No abstract

The binary phase diagram NaBO 2-H 2 O at ambient pressure, which defines the different phase equilibria that could be formed between borates, end-products of NaBH 4 hydrolysis, has been reviewed. Five different solid borates phases have been identified: NaBO 2 •4H 2 O (Na[B(OH) 4 ]•2H 2 O), NaBO 2 •2H 2 O (Na[B(OH) 4 ]), NaBO 2 •2/3H 2 O (Na 3 [B 3 O 4 (OH) 4 ]), NaBO 2 •1/3H 2 O (Na 3 [B 3 O 5 (OH) 2 ]) and NaBO 2 (Na 3 [B 3 O 6 ]), and their thermal stabilities have been studied. The boundaries of the different Liquid+Solid equilibria for the temperature range from-10 to 80 °C have been determined, confirming literature data at low temperature (20 °C ~ 50 °C). Moreover the following eutectic transformation, Liq. → Ice + NaBO 2 •4H 2 O, occurring at-7 °C, has been determined by DSC. The Liquid-Vapour domain has been studied by ebullioscopy. The invariant transformation Liq. → Vap. + NaBO 2 •2/3H 2 O has been estimated at 131.6 °C. This knowledge is paramount in the field of hydrogen storage through NaBH 4 hydrolysis, in which borate compounds were obtained as hydrolysis reaction products. As a consequence, the authors propose a comparison with previous NaBO 2-H 2 O binary phase diagrams and its consequence related to hydrogen storage through NaBH 4 hydrolysis.