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Name: Wendy  C.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 2000


Question:
Can you suggest a quantitative wet analysis technique for measuring the level of caffeine extracted from coffee beans using benzene and/or ethyl acetate?


Replies:
Wendy,

Benzene is to be avoided in lab preparations. Here is a prep from an old lab manual that you might modify for use with coffee:

Isolation of Caffeine from Tea

Caffeine, a powerful stimulant of the central nervous system, is found in tea, coffee, and cola-nuts, but not in "unCola Nuts". It is unquestionably the most widely used psychoactive drug on earth.

In this lab activity you will extract the caffeine from tea and purify it by recrystallization.

Procedure:

Sacrifice a tea bag, cut it open and determine the mass of the tea inside. to determine the ratio of tea to bag. Allow for the mass of the bags and select a number of bags that will provide approximately 20 g of tea. Boil the bags for 15 min. in 300 mL of tap water contained in an 800 mL beaker. During boiling, gently stir the bags about in the hot solution taking care to not puncture them.

Shut off the heat source and then add 80 mL of 10% lead acetate solution to the hot brew. Stir once more, then lift the bags from the beaker and place them in your Buchner funnel fitted with an appropriate size piece of filter paper. Turn on the vacuum and use the bottom of a small clean beaker as a plunger to compress the tea bags and squeeze out the entrained liquid. Turn off the vacuum and saturate the bags with 20 mL of hot water. Restore the vacuum and re-compress the bags.

Transfer the filtrate to your (cleaned) 800 mL beaker and concentrate it by boiling to a volume of approximately 50 mL. Cool the remaining liquid to room temperature and then add 35 mL of methylene chloride. Use a stirring rod fitted with a rubber policeman to stir the immiscible liquids together and to scrape the material adhering to the beaker walls down into the solvent medium. Filter the resulting liquid through a fluted filter paper directly into a separatory funnel.

Separate the layers. The methylene chloride layer is the one on the bottom, it contains the caffeine. However, some caffeine remains in the aqueous. (upper) layer. Extract the aqueous layer twice with 25 mL portions of methylene chloride Filter the combined extracts through a fluted filter paper into a clean, pre-massed, 250 mL beaker. Add boiling chip, use a hot water bath, and boil off the chloroform in the hood. Weigh the beaker and crude product and determine the percent yield. Scrape a small portion of the from the beaker and determine its melting point.

Purification:

Dissolve the impure material in 10 mL of toluene. Use a hot water bath in the hood to warm to solution to the temperature of boiling water. Allow any insoluble material to settle out and carefully decant the clear solution into a 50 mL beaker. Place the beaker on the cool desktop and allow crystallization to proceed. Cool the beaker in crushed ice, then collect the crystals with a Hirsch funnel. Air dry them and determine the melting point and percent yield, based on the mass of tea leaves originally taken.

Follow safe lab procedures and use caution with waste disposal.

Regards,
ProfHoff



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