Water Determination (Karl Fischer Method)
The Water Determination Test (Karl Fischer Method) is designed to determine water content in substances, utilizing the quantitative reaction of water with iodine and sulfur dioxide in the presence of a lower alcohol such as methanol and an organic base such as pyridine, as shown in the following formulae: H2O+I2+SO2 + 3 C5H5N C5H5N SO3 + CH3OH 2(C5H5N+H)I- + C5H5N SO3 (C5H5N+H)O-SO2 OCH3.
There are two determination methods different in iodine-providing principle: the volumetric titration method and the coulometric titration method. In the volumetric titration method, iodine required for reaction with water is previously dissolved in water determination TS, and water content is determined by measuring the amount of iodine consumed as a result of reaction with water in a sample. In the coulometric titration method, first, iodine is produced by electrolysis of the reagent containing iodide ion, and then, the water content in a sample is determined by measuring the quantity of electricity which is required for the electrolysis (i.e., for the production of iodine), based on the quantitative reaction of the generated iodine with water. Hereinafter in the Monographs, such a specification back titration) not more than 4.0% (0.5 g, indicates that when determined by weighing about 0.5 g of the
sample accurately and performing back titration, the water content is not more than 4.0% of the weight of the sample. Method Volumetric titration Generally, the apparatus consists of an automatic burette, a backApparatus
titration flask, a stirrer, and an equipment for amperometric titration at constant voltage or potentiometric titration at constant current. Because water determination TS is extremely hygroscopic, the titration apparatus should be protected from atmospheric moisture. Silica gel or calcium chloride for water determination is usually used for moisture protection. Procedure As a rule, the titration of the sample with water determination TS should be performed at the same temperature as that at the standardization of the TS, while protecting from moisture. The apparatus is equipped with a variable resistor in the circuit, and the resistor is adjusted to apply a definite voltage (mV) between a pair of platinum electrodes
B. GENERAL TESTS immersed in the solution to be titrated. The change in current ( A) is measured during the dropping of water determination TS (Amperometric titration at constant voltage). As titration continues , the abrupt change in current in the circuit occurrs, but returns to the original state within several seconds. At the end of a titration, the change in current persists for a certain time (usually, longer than 30 seconds). The end point of titration is determined at this electric state. Otherwise, by adjusting the resistor, a definite current is passed between the two platinum electrodes, and the change in potential (mV) is measured during dropping water determination TS (Potentiometric titration at constant current). With the progress of titration, the value indicated by the potentiometer in the circuit decreases suddenly from a polarization state of several hundreds (mV) to the nonpolarization state, but it returns to the original state within several seconds. At the end of titration, the non-polarization state persists for a certain time (usually, longer than 30 seconds). The end point of titration is determined when this electric state attains. In the case of back titration, when the amperometric titration method is used at constant voltage, the needle of microammeter is out of scale while an excessive quantity of water determination TS remains. It returns rapidly to the original position when the titration reaches the end point. Similarly, when the potentiometric titration method at constant current is used, the needle of the millivoltmeter is at the original position while an excessive quantity of water determination TS…