Schulze–Hardy rule 

https://doi.org/10.1351/goldbook.S05501
The generalization that the critical coagulation concentration for a typical lyophobic sol is extremely sensitive to the valence of the counter-ions (high valence gives a low critical coagulation concentration).
Source:
PAC, 1972, 31, 577. (Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry) on page 610 [Terms] [Paper]
Citation: 'Schulze–Hardy rule' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.S05501 RIS BibTex EndNote
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Last revised: February 24, 2014