Melting of Tutton's salts studied by DSC

The melting behaviour of 27 Tutton's salts [M ^I ₂M ^II(SO ₄) ₂ · 6H ₂O] has been investigated by DSC. The melting point T _F and the enthalpy of fusion Δ _F H of these double salts were determined by the same method. The Tutton's salt with the cations M ^I= NH ₄ and M ^II = Zn offers a remarkably high value of Δ _F H (285 J/g). From the DSC curves, conclusions have been drawn concerning the mechanism of fusion. The investigated Tutton's salts melt more or less incongruently. The melting process occurs in either one or two steps. In a two-stage melting mechanism the salt first melts with settling of a lower double salt hydrate. The second step seems to be the melting or dissolution of this lower double salt hydrate.