The Purity and Purification of Solids Recrystallization Lab

Topics: Water, Temperature, Phase changes Pages: 5 (1644 words) Published: February 20, 2013
The Purity and Purification of Solids
Recrystallization

ABSTRACT
In this experiment, impure adipic acid is purified though recrystallization. The impure adipic acid is dissolved in a solvent. After cooling it is ran through a vacuum filter to separate any moisture. The dry crystals are then weight for percent yield and the melting point range is taken to test the purity. There were two different trials ran in this experiment. The difference between the two was the addition of charcoal in the initial step to remove color impurities. For part A (without charcoal) the initial impure weight was 1.0256g. After the filtration, the final pure product weighted 0.6063g which is a 59.17% yield. The Melting point range for part A was 151.1°C – 154.9°C. For part B (With charcoal) the initial weight was 1.0008g. After filtration the pure crystals weighted 0.4631g. This came out to be a 46.27% yield. The melting point for part B is also 153.0°C – 156.8°C. INTRODUCTION

Recrystallization is a method that is used to purify solids. To have a good test, two criteria must be met. A high degree of purity is achieved after the purification and there must be a maximum recovery of the sample. This technique of purification is only effective when there is a smaller amount of impurity in one substantial component. EXPERIMENTAL SECTION

Material
Impure adipic acid charcoal water
Erlenmeyer flask filter paper Heat plate
Filter flask 50ml & 250ml Beakers O ring & stand Graduated cylinder Pipette Ice
Scoop Stemless funnel Vacuum filter (assembled)
Spatula Boiling chips Scale
Weighing paper Holding strap Hirsch funnel
Capillaries Spot tray Mel temp device
Procedure
Part A – recrystallization without decolorization (no charcoal). Weight out approximately 1.0g of impure adipic acid and get 2 boiling chips from jar. Add the adipic acid and boiling chips to a 50ml Erlenmeyer flask with about 6ml of water to create slurry. Heat up the mixture on a heat plate until it boils. During this time heat up a stemless funnel, extra boiling water and an extra empty Erlenmeyer flask. As the solvent with the dissolved solute comes to a boil fold 2 filter paper that will be used filter out the solid impurities through the stemless funnel. Swirl the liquid often to get the adipic acid that is stuck to the side wall of the Erlenmeyer flask. Once the mixture comes to a boil, setup the O ring to hold the stemless funnel above the empty Erlenmeyer flask that was heated up. Put the filter paper into the stemless funnel and carefully pour the hot mixture through. With the pipette and boiling water (from the heat plate), pipette some hot water around the edge of the filter paper to make sure all the mixture makes it through. Toss out the filter paper that contains the solid impurities. Let the hot liquid in the Erlenmeyer flask get to room temperature before adding it to an ice bath. As the liquid cools, add another small beaker with about 20 ml of water to an ice bath to cool down for later use. Once in the ice bath for 10 minutes and crystals have formed, set up the Hirsch funnel on filter flask and attach it to the provided vacuum apparatus. Turn the vacuum on and add a small filter paper. Add a squirt of water to keep the filter paper in place. Pour the liquid with the crystals through the vacuum. Add some pre chilled water to the flask to remove any remaining crystals. Make sure not to add too much because the crystals could re dissolve. Keep the crystals on the vacuum for 20 minutes to allow it to dry. Flatten the crystals with a spatula to promote uniform drying. After 20 minutes of drying, the crystals are ready to be weighed. Once the weight is taken the melting point can be taken on the mel temp device. Record these data into the lab notebook. Part B Recrystallization with Decolorization (Charcoal)

The only thing different with part B from part A is the addition of about 5mm of...


References: Organic Chemistry Laboratory I, CH 337M; Department of Chemistry, Portland State University: Portland OR; p 1
Lab Two Question
1.
2. Recrystallization is generally not an effective purification method for a solid which is highly impure because the impure component could recrystallize in the cooling process. The reason why it works when there are smaller amounts of impurities is because a small amount of impurities are dissolved in a larger amount of liquid. When the main component recrystallizes the impurities stay dissolved in the liquid because it is at the point of its solubility. If there was a larger amount of impurities it would give itself a chance to recrystallize.
3. A – It might be advantageous to chill the mixture in an ice bath before suction because the pure major component separates as crystals under lower temperatures. If the temperature is room temperature or higher, the major component will dissolve and not form crystals.
B – This same ice bath could be detrimental if the original sample contained a high proportion of impurities. There would be a higher chance of the impurities also recrystallizing under cooler temperatures. The final product might contain both pure and impure substance.
4. A – Compound A has a solubility of 7.03g per 10 mL at 100°C. It would take 23.61mL of boiling water to dissolve 16.6g of compound A.
B – 0.6847g of compound A would crystalize from the 16.6g at 23°C
5. If we repeated the experiment in Mexico City at an elevation of 7700 ft about sea level. It would take much more boiling ethanol to dissolve the same amount. The temperature of boiling ethanol would decrease at a higher elevation. So what would be 100°C at sea level might only be 80°C at 7700 ft above sea level. Therefore it would need more ethanol to be soluble.
If there is enough ethanol to be soluble, I don’t think that there would be a change in the recovery amount if they are both chilled to the same temperature. The recovery amount would stay the same.
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