Thursday, April 29, 2010

Deliquescence

It means to dissolve gradually by absorbing moisture from the air.

It is derived from the Latin word “deliquescere” literally “to start melting away”, from “liquere” that is “to be liquid”.

Deliquescent:
Deliquescent materials are substances (mostly salts) that have a strong affinity for moisture and will absorb relatively large amounts of water from the atmosphere if exposed to it, forming a liquid solution.

Explanation:
In deliquescence, the final more hydrated state must still exert a lower vapor pressure than that of the water vapor in the surrounding atmosphere.

If this is not so, then the newly formed hydrated state will immediately lose water by efflorescence and revert to the initial state.

Thus, for a liquid phase to be produced by deliquescence, it is necessary that the vapor pressure exerted by a saturated solution of the deliquescent material should be less than 13.33 x 10^2 N/m^2.

Examples:
The behavior of NaOH.H2O may be used as an example of deliquescence.

NaOH.H2O(v.p. is very low at 293K) --> (deliquescence) --> Saturated solution of NaOH (v.p. = 1.33x 10^2 N/m^2 at 293K) --> (further deliquescence) --> Unsaturated solution of NaOH (v.p. = v.p. of atmospheric pressure).

Other deliquescent materials include
Potassium hydroxide,
Sodium lactate,
Ferric ammonium citrate,
Potassium bicarbonate.

Example from everyday life:

In everyday life, people are most likely to come across deliquescence when they spill some instant coffee. This turns from a dry powder to sticky liquid when exposed to air for a few hours.

Reduction of Deliquescence:
Storage precautions for pharmaceutical preparations that are deliquescent are aimed at the maintenance of a moisture free atmosphere inside the container. Well closed containers should be used.

In addition, a well filled container limits the volume of atmosphere in the container and therefore further reduces the uptake of moisture by the product.

In some cases, when the product is particularly susceptible to moisture, a drying agent may be placed inside the container. Silica gel is often used in small packets.

Uses of Deliquescents:
1. Due to their very high affinity for water, these substances are often used as desiccants which is also an application for concentrated sulfuric and phosphoric acids

2. these substances are used in the chemical industry to remove the water produced by chemical reactions to increase the yields.



Multiple Choice Questions (MCQs) from Deliquescence in Pharmaceutics
1     Deliquescent salts use in chemical industry As A _______________

     A     Desiccants
     B     Efflorescent
     C     Anti oxidants
     D     None of the Above
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2     All these salts are deliquescent except ____________

     A     Magnesium iodide
     B     Lithium Sulphide
     C     Sodium Dichloro-isocyanurate dehydrate
     D     None of the Above
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3. The water absorb by the deliquescent salt cause A _____________ in melting point of solid

     A     Decrease
     B     Increase
     C     No change
     D     None of the Above
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4. When A solid absorb water from atmosphere it change into ______________ phase
       A     Liquid
       B     Solid
       C     Gas
       D     semisolid
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5. Solid substances absorb enough water to lower is melting point below ____________

     A     Room temperature
     B     constant temperature
     C     45 C
     D     200 C
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Answers of Multiple Choice Questions (MCQs) from Deliquescence in Pharmaceutics
1. A
2. C
3. A
4. A
5. A

(These MCQs are helpful for the preparation of Pharmacy Exams)
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Further Reading:





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