The Rule of Schulze and Hardy
In a brief historical survey various attempts to explain the rule of Schulze and Hardy are mentioned. It is argued that the inverse proportionality between the coagulation concentration and the sixth power of the charge number of the counterion, as it has been derived from the DLVO theory, cannot be the complete explanation, since as a rule potentials in the Gouy layer are not high enough. It is shown that a combination of van der Waals attraction and electrostatic repulsion can explain the rule of Schulze and Hardy, if adsorption of counterions in the Stern layer is taken into account. The adsorption potential must then increase fairly strongly with the charge of the counterion. Structural forces (due to the molecular structure of the solvent) may be important, but the quantitative theory of these forces has not yet been sufficiently developed to incorporate them in the theory of coagulation.