Precipitation-----Process of precipitation and its applications in Pharmacy.

               PRECIPITATION

Supervised by;

                                   Dr. Khezar Hayat

Prepared by:

                           Dr. Ali Mansoor 
                           Dr.Harry Hamid
                           Dr.Abdullah Yaqoob

 Precipitation

Precipitation is the formation of a solid in a solution during a chemical reaction.

When the chemical reaction occur the solid formed is called, precipitate. This can occur when an insoluble substance, the precipitate, is formed in the solution due to a reaction or when the solution has been supersaturated by a compound. The formation of precipitate is a sign of a chemical change. In the most situations, the solid forms out of solute phase, and sink to the bottom of solution(though it will float if it is less dense than the solvent or form suspension).

Precipitate:

       When the reaction occurs, the solid formed is called precipitate.

Precipitant:

    The agents which cause precipitation from a solution or suspension.

Precipitant either alters the solubility of substance in the solution or decreases or alters the composition of the solvent to lessen the solubility of substance in it.

Supernate:

                      Supernate is the liquid which remains above the solid (precipitate) formed during the precipitation reaction.

Method:  

                                 Natural methods of preipitation include settling or sedimentation. Where a solid forms over a period of time due to ambient forces like gravity or centrifugation. During chemical reaction precipitation may also occur particularly if an insoluble substance is introduced into a solution and the density happens to be greater(other wise the precipitate would float or form a suspension). With soluble substances precipitation is accelerated once the solution become supersaturated.

An important stage of the precipitation process is the onset of the nucleation. The creation of a hypothetical solid partical includes the formation of interphase which require some energy based on the relative surface energy of the solid and the solution. If this energy is not available , and no suitable nucleation surface is available, supersaturation occurs.

Substances produced by precipitation are usually microcrystalline. The process involves the mixing of solutions of the reactants and the interaction of these substances to form a  sparingly souluble substance. When chemically equivalent quantities of the reactants are used. The order of mixing is not usually of importance. When an excess of one reactant is required for complete precipitation of the product this reactant is usually gradually added to the other reactant untill no further precipitate is produced. This procedure avoids waste materials and the precipitate will require less washing in order to obtain it in a pure condition. The process of precipitation in the laboratory is best performed in the conical beaker flask, the form of which is most convinient for washing the precipitate by decantation before collecting it on filter.

Apparatus:

 An apparatus is described for the measurement of the rates of fast precipitation reactions. It consists of a T-tube, where reactants are added to the arms of the T, mixed rapidly at the junction,  and the rates of disappearance of reactants or appearance of products are measured along the stem of the T. The geometry of the system results in a steady state being established in the apparatus, enabling detailed analysis of reaction progress. The apparatus has been used to investigate the kinetics and mechanism of iron sulphide precipitation reactions. It has enabled measurements to be made with relatively high precision and good accuracy of the first 130 ms of reaction time.

Conditions required for precipitation:

                                                                                                        All precipitation phenomenons physical or chemical are based on simple solution chemistry. Solid has a finite solubility in any given solvent when the concentration is exceeded supersaturation occurs this supersaturation condition can be occur by evaporation presence of salt, change of solvent, or by addition of freely soluble salt. This super saturation will eventually give away to precipitation of excess solid.

The process may be hastened by nucleation this is simply induced crystallization and may be achieved by rescratching the inner side of the container or by adding the reed crystal of the expected precipitate.

Physical nature of the precipitate:

                                                              the characteristic of the precipitate often depends upon the conditions under which it has been produced. When the reacting substances are mixed at temperature near to their boiling point the precipitate is usually granular and heavy where the precipitate of  the same substance produced by mixing cold solution may be light take a long time to settle and wash, and will, perhaps pass through the filter. Similarly concentrated slutions produce a precipitate in larger particles than do dilute solutions. A very fine precipitate , once formed, can often be caused to coagulate or form larger particles by boiling or by digesting for sometime on a water bath, for example silver bromide and barium sulphate. Some substances can be precipitated only from cold solutions because of their solubility, acid potassium tatarate and magnessium-ammonium phosphate normally crystallize only from cold solution. Such precipitates can often be induced to form more rapidly by scratching the sides of vessel with the glass rod or by vigorous shaking.

Applications:

v It provides a convenient method of obtainig solid substances in the form of fine particles e.g the precipitation of calcium carbonate.

v one of the important uses of precipitation is the purification of solids, the process as applied to purification is termed as recrystallization, the impure solid is usually dissolved in a suitable solvent at elevated temperature,and on cooling the bulk of the impurities remain solubilized while the purified solid product precipitates.

v Precipitation reactions can be used for making pigments , removing salts from water in water treatment, and in classical qualitative inorganic analysis.

v Purification is also useful to isolate the products of a reaction during work-up.

v Precipitation reactions are used in the preparation of salts; in qualitative analysis reactions; and in the formation of some sedimentary rocks.

References:

·      Pharmaceutics by bently

·      Comprehensive  physical pharmacy by Muhammad Masood Ahmad.

·      Physical pharmacy by Martin

·      apparatus for the study of fast precipitation    reactions

·        Apparatus for the study of fast precipitation reactions by DAVID RICKARD Department of Geology, Cardiff University, Cardiff CF1 3YE, Wales

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