The Thermal Decomposition of Dehydrated d-Lithium Potassium Tartrate Monohydrate: Molecular Modification by a Homogeneous Melt Mechanism

Lithium potassium tartrate decomposes between 485-540 K: nLiKC₄H₄O₆-> nLiKCO₃ + nH₂O + nCO₂ + (C₂H₂)_n. Isothermal fractional reaction (α )-time plots are sigmoid shaped and the kinetic data for single crystal reactants obey the Avrami-Erofe'ev equation {-ln (1-α )}^1/2 = kt, 0.04 < α < 0.96, usually accepted as evidence of a nucleation and growth process. Examination of the dark brown viscous residual product and microscopic observations of partly reacted salt gave evidence that reaction was accompanied by melting. The decomposition rate was increased only slightly by crushing the crystalline reactant hydrate, which underwent rapid initial dehydration before onset of the anion breakdown reaction. Kinetic characteristics of the evolutions of both CO₂ and H₂O were identical and the activation energy for salt decomposition was relatively large, 220 ± 20 kJ mol^-1. The study reported here was predominantly concerned with the d form of the tartrate anion but observations included the decompositions of some related reactants including LiK salts of dl and meso tartaric acids. The reaction mechanism proposed is anion decomposition within an advancing thin layer of molten material that is formally similar (in some respects) to the reaction interface developed during decompositions of solids. Reactant melts or dissolves at one side of the active liquid zone and residual products accumulate at the outer side. Anion breakdown then occurs relatively easily in the molten region after removal from the stabilizing influence of the crystal cohesive forces. Kinetic characteristics are similar to those often found for the reactions of solids except for crushed salt samples where an increase in rate after ca. 50% reaction is ascribed to the onset of more extensive melting. This pattern of kinetic behaviour is so closely similar to that for many reactions of solids that we suggest it to be appropriate to consider the possibility of local or temporary melting in formulating detailed reaction mechanisms for all such rate processes.