Experimental Calorimetric Study of Thermodynamic Properties of Two Magnesium Phosphates, MgHPO4•3H2O and MgKPO4•6H2O

A calorimetric and thermodynamic investigation of two magnesium phosphates, MgHPO4 center dot 3H(2)O(cr) and MgKPO4 center dot 6H(2)O(cr), was undertaken. The synthesis of both phases was performed from magnesium chloride and potassium phosphate solutions at room temperature. The synthetic products were characterized by powder X-ray diffraction and X-ray fluorescence methods. The enthalpies of formation of both phases were determined using hydrofluoric acid solution calorimetry giving Delta H-f(m)o(T = 298.15 K, MgKPO4 center dot 3H(2)O,cr) = -2595 +/- 3 kJ mol(-1) and Delta H-f(m)o(T = 298.15 K, MgKPO4 center dot 6H(2)O,cr) = -3718 +/- 3 kJ mol(-1). The low-temperature heat capacity, C-p,m(o) was measured using adiabatic calorimetry from T = 6.1 to 324.6 K for MgKPO4 center dot 3H(2)O(cr) and from T = 5.9 to 329.0 K for MgKPO4 center dot 6H(2)O(cr). Using these C-p,m(o)(T) data, the third law entropy at T = 298.15 K, S-m(o) is calculated as 212.9 +/- 1.5 J mol(-1) for MgKPO4 center dot 3H(2)O(cr) and 352.7 +/- 2.1 J K-1 mol(-1) for MgKPO4 center dot 6H(2)O(cr). These new experimental results, together with literature data, are used to calculate the Gibbs free energy of formation, Delta(f)G(m)(o) for both phases, giving Delta(f)G(m)(o) (=298.15 K, MgKPO4 center dot 3H(2)O,cr) = -2292 +/- 4 kJ mol(-1) and Delta(f)G(m) (T = 298.15 K, MgKPO4 center dot 6H(2)O,cr) = -3248 +/- 4 kJ mol(-1). Smoothed C-p,m(o)(T) values between 0 and 320 K are presented, along with the values for S-m(o) and the functions [H-m(o)(T) - H-m(o)(0)] and [G(m)(o)(T) - H-m(o)(0)], for both phases.