Chemistry Practical: Purification of compound. Purification? Crystallization?

 Aim: To purify the given sample of impure Benzoic Acid by the re-crystallization.

References:  

• Vogel's text book of practical organic chemistry by Brain S. Furnish, Anatomy J. Hannaford, Peter W. G. Smith & Austin R. Tatchell, fifth edition; page no. 717.

Requirements: 

• Beaker, Funnel, Glass rod, test tube, Filter paper, tripod stand, Bunsen burner, distilled water, Crude sample of Benzoic Acid, Cold water.

Theory: 

Recrystallization is a fundamental technique widely employed in organic chemistry for the purification of solid compounds. It is based on the principle that the solubility of a compound in a given solvent varies with temperature, allowing for the selective separation of the compound from its impurities. This process is particularly valuable when dealing with impure samples, as it can effectively remove contaminants and yield high-purity products.
At its core, recrystallization involves dissolving the impure compound in a suitable solvent at an elevated temperature, followed by controlled cooling to induce the compound to crystallize out of solution. The choice of solvent is critical, as it should readily dissolve the compound at high temperatures while exhibiting minimal solubility for impurities. Through this controlled cooling process, the compound of interest selectively forms crystals, while impurities remain in solution or precipitate separately.

The success of recrystallization relies on several key factors, including the solubility properties of the compound and its impurities, the selection of an appropriate solvent, and the optimization of crystallization conditions such as temperature and cooling rate. Additionally, techniques such as decolorization with activated charcoal and filtration are often employed to further enhance the purity of the final product.

Once crystallization is complete, the purified crystals are isolated from the mother liquor through filtration, washed to remove any residual impurities, and then dried to remove solvent traces. The resulting product is a highly pure crystalline solid suitable for further analysis or use in chemical reactions.

 Calculation: 

Calculation of % recovery for the recrystallization process measures the efficacy of the purification and isolation combined.

% Recovery        =    Final Mass of Pure crystals/ Initial mass of Crude crystals X100

Procedure:

Preparation of solution:  

1. Take about 150ml of distilled water in the beaker and boil the water over a Bunsen burner by placing the beaker over a piece of wire gauze placed over a tripod stand.
2. In another beaker take 2-3g of crude sample of Benzoic acid, while constantly stirring.
3. Gradually add minimum quantity of boiling water just sufficient to dissolve the benzoic acid if required heating can be used to dissolve the benzoic acid.

Filtration of the solution:

1. Take a funnel and fix the filter paper in it and clamp it to a tripod stand.
2. Insoluble impurities will be left in the filter paper.

Cooling the hot saturated solution:

1. Let the filtered solution come to room temperature by itself.
2. Now then cool it by placing it on the cold water through opaque white crystals of benzoic acid begin to separate.

Separation of Crystals:

1. Fix a filter paper in a funnel that is clamped to a stand and separate the crystals by filtration wash the crystals with cold water.
2. transfer the crystals to another filter paper and dry them by pressing gently between the folds of the filter paper.
3. transfer the crystals into a dry test tube and cork it.

Calculation of Percentage (%) Recovery:

1. Measure the mass of the recovered, dry and pure crystals of Benzoic Acid.
2. Measure the melting point for the purified Benzoic scid twice and compare it with melting point of Benzoic acid reported in the literature.

 Report: 

The given sample of crude Benzoic acid was successfully recrystallized using water as recrystallization solvent.

% Recovery for the recrystallization of Benzoic acid was_______

Final mass of pure crystals = 

Initial mass of crude crystals= 

% Recovery =    _____