Sample Preparation

6-5

A sample of water contaminated with pesticide is ectracted with methylene chloride for determination of the pesticide. In experiments with a standard solution, it its found that when a 100 ml water sample is extracted with 30 ml of methylene chloride, 75% of the pesticide it removed. What fraction of the pesticide would be removed if 3 extractions using 10 ml of methylene chloride were done instead?

First, find value of D.

Let C_w^o = 1, then C_wⁿ = 0.25, (amount left after 75% of pesticide is removed.)

Applying above equation, solve for D

D = 10

For 3 extractions:

C_w³ = 0.125 remaining in water

Of the original concentration was 1 and 0.875 was extracted. Therefore 87.5% was extracted after 3 extractions with 10 ml methylene chloride used for each extraction.