Abstract
Heat capacities and phase-transition enthalpies between 270 and 400 K have been measured for commerical samples of sodium carbonate monohydrate and decahydrate by adiabaticshield calorimetry. Enthalpies and entropies at selected temperatures have been derived for the exact stoichiometries. The molar enthalpy of transition of Na 2CO 3 · H 2O to (Na 2CO 3 + aqueous solution) at 380.8 K was found to be (15.00 ± 0.20) kJ · mol −1. The molar enthalpy of transition of Na 2CO 3 · 10H 2O to (Na 2CO 3 · 7H 2O + aqueous solution) at 305.167 is (48.2 ± 0.6) kJ · mol −1. The molar enthalpy of transition of Na 2CO 3 · 10H 2O to (Na 2CO 3 · H 2O + aqueous solution) at 306.11 K is (67.1 ± 0.5) kJ · mol −1. Lattice constants determined from Guinier photographs for Na 2CO 3 · H 2O are a = 646.5, b = 1072.0, and c = 525.0 pm at 300 K; for Na 2CO 3 · 10H 2O: a = 1275.6, b = 898.0, and c = 1346.2 pm, and β = 122.66° at 285 K. Segregation of less-hydrated solid phases might be a problem for the use of Na 2CO 3 · 10H 2O as an energy-storage material.
Published Version
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