Conductivity, Thermal Measurements, X-ray Investigations, and Phase Diagram of the Na2S2O7−K2S2O7 System

Abstract

The SO2 oxidation catalyst solvent system Na2S2O7−K2S2O7 has been investigated by conductometric, calorimetric, and X-ray methods. The phase diagram has been constructed from the phase transition temperatures measured for totally 23 different compositions. The results agree well with previous thermal investigations. The phase diagram has been calculated from classical thermodynamics using all transition temperatures, the heat of liquid−liquid mixing obtained for 18 different compositions, and previously measured fundamental thermal data for liquid and solid Na2S2O7 and K2S2O7. The phase diagram exhibits two eutectics at XK2S2O7 equal to 0.4 (mp 615 K) and 0.6 (mp 613 K), respectively, and a maximum at XK2S2O7 = 0.5 (mp 619 K) corresponding to formation of the compound NaKS2O7. This was confirmed by XRD measurements and the crystal structure shown by Rietveld refinement to be triclinic, space group P-1, with a = 5.9050(1) Å, b = 7.2011(1) Å, c = 7.4190(1) Å, α = 101.707(1)°, β = 90.696(1)°, γ = 94.241(1)°, and Z = 2. The crystal structure of K2S2O7 was confirmed and the unknown structure of Na2S2O7 was similarly shown to be triclinic, space group P-1, with a = 6.8037(1) Å, b = 6.8265(1) Å, c = 6.7846(1) Å, α = 116.361(1)°, β = 95.959(1)°, γ = 84.194(1)°, and Z = 2. It could be excluded that the compound NaKS2O7 was a simple solid solution of the two compounds of the binary system. The measured conductivities in the liquid region of the 18 different compositions have been fitted to polynomials of the form k = A(X) + B(X)(T − Tm) + C(X)(T − Tm)2, Tm = 693 K.