EFFECT OF WATER ON DISSOCIATION AND DISSOCIATION CONSTANT OF PICRIC ACID IN ACETONITRILE

Previously (ref 2) picrate buffers have been recommended to calibrate the glass electrode in acetonitrile. The dissociation constant, Kd, of picric acid used for the calculation of paH involved calculated paH values in other acid-base systems. In the present investigation the degree of dissociation of picric acid was increased by addition of various amounts (up to 1 M) of water, and the value of pKd was found from the paH's measured with the glass electrode and the total picrate ion concentration determined spectrophotometrically. The latter type of determination does not involve any assumption. The pKd found was equal to 10.9 as compared to the previous value of 11.0 ± 0.1. Formation of neither the ion pair H3O+Pi− · 3H2O nor of any other picric acid hydrate ion pair has been found. The predominant species of the proton in the presence of 1 M water are H(H2O)3+ and H(H2O)4+, while a small fraction of the picrate ions is present as Pi− · H2O. From infrared measurements it could be concluded that picric acid is not hydrated in acetonitrile while Kfpi was found equal to 0.3 ± 0.1. © 1969, American Chemical Society. All rights reserved.