Abstract
The design and operation are described of an optical cell and supporting apparatus capable of making precise absorbance measurements of dilute aqueous solutions over the temperature range 25 - 250°C and at pressures up to 1 kbar. The design and operation are similarly described of a multiple optical cell and supporting apparatus capable of making precise absorbance measurements on up to six experimental solutions in one heating cycle over the temperature range 25 - 200°C and at the approximate saturation vapour pressure of the solutions. Both optical cells employed teflon inner cells to isolate corrosive experimental solutions from the main construction material. Results obtained with both cells indicated that a precision of better than +/- 1% in absorbance is obtainable over the temperature range. Using this apparatus, and conventional spectrophotometric techniques, the acid dissociation constants (K[a]) of the 1,10-phenanthrolium ion, 5-nitro-1,10-phenanthrolium ion, 2,9-dimethyl-1,10-phenanthrolium ion and 5,6-dimethyl-1,10-phenanthrolium ion in aqueous solution have been determined with good precision at various temperatures between 25 and 250°C. The temperature dependence of the pK[a] for each phenanthrolium ion has been described by the equation pK[a] = A/T - B + CT, where A, B and C are constants for the particular phenanthrolium ion over the experimental temperature range and T is the absolute temperature. The changes in the thermodynamic functions (enthalpy, entropy, free energy and heat capacity) for the dissociation of the phenanthrolium ions at each experimental temperature have been calculated from the temperature dependence of pK[a]. The increasing dissociation of all four phenanthrolium ions with increasing temperature has been discussed in terms of the relative degrees of hydration of the hydronium ion (H[3]O[+]) and the phenanthrolium ions. The overall stability constants (beta[3]) of the tris complexes of 1,10-phenanthroline, 5-nitro-1,10-phenanthroline and 2,2'-bipyridyl with iron(II) have been determined with good precision at various temperatures between 25 and 160°C by a new spectrophotometric method. The value of beta[3] for all three complexes decreases regularly with increasing temperature as expected. Plots of log beta[3] against l/T have been used to determine average values of the thermodynamic functions (enthalpy and entropy) of formation over the experimental temperature range. The spectrophotometric methods developed for the determination of equilibrium constants were particularly suitable for measurements at elevated temperatures since separate measurements of pH were not required.