Definition:
Evaporation is the free escape of vapour from the surface of a liquid bellow its boiling point.
Factors affecting evaporation:
1) Temperature: The rate of evaporation is directly proportional to temp of liquid.
2) Temperature and time of evaporation: It has been observed that exposure to relatively a high temp for short period of time may be less harmful to the active principles of a drug than a lower temp with exposure for a longer period.
3) Temp and moisture Some drug constituent decomposes more readily in the presence of moisture if heated at high temp. This is due to the hydrolysis of the active constituent to avoid decomposition to the active principle of such material the evaporation is done at low temp and then final drying is done at high temp. When only little moisture remains in it.
4) Types of product required: On evaporation of the liquid the conc. Liquid, semisolid and solid are formed. The selection of the method and the equipment required for the evaporation depends upon the type of the product required
5) Effect of concentration: During evaporation the upper layer of the liquid under evaporation has a tendency to form a film and formation of ppt in the product which results in lowering down of the rate of evaporation. Therefore, efficient steering is required in order to prevent degradation of the product at the bottom due to excessive heat and it will also prevent deposition of solid
6) Surface area: The rate of evaporation is directly proportional to the surface area of the evaporator, in which the liquid is evaporated.
7) Vapour pressure of the liquid to be evaporated: The rate of evaporation is directly proportional to the vapour pressure of the evaporating liquid