Potentiometric Titration
Potentiometric titration is a quantitative analysis that allows the exact concentration of a specific substance in solution (analyte) to be determined. It is based on the stoichiometric reaction between the analyte and another solution of known concentration (titrant), which must be added to the sample being analyzed. For example, the determination of acetic acid in vinegar is possible by stoichiometrically reacting acetic acid (analyte) with soda (titrant):
The titrant must be added to the analyte until the reaction is complete, and therefore the end of the reaction must be “observable” using an appropriate method, for example, optically (evaluating the change using a chemical indicator added to the sample) or potentiometrically (evaluating the change in electrochemical potential during the titration using a suitable electrode or sensor).
For a titration between analyte and titrant to be successful, it must be fast, complete, unambiguous, and observable.
Volumetric Karl Fischer Titration
In the volumetric Karl Fischer reaction, the water contained in the sample to be analyzed reacts quantitatively with Karl Fischer reagent, and the analysis is based on the following reaction:
The extraction solvent introduced into the reaction cell is initially “pre-titrated” with Karl Fischer reagent to eliminate the water it contains. A known quantity of the sample to be analyzed must then be added to this solvent. The water content of the sample is then determined by end-point titration (which is detected by the double platinum electrode with which the instrument is equipped), and the first ml of Karl Fischer reagent consumed (titrant) will be used by the instrument to directly calculate the concentration (e.g., %, ppm).
It is important that the Karl Fischer reagent has a certain factor, which is determined by titrating a standard with a known water content.
Coulometric Karl Fischer Titration
Coulometric titration is the most suitable method for analyzing samples with very low water content because it allows for precise determination of the water content from a minimum of 10 μg up to approximately 5% (the recommended maximum limit). In the Karl Fischer reaction, water reacts with iodine and sulfur dioxide in an alkaline environment and in the presence of alcohol.
In volumetric titration, iodine is added as a titrant, while in the coulometric technique, iodine is generated electrolytically in the anolyte, which contains iodide.
As long as water is present in the reaction cell, the iodine generated will react according to formula (1). When all the water has reacted, excess iodine will form in the anaolite, which will be detected by the double Pt electrode, and iodine production will cease. According to Farady’s law, the amount of iodine produced will be proportional to the current generated. In equation (1), I2 and H2O react in a ratio of 1.1.
One mole of water (18 g) is equivalent to 2×96,500 coulombs or 10.72 coulombs/1 mg H2O. The total amount of water contained in the sample can therefore be determined by measuring the total consumption of current generated during the analysis.
