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Theory Of Precipitation

Precipitation is the formation of an insoluble solid mass in a liquid solution; this insoluble solid mass is called the precipitate. A precipitate is formed when two soluble ionic compounds are mixed.

Soluble ionic compounds can break into their ions in the solution. Then these ions can react with each other to form a precipitate or stay as a solubilized ion in that solution.

The chemical species that cause this precipitation is called precipitants. In addition, precipitates can form when the temperature of the solution is lowered. The low temperature reduces the solubility of salts, causing them to precipitate in the solution.

Colloidal precipitates are solid masses formed in colloidal suspensions. A colloidal suspension is composed of particles having diameters ranging from 10-7 to 10-4 cm. These particles are invisible to the naked human eye

Since the effect of gravity on these particles is very little, they do not tend to settle down at the bottom of the container. Since these particles are very tiny, it is difficult to obtain them via filtering. But by adding a suitable coagulating agent, we can form large particles or a precipitate that is easy to filter. Colloidal suspensions often look like clear solutions due to the scattering of visible radiation.

The formed precipitate may stay as a suspension in the solution if there isn’t sufficient gravity. But later on, the precipitate particles will sediment to the bottom of the container unless disturbed.

Types Of Precipitates

There are two types of suspensions as colloidal suspensions and crystalline suspension based on the particle size in the suspension. Colloidal precipitates are formed in colloidal suspensions while crystalline precipitates are formed in crystalline suspensions. The main difference between colloidal precipitate and crystalline precipitate is  given  in the table below

Preparation of pure precipitates

It is convenient to consider now the influence of digestion on formation of the above mentioned impurities i.e. Co-precipitates and Post precipitates. As a rule, digestion is usually carried out by allowing the precipitate to stand for 12-24 hours at room temperature, or sometimes by warming the precipitate for some time in contact with the liquid from which it was formed. The aim is, of course, to obtain complete precipitation in a form which can be readily filtered.

 During the process of digestion of precipitates, at least two changes occur. The very small particles, which have a greater solubility than the larger ones, will, after precipitation has occurred, tend to pass into solution, and will ultimately re-deposit upon the larger particles; Co-precipitation on the minute particles is thus eliminated and the total Co-precipitation on the ultimate precipitate reduced.

The rapidly formed crystals are probably of irregular shape and possess a comparatively large surface; upon digestion these tend to become more regular in character and also more dense, thus resulting in a decrease in the area of the surface and a  consequent reduction of adsorption. The net result of digestion is usually to reduce the extent of CO-precipitation and to increase the size of the particles, rendering filtration easier.

Conditions of precipitation

1. Precipitation should be carried out in dilute solution, due regard being paid to the solubility of the precipitate, the time required for filtration, and the subsequent operations to be carried out with the filtrate. This will minimise the errors due to CO-precipitation.
2. The reagents should be mixed slowly and with constant stirring. This will keep the degree of supersaturation small and will assist the growth of large crystals. A slight excess of the reagent is al1 that is generally required; in exceptional cases a large excess may be necessary. In some instances the order of mixing the reagents may be important. Precipitation may be effected under conditions which increase the solubility of the precipitate, thus further reducing the degree of supersaturation .
3. Precipitation is effected in hot solutions, provided the solubility and the stability of the precipitate permit. Either one or both of the solutions should be heated to just below the boiling point or other more favourable temperature. At the higher temperature:
4. The solubility is increased with a consequent reduction in the degree of supersaturation,
5. Coagulation is assisted and sol formation decreased,
6. The velocity of crystallisation is increased, thus leading to better-formed crystals.
7. Crystalline precipitates should be digested for as long as practical, preferably overnight, except in those cases where post-precipitation rnay occur. As a rule, digestion on the steam bath is desirable. This process decreases the effect of CO-precipitation and gives more readily filterable precipitates. Digestion has little effect upon amorphous or gelatinous precipitates.
8. The precipitate should be washed with the appropriate dilute solution of an electrolyte. Pure water rnay tend to cause peptisation.
9. If the precipitate is still appreciably contaminated as a result of co- precipitation or other causes, the error rnay often be reduced by dissolving it in a suitable solvent and then re-precipitating it. The amount of foreign substance present in the second precipitation will be small, and consequently the amount of the entrainment by the precipitate will also be small.