Raman studies on the aluminate and carbonate anions in aqueous solutions

Abstract

Sodium aluminate solutions and their precipitates have been investigated by Raman spectroscopy. All Raman data are consistent with the existence of Al(OH)₄⁻ and Al₂O(OH)₆ ²⁻ anionic species in sodium aluminate solutions. In supersaturated solutions, both anions are in equilibrium during the induction process. After the induction period, sodium aluminate solutions produce a precipitate which contains neither the Al(OH)₄⁻ anion nor the Al₂O(OH)₆ ²⁻ anion. -- Quantitative Raman studies with 0.23 M NaNO₃ as an internal standard were employed to elucidate the equilibria in sodium aluminate solutions. The best values of the molar scattering constants Ji are equal to 0.25 ± 0.03 for the Al(OH)₄⁻ and 0.57 ± 0.18 for the Al₂O(OH)₆ ²⁻ species relative to the 1.0 M NO₃⁻ ionic mode at 1052 cm⁻¹. With these Ji values the formation quotient Q and equilibrium constant K are estimated. -- In addition, studies of the effects of temperature on the Raman frequencies and halfwiths of CO₃²⁻, Al(OH)₄⁻ and Al(OD)₄ anions in H₂O and D₂O reveal evidence for the presence of strong hydrogen bonds between these anions and water. There is no significant change in the hydrogen band strength at temperatures up to 150°C.