A Separation and Purification Scheme Essay Example

a decreased solubility at a lower temperature will separate (crystallize) from the solution as it is slowly cooled. The crystals are separated from the solution using vacuum filtration.

This involves use of a Hirsch funnel and a water vacuum system. Materials needed to do this experiment: Most of the glassware and laboratory equipment necessary to perform this lab was located in my lab drawer. Other equipment included:

• Hot plate
• Analytical balance
• Disposable Pasteur pipets
• Centrifuge machine
• Melting point apparatus

The reagents and compounds needed were:

• Unknown 2
• Methylene chloride
• 1. 0M NaOH
• 6.0M HCl
• Anhydrous sodium sulfate
• 95% ethanol

Methanol Procedures: Separation

• Dissolve 0. 5g of unknown mixture in the minimum amount of diethyl ether or methylene chloride oIf more than 4mL of solvent is required, the other solvent should be used
• Once solvent is selected, the same solvent should be used throughout the procedure when an organic solvent is required Isolating Neutral Compound from mixture containing Acid/Base impurity
• Add 0. 150g of unknown to a centrifuge tube
• Add 4mL ether to tube and cap it
• Shake until all solid dissolves Add 2mL 1. 0M NaOH to tube and shake 30 seconds
• Let layers separate
• Remove bottom aqueous layer and place it in a test tube labeled “1st NaOH extract”
• Add another 2mL 1M NaOH to centrifuge tube, shake 30s, let layers separate and remove bottom aqueous layer and put in a second test tube labeled “2nd NaOH extract”
• While stirring, add 6M HCl dropwise to each of the two test tubes until acidic (test with pH paper)
• To the ether layer in the centrifuge tube, add 2mL saturated aqueous sodium chloride
• Shake 30s and let layers separate

Place on ice bath oIf a lot of crystals form, this is a good solvent Crystallization

• Place 0. 3g unknown in a 10mL Erlenmeyer flask
• Add 6mL solvent (chosen above) and a boiling stone to a 2nd Erlenmeyer flask
• Heat solvent on warm hot plat until boiling Add several drops of solvent, heat flask, and swirl
• Continue above step until all solid dissolves
• Remove flask and cool slowly to room temp
• Induce crystal formation by scratching inside surface of flask with glass rod
• Once room temp crystallization ceases, place flask in ice water bath Vacuum Filtration
• Vacuum filter
  

the crystals using a Hirsh funnel

When liquid passes through filter, repeat until all liquid is transferred

• Add 1mL ice cold solvent to flask
• Swirl liquid in flask and transfer to Hirsch funnel
• Repeat above 2 steps if needed
• Wash crystals with 2mL ice cold solvent
• Draw air through funnel for 5 minutes
• Transfer crystals to preweighted watch glass for air drying (10-15 minutes)
• Weigh dry crystals and calculate % recovery
• Obtain melting point

Changes to Original Procedure

When I originally prepared the procedure, I began each technique using the original unknown sample, not realizing that the initial separation separated the acid from the neutral compounds.

The procedure remained nearly the same with the following changes. When isolating the neutral compound from the acid, the aqueous layer (top) was the acid and the organic layer (bottom) was the neutral compounds. To the acid layer, 6M HCl was added until solid stopped forming, and then methylene chloride was added until the solid dissolved. Once the methylene chloride was evaporated off, the acid was in pure state. The neutral layer was dried over anhydrous sodium sulfate, and then solvent was evaporated off. Tiny amounts were used to test for proper solvent for crystallization.

The solvents used were methanol and 95% ethanol.The crystallizion was carried out using methanol and then the crystals were isolated using vacuum filtration.

Results: 1st Separation Acid (benzoic acid)

Melting point: 85-87°C

Weight:u 8. 9106g -8. 7525g 0. 1581g % recovery: (0.1581/0. 5147)(100)=30. 7%

Neutral

(benzin) Melting point: 94-96°C

Weight: 51. 9197g -51. 8289g 0.0908g % recovery: (0. 0908/0. 5147)(100)=17. 6% 2nd Separation Acid (benzoic acid) Melting point: 119-121°C

Weight: 8.8376g -8. 7426g 0. 0950g % recovery: (0. 0950/0. 5076)(100)=18. 7% Neutral (benzin) Melting point: 88-90°C Weight: 51.8743g -51. 8359g 0. 0384g % recovery: (0. 0384/0.076)(100)=7. 6%

Conclusion

Overall, this laboratory experience was successful. Though the percent recoveries were not exact (50% benzoic acid and 40% benzin), product was still obtained to a satisfactory amount. The first separation resulted in a higher yield of both the acid and the neutral compound. I would have expected the opposite to have occurred, but during the second separation, I lost some of the acid component while boiling off the methylene chloride. In order to make this experiment better, I would extract the acid from the neutral compound using two or three portions of 1.0M NaOH as opposed to only one.This could help to recover more pure end products. If I did this experiment again, I would be more careful when boiling solvents off so that my end results would have higher percent recoveries. When the solvent boiled over, product was lost in the water bath.

Other source of error could have occurred when adding 6. 0M HCl to the acid layer. If there wasn’t enough added, then all of the acid would not be present in the end. During the process of drying the neutral layer, if the anhydrous sodium sulfate wasn’t added to the correct extent, then water would contaminate the final neutral compound, skewing the results.