide by iodine in the presence
of water. It is the same reaction as the iodometric titration of sulphur dioxide in
water.
I2 + SO2 + 2H2O 􀃙2HI + H2SO4 (I)
In 1935, Karl Fischer published a description of “a new procedure for the titration of
water” using the above reaction in an anhydrous nonaqueous solvent. However, in
order to shift the equilibrium (I) to the right, it was necessary to neutralise the acids
produced. Originally pyridine was used as the neutralising base. Later on,
diethanolamine followed by imidazole were used as buffers.
Recent studies show that methanol, which is the most commonly used solvent,
contributes in the reaction. The Karl Fischer titration can therefore be described by
the two following reactions:
CH3OH + SO2 + RN => [RNH]SO3CH3 (II)
H2O + I2 + [RNH]SO3CH3 + RN -> [RNH]SO4CH3 + 2[RNH]I (III)
(RN designates the base used)
Ethanol-based reagents have recently emerged. These have the advantage of
being less toxic, offering more stable endpoints and faster kinetics.