The critical coagulation concentration (CCC) is considered as one of the most important parameters to evaluate the particles aggregation and sedimentation behaviors in the environment. Even though there are a few methods for its measurement each method has its limitations especially as those methods to be applied to polydisperse systems like soil. Thus the purpose of this research is to establish a new and more reliable method for its determination.

Two types of polydisperse colloidal materials were adopted: soil and humus in the experimental studies. The dynamic light scattering technique was employed to determine the effective hydrodynamic diameter of the particles or aggregates changing with time under different pH and electrolyte concentrations of CaCl₂ or KCl. In addition the fractal dimension of aggregates was also detected with the static light scattering technique.

A new aggregation rate total average aggregation rate (TAA rate) was defined firstly. We found the defined TAA rate increased linearly with the increase of electrolyte concentration at electrolyte concentrations lower than the CCC value and the TAA rate stayed approximately constant at electrolyte concentrations higher than the CCC value; an intersection point of the two straight lines therefore could be observed and the electrolyte concentration at the intersection point will theoretically be the CCC value. The experimental results for the two materials under different pH conditions indeed meet those theoretical predictions which imply that the CCC value can be determined through the measurement of the TAA rate under different electrolyte concentrations. The comparison of the CCC values obtained between our new method and the widely applied stability ratio method showed that the new method was much better than the stability ratio method for the two polydisperse materials. In addition the static light scattering measurements also showed that the variations of the fractal dimensions of the aggregates with electrolyte concentrations could be well explained by