Experimental methods in chemical engineering: Karl Fischer titration

Abstract

Karl Fischer titration (KF) measures the water content in organic and inorganic compounds. Water reacts with a titrant solution containing iodine and a sulphurous monoalkyl ester to produce a monoalkyl sulphate ester and HI. In volumetric titrators, a burette injects the titrant directly into a cell while coulometric titrators generate titrant in situ. Regardless of the volume of the sample, volumetric titration is limited to 100 mg, while coulometric titration is best suited for $>2\mathrm{mg}$. KF is fast ($<2\min$), considered accurate ($\pm 5\%$), and precise ($<1\%$). However, factors such as humidity, non-standard analytes (hexane, isopropanol, ethylene glycol), tritrant composition, and temperature compromise its repeatability: the % sample standard deviation, $s$ ($n=4$), varied from 2% to 60% ($\overline{s}=22\%,n_{\text{conditions}}=19$). This high variability is, in part, due to the experimental design that included samples and conditions ill-suited for the technique. However, even for the tests for which KF is suitable, $\overline{s}>10\%$. Certain compounds react with the titrant, which increases the variability. The main sources of error affecting the analysis are ambient moisture, pH, solvent, sample handling, and titration speed. As of 2023, the Web of Science has indexed 1332 articles with Karl Fischer as a key word (Topic), but more than 3600 mentioning the technique (All fields). A bibliometric analysis classifies these contributions into five clusters: spectroscopy, stability, temperature, solubility, and mixtures.