How do we Measure Water by Karl Fischer?

Discuss the problems caused by water in oil, including reduced film strength, hydrolysis, acid formation, and additive depletion. Establish the need for accurate water measurement methods.

Describe the simple heating approach to measure water by mass loss, and explain why it fails due to volatilization of light oil molecules along with water. Conclude that a more specific method is needed.

Present the reaction between sulfur dioxide, water, and iodine to produce sulfur trioxide and hydrogen iodide. Explain the 1:1 stoichiometric ratio between iodine and water molecules, allowing quantification of water by measuring iodine consumed.

Explain the coulometric approach using a sample mixed with analyte containing alcohol, sulfur dioxide, and potassium iodide. Describe how electrodes (anode and cathode) oxidize iodide ions at the cathode to form iodine, which reacts with water until electrons stop flowing.

Explain the simpler volumetric approach where iodine solution is added directly to the sample. Describe how a substantial voltage change indicates excess iodine and signals that all water has been consumed.

Specify that coulometric is most accurate for solutions with less than 2% water, while volumetric is best for solutions above 2% water. Explain the different accuracy ranges suited to each method.

Highlight that the method requires only small samples and is generally accurate to within 1%, with an effective range from 10 ppm to 100% water. Note that additives can interfere with the reaction and reduce accuracy.