NOMENCLATURE FOR CONDUCTANCE' RAYMOND M. FUOSS Yale University, New Haven, Connecticut
INATTEMPTING to prepare a compact presentation of some recent work on conductance, the author noticed that the repetition of a number of wordy expressions became monotonous. Consider, for example, how often the phrase, ". . . figure in which the equivalent conductance is plotted against the square root of concentration" occurs in a paper on conductance. It is therefore proposed to refer to this curve as a phoreogram (from +opCw-carry, and rp&$w-write; with obvious ellipsis). I n general, two types of phoreogram are found experimentally: those which approach the Onsager tangent from above as concentration approaches zero, and those which approach it from below. These may be succinctly described as anabatic (d.vaL3aivwgo up) and catabatic (~ardaivw-go down) phoreograms, respectively, while the occasional curve which lies on the tangent for a moderate range of concentration would consistently be called parabatic (sapapaiVU-go by the side of). In discussions of electrolytes, it is frequently necessary to speak of a "solvent of low dielectric constant," when what is implied is that in such a solvent the coulomb potential of a pair of oppositely charged ions is large compared to their average thermal energy, and that the ions therefore cluster together into pairs, triples, or still higher aggregates. The term smenogenic (afitivos-swarm) is suggested for such solvents; conversely, a solvent in which the dielectric constant is high enough to prevent ion association would be called smenocolytic (KWX CU-hinder). Finally, the historical categories of "strong" and "weak" electrolytes often lead to circumlocutions or ambiguities. For example, sodium chloride is called a strong electrolyte and iodic acid a weak electrolyte; nevertheless, their phoreograms in liquid ammonia are strikingly similar, despite the marked difference shown in water. Furthermore, the phoreogram of sodium chloride in ammonia resembles the curve for iodic acid in water much more than it does the curve of a typical strong electrolyte in water. Conventional nomenclature thus forces us into the awkward position of asserting that sodium chloride is a strong electrolyte in water and a weak one in ammonia; the difficulty, of course, lies in the confusion of two distinct phenomena which were not clearly distinguished in Ostwald's day. It is therefore suggested that sodinm chloride be called an ionophore, defined as a substance in which ions and only ions are present in the crystal lattice. The contrasting category would then be the ionogens, those substances with molecular crystal lattices which can produce electrolytes by reaction when dissolved in
ACKNOWLEDGMENT
Contribution No. 1305 of the Sterling Chemistry Laboratory, Yale Univemity, New Haven, Connecticut.
Acknowledgment is made to N. N. T. Samaras of Monsanto and to Ralph L. Ward of Yale for their help.
appropriate solvents. We might then properly refer to iodic acid as a weak ionogen in aqueous solution and as a strong ionogen in ammonia; weak and strong thus refer to the acid-base equilibria between solute and solvent. The traditional meanings of these two words can therefore be preserved if they are reserved for the above usage and are not forced to do double duty. The familiar term "associated," with suitable qualifiying adverbs, can then be used to describe the relative magnitude of the conductance in various solvents. The usefulness of the proposed terminology is shown by the following equivalent statements: (1) The conductance curve of sodium chloride in water approaches the Onsager tangent from above, because sodinm chloride is a strong electrolyte in water; that is, the potential energy of a (hydrated) sodiumion and a (hydrated) chloride ion in water is a t best of the order of IcT, and therefore ion pairs are unstable. For iodic acid in water, the condnrtance curve is concave down, as a consequence of the presence of molecular HI03 in solution. The latter species is in equilibrium with a molecular addition compound HI0,HI03, which rearranges to the ion pair H30+.IO3', which then dissociates into free ions. Disregarding the water (which is in excess a t substantially constant concentration), we may then write [H+] [I0,']/[HI03] = K. We therefore say that iodic acid in water is a weak electrolyte, compared t o sodium chloride in the same solvent. I n liquid ammonia, however, both solutes give conductance curves which approach the limiting tangent from below. This downward deviation from the limiting law at nonzero concentrations is caused by the formation of ion pairs; in ammonia, with a dielectric constant only one-quarter that of water, ion pairs are stabilized by their high electrostatic potential energy. Iodic acid is as strong an electrolyte in ammonia as sodium chloride, because ammonia is a much stronger base than water and the solute is therefore completely ionized, although only partially dissociated into free ions. (2) The phoreogram of sodium chloride in water is anabatic, because the solute is an ionophore which naturally is unassociated in this smenocolytie solvent. Iodic acid is only a fairly strong ionogen in water and hence exhibits a catabatic phoreogram. I n ammonia, both solutes give catabatic phoreograms, the former because the ionophore is moderately associated in the smenogenic solvent ammonia, the latter because it is a very strong ionogen in ammonia and the resulting ions are likewise moderately associated.
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