Isolation of Caffiene experiment a. What is sublimation? How can this affect the

Question

Isolation of Caffiene experiment

a. What is sublimation? How can this affect the obtained results and the recover yield?

b. Although we are not purifying the extracted caffeine by recrystallization, we still need to test for its purity. Melting Point will be used in this experiment as a way to analyze the purity of the recovered caffeine. Discuss how the MP measurements of MP be used as an indication of purity?

c. A student neglected to add sodium carbonate when extracting the tea leaves with hot water. Yet a very high yield of caffeine was obtained. The crystals melted at 202-214oC. Explain.

d. Is it better to do two extractions with 10 mL of methylene chloride or one extraction with 20 mL of methylene chloride? Explain.

Explanation / Answer

A)Sublimation is the direct transition of any substance from solid to gaseous phase without passing through the liquid phase.

The sublimed caffeine is pure as the impurities do not vaporize and is left behind. Sublimation is generally used to extract pure caffeine from the impure crude caffeine.

Although the recovered yield is lesser than the crude, but the sublimed caffeine had a melting point range of 231-234C close to pure form

B) Pure caffeine has a melting point of around 2380C. The impure caffeine had a melting point range of 223.6-229.30C.

So by determining the m.pt we can estimate the purity of the extracted caffeine.

C) The caffeine obtained in this case ,is a mixture of caffeine and gallic acid as impurity so melting at low temp.

Caffeine has a good solubility in methylene chloride compared to water.However caffeine is not the only organic substance found in tea that dissolves in methylene chloride. It has organic substances called tannins, or gallic acid that equally dissolves in water.

But comparatively caffeine has a stronger attraction to water due to its greater polarity of molecule( hydrogen bonds are formed with water)

The structure of gallic acid to induce a stronger dipole-ion interaction by adding sodium carbonate, that converts gallic acid into phenol salt: a polar substance insoluble in methylene chloride.The organic layer than contains only caffeine and no gallic acid.

D) Two extractions with 10 mL of methylene chloride gives a better yield than one extraction with 20 mL of methylene chloride.

According to principle of solvent extraction, percent yield is given by,

% solute extracted=[(Vaq/Vaq+kd Vo)^n]*100 where n=number of extraction and Vo is the organic solvent used for extraction

So for more number of extraction with less amount of solvent more is the extraction.

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.