The pH value - an important parameter in analytical separation methods
Owing to the fact that pH is one of the most important factors governing separations, its concept is more deeply investigated. It is shown that the true (or non-isothermal) temperature coefficient of the electrode potential of an electrode is an absolute physical quantity depending on the individual activity of the electrochemically active ionic species determining the electrode potential of an electrode, This temperature coefficient is experimentally measurable and a way is therefore given of obtaining the individual activity coefficient (and thus also the PH) of ionic species provided that the correction term arising from the ionic heats of transport is correctly evaluated. The first experimental results obtained show activity coefficients that agree within some units per cent with those theoretically calculated following the Debye—Huckel theory. Many analytical processes in solution require as a necessary condition for the success of the qualitative identification or of the quantitative determination, that the pH of the medium be rigorously fixed, otherwise uncertainties or even important systematic errors may arise. Moreover, the pH is of great importance in a broader analytical sense for many other determinations, for example that of the stability constant of complexes. The pH is considered an obvious and easily reproducible quantity and this is sometimes true from a purely operational point of view. But a deeper consideration of the real meaning of pH immediately shows that this opinion can no longer be supported. In fact, an unambiguous theoretical definition of pH must be based on the concept of the individual activity of the hydrogen ions, a quantity that up to now is thought to be experimentally inaccessible. Therefore an operational definition1 of pH (not of the hydrogen ion activity) was adopted and is currently used. It is known that this operational definition is based on the potentiometry of particular galvanic cells using dilute solutions of some selected buffers. This approach is the best one at our disposal today, but in the case where the technique is applied to not too dilute solutions, using galvanic cells with two different electrolytes, i.e. where a considerable potential difference due to the electrolytic junction may occur, highly reliable and reproducible results can no longer be expected (± 1 mY ± 0.015 pH at 25°C). This difficulty is still more exaggerated in the case of non-aqueous solutions for which no operational correlation can be made with the aqueous solutions. The problem is therefore precisely located in the knowledge of the activities