Isopiestic measurement of the osmotic and activity coefficients for the NaOH-NaAl(OH) 4-H 2O system at 313.2 K

Abstract

By using a specially designed and constructed isopiestic apparatus, we measured the osmotic coefficients at 313.2 K for the NaOH-NaAl(OH) 4-H 2O system with the total alkali molality, m NaOHT ( m NaOH + m NaAl[OH]4), from 0.05 mol/kg H 2O to 12 mol/kg H 2O and α 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-H 2O 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: β (0) NaOH: 0.08669, β (1) NaOH: 0.31446, β (2) NaOH: −0.00007367, C Φ NaOH: 0.003180, β (0) NaAl(OH)4: 0.03507, β (1) NaAl(OH)4: 0.02401, C Φ NaAl(OH)4: −0.001066, θ OH − Al(OH)4 − : 0.08177, Ψ 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 = (γ NaAl(OH)4,cal 2 · m Al(OH)4 −,exp )/(γ NaOH,cal 2 · m OH −,exp ) 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 H 2O. We also calculated the stoichiometric activity coefficients of NaOH and NaAl(OH) 4 and the activity of H 2O for the NaOH-NaAl(OH) 4-H 2O system, and these calculations establish their variations with m NaOHT and α 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 H 2O is weaker than that between OH − and H 2O.