The Karl Fischer method is divided into Coulomb method and volumetric method.
The principle of Coulomb method is that Karl Fischer solution is composed of a mixture of dominant iodine and sufficient sulfur dioxide such as pyridine, methanol, etc. In the presence of water, iodine is reduced by sulfur dioxide to form pyridine Hydroiodic acid and pyridine methyl bisulfate in the presence of pyridine and methanol. The reaction endpoint is indicated by a detection unit composed of a pair of platinum electrodes. According to Faraday's laws of electrolysis, the number of iodine molecules participating in the reaction is equal to the number of water molecules, and the through charge is proportional. Simply put, the number of electron moles consumed by electrolysis is equal to the number of electron moles obtained by the decomposition of water molecules into hydrogen and oxides. Determine the amount of water required for electrolysis based on the amount of electrons consumed.
The principle of volumetric method is to titrate a water containing sample with a known concentration of reagent. When the titration reaches the endpoint, the water content in the sample is determined based on the amount of reagent consumed.
The advantage of the Coulomb method is that the testing accuracy is extremely high, usually reaching an accuracy of 0.1 micrograms. Suitable for measuring samples with extremely low water content, such as oils, alcohols, esters, benzenes, ethers, resins, and various chemical raw materials. And Karl Fischer's usage is minimal. The current market price varies from over 10000 to 30000 to 40000 yuan depending on the grade. The instrument is characterized by a host with a Electrolytic cell bottle, and the Electrolytic cell bottle has two pure platinum electrodes.
At present, the instrument testing accuracy of the capacity method in China is relatively low, and it belongs to manual testing devices. Of course, if we increase our research and development efforts on instruments, we can also achieve high accuracy, such as in Switzerland's Wantong. Currently, domestic volumetric moisture meters can roughly measure samples with high water content, and the test results for ppm level samples are not reliable. The advantage is that the price is low, and the current market price is around a few thousand yuan. The instrument is characterized by two glass Burette with scales, and the host part has an indicating pointer.
The note is that the Karl Fischer reagent for Coulomb method requires shading, sealing, and refrigeration. Otherwise, it will shorten the lifespan. Special attention should be paid to the sealing of volumetric reagents, as poor sealing directly affects the accuracy of test results.