Dissociation of perchloric acid in aqueous solution at 25 °C

Abstract

Quantitative Raman spectroscopy of electrolyte solutions can furnish direct evidence of the species present in concentrated solutions and their concentrations, but it is by no means easy to develop the photoelectric recording of the Raman intensities to yield measurements of sufficient accuracy. This has been achieved by Young at Chicago between 1948 and 1959; work which has been largely unpublished. The present paper describes the development and subsequent testing of apparatus in Newcastle for the photoelectric recording of Raman spectra to the high precision and reproducibility necessary. The principal requirements are a highly stable mercury arc light source and stable electronic amplification of the d.c. signal from the photomultiplier tube. Perchloric acid was chosen as the first system for quantitative study in view of doubts about the low value for the dissociation constant of 38 moles 1. -1 obtained by Redlich by n.m .r. spectroscopy, despite the apparent agreement with his earlier values of the degree of dissociation from photographic Raman studies. N.m .r. studies yield similar dissociation constants for nitric acid and perchloric acids. This is not in keeping with ideas about the strengths of these acids, and in particular conductivity measurements (Murray-Rust & Hartley 1929) show that perchloric acid is highly ionized in methanol whereas nitric acid is not. From studies of the intensity of the 931 cm -1 perchlorate ion line in perchloric acid solutions up to 11 M, values for the degree of dissociation (α) in the equilibrium H 2 O + HClO 4 ⇔ H 3 O + + ClO - 4 (1 - α)c αc αc were obtained by assuming that α is unity in the most dilute solution studied (0·3 M). This method obviates the use of a standard salt such as sodium perchlorate for comparison of the measured Raman intensities. The distinction is important in view of recent measurements which show that the integrated intensity of the nitrate ion band in aqueous solutions of metal nitrates is not strictly proportional to nitrate ion concentration (Vollmar 1963). The values of α obtained do not deviate significantly from unity until the molarity is greater than about 10 M, in which solutions there is insufficient water present to solvate the proton as H 9 O + 4 as in dilute solutions. The values obtained differ markedly from those from the n.m .r. studies, but it is suggested that the latter are incorrect being based on the naive assumption that the proton chemical shift does not vary as its solvation changes in solutions of increasing acid concentration.