16.3 What does the Karl Fischer titration measure?
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This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
16.3 What does the Karl Fischer titration measure?
In 1935 Karl Fischer used the reaction between iodine, sulfur dioxide, and
water to produce a technique for quantifying water [11]. In aqueous solution,
the reaction can be presented as I2 + SO2 + 2H2O <=> 2HI + H2SO4.
He used anhydrous methanol to dissolve the I2 and SO2, and added pyridine
to move the equilibrium to the right by reacting the acidic products.
Fischer assumed his modifications did not change the reaction and one mole of
iodine was equivalent to two moles of water. Smith et al.[12], demonstrated
that both the methanol and pyridine participate in the reaction and one mole
of iodine is equivalent to one mole of water. They suggested two steps:-
(1) SO2 + I2 + H2O + 3RN -> 2RN.HI + RN(SO2)O
(2) RN(SO2)0 + CH3OH -> RN(SO4CH3)H (where R = base = C5H5 for pyridine)
This was further investigated by E.Scholz [13], who proposed:
(1) CH3OH + SO2 + RN -> (RNH)SO3CH3
(2) H20 + I2 + (RNH)SO3CH3 + 2RN -> (RNH)SO4CH3 + 2(RNH)I (where R = Base)
The advantage of the Karl Fischer titration is that it has few interferences
and can quantify water from < 1ppm to 100% in diverse samples, ranging from
gases to polymers. It will measure all water that is made available to the
reagent. the endpoint is usually ascertained using a dead-stop endpoint,
and for low water levels coulometric techniques are used to quantitatively
produce the iodine by anodic oxidation of iodide. The procedures are
described in detail in ASTM, AOAC etc.
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