CBSE Class 12-science Answered

Fe complex to have better coagulating power.However it does not take into account the mass of the complex compared to phosphate ion.This is why the "Gold number "concept has been brought in.

Removal of the electrostatic barrier that prevents aggregation of the particles. This can be accomplished by the addition of salt to a suspension or changing the pH of a suspension to effectively neutralize or "screen" the surface charge of the particles in suspension. This removes the repulsive forces that keep colloidal particles separate and allows for coagulation due to van der Waals forces

The rate of aggregation is in general determined by the frequency of collisions and the probability of cohesion during collision. If the collisions are caused by Brownian motion, the process is called perikinetic aggregation; if by hydrodynamic motions (e.g. convection or sedimentation) one may speak of orthokinetic aggregation.

In hydrophobic sols, coagulation can be brought about by changing the electrolyte concentration to the critical coagulation concentration (c.c.c.) (preferably expressed in = ). As the value of the critical coagulation concentration depends to some extent on the experimental circumstances (method of mixing, time between mixing and determining the state of coagulation, criterion for measuring the degree of coagulation, etc.) these should be clearly stated.

The generalization that the critical coagulation concentration for a typical lyophobic sol is extremely sensitive to the valence of the counterions (high valence gives a low critical coagulation concentration) is called the Schulze-Hardy rule.

If the critical coagulation concentration of a mixture of two electrolytes  and  corresponds to concentrations of the two components of  and  whereas the c.c.c.'s of  and  taken separately are  and  then the effects of the electrolytes are said to be additive if ; they are synergistic if ; and antagonistic if . It is often found in the latter case that the individual values of  and/or  exceed unity.

Addition of small amounts of a hydrophilic colloid to a hydrophobic sol may make the latter more sensitive to flocculation by electrolyte. This phenomenon is called sensitization. Higher concentrations of the same hydrophilic colloid usually protect the hydrophobic sol from flocculation. This phenomenon is called protective action. Colloidally stable mixtures of a lyophobic and lyophilic colloid are called protected lyophobic colloids; although they may be thermodynamically unstable with respect to macroscopic phase separation, they have many properties in common with lyophilic colloids.

Sedimentation is the settling of suspended particles under the action of gravity or a centrifugal field. If the concentration of particles is high and interparticle forces are strong enough, the process of sedimentation may be better described as compaction of the particle structure with pressing out of the liquid. This particular kind of settling is also called subsidence.