Law of matching water affinities is intuitive law postulated by K. D. Collins. This law assumes that pairs of ions of the opposite charge with identical or very similar water affinity form contact ion pairs (CIPs) while no ion pairs are formed between ions with different water affinity.
In the present master thesis, due to the Collins law, mixing of aqueous alkali halides solutions has been examined. The main focus was on analyses of heat effects, which arise by the isothermal mixing of different pairs of aqueous alkali halides solutions of the same concentration, determined by isothermal titration calorimetry (ITC). In order to validate the results measured on ITC, we also measured the electrical conductivity and density of such aqueous alkali halide mixtures.
Primary objective of the research was to discover, if formation of contact ion pairs occur when certain pairs of salts (according to Collins’ law) are mixed. The measured values of the enthalpy of mixing ions of the same (very similar) sizes do not indicate extensive formation of CIPs. This was confirmed by electrical conductivity measurements.
From the measured heats of mixing, I have found due to low thermal effects and linear increase (decrease) of heat by changing the composition of the salt mixture that when mixing most of the salt pairs there are no other unexpected processes. Exceptions were made when adding a solution of CsF (and also LiF) to the solutions of second salt, where is in the graph ΔHint. (X(MX)) the value of ΔHint. on two or three parts of each curve settle down. The reason for the given function of thermal effects is at least in part due to the hydrolysis of the fluoride ion and possibly also due to the interaction of fluoride ions with the wall of titration cell. Measurements of the enthalpy of mixing has also showed that the enthalpy of mixing for a given salt pair is increasing with initial concentration of involved aqueous salt solutions.
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