HYDROGEN PEROXIDE: THE LOW TEMPERATURE HEAT CAPACITY OF THE SOLID AND THE THIRD LAW ENTROPY

The heat capacity of crystalline hydrogen peroxide between 12° K. and the melting point has been determined with a low temperature adiabatic calorimeter. The heat of fusion was also measured and found to be 2987 ± 3 cal./mole. The two samples of hydrogen peroxide used were 99.97 mole % pure as deduced from behavior on melting and from premelting heat capacities; the triple point was estimated to be 272.74 K.The only anomaly observed in the heat capacity measurements was the absorption of 1.3 cal./mole at 216.8 ± 0.15° K., the lower eutectic temperature of H2O-H2O2 solutions. Such an effect is to be expected if the only significant impurity is water. The entropy of hydrogen peroxide as an ideal gas at 1 atm. pressure and 25° C. computed from the thermal measurements is 55.76 ± 0.12 cal./mole deg. Comparison of this datum with the recalculated statistical entropy leads to a value of 3.5 kcal./mole for the height of a hypothetical single barrier hindering internal rotation in the molecule. From these results it is concluded that hydrogen peroxide does not consist of two tautomeric modifications.