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Heat capacity measurements were made from 5 to 300°K on salts of the type RCl3·6H2O, with R being Gd, Tb, Ho, and Lu. The lattice contributions to the heat capacities of the magnetic salts were obtained by using an interpolation between the heat capacities of the Gd and Lu salts. The crystal field splittings of the lowest J levels of the Tb3+ and Ho3+ ions were studied by comparing the calorimetrically determined magnetic entropies with those calculated on the basis of extant optical spectral data. This comparison indicated that the spectral levels were identified correctly in the case of the Ho salt but incorrectly in the case of the Tb salt. The total entropies of the Gd, Tb, Ho, and Lu salts at 298.15°K are 400.80, 403.84, 403.51, and 374.54 J mole−1·deg−1, respectively. The standard state partial molal entropies of the solutes for the above salts in aqueous solution were determined using the present measurements and previous data from solution calorimetry. The heat capacity of the Lu salt contains an anomalous peak between 279 and 285°K. The enthalpy change associated with the peak is 13.8±1 J. The entropy change is less than 0.3 J mole−1·deg−1. Possible causes of the anomaly are discussed.

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