Thermodynamic investigation of the ternary mixed aqueous electrolyte (MgCl 2 + MgSO 4) system by potentiometric method at T = 298.15 K

Abstract

This work reports the results of thermodynamic investigation of the ternary mixed-electrolyte system (MgCl 2 + MgSO 4 + H 2O). The investigation was performed based on Pitzer ion interaction model by using the potentiometric technique at T = 298.15 K. The mean activity coefficients of MgCl 2 in the mixed aqueous electrolyte system were determined on the galvanic cell without liquid junction of the type: Mg-ISE|MgCl 2 ( m A ), MgSO 4 ( m B ), H 2O|Ag/AgCl over total ionic strengths from (0.001 to 8.000) mol · kg −1 for different series of salt ratio r ( r = m MgCl 2 / m MgSO 4 = 2.5 , 5.0 , 7.5 , 10.0 , 15.0 and pure MgCl 2). The PVC based magnesium ion-selective electrode (Mg-ISE) and the Ag/AgCl electrode used in this work were made in our laboratory and had a reasonably good Nernst response. The experimental results obeyed the Harned rule, and the Pitzer model could be used to describe this ternary system satisfactorily. Pitzer ion-interaction parameters for mixed salts were determined for this system. Then, these parameters ( θ ClSO 4 = 0.0252 ± 0.0042 , ψ MgClSO 4 = - 0.0049 ± 0.0003 ) were used to calculate the values of the mean activity coefficients of MgSO 4, the osmotic coefficients of water ( ϕ) and the excess Gibbs free energies of solution ( G E ) for the whole series of the studied electrolyte systems.