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Answer Posted / lalitha
What is the Karl Fischer Reaction?
ROH + SO2 + R’N �� [R’NH]SO3R + H2O + I2 + 2R’N �� 2[R’NH]I +
[R’NH]SO4R
[alcohol] [base] [alkylsulfite salt] [water] [iodine]
[hydroiodic acid salt] [alkylsulfate salt]
The alcohol reacts with sulfur dioxide (SO2) and base to
form an intermediate alkylsulfite salt, which
is then oxidized by iodine to an alkylsulfate salt. This
oxidation reaction consumes water.
The reactive alcohol is typically methanol or 2-(2-
Ethoxyethoxy)ethanol, also known as diethylene
glycol monoethyl ether (DEGEE), or another suitable alcohol.
Classic Karl Fisher reagents contained pyridine, a noxious
carcinogen, as the base. The reagents most
frequently used today are pyridine-free and contain
imidazole or primary amines instead.
Water and iodine are consumed in a 1:1 ratio in the above
reaction. Once all of the water present is
consumed, the presence of excess iodine is detected
voltametrically by the titrator’s indicator
electrode. That signals the end-point of the titration.
The amount of water present in the sample is calculated
based on the concentration of iodine in the
Karl Fisher titrating reagent (i.e., titer) and the amount
of Karl Fisher Reagent consumed in the
titration.
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