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. Some of the common positively and negatively charged colloids are given below:

`text(Positively charged)`

`Fe(OH)_3` sol, `Cr(OH)_3` sol, `Al(OH)_3` sol, `Ca(OH)_2`, `TiO_2`, dyes like methylene blue and haemoglobin.

`text(Negatively charged)`

`As_2S_3` sol, `Sb_2S_3` sol, `CdS` sol, `Au` sol, `Cu` sol, `Ag` sol and acid dyes like 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.

`text(Electro-osmosis)` : If the dispersed phase is prevented from moving, the application of an EMF will, however result in movement of the dispersion medium. This is the basis of the phenomenon of electro-osmosis i.e., the passage of dispersion medium of a colloid through a porous diaphragm under the influence of an applied electric field. This can be demonstrated in the following way. In the figure given, the plug of clay provides stationary negatively charged colloidal particles. Water, in contact with clay, must therefore carry a positive charge. Under the influence of an electric field, water will be found to move towards the cathode.

A colloidal solution is stabilised by small amount of an electrolyte. But if the electrolyte is present in higher concentration, then the ions combine with colloidal particles and neutralise them. Once discharged, the colloidal particles unite together to form bigger particles and hence the coagulation takes place. The precipitation of a colloidal solution through induced aggregation by the addition of a suitable electrolyte is called coagulation or flocculation.

Certain minimum concentration of an electrolyte is needed to cause coagulation of a particular sol. The minimum amount of an electrolyte (in millimoles) that must be added to one litre of a colloidal solution so as to cause its complete coagulation is called coagulation or flocculation value of the electrolyte.

The coagulation value of an electrolyte varies from electrolyte to electrolyte. Hardy and Schulze studied the coagulation behaviour of various electrolytes and presented their observations in the form of `text(Hardy-Schulze rule)`, which states that :

(i) The ions carrying charge opposite to that of sol particles are effective in causing the coagulation of the sol.

(ii) Coagulating power of an electrolyte is directly proportional to the valency of the ions causing coagulation.

Thus, for the coagulation of negatively charged sols such as `As_2S_3`, `Al^(3+)` ions are more effective than `Ba^(2+)` ions, which are more effective than `Na^+` ions. In the same way, for the coagulation of positively charged sols such as `Fe(OH)_3`, `PO_4^(3-)` ions are more effective than `SO_4^(2-)` ions which are more effective than `Cl^-` ions. Other methods used to cause coagulation of colloidal solution are as follows:

(i) By Mutual Precipitatiion : When two oppositely charged sol such as `Cr(OH)_3` and `Sb_2S_3` are mixed in equimolar proportion, they
neutralise each other and get coagulated.

(ii) By Electrophoresis : During electrophoresis of a sol, the colloidal particles move towards oppositely charged electrode. The particles touch the electrode, lose charge and get coagulated.

(iii) By Repeated Dialysis : The stability of a sol is due to the presence of a small amount of electrolyte. If the electrolyte is completely removed by repeated dialysis, the sol will get coagulated.

(iv) By Heating : Even simple heating may coagulate the sol.

Lyophilic sols are reversible and are practically not affected by the electrolytes. Even if coagulated due to evaporation of dispersion medium, they can easily be brought back to colloidal solutions simply by shaking with some quantity of dispersion medium. Lyophobic sols, on the other hand are irreversible and are highly sensitive to the presence of electrolytes. They can be easily coagulated by the addition of electrolytes beyond a certain minimum concentration. Once coagulated they cannot be revived back to colloidal solutions. However, addition of a small amount of lyophilic sol to a lyophobic sol, the former exerts a protecting influence on the latter against coagulation by electrolyte. For example, a gold sol or a silver sol protected by gelatin, gum-arabic or egg albumin, may not be coagulated easily. Different lyophilic sols exert protective influence to different extent. The protecting power of a protective colloid is expressed in terms of Gold number defined as below:

The number of milligram of the protective colloid that just prevents coagulation of `10` ml of standard gold sol when `1` ml of `10%` solution of `NaCl` is added to it is called Gold number of the protective colloid.

Thus, smaller the gold number of a protective colloid, greater will be its protecting power. The gold numbers of a few protective colloids are given below:

Potato starch : `25`
Egg albumin : `0.15 - 0.25`
Gum Arabic : `0.15 - 0.25`
Haemoglobin : `0.03`
Gelatin : `0.006`