Complexation of aluminate anion by bis-tris in aqueous media at 25–50°C

Abstract

Gibbsite, Al(OH)₃, solubility studies in aqueous NaCl solutions at pH 9 demonstrate that the aluminate anion forms a strong complex with the neutral Bis-tris molecule 2,2-Bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol, at low temperatures. The logarithm of the molal concentration quotient for the reaction

$$Al(OH)_4^ - + H_2 [Bis - tris] _ \leftarrow ^ \to Al(OH)_2 [Bis - tris]^ - + 2H_2 O$$

in aqueous NaCl solutions at 0.1 molal ionic strength is 2.63±0.03 at 25°C and 2.6±0.1 at 50°C. Raman spectroscopy of NaOH−NaAl(OH)₄ solutions at 25°C quantitatively verifies these results and indicates that a complex with symmetrical vibrational frequencies at 498 and 574 cm⁻¹ is stabilized in the presence of excess Bis-tris in these solutions. Gibbsite solubility and Raman spectroscopy also indicate that a similar complexation reaction between the aluminate anion and dextrose occurs, with a molal equilibrium quotient of 10^0.65 at 25°C, 0.1 molal ionic strength. These complexation reactions are suggested to be equivalent to the wellknown reactions involving the borate anion and polyhydroxyl sugar-like molecules. Experiments with ethylene glycol, glycerol, mannitol and sucrose indicate no significant complexing of these compounds with the aluminate anion.