Karl Fischer reagent requires a high degree of freshness. When purchasing Merck Karl Fischer reagent, pay attention to the production date and use it immediately according to the amount of use. It should be kept away from light to extend the storage period.
At present, Merck Karl Fischer reagent without pyridine has come out, which has solved the problem that pyridine reagent has pungent odor. However, in the determination, it is found that the end point mutation of pyridine containing Karl Fischer reagent is obvious, and the color of the reagent at the end point is slightly brown yellow. According to experience, it can be predicted by the naked eye that the end point is coming, while the end point mutation of pyridine free Karl Fischer reagent is not obvious, and the color of the reagent at the end point is dark brown.
Applicable scope of Merck Karl Fischer reagent without pyridine.
Anhydrous methanol is widely used as a sample solvent. General organic compounds, saturated or unsaturated hydrocarbons, general inorganic compounds, acidic oxides, and some organic and inorganic salts can be applied. However, some ketones and aldehydes cannot be reacted with methanol. If it is found that the reaction cannot be interrupted, there is no end point, and the reaction continues, the question of whether there are side effects should be considered. When there is a side reaction, it only takes a few minutes, but it has been going on. At this time, glycol methyl ether can be used instead of methanol to obtain a more constant titration volume, and the water content of some ketone and aldehyde chemical products can be determined without any special technology.
Too high pH value and too high alkalinity of the sample will also cause side reactions, that is, continuous reactions without end points. At this time, if the pH value is too high, the buffer solution can be used to adjust the pH value. If the alkalinity is too high, toluenic acid and salicylase can be added to ease the alkalinity solution, but acetic acid cannot be used.
During methanol water titration (commonly known as blank titration), if the color in the reaction bottle gradually changes from colorless to dark brown, and the instrument still has no end point, it should be considered that the Karl Fischer reagent has failed, and the reagent should be replaced.