A Physico-Chemical Study Of Pyridine-Acetic Acid System

Abstract

Equivalent conductivity, viscosity, contraction in volume and refractive index of mixtures of pyridine and acetic acid at different percentage compositions have been determined. In all cases the above properties exhibit a maximum at a composition of 83 mole % of acetic acid. Previous workers who investigated some of these properties ascribe the maxima to the formation of a complex. We have explained on a different basis that no such compound is formed, and that instead at that composition acetic acid attains maximum ionization. At least 17 mole % of pyridine has been shown to be necessary for complete ionization of acetic acid in pyridine.

The origin of the conductivity maximum vs. mixing ratio in pyridine/acetic acid and water/acetic acid

“…A crystalline form exists at this ratio but melts incongruently. 28 Plots of refractive index 10 and pH 29 versus composition feature significant changes in a slope at this ratio. Mixtures of acetic acid with other organic nitrogen bases have also exhibited maxima in conductivity and viscosity, although not always at 1:5 ratios.…”

“…Mixtures of amines with carboxylic acids are "poorer than poor"-with conductivities usually sufficiently low (∼mS cm −1 ) that a classification as a PIL (even a poor one) would be unwise. However, they are well-studied materials with well-tabulated data [9][10][11][12][13][14][15] and hence a good testing ground for developing quantitative theories of ionicity a) Author to whom correspondence should be addressed: allan.east@uregina.ca and conductivity. Such theories may then find applicability to higher-conducting "poor PILs" to better understand their partial ionicity.…”

“…It is also curious that σ max (C 5 H 5 N/CH 3 COOH) is less than triple that of H 2 O/CH 3 COOH, given the expectation that pyridine should generate considerably more ions than the mild base H 2 O. This system exhibits other strange phenomena: viscosity 10,26 and density, 10,27 like conductivity, are all maximal at the same mixing ratio (x A = 0.82-0.86). A crystalline form exists at this ratio but melts incongruently.…”

Anhydrous Acetic Acid as Nonaqueous Solvent

“…B + HOAc^±BH + OAc-^±BH + + OAc" (96) and the respective overall dissociation constants are : It is seen that for very weak acids K x > 1 a n d (99) Similar considerations hold for bases.…”

“…Swearinger and Ross 92 report that melting point-composition diagram shows evidence for the formation of Py · HO Ac and Py · 4HOAc complexes in the binary mixture with respective mp's of -59° and -42°C, but a later paper claims that the electrical conductance measurements indicate only the latter compound, 93 and viscosity measurements do not indicate any compound formation. [98][99][100] Many other systems have been studied by similar techniques. 96 Naumova 97 postulates the existence of the complexes 3 P y 2HOAc, 2 P y 3HOAc, Py4HOAc and P y H O A c from electrical conductance data at 28° and 50°C.…”