Heat ~ 40 mL of water in a beaker to almost boiling, Place two tea bags (~2.4 g

Question

Heat ~ 40 mL of water in a beaker to almost boiling, Place two tea bags (~2.4 g of tea per bag) in the water so as to be completely submerged and heat for ~15 minutes. Gently push (without breaking) the bags around several times to optimize the extraction of the tea. Keep water volume constant by adding small portions as it evaporates. Transfer all the liquid to a separator funnel, pressing all the water out of the bags. Repeat the extraction with an additional 30 mL fresh water and add the tea extract to the separator funnel. While the solution is still warm, add 2.0 g potassium carbonate K2CO3. Cap the separator funnel. Shake and vent frequently until ALL carbonate dissolves. Cool to room temperature. Add 10 mL methylene chloride (dichloromethane), shake and vent the mixture for one minute. Drain the organic layer into a 50 mL Erlenmeyer flask. Keep the aqueous layer in the same separator funnel. Repeat the extraction by adding 10 mL fresh methylene chloride and combine both organic fractions in the same Erlenmeyer flask. Dry the organic layer over anhydrous sodium sulfate (see Note). Instead of sublimation, transfer the DRIED organic fraction -by avoiding the drying agent- into a clean 400 mL beaker for the final evaporation of solvent at ambient conditions (OK to store in locker as is). Collect the crude caffeine crystals on Day 2. What was the purpose of adding the following reagents in this experiment? (a) Potassium carbonate (b) anhydrous sodium sulfate?

Explanation / Answer

a) The purpose of the K2CO3 is to convert any extracted acids into anions.
In leaves these acids exist as polar molecules, which are electrically neutral and tend to
dissolve in both water and CH2Cl2. This way purification process can be tough, increasing the
tendency to form an emulsion in the next step, which is undesirable
As anions they are not soluble in CH2Cl2, and this helps avoid an emulsion. The salt K2CO3 will
cause precipitation of some substances that tend to clog the filter paper in the next step and
also help avoiding formation of emulsions.

b) The solvent is dried with sodium sulfate which can absorb water molecules into its crystal
structure. When anhydrous (dry) sodium sulfate is added to a solvent containing dissolved water,
   it clumps up and sticks together as it absorbs the water.

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