Densities of Concentrated Alkaline Aluminate Solutions at Temperatures from (323 to 573) K and 10 MPa Pressure

Abstract

Densities of highly alkaline sodium aluminate solutions have been measured by vibrating-tube densimetry over the temperature range 323 ≤ T/K ≤ 573 at a pressure of 10 MPa. Ionic strengths, I, of these mixed solutions were 1 ≤ I/mol·kg−1 ≤ 6, and the degree of substitution of hydroxide by aluminate, α, ranged from 0.1 ≤ α ≤ 0.4, where α = mA/m and mA and m (= I) are the aluminate and total molalities, respectively. Apparent molar volumes, Vϕ, derived from the density data for the (NaOH + NaAl(OH)4)(aq) solutions, were found to follow Young’s rule; that is, they depended linearly on the level of substitution of aluminate for hydroxide at all temperatures. The slopes of the Young’s rule plots were independent of ionic strength at a given temperature but depended linearly on temperature. It is therefore possible to model Vϕ (and hence the densities) of ternary (NaOH + NaAl(OH)4)(aq) mixtures using only two parameters in addition to those needed for the correlation of the binary NaOH(aq) solutions. This also means that the standard state (infinite dilution) partial molar volumes, V2°, of pure, hypothetical NaAl(OH)4(aq) can readily be obtained from V2° for NaOH(aq) by linear extrapolation using the two Young’s rule parameters.