The decomposition of potassium hydrogen oxalate hemihydrate

In a previous communication, Topley and Hume described the results obtained in an experimental study of the decomposition of calcium carbonate hexahydrate and showed that they could be most reasonably explained by assuming that reaction occurred only at the interface separating the two solid phases, CaCO3. 6H2O and CaCO3. In addition it was assumed that this interface advanced through the crystalline particle with constant velocity in all directions; that is to say, the interface advanced as an expanding spherical surface, described about the nucleus as centre. Since the latter assumption is inherently improbable, it was decided to investigate a reaction the interfacial nature of which would be beyond question and which would be susceptible to direct observation. After considerable search a suitable reaction was found in the decomposition of potassium hydrogen oxalate hemihydrate, i. e., 2KHC2O4.H2O → 2KHC2O4 + H2O. The transition temperature of the hemihydrate, according to Hartley and others, is in the neighbourhood of 6⋅4° C.