Abstract
Controversial solubility data put forward by other workers for aqueous lanthanum ferricyanide solutions are satisfactorily explained; solutions over a wide concentration range are in apparent equilibrium with the crystalline salt, and these solutions are thought to constitute a single phase in which fluctuations of structure and concentration can occur. It is possible that the solubility limit reached when crystals of LaFe(CN)[6] are dissolved in water may represent the maximum amount of salt that can be accommodated in water without a radical change in the water structure; the higher concentration of the salt that can be prepared by indirect methods would involve a change in the water structure; the theoretical evidence suggests that the primary cause of the unusual features is the rapid increase in ionic-strength with rising concentration. A negative temperature coefficient is found for the solubility of lanthanum ferricyanide in water, and this may be connected with the tendency of the water structure characteristic of dilute solutions to be gradually broken up as the temperature is increased. Several methods of preparing a concentrated solution free from impurity and suitable for conductivity measurements have been discussed in detail. Concentrated H[3]Fe(CN)[6] solutions can be prepared by ion-exchange, but their neutralisation, either by La(OH)[3], or by the lanthanum salt of a weak acid resin, is never quite complete and the resulting solutions show a high conductivity due to free acid. A satisfactory method is to convert the acid to its barium salt by means of Ba(OH)[2], and then to form the lanthanum salt by double decomposition with lanthanum sulphate. The latter stages were controlled conductimetrically. A new method of estimating lanthanum by conductimetric titration with E. D. T. A. solution was developed during the course of the work. The properties of lanthanum ferricyanide solutions were studied by spectrophotometry, conductimetry and osmometry. The spectrophotometric measurements gave some evidence of ionpair formation in lanthanum ferricyanide solutions, and also in concentrated solutions of lanthanum chloride, but the photosensitive nature of the ferricyanide solutions made the technique unsuitable for a detailed study. The osmometer measurements enabled the ionic activities of lanthanum ferricyanide solutions to be calculated, and gave further evidence that the degree of dissociation of lanthanum ferricyanide passes through a minimum value. This conclusion is strongly supported by the equivalent conductances, which pass through a flat minimum, followed by a maximum, at moderate concentrations, and thus show a region in which the equivalent conductance is increasing with rising concentration. This is the first instance reported of this behaviour in an aqueous salt solution.