Ftir-spectroscopic investigation of sodium tungstate and sodium molybdate solutions in wide range of pH

Abstract

A FTIR spectroscopic study of aqueous solutions of sodium tungstate and molybdate (solu-tion concentration was 0.1 mol/l) over a wide pH range (factor (level) of acidity Z, Z = C (H+)/C (WO42-) or Z = C (H+)/C(MoO42-)) was carried out. In solutions of sodium tungstate complex fre-quency band at 885-865 cm-1 correspoding to the stretching vibrations v(W-O-W) was fixed. The fre-quency bands of 1720-1700 cm-1, 990, 985 and 1025 cm-1 corresponding to bending vibrations δ(W-OH) were fixed that indicates a significant change in composition of the solution as a result of hy-drolytic and polycondensation processes. The sodium molybdate solution has not bands correspond-ing to the stretching vibrations v(Mo-O-Mo). Only the characteristic bands of the deformation vibra-tions d(Mo-OH) were recorded. The low intensity complex band in the area of 885-865 cm-1 corre-sponding to the stretching vibrations δ(W-O-W) even for freshly prepared 0.1 mol/l sodium tungstate solution was appeared as well as the band at 1720-1700 cm-1 attributed to deformation vibrations δ(W-OH) that indicates a fast change in the solution composition. For solutions of sodium molyb-date bands of stretching vibrations v(Mo-O-Mo) are fixed at a pH less than 6 after standing for sev-eral days. With Hydra/Medusa program diagrams of distribution of molybdate and tungsten particle depending on the pH were calculated. In relatively dilute solutions, the diagrams received with Hy-dra/Medusa program showed the only protonated (monomeric) form of molybdate ions, where as in the sodium tungstate solution until pH of 9 W6O216- and HW6O215- particles exist that agrees with the results of IR spectroscopy. The results of IR spectroscopy and modeling with Hydra/Medusa pro-gram will be used to support the pH operating range for molybdate and tungstate-selective elec-trodes, since they are an important feature of any analytical ion-selective electrodes. For tungstate-selective electrode it is necessary to maintain the pH less than 9, for molybdate-selective electrode - less than 8 (with dilute ammonia). Considering the changes in the composition of sodium molybdate and tungstate solutions, for the design of molybdate and tungstate-selective electrodes the freshly prepared solutions have to be only used, rather than stored for more than two days.