A microcalorimetric study of the influence of lead ions on the heat of dilution of potassium halides, and the derived thermodynamics of ion association in solutions of the lead halides. (1956)
AuthorsAustin, J. M.show all
Recently the association constants for the formation of the complex ions PbCl+, PbBr+, PbI+ have been determined by measurements of the ultraviolet absorption spectra of the corresponding lead halide solutions. In determining these association constants no assumptions are made about the nature of the complex ion except that a chemical potential may be assigned to it and that the law of Mass Action is applicable to the equilibrium Fb++ + X-  PbX+ where X- = Cl=, Br-, or I-. According to the Bjerrum theory of ion association it is considered that complex ions are formed by electrostatic forces between pairs of ions separated by any distance between that of the minimum distance of closest approach å of a pair of ions and the Bjerrum minimum distance q. When the association constants for the above reaction were used in conjunction with Bjerrum theory to calculate values of the minimum distance of closest approach å they were found to give results much smaller than the sum of the ionic radii of the lead and halide ions. It has been usual to explain such an observation by suggesting that the bonding in the complex is predominatly covalent. From a review of the theory of electrolyte solutions with reference to ion association, however, it is concluded that such an assumption is not valid because of the rather arbitrary value of the Bjerrum minimum distance q. In order to find more evidence for the formation of complex ions in solution and to try and elucidate the nature of such ions, a microcalorimeter has been used to investigate the influence of lead ions on the heat of dilution of potassium halide solutions. If the complex ion PbX+ exists in lead halide solutions then the complex should be formed when a solution of lead perchlorate is mixed with a potassium halide solution. From the properties of lead perchlorate, potassium halide and perchloric acid solutions one would expect that any difference between the heat of dilution of a potassium halide solution in lead perchlorate and perchloric acid solutions should be due to the formation of complex ions, provided the ionic strength of the undiluted and final solutions are the same in each case. A twin differential microcalorimetric system has been designed and built to measure the heat effects associated with the above dilutions. Because of the small heat effects usually associated with the dilution of an electrolyte, temperature changes were measured by an eighty junction copper-constantan thermal and a Paschen galvanometer circuit sensitive to 2x10-6°C or 10-3 cal. The calorimeters were housed in an air-bath which was immersed in a thermostat electronically regulated to maintain the temperature constant to within 0.001°C. Measurements of the heat of solution of potassium chloride crystals were made to check the apparatus and calorimetric technique.