Investigation of the Polyborate Equilibria in Aqueous solutions by 11B-NMR and Raman spectroscopy

Abstract

The aim of this work was to extend our knowledge of the ions which are involved in equilibria in concentrated aqueous solutions of borates [1, 2]. The 11B nuclear magnetic resonance spectra of sodium, potassium and ammonium polyborates in D 2O solutions, - as a function of concentration and pH value -, provide information of the hydrolysis equilibria. Two NMR signals are observed in the aqueous solutions of the pentaborates Na[Ba 5O 6(OH) 4], K[B 5O 6(OH) 4] and NH 4[B 5O 6(OH) 4], while only one NMR line is recorded in the solutions of the tetrabirates Na 2[B 4O 5(OH) 4] and (NH 4) 2[B 4O 5(OH) 4]. A solution of (NH 4) 2[B 4O 5(OH) 4] at pH 7 shows two lines again [3]. The next step was to study the Raman spectra of solid H 3BO 3, Na 2[B 4O 5(OH) 4]·8H 2O, K[B 5O 6(OH) 4]·2H 2O and of Na[B(OH) 4] as well as of borax in aqueous solutions as a function of concentration and pH, recorded between 300 and 1500 cm −1. For comparisons the Raman spectra of solid (NH 4) 2[B 4O 5(OH) 4]·2H 2O, β-NH 4[B 5O 6(OH) 4]·2H 2O, NH 4[B 5O 6(OH) 4]·0.67H 2O and NH 4[B 5O 6(OH) 4] were considered. In solution, all lines in the Raman spectra could be assigned and the degree of depolarization measured. The assigned Raman frequencies can be used for the identification of unknown BO lines and therefore of unknown borate structures [4]. In the last step the influence of some series of foreign electrolytes in various concentrations of the equilibria of the borate species in a 0.5 M solution was investigated by Raman spectroscopy. On the one hand, addition of the foreign electrolyte solutions has a dilution effects, so that in many of these systems there exists only the [B(OH) 4] −/other hand, the effect is sometimes a forcing of the equilibrium in favour of the polyborate formation with the triborate ions [B 3O 3(OH) 4] − and [B 3O 3ion [B 4O 4(OH) 5] 2− is present only to a minute extent, while the formation of the pentaborate ion [B 5O 6(OH) 4] − is presented the most [5].