Conductivity, NMR, Thermal Measurements, and Phase Diagram of the K₂S₂O₇−KHSO₄System

Abstract

The phase diagram of the catalytically important K2S2O7−KHSO4 solvent system has been investigated by means of electrochemical, thermal, and spectroscopic methods. The phase diagram exhibits a eutectic at ΧKHSO 4 = 0.94(1) with a temperature of fusion of 205 °C. No compound is formed in the system, but the strong α → β solid−solid transition of K2S2O7, found at 318 °C with ΔH tr = 21.8 kJ/mol, gives rise to a marked change in the slope of the liquidus curve at this temperature. The experimental phase diagram is in very good accordance with a calculated diagram based on the assumption of an ideal liquid mixture. 39K, 1H, 17O, and 33S NMR measurements on the molten K2S2O7−KHSO4 mixtures up to 540 °C show that a fast ionic exchange takes place in the melt at all compositions. The conductivities of the solid and molten K2S2O7−KHSO4 systems were measured at 13 different compositions in the whole composition range, ΧKHSO 4 = 0−1. For each composition in the temperature range examined , the conductivity of the molten mixtures has been expressed by equations of the form κ = A(Χ) + B(Χ)(T − 600) + C(Χ)(T − 600)2. The measurements indicate an enhanced molar conductivity of the binary system, probably due to delocalization of the conducting ions compared to the pure molten components.

Thermodynamic evaluation and optimization of the (Na2SO4+ K2SO4+ Na2S2O7+ K2S2O7) system

“…The melting temperature is 692 K and the enthalpy of melting is 21.2 kJ AE mol À1 . The measured melting temperatures in other studies are in agreement with Erikssen et al [16]: 673 K [14], 683 K [17], 692 K [18], 687 K [15], 687 K [11]. The enthalpy of melting is in good agreement with cryoscopic measurements by Hansen et al [18], who reported a value of 19.93 kJ AE mol À1 .…”

“…Others have also recorded a solid-solid transition at similar temperatures (603 K [17,19], 588 K [11,15]). Erikssen et al [16] and Meehan et al [17] have also recorded a transition at T = 473 K. No enthalpy data for this transition have been reported. Similarly as for Na 2 S 2 O 7 , the low-temperature solid-solid transition is very close to the eutectic temperature of (K 2 S 2 O 7 + KHSO 4 ), which is reported to be 478 K [16].…”

“…For K 2 S 2 O 7 , the C p -values, transformation/melting temperatures and enthalpy of transformation/melting reported by Erikssen et al [16] were used in this study. Tabulated values of the measured C p in [16] are given in [8].…”

“…Tabulated values of the measured C p in [16] are given in [8]. The C p -functions in [16] are erroneously given as a function of°C [3], but this was corrected in their subsequent publications [3,8]. The melting temperature is 692 K and the enthalpy of melting is 21.2 kJ AE mol À1 .…”

“…Two solid-solid transitions have been reported. Erikssen et al [16] noted a solid-solid transition at T = 591 K, with a corresponding enthalpy of transition of 21.8 kJ AE mol À1 . Others have also recorded a solid-solid transition at similar temperatures (603 K [17,19], 588 K [11,15]).…”