Experiment 10 - The extraction of Caffeine from Cola drinks >> Partition Coefficients and Phase Separation

Experiment 10 - The extraction of Caffeine from Cola drinks

Partition Coefficients and Phase Separation: How Many Extractions?

As shown in the walk-through in the previous page, rocking a separatory funnel containing two immiscible solutions will allow the solutes in those solutions to preferentially migrate into the solvent in which they have the higher solubility. This redistribution of the solutes is called a "partitioning".

However, because the solutes involved have to be soluble at least slightly in both solvents, this procedure will never result in 100% of a solute being transferred from one solvent into another. How do we work around this problem to extract the largest possible amount of solute into a new solvent?

Let's go through an example. You are a student in Chemistry 123 trying to extract caffeine from an aqueous cola solution into a dichloromethane (CH₂Cl₂) solution. To start, you need to look up the solubility of caffeine in water and in dichloromethane. You find that approximately 2.18 g of caffeine will dissolve in 100 mL of water, while 18.2 g will dissolve in 100 mL of dichloromethane. Both measurements must be done at the same temperature (remember, K_sp depends on temperature!). The measurements reported above have been made at 25°C.

The partition coefficient for caffeine in this solvent mixture is simply the ratio of its relative solubilities in the two solvents:

18.2 g/100 mL ÷ 2.18 g/100 mL = 8.35.

Assume that your 50 mL sample of cola (aqueous solution) contains 20 mg of caffeine, a realistic value for that experiment. You are given 30 mL of dichloromethane to carry out your extractions. How to use those 30 mL? You have several options:

• Carry out a single dichloromethane extraction of the aqueous solution, using all 30 mL of dichloromethane at once.
• Perform two sequential dichloromethane extractions of the aqueous solution, using two portions of 15 mL dichloromethane each.
• Perform three sequential dichloromethane extractions of the aqueous solution, using three portions of 10 mL dichloromethane each.

Let us do some math 

Which choice will extract the most caffeine from the cola into dichloromethane? Let's do some math to find out.

Assume that x mg of caffeine (from the starting 20 mg) migrate from the cola layer into dichloromethane. That leaves behind 20 - x mg in the cola layer. Knowing the relative volumes of the two layers, and the partition coefficient of 8.35, we can solve for x in each case. Remember that the partition coefficient is a unitless ratio! It only changes with temperature, and not with volume. It will always be true, at 25°C, that:

Therefore, in our case:

Let us try the calculations: