Acid-Base Equilibria in Acetonitrile. Spectrophotometric and Conductometric Determination of the Dissociation of Various Acids

The indicators o-nitroaniline, o-nitro-p-chloroaniline and o-nitrodiphenylamine are about 5 × 10⁴ as strong in acetonitrile as in water. Perchloric acid behaves as a strong acid, other acids denoted by HA are weak. Except in extremely dilute solutions hydrobromic, sulfuric, nitric and hydrochloric acids dissociate according to 2HA ⇆ H⁺ + AHA⁻. The over-all dissociation constants of these acids have been determined spectrophotometrically with the aid of indicator bases and conductometrically for sulfuric and picric acids. By other suitable methods the stability constants 2 have been evaluated. The value of 3 decreases from 1.3 × 10³ for sulfuric acid to 2.5 × 10² for hydrobromic, and 2 × 10² for nitric and hydrochloric acids at 25°. From the above data the simple dissociation constants 4 could be evaluated. These values for pK_HA are reported: hydrobromic, 5.5; sulfuric, 7.25; nitric and hydrochloric acids, 8.9. Picric acid at concentrations smaller than 0.1 M dissociates into H⁺ and Pi^™, and pK_HPI = 8.9. At higher concentrations the data fit a dissociation 3HPi ⇆H⁺ + Pi(HPi)₂⁻, pK_3(HPI) being 6.1. The ion pair dissociation constants of tetraethylammonium bisulfate and of lithium nitrate were found to be 1.4 × 10 ⁻³ and 4.1 × 10 ⁻⁴, respectively.