Detection of Hydrate Forms of Lithium and Sodium Perchlorates in Aqueous Solutions Using near Infrared Spectroscopy

Abstract

A new approach to the study of concentrated electrolyte water solutions is proposed. NIR spectra of aqueous solutions of LiClO 4 and NaClO 4 were studied. The pure spectra and concentration profiles of spectral forms in the solutions were found by deconvolution of an experimentally measured data matrix. The chemometric analysis and the generalised phenomenological model of the structure of aqueous electrolyte solutions were used to establish that the variation in spectra of LiClO 4 solutions with concentration is due to antibatic changes in the fractions of water that is unbonded with ions and of water in the structure of the cybotactic group of lithium perchlorate trihydrate. Corresponding changes in NaClO 4 solutions are also associated with interconversion of sodium perchlorate forms of di-and monohydrates. Hydrate forms of lithium and sodium perchlorates in aqueous solutions are described, and a cybotactic group in a saturated solution is found to be equivalent to a solid-phase structure fragment. A strategy for selecting the nature of a background electrolyte is recommended. IntroductionAqueous solutions of alkali metal salts are of interest from the viewpoint of their use as background electrolytes. A number of studied parameters of chemical processes (the constants of complex formation, hydrolysis etc.) depend on the nature and concentration of the background electrolyte. 1 From a chemical viewpoint, the cations of sodium and lithium differ in the nature of their interaction with water, e.g. the free energies of Li + and Na + ion hydration are 114.6 and 89.7 kcal mol -1 , respectively. 2 The energy of the ion-dipole interaction of singly charged ions with water exceeds that of water-structure destruction by a quantity necessary for ion hydration. Therefore, lithium and sodium ions cannot cause strong polarisation of coordinated water molecules in the solution or change the spectral characteristics of such molecules to any considerable degree. 2 This makes the interpretation of spectral data more involved.The perchlorate anion is convenient because of its weak complexing, electron-donor and proton-acceptor properties. 3 NaClO 4 and LiClO 4 aqueous solutions have been studied using several methods: viscometry, 4 cryoscopy, 5 proton relaxation, 6 Raman spectroscopy in the frequency range of totally symmetric stretch vibration, n Cl-O of the ClO 4 -ion, 7 Fouriertransform infrared attenuated total reflection spectroscopy in the frequency range of the fundamental harmonic of stretch vibrations of water in perchlorate solutions up to the salt concentration 3 M 8 and absorption spectroscopy in the near infrared (NIR) region of 2 M sodium perchlorate water solution . 9 The special features of the physical and chemical properties of concentrated solutions are attributed to ion-ion interactions, in particular, the formation of solvent-separated and contact ion pairs. 7The aim of this paper is to detail a study of hydrate forms of lithium and soldium perchlorates in aqueous solutions using NIR spectroscopy. The prese...

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