The distribution coefficient of caffeine describes how it dissolves when it's added to two unmixable solvents. Solvents with different chemical properties, such as water and methylene chloride, won't mix together. When they're combined, they form two distinct layers that sit on top of each other in the container. Caffeine will be able to dissolve in both layers, but different amounts of caffeine will be found in the two layers based on caffeine's distribution coefficient for the two different solvents.
Step 1
Calculate the solubility of caffeine in the top layer, hereafter referred to as "solvent 1." One way to do this is to see how much of solvent 1 it takes to dissolve 1 g of caffeine. The solubility of caffeine can be calculated by dividing the amount of caffeine you added by how much solvent you needed to completely dissolve the caffeine. For example, if it takes 22 mL of solvent 1 to dissolve 1 g of caffeine, the solubility of caffeine in solvent 1 is 1/22 or approximately 0.0455.
Step 2
Measure the solubility of caffeine in solvent 2 using the same technique as solvent 1. For example, if you need 40 mL of solvent 2 to dissolve 1 g of caffeine, your solubility is 1/40 or 0.025.
Step 3
Divide the solubility of caffeine in solvent 1 by its solubility in solvent 2 to calculate the distribution coefficient of caffeine in these two solvents. Using the above sample calculations, the distribution coefficient of caffeine would be 0.0455/0.0250 or 1.82.
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