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In the ideal pH range 5-7, the Karl Fischer reaction runs quickly and stoichiometrically. At higher pH values a side reaction occurs which consumes iodine and leads to vanishing end points. In strongly acidic conditions the reaction constant of the Karl Fischer reaction decreases and the course of the titration is slower [1]. In practice, a pH range of 4-7 is acceptable.
The titration of water produces acids that must be neutralized. The HYDRANAL ® reagents contain imidazole or diethanolamine for buffering the titration system. The pH is thus stabilized around the ideal value of 5, ensuring a rapid and stoichiometric course of reaction.
Large amounts of strong acids or bases from the sample can interfere with the pH balance of the titration [1]. These substances must be neutralized by adding an appropriate amount of weak base, weak acid or buffer solution to the working medium.
The log K vs. pH curve for a Karl Fischer titration is shown in figure 1.1. Note the optimal range of pH 5-7 over which the log K does not change significantly.
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Figure 1.1: Dependence of the reaction rate constant K on the pH (Verhoef, J. C. and Barendrecht, E.; Mechanism and Reaction Rate of the Karl-Fischer Titration Reaction. J. Electroanal. Chem. 1976, 71, 305-315.) |
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