The distribution coefficient for caffeine in dichloromethane and water is 4.6. A

Question

The distribution coefficient for caffeine in dichloromethane and water is 4.6. Assume that your local 100-mL tea solution contained 0.30 g of caffeine. If you had extracted with only one 15-mL portion of dichloromethane, how much caffeine would have been left in the water solution? How much would be left in the water after the second 15-mL dichloromethane extraction? How much would be left in the water solution after if only one extraction with 30-mL of dichloromethane were performed? Is it better extract 2 times with 15-mL or 1 time with 30-mL?

Explanation / Answer

distribution coefficient, K= conc. in dichloromethane/ conc. in water              [c is in g/ml ]
Let w g be the weight in dichloromethane, then

4.6 =( w / 15 ) / ( [0.3 - w] / 100) where w is the wt extracted

w = 0.122 g extracted

weight left in water after 1st 15 ml extarction = 0.30 - 0.122 = 0.178 g

For the next 15 ml extaraction

4.6 =( w / 15 ) / ( [0.178 - w] / 100)

w = 0.073 g

weight left in water after 2nd 15 ml extarction = 0.178 - 0.073 = 0.105 g

If only one extraction with 30-mL of dichloromethane was done , then

4.6 =( w / 30) / ( [0.3 - w] / 100) where w is the wt extracted

or w = 0.174 g

therefore, weight left in water after one 30 ml extarction = 0.30 - 0.174 = 0.126 g

It is better to extract 2 times with 15 ml than with 1 time 30 ml as seen from the final weight left in water.

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