Enthalpies of formation and standard entropies for some potassium Tutton salts

Abstract

Differential scanning calorimetry (DSC) was used to measure the enthalpies of reaction for the dehydration of the potassium Tutton salts, K₂M(SO₄)₂ ^. 6 H₂O with M = Mg, Co, Ni, Cu and Zn. The values determined ranged from 335 kJ mol⁻¹ for K₂Mg(SO₄)₂ 6 H₂O to 355 kJ mol⁻¹ for K₂Ni(SO₄)₂^. 6 H₂O with a measured standard deviation of ± 5 kJ mol⁻¹. Although the information needed to obtain precise values for the enthalpies of formation is not available in the literature for all of these salts, values calculated by modeling the amorphous dehydrated compound as an ideal solid solution produced values within 10 kJ mol⁻¹ of the values determined for K₂M(SO₄)₂^. 6 H₂O (M= Mg, Cu, and Zn) where the information needed for this calculate was available. DSC was also used to determine the entropies of reaction for the dehydration of these salts. Since there is little information about the entropies of these compounds in the literature, the entropies of reaction were used with the ideal solution model for the amorphous compound to estimate the standard molar entropies for these salts. The values determined ranged from 490 J K⁻¹ mol⁻¹ for K₂Cu(SO₄)₂ ^. 6 H₂O to 540 J K⁻¹ mol⁻¹ for K₂Co(SO₄)₂ ^. 6 H₂O. Since these values are based upon estimated values, they have an estimated error of ± 5 percent.