Titration, a form of chemical analysis that measures the per cent content of a substance, works by adding some amount of a reagent of known concentration to the sample until a chemical reaction occurs. The Karl Fischer titration method, in particular, is a widely used and popular method used to measure the water content in a given substance.
Early History of Karl Fischer Titration
In 1935, the German chemist Karl Fischer developed a titration method to determine trace amounts of moisture in a sample of sulphur dioxide, known today as the Karl Fischer (KF) titration. However, as noted by Sigmaldrich, because lab technicians “disliked [the method] since it was difficult to determine the end-point” and due to residual “malodorous” pyridine in the reaction, the KF titrationnot gain any popularity in the scientific community.
Recent Modifications of the Karl Fischer Titration Method
After Eugene Scholz modified the method in the 1980s by replacing pyridine with imidazole, not only did the original malodorousness of the KF titration method disappear, but the increased basicity of the imidazole rendered more quickly obtainable and accurate end point. Due to this breakthrough, variants of the KF titration--the one-component, two-component and coulometric titration methods--quickly developed.
More recent modifications of the method have decreased or altogether eliminated use of carcinogenic solvents in KF titrations.
Applicability of the Karl Fischer Titrations
In industrial settings where even trace amounts of water can be extremely detrimental to equipment or chemical reactions, KF titrations serve as a useful and accurate determinant of water content. The basic principle of KF titrations relies on iodine with water in an alcoholic solution in the presence of sulphur dioxide and a base.
The main difference between the two variants of KF titrations--volumetric and coulometric--lies the application of the titrant. Due to this difference, coulometric titrations can detect far smaller quantities of water than volumetric titrations can.
In volumetric titrations, titrant--usually a mixture of known concentrations of ethanol, base, sulphur dioxide and iodine--is added directly to the sample of unknown moisture content. In this variant of the KF titration, the amount of titrant needed to convert the sample is used to determine the moisture content of the sample, where one mole of titrant is consumed per one mole of water.
Machinerylubrication.com describes coulometric titrations as containing a set-up of an anodic and cathodic compartment, where the “anodic compartment contains the anolyte solution which includes sulphur dioxide (SO2), iodide (I-) and imidazole” with a solvent of either methanol or ethanol. Water is titrated when it comes into contact with the iodine. Ultimately, the current required for iodide to reach the end point (when excess iodine lowers the voltage) can be used to measure the water content of the original sample, as one mole of iodine is consumed per two moles of water.
Advantage of Karl Fischer Titrations
Karl Fischer titration methods are preferred over other methods of titration due to a relative lack of interferences, its ability to quantify a range of water (from less than 1 ppm to 100 per cent), its accuracy and precision, and its versatility in measuring the moisture content in liquids, gases, as well as solids.