The critical coagulation concentration of a colloidal dispersion for the case of a general electrolyte solution is estimated under the conditions of constant surface potential. By combination of the Langmuir approximation and the Debye-Huckel approximation, explicit expressions for the electrical potential, the osmotic pressure, and the electrical energy are derived, and the results are used to estimate the critical coagulation concentration of ionic species. The influence of the valence of ionic species on this concentration is discussed. We show that the Schultz-Hardy rule is invalid, in general, and that the effect of co-ions can be too large to be ignored.