Dissociation of Acetic Acid in Water

Abstract

IN the measurements of Maclnnes and Shedlovsky¹ and Jeffery and Vogel² on the conductivity of acetic acid and its salts in water, which have recently been the subject of correspondence in these columns³, the most noticeable divergence is in the values for dilute solutions of sodium acetate. Jeffery and Vogel find that their values, after deducting the conductivity of the solvent, are lower than those of Maclnnes and Shedlovsky, and that a correction for hydrolysis according to the equation Ac′ + H₂O HAc + OH′ reduces their figures still further, which leads them to the view that Maclnnes and Shedlovsky's values are in error. The reason for this discrepancy is that in Jeff ery and Vogel's experiments the hydrogen with which the acetate ion unites comes not from the water but from the carbonic acid present in it. The proper solvent correction for the salt of a weak acid has been discussed elsewhere⁴ and can be calculated if the amount of carbon dioxide dissolved in the water is known. Applying it to Jeffery and Vogel's figures on the assumption that their water was saturated with carbon dioxide at a partial pressure of 0.00035 atm. gives values far greater than those of Maclnnes and Shedlovsky, but the figures obtained in this way are undoubtedly too high, since Jeffery and Vogel's water (like that of the American workers) probably contained saline impurities as well as carbon dioxide. Without precise information on this point the correct extrapolation of Jeffery and Vogel's figures is impossible, but it can safely be concluded that the uncertainty fully covers the discrepancy of 1·6 units between the limiting conductivities derived in the two investigations.