On the effect of impurities on the metastable zone width of phosphoric acid

The experimental data published in the literature on the metastable zone width, as determined by the maximum supercooling Delta T(max) using the conventional polythermal method, of phosphoric acid aqueous solutions containing impurities were analyzed to understand an increase in Delta T(max)/T(0) with an increase in saturation temperature T(0) of solute-solvent system and the effect of impurities on the metastable zone width. For the analysis the following relations were used: ln(Delta T(max)/T(0))=Phi+beta ln R (K. Sangwal, Cryst. Res. Technol. 44, 2009, 231-247) and (T(0)/Delta T(max))(2)=F(1-Zln R) (K. Sangwal, Cryst. Growth Des. 9, 2009, 942-950: J. Cryst. Growth 311, 2009, 4050-4061), where Phi, beta, F and Z are constants. Analysis of the experimental data revealed that: (1) the parameters Phi and F strongly depend on saturation temperature T(0) and concentration c(i) of impurities, but the constants beta and Z are independent of T(0) and depend on c(i), (2) the dependence of the parameters Phi and F on T(0) follows an Arrhenius-type equation with activation energy E(sat) (3) the activation energy E(sat) for diffusion of ions/molecules of phosphoric acid containing impurity ions is equal to the differential heat of adsorption Q(diff) for these impurities, (4) the effectiveness of an impurity is directly connected with the values of their differential heat of adsorption Q(diff); the lower the values of Q(diff) for an impurity, the lower is its effectiveness in promoting nucleation, (5) the activation energy E(sat) is not related with its heat of dissolution Delta H(s) and (6) the increase in Delta T(max)/T(0) with an increase in T(0) for phosphoric acid is associated with the activation energy E(sat) for diffusion of solute molecules in the solution such that E(sat)< 0. (C) 2010 Elsevier B.V. All rights reserved.