Isopiestic measurement of the osmotic and activity coefficients for the NaOH-NaAl(OH)(4)-H2O system at 313.2 K

摘要

By using a specially designed and constructed isopiestic apparatus, we measured the osmotic coefficients at 313.2 K for the NaOH-NaAl(OH)(4)-H2O system with the total alkali molality, m(NaOHT) (m(NaOH) + m(NaAl[OH]4)), from 0.05 mol/kg H2O to 12 mol/kg H2O and alpha(K) (m(NaOHT)/m(NaAl(OH)4)) from 1.64 to 5.53. The mean standard deviation of the measurements is 0.0038. Several sets of the Pitzer model parameters for NaOH-NaAl(OH)(4)-H2O system were then obtained by regressing the measured osmotic coefficients with the Pitzer model and the Pitzer model parameters for NaOH(aq). One set of the results is as follows: beta(NaOH)((0)): 0.08669, beta(NaOH)((1)): 0.31446, beta(NaOH)((2)). -0.00007367, C-NaOH(Phi): 0.003180, beta(NaAl(OH)4)((0)): 0.03507, beta(NaAl(OH)4)((1)): 0,02401, C-NaAl(OH)4(Phi): -0.001066, theta(OH)-(Al(OH)4)-: 0.08177, Psi(Na)+(OH)-(Al(OH)4)-: -0.01162. The mean standard difference between the calculated and the measured osmotic coefficients is 0.0088. With the obtained Pitzer model parameters, we calculated the values of K-1 = (gamma(NaAl(OH)4,cal2)(.)m(Al(OH)4)-(,exp))/(gamma(NaOH,cal2)(.)m(OH)-(,ex p)) for the gibbsite solubility. The results show that the obtained Pitzer model parameters are reliable, and the relative error of the calculated activity coefficients should be < 2.1%. We also compared the calculated gibbsite solubility data among several activity coefficients models over a range of m(NaOHT) at various temperatures. The comparison indicates that our activity coefficients model may be approximately applied in the ranges of temperature from 298.2 to 323.2 K and m(NaOHT) from 0 to 8 mol/kg H2O. We also calculated the stoichiometric activity coefficients of NaOH and NaAl(OH)(4) and the activity of H2O for the NaOH-NaAl(OH)(4)-H2O system, and these calculations establish their variations with M-NaOHT and alpha(K). These variations imply that the strengths of the repulsive interactions among various anions are in the following sequence: Al(OH)(4)(-)-Al(OH)(4)(-) < Al(OH)(4)(-)-OH- < OH--OH-, and the attractive interaction between Al(OH)(4)(-) and H2O is weaker than that between OH- and H2O. Copyright (C) 2003 Elsevier Ltd. [References: 41]