The dissociation constant of hydrogen cyanide in aqueous solutions containing three ionic media at 25-degrees-C has been determined by a combination of potentiometry and C-13 NMR. The dissociation constants obtained are (in molar units): pK(a) = 9.09 (in 1.00 M NaClO4); 10.00 (in 3.00 M LiClO4) and 10.11 (in 1.00 M NaClO4 - 3.00 M LiClO4). The advantage of the combination of potentiometry and NMR as compared to potentiometry alone is discussed. It is suggested that in many cases the former method is superior to the latter, since some possible error sources are eliminated. The constants obtained were used to calculate pK(a) values for HCN in aqueous solutions containing several other ionic media by means of the specific interaction theory. The calculations showed that the majority of the dissociation constants reported in the literature can be predicted. Some of the recently reported values, however, are not in concordance with our calculations and are probably erroneous. The specific interaction coefficients, epsilon(Na+, CN-) = 0.07(1) and epsilon(Li+, CN-) = 0.22(2) 2-sigma in parentheses), obtained in this work can be used to predict the value of the dissociation constant for HCN in water in other ionic media.
