UV/Vis spectroscopy in determining water content in solvents

Abstract

Background: Karl Fischer titration is one of the main methods used to determine water content in solvents. Adopting UV/Vis spectroscopy could be a simpler alternative to the Karl Fischer method. Purpose: To develop an alternative method to standard Karl Fischer titration for the determination of water content in tetrahydrofuran (THF) as an example of a solvent. Method: A method to determine the water content in alcohol-based solvents using cobalt chloride (CoCl₂) as an indicator and a UV/Vis spectrometer was adapted for THF. Three solutions of CoCl₂ in anhydrous THF at concentrations of 1.69 × 10⁻³ mol/L, 3.42 × 10⁻³ mol/L, and 6.98 × 10⁻³ mol/L were prepared. For each concentration, eight dilutions making up a volume of 1 mL were prepared in triplicate by adding 0.1, 0.5, 1, 2, 5, 8, 10, and 15 μL, respectively, of HPLC-grade water. Analysis of the solutions was conducted using UV/Vis spectrometry (200–800 nm). Results: THF showed absorbance between 200–320 nm. CoCl₂ showed absorbance between 480–720 nm, with a maximum absorbance at 672 nm observed in the 1.69 × 10⁻³ mol/L and 3.42 × 10⁻³ mol/L solutions, and at 669 nm in the highest THF/CoCl₂ solution. The average absorbance for each dilution was calculated at 672 nm and plotted. From the respective polynomial equations, an initial increase in absorbance followed by a decrease with further addition of water was observed. This variance in absorption could be due to incomplete dissolution of CoCl₂ in THF, forming a very fine suspension that escapes detection. Conclusion: UV/Vis spectroscopy may present an alternative method for the determination of water in solvents. However, Karl Fischer titration remains the method of choice.