Dissociation, electrolytic Distillation

For most organic chemicals the solubility is reported at a defined temperature in distilled water. For substances which dissociate (e.g., phenols, carboxylic acids and amines) it is essential to report the pH of the determination because the extent of dissociation affects the solubility. It is common to maintain the desired pH by buffering with an appropriate electrolyte mixture. This raises the complication that the presence of electrolytes modifies the water structure and changes the solubility. The effect is usually salting-out. For example, many hydrocarbons have solubilities in seawater about 75% of their solubilities in distilled water. Care must thus be taken to interpret and use reported data properly when electrolytes are present. 

Before we discuss the next major definition of acid-base behavior, let s examine a crucial property of water that enables us to quantify [H30 J in any aqueous system water is an extremely weak electrolyte. The electrical conductivity of tap water is due almost entirely to dissolved ions, but even water that has been repeatedly distilled and deionized exhibits a tiny conductance. The reason is that water itself dissociates into ions very slightly in an equilibrium process known as autoionization (or self-ionization) ... 

Water was born to conduct protons (see Special Issue Is life possible without water [67]). The conductance of distilled water is miserable due to a negligible concentration of free protons (10 mol/liter), but the proton mobility in water is approximately five times higher than the mobility of an alkali cation (e.g. Na" ), an object of similar size as the hydronium (HaO ) ion [68]. So, donated protons can run fast through the aqueous phase. Excess protons result from dissociation of acidic molecules or molecular groups, e.g. in solutions of strong acids, hydrated polymer-electrolytes, or proteins. In acidic solutions both the protons and counter-anions are mobile. In polymer-electrolyte membranes and in proteins only protons are mobile in the connected aqueous phase while the counter anions are mostly a part of an immobile skeleton. 

Even traces of dissolved electrolytes considerably increase the very small conductance of water. This effect is often used to continuously monitor the purity of distilled or demineralized water. The fundamental importance of conductance measurements became obvious when they were used by Svante Arrhenius (1859-1927) to develop his theory of electrolytic dissociation (12). 

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