Purification By Recrystalization Flashcards
What are the characteristics of benzoic acid?
- it has antibacterial/and antifungal properties
- used in food preservative
- synthesizes large numbers of chemicals
- used in toothpastes, mouthwashes, cosmetics & deodorants
What is meant by recrystallization?
The separation of a mixture of substances into pure components on the basis of their solubilities, by dissolving the substance in a appropriate hot solvent leaving it to cool down saturating the solution which then crystallizes.
- insoluble impurities are removed by 1) hot gravity filtration, 2) the soluble impurities stays in the solution as they are not concentrated enough to crystallize 3) the crystals are collected and washed with cold water during suction filtration to remove the adhering impurities
- charcoal is added at the beginning to remove any colored impurities
What are the steps of the recrystallization process?
1) Choosing the solvent
2) Dissolving the solute in an appropriate hot solvent (minimum volume required)
3) Removing of some colored impurities by adding a decolorizing agent (charcoal) to the hot solution (if necessary)
4) Removing of suspended solids by gravity filtration (if necessary)
4) Allowing the substance to crystallize (precipitate) as the temperature of the solution falls down (cooling step)
5) Collecting and washing the crystals of the solute using water or air suction filtration
6) Drying the resulting purified crystals (product) and determine the purify
7) Calculate the percentage yield of the purified product
On what basis is the solvent chosen?
1) The solvent must dissolve the solute when its hot and precipitate the solute when its cold.
2) Either it doesn’t dissolve the impurities at all or it dissolves them very well.
3) It doesn’t react with the solute.
4) It must be very volatile to remove easily during drying step.
5) Nonflammable, nontoxic and inexpensive.
How does the crystals forms?
The dissolved solute starts precipitating from the solution while the temperature of the solution decreases, If the crystal growth is slow and selective (crystallization) it makes the impurities leave as a compound with a more suitable geometry comes in to take their place, on the other hand the size of the crystal will be larger than when the process is rapid and nonselective (precipitation) trapping the impurities are inside the crystals being formed, therefore the crystals isolated are impure.
How to calculate the percentage yield of the crystallized product?
To calculate how much of the pure substance was present in the initial impure sample, the percentage yield is calculated:
% Yield = (Mass of pure sample / Mass of the impure sample) × 100%
Indicates the purity of the initial sample, A high percentage yield implies a low concentration of impurities, whereas a low percentage yield implies a high concentration of impurities.
