Measuring Principle of Volumetric and Coulometric Moisture Meter
The volumetric method moisture meter calculates water content by measuring the volume of Karl Fischer reagent consumed during the reaction.
The Karl Fischer volumetric method of determining water content is based on the electrochemical reaction: I2 + 2eó2I- When both I2 and I- are present in the solution of the reaction cell, the reaction takes place simultaneously at the positive and negative ends of the electrodes, i.e., at one electrode I2 is reduced and at the other I- is oxidised, and therefore a current passes between the two electrodes. If there is only I- and no I2 in the solution at the same time, no current passes between the two electrodes. Karl Fischer's reagent contains active ingredients such as pyridine and iodine, which are measured by dropping them into the reaction cell and can react with water in the solution to be tested as follows:
H2O + SO2 + I2 + 3C5H5N →2C5H5N-HI + C5H5N-SO3
C5H5N-SO3 + CH3OH → C5H5N-HSO4CH3
C5H5N-HI→C5H5N-H++I- The reaction continues, constantly consuming water and generating I-, until the end of the titration of the reaction, when the water is consumed. At this point, the solution has a trace amount of unreacted Karl Fischer reagent exists, in order to occur at the same time I2 and I-, the solution between the two platinum electrodes began to conduct electricity, indicated by the current to reach the end point, stop the titration. The water content of the solution is thus calibrated by measuring the volume of Karl Fischer reagent that has been consumed (volumetric).
Principle of the Karl Fischer Coulometric Method (Volumetric Method)
The coulometric moisture meter calculates the water content by measuring the amount of current passed during the reaction.
The galvanic method, is based on the dissolution of the sample in a special solvent containing a certain amount of iodine in the electrolyte, water that is consumed iodine, but the required iodine is no longer calibrated iodine-containing reagents to carry out the titration, but through the process of electrolysis, so that the iodine ions in the solution in the anode oxidation of iodine: 2I - - 2e - - → I2 produced by the iodine and the reaction of the sample of the water. The end point is indicated by a double platinum electrode. When the iodine concentration in the electrolyte returns to the original concentration, electrolysis is stopped. Then according to Faraday's law of electrolysis: calculate the moisture content of the sample to be tested.
