Deuterium isotope effect on the dissociation of weak acids in water and deuterium oxide

Abstract

The dissociation constants of <i>o</i>-nitroanilinium ion, <i>m</i>-nitroanilinium ion, and 4-chloro-2,6-dinitrophenol in deuterium oxide at 25 °C have been determined by a spectrophotometric method, and an emf method has been used to obtain (<i>pK</i> ₁ + <i>pK</i> ₂)/2 for citric acid in deuterium oxide. In addition, data for the dissociation constants of other weak acids in ordinary and heavy water have been critically examined with a view to clarifying the relationship between the deuterium isotope effect and the intrinsic strength of the acid. The difference Δ<i>pK</i> between the <i>pK</i> value in deuterium oxide and that in water varies linearly with <i>pK</i> above <i>pK =</i> 7. Two stronger inorganic acids (sulfuric and phosphoric) also appear to lie on an extension of this same line. On the contrary, a considerable group of organic acids with <i>pK</i> less than 7 have values of Δ<i>pK</i> that are more or less constant near Δ<i>pK</i> = 0.55. It appears, therefore, that the isotope effect is more complex than has heretofore been assumed.