Development of Karl Fischer Reagents

II-1 Organic Compounds
21. Sulfur Compounds
The moisture content of most sulfur compounds, including sulfides,
disulfides, thiocyanic acid and sulfonic acid, can be measured by
means of direct titration using standard procedures. As with
hydrocarbons, these compounds have a tendency to slow the rate of
the reaction between water and Karl Fischer reagent, so they should
always be measured in excess methanol.
For volumetric titration, the substances are measured after they have
been dissolved in a general-use dehydrated solvent.
For coulometric titration, the sample is added directly to a general-use
electrolyte (anolyte).
The only substances that cause interference are mercaptan, thiourea,
thioacetic acid and dithio acid. These consume Karl Fischer reagent by
reacting with iodine as shown in the following formulae.
2RSH+I2 � RSSR+2HI
2RCSSH+I2 � RCSSSSCR+2HI
Before these substances can be titrated, they must be neutralized using
special procedures.
Dimethyl sulfoxide (DMSO) exhibits unusual behavior. Karl Fischer
titration appears to move toward a stable end point without any
difficulty, but with sequential addition of the sample the moisture
content declines with each analysis. It is important to note that the
presence of DMSO reduces the percentage of water recovered.
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Key Points
Examples of substances that can be measured directly:
Sulfides
Carbon disulfide, allyl sulfide, n-butyl sulfide, n-butyl disulfide, thiophene,
benzothiophene
Thiocyanates, etc.
Ethyl thiocyanate, ethylene thiocyanate, n-butyl thiocyanate, ethyl
isothiocyanate, 1-naphthyl isothiocyanate, ethyl thioacetate, benzothiazole,
1-acetyl-2-thiourea, methyl isothiourea