Solvent extraction

You now know that it is possible to extract some of a solute in a solution by shaking with another liquid in which the solute is more soluble. This is the basis of solvent extraction; i.e. separating a substance from a mixture by using its ability to dissolve to different extents in different solvents. When you answer SAQ, you should discover that more iodine could be removed from the original solution by shaking with two separate 20 cm³ portions of tetrachloromethane rather than by shaking with one 40 cm³ portion. This is a general rule in solvent extraction:

Repeated extractions with smaller volumes solvent are more efficient than one extraction with a larger volume.

Suppose that 50cm³ of a 0.1 moldm^-3 solution of iodine in potassium iodide was shaken with 10 cm³ of an organic liquid. The partition coefficient between the organic liquid and water was 100.

Use the correct version of the partition law to calculate:

a. the concentration of iodine in the organic, layer at equilibrium;

b. the mass of iodine in the organic layer at

equilibrium; and

c. the mass of iodine left in the aqueous layer

You will need to know that

M(I₂) = 254gmol^-1.

[Hint: The safest way to do this problem is to

work in terms of the number of moles ofiodine

in a given volume of liquid.

In general, for a solute that is in contact with two solvents we can write the partition law as:

(The decision about which is the first solvent, and which is the second, is made so as to make the value of K_pc as large as possible, e.g. K_pc = 99, rather than putting the values the other way round and aking K_pc = 1/99 = 0.0101, as such small values are less easy to use.)

Worked example 4

A solution contained 1 g of iodine dissolved in 40 cm³ of aqueous potassium iodide. If we shook this solution with 20 cm³ of tetrachloromethane, what mass of iodine would be transferred into the tetrachloromethane? (Tetrachloromethane is an organic liquid, also known as carbon tetrachloride, that has been used as a dry cleaning fluid. It has a very harmful vapour and is no longer widely available.)

However, no matter how many times an extraction is done, there will always be a tiny amount of the solute left in the original solution. This is because it is impossible to move an equilibrium completely (100%) to one side or the other.

SAQ

Using the same data as in Worked example, suppose that the remaining aqueous layer was separated after the first extraction of iodine.

a This layer was then extracted with another 20 cm³ of tetrachloromethane. What mass of iodine would have been left in the aqueous layer after the two extractions?

b If the original aqueous iodine solution had just been shaken with 40 cm³ of tetrachloromethane, what mass of iodine would have been extracted? c Is it best to extract the iodine with two smaller volumes or one larger volume of solvent?

SUMMARY

● The solubilities of gases in water follows
these rules:

- A gas that reacts with water usually has a
high solubility.

- The solubility decreases as the
temperature increases.

- The solubility increases as the pressure
of the gas increases.

● Henry's law states that: The concentration
of gas dissolved in a liquid at a constant
temperature is proportional to the partial
pressure (p) of the gas.

● In symbols Henry's law is

where K_n is Henry's law constant. Like all equilibrium constants K_h is a constant at a given temperature.

Immiscible liquids do not mix. and form
two layers when pin together.

· A partition coefficient is an equilibrium
constant that describes the ratio in which a
solute will be distributed between two
immiscible liquids in contact with one
another. The general formula for the
partition coefficient is

● Solvent extraction can be used to separate a substance that dissolves more in one of a
pair of immiscible liquids than in the
other; for example, iodine preferentially
dissolves in cyclohexane rather than in
aqueous potassium iodide, so cyclohexane
can be used to extract iodine from the
aqueous solution.

QUESTIONS

The solubility of sulphur trioxide in water is too high to measure. Find out what is made when this gas dissolves in water.

1 Ammonia is also listed in table 3.1 as a very soluble gas. The solution it makes is sometimes called ammonium hydroxide, and given the formula NH4OH. In fact, there is very little NH4OH in a solution of ammonia, and the solution is best called 'aqueous ammonia' - it simply contains
very large amounts of ammonia dissolved in, rather than reacted with, the water.

a What types of intermolecular force can exist in water, and in ammonia?

b Why would you expect ammonia and water to mix easily (i.e. why would you expect ammonia to be very soluble in water)?

2 In the table below, there is a set of data showing the solubility of carbon dioxide in

water at 25 °C for a range of pressures of the gas. Plot a graph of the results use the graph to estimate a value for the Henry's law constant for carbon dioxide.

Partial pressure of CO2(kPa)	Solubility (mol dm^-3)
	1.72 ×10^-3
	3.44 ×10^-3
	6.83 ×10^-3
	1.03 ×10^-2
	1.37 ×10^-2
	1.71 ×10^-2
	3.41 ×10^-2

3. Make up the glossary of physical terminology.

Дата добавления: 2015-11-16