Distribution between Phases

• A phase is a state eg solid, liquid and gas
• we can also have two [phases in the same state eg two phases in the liquid state which can be found between two immiscible liquids

Partition law

Partition

• to divide into into parts with a boundary/interface
• used when a solute distributes itself between two immiscible liquids

Partition Coefficients

• is the ratio of concentrations of a solute in two immiscible solvents at equilibrium
• partition coefficient has no units
• The partition or distribution coefficient of a solute X between two phases Phase-1 and Phase-2 is written as:
• the value of partition coefficient is affected by temperature
• If a solute X is added to a system of two immiscible solvents, it will distribute itself between the immiscible liquids so that the ratio of the solute concentrations remains the same at constant temperature independently of the total quantity of X in the same molecular state. That is called the partition equilibrium or distribution law.
• Therefore when temperature changes, the Kpc law still holds, only the value of Kpc changes
• The partition coefficient is a measure of the differential solubility of the solute between these two solvents

Factors affecting the Partition law

• there is no dissociation of solute particles in one solvent and not the the other
• there is no association of solute particles in one solvent and not the the other
• eg (CH₃CO₂H)₂ in ether and (CH₃CO₂H) in water

 

Application of the partition law

Solvent Extraction

• this is a process whereby a solvent is used to obtain or extract a solute from the dilute aqeuous solution of that solute
• a good extracting solvent is used
• Partition coefficients are useful in calculating how much solute can be extracted from a solvent

 

Characteristics of a good extracting solvent

1. it must be immiscible with the (aq) solution
2. it must have high solubility for the solute which is to be extracted
3. it must be inert
4. it must be very volatile

Examples of good extracting solvent solvents

• non-polar solvents eg ether and tetrachloroethene

 

Procedure for solvent extraction

1. place a dilute solution of a solute in a separating funnel followed by the extracting solvent
2. close the separating funnel and shake the two solutions for some time, an equilibrium is going to be established
3. separate the two solvents i.e the extracting solvent and the dilute aq solution using separating funnel

Precautions

• it must be done in a fume cupboard because most extracting solvents are carcinogenic
• naked flames must be avoided because the extracting solvents are very flamable
• the vapour of ether is harmful to people

Uses of solvent extraction

to extract natural products from dilute (aq) solutions eg extraction of carfain from tea

Improving efficiency of the solvent

Extracting method

the efficiency of the solvent extraction process cab be improved by the use of smaller multiple portions of the extracting solvent