Wednesday, March 9, 2011

Efflorescence

It is the loss of water from a crystal.


It means “to flower out” in French.

It is the spontaneous loss of water (or solvent) from a hydrated or solvated salt to the atmosphere on exposure to air, which occurs when the aquous tension of the hydrate is greater than the partial pressure of the water vapor in the air.

Efflorescent:
Denoting a crystalline body that gradually changes to a powder by losing its water of crystallization on exposure to a dry atmosphere.

Explanation:
If the vapor pressure of a hydrated salt is greater than the pressure exerted by the water vapor in the surrounding atmosphere than the salt will attempt to attain equilibrium with its surroundings and therefore tend to lose water to form a lower hydrate or an anhydrous salt.

This phenomenon is known as efflorescence.

The pressure of water vapor in the atmosphere is about 13.33 x 10^2 N/m^2 at 293 K.

Therefore hydrates with vapor pressure greater than this will tend to exhibit efflorescence and be unstable provided that the lower hydrate that if formed still exerts a vapor pressure greater than the surrounding atmosphere.

If this is not so then water will be taken up from the atmosphere by the lower hydrate as fast as it is formed and the final equilibrium will depend on the rates at which water is lost or taken up the two hydrates.

Examples:

The behavior of the various forms of sodium carbonate may be represented by the following scheme:

Na2CO3.10H2O (v.p = 32 x 10^2 N/m^2 at 293K) --> (Spontaneous dehydration i.e. efflorescence)  Na2CO3.H2O (v.p. = 16 x 10^2 N/m^2 at 293K)--> (efflorescence not observable because anhydrous salt is rapidly hydrated) --> Na2CO3 (anhydrous) (v.p. = 0)

Since the vapor pressure exerted by the decahydrate is much greater than that of normal atmosphere. It loses water by the process of efflorescence and is converted to the monohydrate.

The vapor pressure of the later is still above that of the atmosphere but further apparent loss of water does not occur. Since the anhydrous salt is rehydrated at a faster rate than dehydration of the monohydrate.

Similarly, vapor pressure of Glauber’s salt (Na2SO4.10H2O) normally exceed that of the water vapor in the atmosphere these salts effloresce and their surface assumes a powdry appearance. Blue stone or blue vitriol (CuSO4.5H2O)is a blue crystalline solid that when exposed to air slowly loses water of crystallization from its surface to form a white layer of anhydrous copper (II) sulfate.

Factors affecting efflorescence:
The vapor pressure of hydrated salts, and therefore the rate of efflorescence increases with rise in temperature.

Pressure of vapors.

Reduction of efflorescence:
Since the instability that arises from efflorescence is caused by the loss of water vapor. The common method of minimizing such deterioration involves the use of containers that present the loss of water vapor.

The additional precautions of using well filled containers with a minimum amount of atmosphere above the efflorescent material and storage in a cool place are also advisable.
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