Acid-Base Equilibria in Glacial Acetic Acid. II. Spectrophotometric Determination of the Ionizationand Dissociation Constants of p,p′-Dimethylaminoazobenzene and Pyridine. The Abnormal Effect of Water on Indicator Bases

The following constants were determined spectrophotometrically at 25°: K^I_i (ionization constant of p,p′-dimethylaminoazobenzene, (I)) = 0.100; K^I_d (ion-pair dissociation constant of I) = 5.0 X 10^-6, K_I (over-all dissociation constant of I) = 4.6 X 10^-7; Km = 8.4 X 10^-11; K_i^py = 5.37 (Py = pyridine); K^py_d = 9.4 X 10^-7; K_Py = 7.9 X 10^-7. Upon addition of water to a solution of an indicator base I like p,p′-dimethylaminoazobenzene in acetic acid the color changes in favor of that of the acid species, even though water is a base. This abnormal effect of water is attributed to formation of ion quadruplets and triplets: [Formula omitted], where Q can dissociate into an ion triplet and a simpleion. Other bases instead of I like pyridine and, -diethylaniline exhibit a similar behavior with water. The formation constant of Q from IH⁺Ac^- and water was found equal to 2.0 ± 0.1, and its dissociation constant 3.3 (±1) X 10^-5. The formation constant of Q from IH⁺Ac^- and pyridine is reported equal to 1.1, and that from, -diethylaniline and water to 2.65 ± 0.10.