Karl Fischer Method

The Karl Fischer method, abbreviated as the Fischer method, is a volumetric analysis method for determining moisture proposed by Karl Fischer in 1935. Among all chemical methods for determining moisture in substances, it is the most specific and accurate for water. Improved in recent years with greater accuracy and an expanded measuring range, it has been adopted as the standard method for moisture determination in many substances.

The Fischer method has two variants: titration (volumetric) and coulometric. It is suitable for determining water content in many inorganic and organic compounds and is a world-recognized classic method for determining moisture content. It can quickly determine moisture in liquids, solids, and gases; it is the most specific and accurate chemical method and a universally used industry-standard analysis method. It is widely applied in the petroleum, chemical, power, pharmaceutical, food, testing, and pesticide industries, as well as in universities and scientific research.

Differences Between Coulometric and Volumetric Karl Fischer Methods

The principle of Karl Fischer titration involves two reaction steps, as shown below:

(1) CH₃OH + SO₂ + RN → [RNH]SO₃CH₃ (RN = organic base)

(2) H₂O + I₂ + [RNH]SO₃CH₃ + 2RN → [RNH]SO₄CH₃ + 2[RNH]I   (RN = organic base)

During the reaction, iodine and water are consumed in a 1:1 molar ratio; by measuring the amount of iodine consumed, the water content in the sample can be calculated.

Karl Fischer moisture determination is an electrochemical method that can be divided intovolumetricandcoulometrictwo types.

When the volumetric method determines moisture content, it is mainly based on the electrochemical reaction:

Anode: 2I^- - 2e → I₂ ;   Cathode: I₂ + 2e → 2I^-;   2H⁺ + 2e → H₂↑

When both I₂and I^-are present in the reaction cell, the above reactions proceed simultaneously at both electrodes — I₂is reduced at the positive electrode and I^-is oxidized at the negative electrode; therefore, current flows between the two electrodes. If only I^-is present in the solution, without I₂, no current flows between the two electrodes.

Karl Fischer reagent contains active components such as iodine and an organic base (e.g., pyridine). When metered into the reaction cell, it undergoes the above reactions (1) and (2) with the water in the sample.

As the reaction proceeds, water is continuously consumed and I^-is generated until the titration endpoint, when all water is consumed. At this point, a trace of unreacted Karl Fischer reagent remains in the solution, with I₂and I^-coexisting; current begins to flow between the two electrodes, indicating the endpoint, and titration stops. The moisture content of the sample is determined by metering the volume of Karl Fischer reagent consumed.

coulometric

In the coulometric method (also called the electrometric method), the sample is dissolved in an electrolyte containing a special iodine-based solvent; the water in the sample reacts with and consumes iodine. However, the required iodine is no longer a standardized iodine reagent, but is generated by anodic oxidation of I- in the solution. The generated iodine reacts with the water in the sample and is consumed, then regenerated by electrolysis. When all water has reacted and the iodine concentration in the electrolyte returns to the preset level (excess generated iodine causes a potential change), electrolysis stops. The moisture content of the sample is then calculated according to Faraday's law of electrolysis.