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The solubilities and densities of solid–liquid equilibria in the quaternary system KCl–KBr–K2SO4–H2O at 348 K were determined experimentally by using an isothermal solution saturation method. According to the experimental data, the phase diagram of the quaternary system was plotted. In the equilibrium phase diagram of the quaternary KCl–KBr–K2SO4–H2O system at 348 K, there are one univariant dissolution curve and two crystallization fields corresponding to K(Cl, Br) and K2SO4, and no invariant point has been found in this quaternary system. The quaternary system contained solid solution K(Cl, Br). The density values in the equilibrium solution increase with the increase of potassium bromide concentration and decrease with the increase of potassium chloride concentration.

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The mean activity coefficients for NaBr in the (NaBr + SrBr 2 + H 2 O) ternary system were determined by electromotive force (EMF) measurements of the following cell without liquid junction: Na + − ISE−NaBr (m A ), SrBr 2 (m B )−Br − −ISE At the total ionic strengths ranging from 0.01 mol•kg −1 up to 2.00 mol•kg −1 at 298.15 K for different ionic strength fractions y B of SrBr 2 with y B = (0.0, 0.2, 0.4, 0.6, and 0.8). The activity coefficient results were interpreted based on the Harned rule and Pitzer model. Furthermore, the Pitzer ion interaction parameters θ Na + Sr 2+ and φ Na + Br − Sr 2+ were calculated. those parameters obtained with the Pitzer model were used to calculate the values of the mean activity coefficients of SrBr 2 , the osmotic coefficients, solvent activity, and the excess Gibbs free energy for the whole series of the mixed electrolyte system that was studied.