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

Abstract

A calorimetric and thermodynamic investigation of two magnesium phosphates, MgHPO4·3H2O­(cr) and MgKPO4·6H2O­(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 Δf H m o(T = 298.15 K, MgHPO4·3H2O,cr) = −2595 ± 3 kJ mol–1 and Δf H m o(T = 298.15 K, MgKPO4·6H2O,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 MgHPO4·3H2O­(cr) and from T = 5.9 to 329.0 K for MgKPO4·6H2O­(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 K–1 mol–1 for MgHPO4·3H2O­(cr) and 352.7 ± 2.1 J K–1 mol–1 for MgKPO4·6H2O­(cr). These new experimental results, together with literature data, are used to calculate the Gibbs free energy of formation, Δf G m o, for both phases, giving Δf G m o(=298.15 K, MgHPO4·3H2O,cr) = −2292 ± 4 kJ mol–1·and Δf G m(T = 298.15 K, MgKPO4·6H2O,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.

Benzo[a]pyrene: Standard Thermodynamic Properties from Adiabatic and Combustion Calorimetry and Density Functional Theory

“…I. Mendeleyev Institute for Metrology (VNIIM), Russia) and described in detail in our previous work. 39 According to those results, the standard heat-capacity measurements using this calorimeter and our experimental setup can be performed with a relative expanded uncertainty, U c,r (C p,m °) = 0.02 at T ≤ 50 K and 0.005 at T > 50 K at a 0.95 confidence level (k = 2).…”

“…The experimental uncertainty was determined from heat-capacity measurements made on crystalline benzoic acid (NIST SRM 39j, certified reference material for calorimetry, mass fraction purity 0.999996) and aluminum oxide (corundum, α-Al 2 O 3 , mass fraction purity 0.9999, manufacturerD. I. Mendeleyev Institute for Metrology (VNIIM), Russia) and described in detail in our previous work . According to those results, the standard heat-capacity measurements using this calorimeter and our experimental setup can be performed with a relative expanded uncertainty, U c,r ( C p,m ° ) = 0.02 at T ≤ 50 K and 0.005 at T > 50 K at a 0.95 confidence level ( k = 2).…”

Thermodynamic Properties from Adiabatic and Combustion Calorimetry of Two Polycyclic Aromatic Hydrocarbons, Benz[a]anthracene and Chrysene

“…The experimental uncertainty was determined from heat capacity measurements made on crystalline benzoic acid (NIST SRM 39j, certified reference material for calorimetry, mass fraction purity of 0.999996) and aluminum oxide (corundum, α-Al 2 O 3 , mass fraction purity of 0.9999, manufacturer, D. I. Mendeleyev Institute for Metrology (VNIIM), Russia) and is described in detail in our previous work . According to those results, the standard heat capacity measurements using this calorimeter and our experimental setup can be performed with a relative expanded uncertainty, U c, r ( C p , m o ) = 0.02 at T ≤ 50 K and 0.005 at T > 50 K at the 0.95 confidence level ( k = 2).…”

Standard Thermodynamic Functions from Experimental Heat Capacity Measurements of Two Polycyclic Aromatic Hydrocarbons, Indeno[1,2,3-cd]pyrene and Benzo[ghi]perylene

“…The experimental uncertainty was determined from heat-capacity measurements made on crystalline benzoic acid (NIST SRM 39j, a certified reference material for calorimetry, mass fraction purity = 0.999996) and aluminum oxide (corundum, α-Al 2 O 3 , mass fraction purity = 0.9999) and described in detail in our previous work . According to those results, the standard heat-capacity measurements using this calorimeter and our experimental setup can be performed with a relative expanded uncertainty, U c, r ( C p , m o ) = 2% at T ≤ 50 K and 0.5% at T > 50 K at the 0.95 confidence level ( k = 2).…”