Method——Coulometric titration
Apparatus Usually, the apparatus is comprised of an electrolytic cell for iodine production, a stirrer, a titration flask, and a potentiometric titration system at constant current. The iodine production device is composed of an anode and a cathode, separated by a diaphragm. The anode is immersed in the anolyte solution for water determination and the cathode is immersed in the catholyte solution for water determination. Both electrodes are usually made of platinum-mesh.
Because water determination TS is extremely hygroscopic, the titration apparatus should be protected from atmospheric moisture. For this purpose, silica gel or calcium chloride for water determination is usually used.
Procedure Take a suitable volume of an anolyte for water determination in a titration vessel, immerse in this solution a pair of platinum electrodes for potentiometric titration at constant current. Then, immerse the iodide production system filled with a catholyte for water determination in the anolyte solution.
Switch on the electrolytic system and make the content of the titration vessel anhydrous. Next, take an accurately weighed amount of the sample containing 1 to 5 mg of water, add it quickly to the vessel, and dissolve by stirring. Perform the titration to the end point under vigorous stirring. When the sample is insoluble in the anolyte, powder it quickly, and add an accurately weighed amount of the sample to the vessel. After stirring the mixture for 5 to 30 minutes, while protecting from atmospheric moisture, perform the titration with vigorous stirring.
Determine the quantity of electricity (C) required for the production of iodine during the titration, and calculate the content (%) of the water in the sample by the formula below.
When the sample interferes with the Karl Fisher reaction, water in the sample can be removed by heating under a stream of nitrogen gas and introduced into the titration vessel by using a water-evaporation device.
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