Question

The distribution co-efficient of butanoic acid (pKa = 4.8) between water and two organic solvents are given below;

Benzene D= 3.0

Octanol D= 6.2

a) which organic solvent is expected to extract the acid most efficiently from the water? explain your answer.

I understand that D is a measure of extraction efficiency but I am having trouble answering this question.

b) When extracting butanoic acid into benzene is it more effective to do one extraction with 300mL or three 100mL extractions? Support your answer with calculations.

Answer 1

Distribution coefficients = K_o/a so large K values occur as the result of higher solubility in the organic phase than in the aqueous phase. In contrast, lower K values result from a higher concentration in the aqueous phase than in the organic phase. Therefore, ......can you take it from here?

B. You can go through the calculation (with a made up problem if necessary) to see how this works, but in general three extractions ALWAYS does a better job than a single extraction of the same volume; i.e., three 100 mL extractions do a better job than a single 300 mL extraction. Post your work if get stuck on the calculation or on the formula.