Thermodynamics of the system HCl+SmCl3+H2O from 5 to 55 degrees C. Application of Harned's rule and the Pitzer formalism

A comprehensive array of electrochemical cell measurements for the system H Cl + SmCl3 + H2O was made From 5 to 55 degreesC using a cell without liquid junction of the type: Pt: H-2(g. 1 atm)\ HCl (m(A)) + SmCl3 (m(B))\ AgCl. Ag (A) The present study, unlike previous studies of trivalent ions. are no t complicated by hydrolysis reactions. Measurements of the emf were perform ed for solutions at constant total ionic strengths of 0.025, 0.05, 0.1, 0.2 5, 0.5, 2.0, 1.5, 2.0, 2.5. and 3.0 mol-kg(-1). The mean activity coefficie nts of HCl (gamma (HCl)) in the mixtures were calculated using the Nernst e quation. All the experimental emf measurements (about 850) were first treat ed in terms of the simpler Harned's rule. Harned interaction coefficients ( alpha (AB) and beta (AB)) were calculated. The linear form of Harned's rule is valid for most ionic strengths. hut quadratic terms are needed at I = 1 .5 and 3 mol-kg(-1). The Pitzer model was used to evaluate the activity coe fficients using literature values. beta ((0)), beta ((1)), and C-phi, for H Cl from 0 to 50 degreesC and 25 degreesC for SmCl3, The effect of temperatu re on the parameters for SmCl3 has been estimated using enthalpy and heat-c apacity data. The mixing parameter Theta (H,Sm) was determined at 25 degree sC. The addition of the Psi (H,Sm,Cl) coefficient did not improve the fit s ignificantly and no temperature dependence was found to he significant. The value of Theta (H,Sm) = 0.2 +/- 0.01 represented the values of gamma (HCl) with a standard deviation of sigma = 0.009 over the entire range of temper atures and ionic strength. The use of higher-order electrostatic effects (( H)Theta (H,Sm), (b)Theta (H,Sm)) was included as it gave a better fit of th e activity coefficients of HCl.