Abstract
The water content is one of the mandatory criteria for the quality of pharmaceutical substances. Violation of the technological process for the manufacture of substances or their storage conditions can lead to undesirable outcomes: loss of crystallization water or dampness, which can cause processes such as hydrolysis, oxidation, decomposition, the formation of undesirable impurities that can have a toxic effect. The regulatory documentation regulates either only the upper limit of the water content, or the upper and lower limits (for substances containing water of crystallization in their structure), depending on the properties of the pharmaceutical substance. Coulometric Fischer titration is a promising method for the determination of water. The titrant is electrogenerated iodine, which is formed during the anodic oxidation of iodide ions contained in the anode solution. Further, iodine oxidizes sulfur (IV) in the presence of water according to the Fisher reaction and is reduced to iodide ions, from which iodine can be generated again. Measuring the current and time duration for electrolysis, on which the amount of iodine electrogenerated at the anode depends, makes it possible to calculate the water content in the pharmaceutical substance. Preliminary removal of moisture from the anolyte and the possibility of water determination only in the injected sample significantly increase the accuracy of the analysis. The advantages of this method are specificity, no need for preliminary standardization of the titrant and creation of calibration graphs, rapidity and simplicity of the experiment, possibility of full automation. A unified method for the determination of water by coulometric Fischer titration in substance of nicotinic acid, bendazole hydrochloride and lizinopril dihydrate has been developed. The relative standard deviation does not exceed 2%, the relative error of the mean is 1.8%. The methods can be used in the practice of analytical laboratories to assess the "Water" parameter in these pharmaceutical substances.
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