Karl Fischer titration

The elementary reaction responsible for water quantification in the Karl Fischer titration is oxidation of sulfur dioxide (SO2) with iodine: This elementary reaction consumes exactly one molar equivalent of water vs. iodine.

Iodine is added to the solution until it is present in excess, marking the end point of the titration, which can be detected by potentiometry.

The reaction is run in an alcohol solution containing a base, which consumes the sulfur trioxide and hydroiodic acid produced.

The anode solution consists of an alcohol (ROH), a base (B), sulfur dioxide (SO2) and KI.

Typical alcohols that may be used include ethanol, diethylene glycol monoethyl ether, or methanol, sometimes referred to as Karl Fischer grade.

The popularity of the Karl Fischer titration (henceforth referred to as KF) is due in large part to several practical advantages that it holds over other methods of moisture determination, such as accuracy, speed and selectivity.

However, the strong redox chemistry (SO2/I2) means that redox-active sample constituents may react with the reagents.

Although KF is a destructive analysis, the sample quantity is small and is typically limited by the accuracy of weighing.

However, KF suffers from an error called drift, which is an apparent water input that can confuse the measurement.

The major disadvantage with solids is that the water has to be accessible and easily brought into methanol solution.

Many common substances, especially foods such as chocolate, release water slowly and with difficulty, requiring additional efforts to reliably bring the total water content into contact with the Karl Fischer reagents.