Hofmeister Series

HOFMEISTER SERIES (Hofmeister), a sequence in which various anions or cations can be arranged according to the strength of their colloid-chemical or colloid-biological action. Hofmeister, acting on colloids with various neutral salts, noted characteristic and constant differences among them. Thus, the coagulation of a protein solution by sodium salts, manifested by its noticeable clouding, is most easily caused by citrate, as well as tartrate and sulfate; the least effect is produced by iodide and rhodanide (thiocyanate), while other anions occupy an intermediate position. The ability of anions to coagulate a protein solution increases in the following sequence: SCN << J < ClO₃ < NO₃ < Cl < CH₃COO < SO₄ < tartrate < citrate. When comparing salts having a common anion, analogous inequalities are found between cations, although the differences here are much less sharply expressed. Such series, named Hofmeister series, are also observed in the precipitation of other colloids, in the influence of electrolytes on thermal coagulation, on the swelling of gels, etc., as well as in their action on physiological processes, for example, on the excitability of muscles and nerves, on the viability of spermatozoa, and many others. There is almost no physiological process in an animal or plant organism upon which various neutral salts act in identical concentrations in the exact same manner. In the vast majority of cases, quantitative differences are discovered between them, making it possible to construct physiological ion series similar to those established for the action of ions on colloids. The similarity between physiological and colloidal ion series proves that biocolloids serve as the site of application of ion action in the living organism as well. Loeb drew attention to the fact that many salts (e.g., citrates and tartrates) can significantly alter the active reaction (pH) of a solution; as is known, the reaction exerts a powerful influence on colloidal processes. The colloidal action of salts often depends merely on such an indirect influence. The latter was frequently disregarded, and many results described as examples of the Hofmeister series were actually caused by this methodological error uncovered by Loeb. However, in other cases, the real existence of Hofmeister series is beyond any doubt, since they can be established even with the most precise maintenance of constant reaction. Of great importance for understanding the nature of Hofmeister series is the ability of salts to lower the solubility in water of very various substances, including gases and crystalloids. This precipitating action finds its explanation in the relation of salts to the solvent itself, in their affinity for it and the ability to strip it away from the dissolved substance. Such an influence on water binding, on the affinity between the colloid and the solvent (which varies for different ions), forms the basis of Hofmeister series according to Freundlich. Freundlich proposed calling this influence on the relationship of the colloid (or dissolved substance in general) to the solvent lyotropic, and the ion series themselves lyotropic series. Ions possessing a lesser coagulating ability constitute the so-called lyophilic end of the series, while the opposite end is called lyophobic.

D. Rubinstein.