Polyborates in Aqueous Sodium Borate Solution at 298.15 K

Abstract

pH of aqueous sodium metaborate and pentaborate solutions were precisely measured as functions of concentration from dilute to super-saturation at 298.15 K. Coupling with Raman spectra of some super-saturation samples, a clear picture of the main polyborates and their equilibria was presented. At least five polyborates B(OH)₃, B(OH)₄^–, B₃O₃(OH)₄^–, B₄O₅(OH)₄^2- and B₅O₆(OH)₄^– exist in aqueous sodium borate solutions. In supersaturated aqueous sodium pentaborate solutions B₅O₆(OH)₄^–, B₃O₃(OH)₄^– and B(OH)₃ are the main polyborates, B(OH)₄^–, B₄O₅(OH)₄^2- present in a minor concentration. While, in aqueous sodium metaborate solutions, covering all the concentration range, B(OH)₄^– is the most important species other polyborate anions are less than 5 %. Relevant interactions mechanisms among polyborates were also suggested.