Preparation of highly concentrated aqueous solution of sodium borate

A highly concentrated aqueous solution of sodium borate with 5.24 mol/kg of boron was prepared at 20 °C by adjusting the Na/B ratio at 0.22 (pH 6.9). This is more than twice more concentrated than the most soluble known borate solution (Na2O · B2O3 · 8H2O, 2.49 mol/kg at 20 °C). Analysis using the Raman spectroscopy and the APCI/MS (atmospheric pressure chemical ionization/mass spectrometry) suggested the formation of polyborate ions such as image, image, image, image.