Question: 1. Explain why and how you need to use the glass rod to promote the cr

Question

Question: 1. Explain why and how you need to use the glass rod to promote the crystallization.
2. Which appartus need to be cleaned & dried before use?
3. List possible ecperimental errors that may affect thr product purity.
Please please keep the answers clearly. Thanks a lot.
Experiment A Preparation and Purification of Aspirin AIMS Demonstrate the synthesis of aspirin and recrystallization of aspirin. INTRODUCTION Aspirin Gacetylsalicylic acid) belongs to a class of compounds known as analgesics (painkiller). one of the most widely manufactured drugs in the world. It may be easily It is prepared in the laboratory by heating salicylic acid and acetic anhydride using phosphoric acid as the catalyst. COOH COOH H3PO (CH3CO)20 OCOCH CH,COOH OH Aspirin Salicylic acid Acetic anhydride MATERIALS Salicylic acid, acetic anhydride, concentrated phosphoric acid, 95% ethanol PROCEDURE 1) Add salicylic acid (3 g) in a 125 ml Erlenmeyer flask (conical flask). 2) Demonstrator will distribute acetic anhydride (6 m) to the flask for you. 3) Add concentrated phosphoric acid (5 drops) to the mixture. 4) Swirl the mixture until all the salicylic acid dissolved. 5) Heat the flask on the steam bath for 10 minutes. 6 Allow the mixture cool to room temperature and stir the mixture with a glass rod to promote crystallization. (Record the shape of the crystal) Add water (50 ml slowly to the ice-cold mixture containing the crystals. 8) Separate the crystals by water aspirator using the Buchner funnel. 9 the crystals by sucking air through it. (Save a small amount of sample (1 mg for determining the melting point of the crude sample) 10) Transfer the crystals to a 125 ml Erlenmeyer flask and dissolve the crystals with minimal amount of hot ethanol. (Not more than 8 ml). 11) Add water (25 m) to the alcoholic solution and heat the mixture to almost boiling on a steam bath. 12) Cool the solution at room temperature followed by ice bath and separate the crystals by vacuum filtration and air-dry the crystals by sucking air through it. 13) Dry the crystals in an oven set at 70 oC.

Explanation / Answer

Ans. 1. Crystallization in a super saturated solution exhibits cooperativity, i.e. crystallization of one particle promotes crystallization of others in the solution. However, for first crystallization to occur, sometime scratching the inner surface of the reaction vessel is needed. Scratching with glass rod removes some glass particles simultaneously with making the surface a bit rough. The free glass particles serve at the points of nucleation (crystallization) onto which the crystallization process is initiated. Once initiated, the process proceeds with gradually increasing rate.

Ans. 2. All apparatus need to be cleaned. And preferably pre-rinsed with the solution to be handled in. Most of the glassware shall also be dried.

Erlenmeyer flask, pipettes, glass rod, etc. must be cleaned and dried. Presence of fluid in flask may affect the volume of reaction mixture and the reaction, too. A moist pipette may introduce undesirable fluid to the solution being pipetted out. A contaminated glass rod may alter the reaction environment depending on type of contamination.

Standard volumetric flasks need not to be dried because it is used to make the final volume upto the mark. So, even if there is presence of some distilled water, it won’t affect the result. The same holds true for measuring cylinders and pipettes if they handle water or same kind of solution as previous use.

Ans. 3.

I. Contaminated glassware and other apparatus undesirable affect the reaction. For example, a pipette not thoroughly rinsed after taking NaOH solution may introduce traces of NaOH the next solution being handled and the reaction mixture, too.

II. Drying: Drying at specified temperature must continue till a constant weight is obtained. Once a constant weight noted, the filter paper or other to be dried shall be transferred to desiccator for cooling overnight.

Underheating (heating as low temperature) gives a higher weight value than the actual. Overheating gives a lower weight value than the actual to be.

Keeping the desiccator dried precipitate in open environment for longer time before measuring may give higher weight value due to moisture absorption by precipitate.   

III. Chemical reactions are very sensitive to heat. For example, rate of reaction nearly double for increase of every 100C. Colling may enhance optimum crystallization, etc. So, all heating and cooling procedures must be carried out at specified temperature.

IV. Filtration: Be gentle with vacuum filtration. Application of greater pressure may rupture the membrane and so, cause loss of the precipitate.   

Etc.

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