Chemistry - Recrystallization & Melting Point

Recrystallization & Melting Point

INTRODUCTION

The purpose of this experiment was to understand the process of purifying a crystal through recrystallization and observe the relation between purity level and the melting point.

These are the two main experiments that are used in chemical labs, pharmaceutical departments and many more labs to purify impure substances and determine their melting points.

The Fundamental Principle or Theory behind the Experiment:

The theory behind this experiment was to become exposed with the knowledge that a crystal can be purified through recrystallization. Through dissolving the impure crystal (which in this lab we used Naphthalene) with the best solvent at boiling temperature, impurities are able to separate from the crystalline substance after filtration. Adding charcoal can purify the substance’s color and from remaining impurities as well. However, use with caution since excess charcoal can actually make the solution even more impure. At the end, pure Naphthalene will be produced and the melting point will have to be determined. The purer the substance, the smaller the melting point range because there is less of a variety of substances in.

Detailed Procedure with observations of Recrystallization:

1. Prepare 3 Erlenmeyer flasks or beakers, filtering flask, stopper, Buchner funnel, filter paper, graduated cylinder, spatula, watch glass, heating plate and stirring rod.
2. Collect 2g of impure Naphthalene provided by lab tech and add 3ml of methanol.
3. Add about 7 to 10 boiling chips.
4. Use the hot plate to heat it up gently until boiling point.
5. Cool it down for 5 to 7 minutes and then add 15ml of methanol.
6. Use the hot plate to heat it again until it boils.
7. Add 3.5ml of H2O (distilled water) dropwise.
8. Decant the solution into another beaker or flask.
9. Cool it down in ice.
10. Use the filtering flask attached to the vacuum to filter. (You may filter more than once to collect the most of the solids.)
11. Collect the solids and discard the liquid.

I used exactly 2g impure naphthalene (see picture bellow), after the experiment was done and filtered it twice the collect pure naphthalene solid was 1.4g.

[pic 1]                                [pic 2]

Figure 1 Impure Naphthalene                                        Figure 2 Pure Naphthalene

% Yield ➔ (Actual/Accepted)100% ➔ (2.0g/1.4g) * 100% = 1.43%

Key steps to the procedure:

1. Weigh the impure substance correctly and write down the right weight of it.
2. Use the right amount of liquid and the right solvent.
3. Be careful with the boiling, not to boil it too much. The hot plate should be on the low heating point.
4. Adding H2O is advised to be done dropwise not at once.
5. Filter it twice or three times to get the most of the pure solids.
6. Make sure you weigh the solid and subtract the weight of weighing paper you use.

Questions:

5. Under which circumstances is it wise to use a mixture of solvents to carry out a crystallization?