Electrophoresis or Cataphoresis
The particles of the colloidal solution carry same type of charge, either positive or negative. The dispersion medium carries an equal and opposite charge. The colloidal solutions as a whole are electrically neutral. The origin of electrical charge on colloidal particles could be due to frictional electrification, electron captures, or preferential adsorption of ions from solutions depending on the method used for the preparation of colloidal solutions. Due to similar nature of the charge carried by the particles, they repel each other and do not combine to form aggregates. This makes a colloidal solution stable and the colloidal particles do not settle down.
Positively charged: Fe(OH)3 sol, Cr(OH)3 sol, Al(OH)3 sol, Ca(OH)2, TiO2, dyes such as methylene blue and hemoglobin.
Negatively charged: As2S3 sol, Sb2S3 sol, CdS sol, Au sol, Cu sol, Ag sol, and acid dyes such as congo red.
The existence of charge is shown by passing electric current through two electrodes when all the colloidal particles move towards the same electrode, either cathode or anode. Positively charged colloidal particles move towards negatively charged cathode, whereas negatively charged colloidal particles move towards positively charged anode. The movement of colloidal particles under the influence of electric field is called electrophoresis or cataphoresis.