Conductivity, Thermal Measurements, X-ray Investigations, and Phase Diagram of the Na_2S_2O_7-K_2S_2O_7 System

摘要

The SO_2 oxidation catalyst solvent system Na_2S_2O_7-K_2S_2O_7 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 Na_2S_2O_7 and K_2S_2O_7. The phase diagram exhibits two eutectics at X_(K_2S_2O_7) equal to 0.4 (mp 615 K) and 0.6 (mp 613 K), respectively, and a maximum at X_(K_2S_2O_7) = 0.5 (mp 619 K) corresponding to formation of the compound NaKS_2O_7. 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) A, b = 7.2011(1) A, c = 7.4190(1) A, 250L?= 101.707(1)°, β = 90.696(1)°, γ = 94.241(1)°, and Z = 2. The crystal structure of K_2S_2O_7 was confirmed and the unknown structure of Na_2S_2O_7 was similarly shown to be triclinic, space group P-1, with a = 6.8037(1) A, b = 6.8265(1) A, c = 6.7846(1) A, α = 116.361(1)°, β = 95.959(1)°, γ = 84.194(1)°, and Z = 2. It could be excluded that the compound NaKS_2O_7 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 - T_m) + C(X)(T - T_M)~2, T_m = 693 K.