The subject of the present thesis involves four main lines which may be summarised as follows. 1- The study of the physical properties of three mixed solvents, which are: a)Ethylene glycol-water (EG-H2 0) ; b) glycerol-water (GL-H20) , and c) Urea- -water (urea-H2O) . The study covered the measurements of the specific conductances , densities , viscosities and dielectric constants of the solvent mixtures at four different temperatures in the range 293.15 -323.15 K and at a number of compositions . The deviation of the solvent mixtures from an ideal volume mixture relation was examined from the dielectric properties of the mixtures and of the individual constituents. The volumetric behaviours of EG-H2O and GL-H2O mixtures
have been investigated in terms of the excess molar volume. The variation of the dielectric constant deviation function ( ΔD) with the volume fraction of the organic component of the mixture was interpreted in terms of the change in the degree of alignmant of the dipoles with changing the composition as a consequence of the destruction caused by EG or GL of the three-dimentional hydrogen-bonded water structure. The variation in the relative excess molar volumes of the mixtures was due to a volume contraction (negative ΔV) on mixing the constituents of the mixture. The results were rationalized in terms of the two models for liquid water; one maximally hydrogenbonded and voluminous, the other non-hydrogen-bonded and dense. The effect of the added organic co-solvent to water was to shift this equilibrium in one direction other The possibility of ion-associatio n in the solvent mixtures was examined at a number of solvent compositions and temperatures. The results have been expressed in terms of the association constant (K,A)of the electrolytes (AgNO3 , KC1, KBr, KI, NaCl and LiCl)-It was found that the factors which increase the value of KA involved the (a) law dielectric constant of the medium in which the electrolyte is dissolved, (b) the small ionic radii which lead to a small value of the ion-size parameter, and (c) the large charges which are carried by the ions. The ideas have first been expressed on Bjerrum's picture of ion-pair formation. The mean activity coefficients (f±) of the 1:1 electrolytes at various concentrations could be estimated from the extended Deby-Huckel equation. Values of KA, for the same electrolytes have also been determined from the experimentally measured conductance data and compared with those derived from Bjerrum picture of ion-pair formation. The agreement of the values obtained by the two sources was sbstantially high. The association constant and the limiting molar conductance in all cases have been iteratively calculated by a least-square treatment with an appropriate computer programe using Shedlovsky method. The results indicated a good agreement between the values of KA which have been derived from Bjerrum equation and those obtained by inserting the experimentally measured. conductances, dielectric constant and viscosity date into a set of equations using Shedlovsky method. The possibility of the formation of triple-ions in the electrolyte solutions in the mixed solvents has also been examined with the aid of Fouss and Kraus plots. The results indicated the formation of such ions at certain compositions of the solvent mixtures. The thermodynamic functions for ion-association in KC1 solutions at each experimental temperatures have also been determined. Ionassociation was found to be quite unfeasible on hermodynamic grounds. 3- The cation transference numbers (t± ) of several alkali halides in urea-H2 O mixtures at two different compositions have been determined from E.M.F. measurements of cells with and without transference. Attempts have also been made to determine the liquid-junction potential values in cells with transference where two similar electrolytes of different concentrations were in contact. 4- The transference numbers of a number of alkali halides as well as of AgNO3 in the various solvent mixtures have been r determined from the potential measurements using cells with ransference. The dependence of the transference number on concentration of each electrolyte was also investigated in an attempt to elucidate the values of the limiting transference numbers, The Longsworth method has been used in many cases of the extrapolation of cation transference numbers in the solvent mixtures. The cation (and anion) Longsworth function was then calculated by the method reported by Kay and Dye)(108) . Using the values of the limiting transference number and the appropriate values of the limiting molar conductance it was possible to determine the correspoding values of the limiting ionic conductances. The Walden product for each type of ions in the different solvents could then be obtained from the product of the limiting ionic conductance and the viscosity of the solvent mixture.