In an experiment, some butanedioic acid (succinic acid) was added to a mixture of water and ethoxyethane. Water and ethoxyethane are immiscible. The mixture was shaken and allowed to settle. The two layers were separated and the concentration of buatenedioic acid was determined in each layer. The experiment was repeated using different amounts of butanedioic acid. The results of the experiment are shown in the following graph.

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How might the concentration of butanedioic acid in each layer be determined?

The concentration of butanedioic acid can be determined by titration with a standard solution of an alkali such as sodium hydroxide.

Is the butanedioic acid more soluble in water or in ethoxyethane? How can you tell?

The butanedioic acid is more soluble in water than in ethoxyethane. Any point on the graph corresponds to a higher concentration of the solute in the water than the concentration in ethoxyethane.

The graph is a straight line through the origin. What does this tell you about the ratio of the concentration of the butanedioic acid in the two liquids?

The ratio of the concentration of the solute in the two liquids is constant.

In general, if a solute is added to two immiscible liquids, a dynamic equilibrium is formed as the solute moves from one liquid to the other in both directions. The concentrations of the solute in each liquid do not change. The concentrations are linked through the following expression:

K is known as the partition coefficient. It is a type of equilibrium constant.

It is important to show which liquid is which when you write down an expression for a partition coefficient. Where one liquid is water and the other is an organic liquid, it is common to put the concentration of the organic liquid on the top of the expression.

The partion coefficient is a constant provided that:

• the temperature is constant;
• the liquids are immiscible and don't react with each other;
• the solute does not react, ionise or dissociate in the liquids.

The concentrations can be given in any units as long as they are the same for both top and bottom of the fraction.

What units does the partition coefficient have?

The partition coefficient does not have any units because they cancel out.

What does the value of the partition coefficient tell you about the solute and the two immiscible liquids?

The partition coefficient provides a measure of the relative solubility of a solute in the two liquids.