Water analysis based on coulometric methods is ideal for lubricants with a water content in a trace range of 10 ppm up to several thousand ppm (mg/kg). In this way, water contained in the sample is quasi „boiled out“. For determination using water vapour, additives and other sample components which could disturb titration, as when using a direct method, are retained in the sample.
Titration is a procedure used in quantitative analysis in chemistry. A known substance with an unknown concentration is converted in a chemical reaction using a standard solution with a precise concentration. The volume of the spent standard solution is measured and the unknown concentration of the sample solution calculated. Quantitative water analysis by titration was named after the German chemist Karl Fischer, who developed the method.
A method is described as coulometric if it is used to determine the quantitative amount of an oxidable or reducible compound. It is based on measurement of the electrical charge or amount of electricity converted at a working electrode. In the indirect coulometric determination of the water content, about 2 g of the lubricant sample is weighed into a glass vessel and hermetically sealed with a septum. The sample is subsequently heated to 140°C in an oven integrated in an autosampler. At the same time, the septum is pierced with a hollow needle and the sample vessel flushed with nitrogen. The vaporised water is transferred in this nitrogen stream into the actual measuring cell where it is initially titrated. This method is therefore classified as an indirect titration procedure.
A measuring cell contains a special Karl Fischer reagent from which iodine is produced by means of a generator electrode. As long as the nitrogen stream flushes water vapour from the sample glass into the measuring cell, it immediately reacts there with the iodine. The generator electrode must permanently compensate this loss of iodine and uses electrical current in the process. Once the water from the sample has reacted, the produced iodine remains unused in the measuring cell.
This condition is detected by a second electrode, the indicator electrode, and titration is ended. The water content of the sample can be calculated based on the amount of electricity used.
The coulometric method is extremely sensitive. The supply of electricity to the generator electrode is precisely regulated so that iodine is only produced in very small quantities. This enables precise determination of the water content from 10 mg/kg.