A technique for analyzing inorganic cations using Fourier transform infrared spectroscopy

A technique utilizing Fourier transform infrared (FTIR) spectroscopy in the multiple internal reflection (MIR) mode was developed for analyzing inorganic cations in solution. By using a liquid cell assembly, 1M solutions of NaCl, KCl, MgCl$\sb2$, and CaCl$\sb2$ were in contact with a stearic acid film. Ionization of the $-$COOH group by the cations resulted in an intensity decrease of the C=O group at 1700 cm$\sp{-1}$ and the development of a carboxylate peak at 1556 cm$\sp{-1}$ for NaCl and KCl, 1560 cm$\sp{-1}$ for MgCl$\sb2$, and 1577 and 1540 cm$\sp{-1}$ for CaCl$\sb2$. Monovalent cations had a reaction rate 2-5 times that of divalent cations. Calcium formed double carboxylate peaks; sodium, potassium and magnesium all formed a single carboxylate peak. The formation of an infrared active carboxylate group, by cations in the presence of a carboxylic acid, allows the use of FTIR as an analytical tool for studying cations in solutions.