Cobalt Chloride coupled with UV/Vis spectroscopy in the determination of water content

Abstract

The Karl Fischer equipment is an established state of the art method of determining of water content. It is established that the UV/Vis spectrometer is readily available in laboratories. This dissertation seeks to investigate the possible option of using a UV/Vis spectrometer to determine water content. A method developed by Bai and Yang (2007) to determine the water content in alcohol-based solvents, using cobalt chloride (CoCl2) as an indicator and UV/Vis spectrometry to monitor changes in absorbance in relation to water content was adapted to determinate water content in tetrahydrofuran (THF). The method used different concentrations (1.69x10-3 mol/L, 3.42x10-3 mol/L and 6.98 x10-3 mol/L) of CoCl2 in anhydrous THF. Eight dilutions were prepared, in triplicates, for each CoCl2/ THF concentration by adding HPLC grade water (0.1, 0.5, 1, 2, 5, 8, 10, 15 µl) to reach a total volume of 1ml. THF has an absorbance between 200-320nm while CoCl2 has an absorbance between 480-720nm. The average absorbance for each dilution was calculated at 672nm, was plotted against the wavelength and their respective polynomial equations derived. An initial increase in absorbance was unexpectedly observed with the addition of water. This unexpected increase was however followed by a decrease in absorption on further addition of water. Two hypotheses are proposed to explain the initial increase in absorbance followed by a decrease in absorbance on further addition of water. The first hypothesis is that there is an incomplete dissolution of CoCl2 in THF which forms a very fine suspension escaping detection. The second hypothesis proposed is a possible incompatibility of CoCl2 with residues of molecular sieves. In order to test the second hypothesis, THF was dehydrated by MgSO4 and CaCl2 and UV/Vis analysis was performed. The dehydrations with MgSO4 and CaCl2 resulted in a cloudy THF solution which brought about ambiguously high absorbance results when the solution with CoCl2 was prepared and analysed by UV/Vis spectroscopy. UV/Vis absorbance of dehydrated THF by MgSO4 was lower when nano filtration was applied. The method for determination of water investigated in this research requires further investigation and elucidation before it could be considered as a possible substitute for the established Karl Fischer method.