Thermochemical investigation of the (phosphonic acid + N,N-dimethylformamide) system at T = (298.15 to 328.15) K

• The solution and dilution enthalpies of H3PO3 in DMF were measured over the wide concentration range.
• The heat capacity of crystalline phosphonic acid was determined.
• The standard solution enthalpies of ionic and molecular H3PO3 in DMF are negative and depend weakly on the temperature.

For the first time the solution enthalpies of crystalline phosphonic acid (H3PO3) in N,N-dimethylformamide (DMF) and dilution enthalpies of the acid solutions in DMF (H3PO3 concentrations: x2 < 0.781 molar fraction or m < 49 mol/kg) in pure DMF have been measured at T = (298.15, 308.15, 318.15, and 328.5) K. The temperature dependence of the heat capacity of crystalline H3PO3 was obtained also for the first time. The standard solution enthalpies of H3PO3 in DMF have been calculated at different temperatures according to two different models considering the phosphonic acid either as a weak electrolyte dissociating by the first stage or as a non-electrolyte able to form dimmers. A number of thermodynamic properties have been evaluated.