Desiccation ,sublimation & tirturation By: Dr. Tahseen Ismail

Desiccation Desiccation is the process of removing adhered moisture from liquid or solid substances. The term "desiccated" should be used for those substances from which water has been completely removed.

Desiccation

Desiccation On laboratory scale desiccation can be carried out in a desiccator, which consists of a tightly closed glass vessel containing a drying agent at its bottom, which absorbs moisture from the substance being desiccated. The commonly used drying agents include concentrated sulphuric acid, phosphorous pentoxide, exsiccated calcium chloride and silica gel. The drug to be dried is taken in a dish, which is placed inside the desiccator above the surface of drying agent. For continuous operation the desiccator may sometimes be connected to a vacuum pump.

Desiccation The substances which are very sensitive to moisture are formulated as tablets or capsules which are further protected by enclosing them in sealed vials, on the bottom of which a small cloth bag containing silica gel is placed which acts as a desiccant.

Desiccation Applications Desiccation helps in preserving the vegetable and animal drugs which get destroyed in the presence of moisture. 2. Comminution of drugs is difficult if they are wet but if they are dry, can be easily powdered. 3. Desiccation reduces the bulk and weight of the substances due to loss of adhering moisture in them therefore they become easy to handle. 4. Stability of certain drugs is also increased.

SUBLIMATION Sublimation is the process in which transforming the solid to vapors without intermediate passage through the liquid state. Triple point. Triple point is the point having a definite temperature and pressure at which the solid, liquid and vapor phases of a chemical substance co-exist Triple point is a point in a graph between temperature and pressure at which three phases like solid, liquid and vapours may co exist.

Method of sublimation The pressure below the triple point is called as Sublimation pressure. In the diagram the line (AO) is known as sublimation pressure. At this (O) point solid and liquid are at equilibrium. If the sublimation pressure is below this triple point the solid will directly converted into vapor phase and the vapor phase is directly change into solid. Two factors in the diagram indicate that: When the vapor pressure of the substance is less then that of sublimation pressure, it will directly change into solid or vapor phase without changing into liquid phase.

Method of sublimation When the vapor pressure of the substance is high then that of the sublimation pressure, then it will first convert into liquid phase and then into solid state on further cooling. The common sublimed substances are as follows: Iodine Naphthaline Camphor Para-dichloride benzene Thus the vapor pressure of Iodine, camphor, Naphthaline and p-dichloride benzene are less triple point.

Method of sublimation In laboratory it consists of a china dish, a perforated filter paper, funnel, and a cotton plug. The material to be sublimed is placed in the china dish; the china dish is covered with a perforated filter paper. A glass funnel is placed in the inverted position over the filter paper; the open end of the funnel is plugged with cotton to prevent the escape of sublimed product. The china dish is heated and vapors pass through the perforations of the filter paper, and are collected on the inner surface of the glass funnel which is kept cool from out side by wrapping a wet filter paper or cotton wool. The vapors on cooling are condensed into solid which is collected.

APPLICATIONS This process is used in the purification of many pharmaceutical substances such as iodine, camphor, naphthalene, benzoic acid, mercuric chloride, ammonium chloride. This process is also employed to purify volatile solids contaminated with non volatile impurities.

Trituration By triturating the grinding of a solid substances is done to fine powder by continuous strike in or rubbing the particles in a mortar with a pestle. The term trituration is also emloyed to designed the process by which two or more powders are intimately mixed with each other. For this purpose a pestle and mortar made of, porcelain or glass is used. For hard materials steel mortar and pestle may be used. Trituration may be carried out with a tile and spatula or using a mortar and pestle.

Trituration using tile and spatula Small quantities of finely powdered solids may be mixed on a tile by means of a spatula. Tiles are usually made of glass and should be large enough for the quantity of powder to be mixed or ointment to be prepared. Usually for small scale work 300mm square is a useful size for a tile. Spatula is made of stainless steel except for the few medicaments those react with stainless steel (iodine), should be flexible and long blade (25mm by200mm) to provide a large rubbing surface. Powders for Trituration are placed on the tile and gently mixed until the mixture is smooth and homogeneous, but in the case of ointment if base is very soft it may be helpful to warm the tile but overheating should be avoided because the base will become too fluid and may run off the edge of the tile. The dispersion is then diluted with increasing amount of base, doubling the quantity each on the tile of each dilution. Finally any liquid ingredient is incorporated.

Trituration using mortar & pestle A mortar should be used when the quantities are too large . A mortar with a fairly flat base and a pestle with a flat head will give best results. It is impossible to ensure intimate dispersion of one powder in another by mixing the two substances all at once. The purpose is to add a substance that is present in greater amount to the whole of the substance present in lesser amount. Substance present in greater amount is introduced into the mixture in very small quantities at first, but gradually increasing the quantities, until the whole of the substance has been added..