Development of Karl Fischer Reagents

Chapter II: Applications - 1
1-Phenyl-2-thiourea, 1, 3-diphenyl thiourea and thiobenzamide also react in
the same way as thiourea. However, N-methyl thiourea and 1-acetyl-2thiourea
can be titrated directly without causing a reaction.
When thioacetatic acid is titrated directly, it reacts quantitatively with Karl
Fischer reagent and consumes 1 mole of iodine per mole. Dithio acid
causes a similar quantitative reaction. This interference can be prevented by
taking the following preliminary steps.
First, add an equivalent amount of n-butanol to the sample and treat it with
olefin using boron trifluoride as the catalyst. It appears that the butyl
mercaptan produced in this way combines with the olefin as shown in the
following formulae.
CH3COSH+C4H9OH � CH3COOH+C4H9SH
RCSSH+C4H9OH �RCSOH+C4H9SH
If excess butanol is used it will react with the acid during titration and
produce ester and water. The results must be corrected to allow for this.
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