Download Distribution Coefficients

Distribution Coefficients
1. If 0.500 grams of caffeine is dissolved in 100 mL of water, what percentage of caffeine can be
separated from the water using a single 40 mL sample of methylene chloride. P = 4.6
2. Benzoic acid can be separated from water using octanol as the organic solvent. The
distribution coefficient for this water/octanol system is P = 1.87. Assuming that 1 gram of
benzoic acid has been dissolved in 100 mL of water, how many 20 mL extractions must be done
to extract 60+ percent of the benzoic acid from the water?
3. A 0.100 gram sample of phthalic acid was dissolved in 100 mL of water. When 25 mL of
diethyl ether was used to extract the phthalic acid, 0.042 grams of phthalic acid were recovered.
What is the distribution coefficient for this extraction?
4. Draw a graph (using Excel) that shows the percentage of solute (extractable compound)
extracted from water using increasingly large volumes of organic solvent. Express the amount
extracted as a percent of the total. Make the spreadsheet very general. Allow for the input of
various distribution coefficients, amount of dissolved compound, and volume of aqueous
solution.
1.
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0.92 – 1.84x = x
0.92 = 2.84x
2. First extraction
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0.374 – 0.374x = x
0.374 = 1.374x
Second extraction
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0.2722 – 0.374x = x
0.2722 = 1.374x
Third extraction
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0.1979 – 0.374x = x
0.1979 = 1.374x
3.
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4. You should begin by solving the extraction equation so that the grams of extracted solute is
in terms of the volume of extracting solvent.
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It will be difficult to solve this equation using the long words to describe each part so let’s
simplify the variables,
g extracted solute = gx
g solute = gs
vol. extracting solvent = vx
vol. water = vw
Our equation is now,
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We want to rearrange this equation so that we have gx in terms of vx. So, rearranging,
This is our equation. We can test is by using the number given in problem #1.
P = 4.6
vx = 40 mL
gs = 0.500 gram sample
vw = 100 mL
Finally, we can calculate the percentage directly by dividing our answer (gx) by the grams of
sample (gs) and multiplying by 100.
So now, we need to make an Excel spreadsheet to do this calculation for us for various volumes
of extracting solvent. The Excel file that does this for you is online with this problem set. You
will notice that the amount extracted increases with the volume of extracting solvent but it also
increases when you lower the volume of the water. So, it is easier to extract stuff out of a more
concentrated water solution. You will also get more extracted if the distribution coefficient
increases.