Karl Fischer Moisture Analyzer Principle

Principle and Development of Karl Fischer Moisture Analyzers

Proposed by Karl Fischer in 1935, this volumetric method is the most specific and accurate for water—now the standard method for many substances.

Its titration and coulometric forms suit many inorganic and organic compounds—the globally recognized classic for rapidly determining moisture in liquids, solids, and gases, widely used in petroleum, chemicals, power, pharmaceuticals, pesticides, and research.

Beyond a good instrument, check for interferences, size samples properly, and operate carefully. In 1935 Karl Fischer proposed the volumetric (visual) method of GB6283 for colorless liquids; the electrolytic method followed, and combining the coulometer with volumetric analysis produced the coulometric method of GB7600. Karl Fischer thus divides into volumetric and coulometric branches, both standard methods for calibrating other methods and instruments.

Moisture Reaction Principle

When iodine oxidizes sulfur dioxide, a certain amount of water participates in the reaction: I₂+SO₂+2H₂O=2HI+H₂SO₄ The reaction is reversible; an alkaline substance must be added, and the reagent must include methanol or another solvent with active OH groups so pyridine sulfuric anhydride converts into stable methyl sulfuric acid pyridinium hydrogen.

In KF coulometric titration, the required iodine is generated directly by electrolysis; water content is calculated from the charge-iodine relationship. When free iodine appears, the voltage across the Pt indicator electrodes drops sharply—marking the endpoint.

Electrolysis Reaction (Faraday's Law of Electrolysis)

During electrolysis, the electrode reactions are:

Anode: 2I--2e→I₂

 Cathode: I₂+2e→2I- ，2H++2e→H₂↑

From the above, 1 mol of iodine oxidizing 1 mol of sulfur dioxide requires 1 mol of water. Electrolyzing 1 mol of iodine requires 2×96493 C (Faraday constant 9.6493×10000 C/mol), consuming 18 g of water.

From Faraday's law of electrolysis, M×10-6/18=Q×10-3/2*9.6493, so M=Q/10.722

Where: W--moisture content (μg); Q--electrolysis charge (mC); 18--molecular weight of water.