Development of Karl Fischer Reagents

2. Inorganic Acids
Key Points
Chapter II: Applications - 1
The following problems arise when methanol is used as the titration
solvent for inorganic acids.
1) The pH of the Karl Fischer titration system is shifted to the acid side.
This lowers the reaction rate.
2) An esterification reaction occurs with acids at high temperatures.
This produces water, raising the measurement value.
To suppress these interference reactions the sample should be
neutralized with an amine, such as pyridine, before Karl Fischer
titration.
Since these substances commonly have a high moisture content,
volumetric titration is used. It is convenient to use Dehydrated Solvent
MS (manufactured by Mitsubishi Chemical). As this product contains a
relatively high percentage of pyridine as a reaction accelerant, it is not
necessary to add more pyridine.
In the case of hydrogen chloride gas, moisture is condensed in special
apparatus at -78 � C. The main constituent, hydrogen chloride, is then
separated so that the condensed water can be dissolved in pyridinemethanol
and titrated.
Reference:Muroi, Ono: Microchem.J., 18,234 (1973)
"Determination of Trace Moisture in Hydrogen Chloride Gas by
Karl Fischer Titration."
Examples of test methods
1) Sulfuric acid
Concentrated sulfuric acid (95% or higher) causes an esterification
reaction in methanol and produces water. By adding 50ml of Dehydrated
Solvent MS and using a small sample (approx. 0.5g or less), it is possible
to carry out direct titration without any interference reaction.
2) Nitric acid
The reagent nitric acid can be titrated directly by adding about 0.3g of
sample to 50ml of Dehydrated Solvent MS. In the case of fuming nitric
acid, the sample will have to be neutralized with pyridine.
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