Acetonitrile (AN) is widely used as a solvent for titration in nonaqueous media, because many organic and inorganic compounds are soluble in it. Moreover the solvent has a long pH-scale and pronounced differentiating properties. It can be purified easily and has a low toxicity.
The acid-base equilibria in AN have been studied most thoroughly by Kolthoff et al. 1 3. These authors have defined a pH-scale, have determined pKvalues of several acids and indicators and have performed titrations of bases, using conductometric, potentiometric and spectrophotometric measurements. The more acidic properties of AN compared with water, make it an appropriate medium for titrations of bases. Unfortunately the hydrogen ion activity of perchloric acid, the only acid, which is completely dissociated in AN, decreases on aging. The aging effect is noticeable 1 h after preparation of the solution 3 and the hydrogen ion activity decreases more than a thousand times after long standing. It was found at the same time that the potential of the glass electrode in solutions of this acid was poorly reproducible 4. That is why the readings of a glass electrode are not reliable and the visual titration of weak bases can not be performed because of the shortened titration curve in the acid region.
Perchloric acid has been used by some authors 3 as a titrant in AN after stabilizing of its solution with other nonaqueous solvents such as acetic acid, nitromethane, dioxane etc. Regardless of the fact that water has indisputable advantages, it is not used so far as a cosolvent of AN for the purposes of the acid-base titration*. In the first place water has a good solvating ability and that is why its perchloric acid solutions are stable. Secondly, when the amount of water in AN increases, the glass electrode readings become more stable. Taking into account the solvating ability of water, it can be assumed that a smaller amount of water in AN will provide a greater stability of the perchloric acid solution compared with the above mentioned solvents. This assumption is based on the fact s , that in AN solutions of perchloric acid which contain ! M of water, most of the protons are present in the form of tri-and tetrahydrates, H(H20)~-and H(H20)~.
The fraction which is present as unhydrated protons is 2 x 10-5 from the total concentration of protons. A drawback of water as a cosolvent is its pronounced * Some investigations 5-7 were carried out in AN-water mixtures which have a physico-chemical character.
SUMMARY
The acid-base equilibria in "AN-water mixtures with decreasing content of water: 30, 10, 5 and l~-wt., were investigated potentiometrically. It was concluded that l~-wt, water in pure AN is enough to provide the stability of perchloric acid solutions. The perchloric acid behaves as a strong acid in the four mixed soh,znts. The protolysis constants of several uncharged bases were determined by means of potentiometric titration with a glass electrode. The titration conditions of bases of this type improve compared with water when the AN content increases and for the mixture with l~o-Wt, water the improvement is approximately of the same order as in glacial acetic acid.