Volumetric or Coulometric – What is the difference?

Detection system is the same for each method – only difference is the means by which the “active ingredient” (Iodine) is introduced. In the volumetric technique the iodine is introduced via a burette or similar dosing system. In the coulometric technique the iodine is produced in-situ by electrolysis.

Volumetric technique is generally better suited to high water contents and is most widely used in food, agriculture, industries.

Coulometry, being 1000 times more sensitive, is better for low water content determination.

The volumetric technique involves dissolving a sample in a suitable solvent and adding measured quantities of a reagent containing iodine until an end point is reached. This end point is determined potentiometrically using a platinum electrode. The iodine concentration of volumetric Karl Fischer reagents must be checked using standards.

In the coulometric technique the required amount of iodine is produced at the anode which then reacts with any water present. The production of iodine is directly proportional to the amount of electricity. No reagent calibration / standardisation is required.

According to the stoichiometry of the reaction, 1 mole of iodine will react with 1 mole of water, and combining this with coulometry, 1 milligram of water is equivalent to 10.71 coulombs of electricity. It is therefore possible to directly determine the amount of water present in a sample by measuring the electrolysis current in coulombs.

Posted in: Theory