25.2 Partition coefficients
- Syllabus
- 9701–2028–2029
- Topic
- 25.2
- Level
- A2
Learning objectives
25.2.1Terms: partition coefficient, Kpc• Define: partition coefficient, Kpc25.2.2And use a partition coefficient for a system• Calculate/use: a partition coefficient for a system in which the solute is in the same physical state in the two solvents25.2.3Factors affecting partition coefficient• Understand the factors affecting the numerical value of a partition coefficient in terms of the polarities of the solute and the solvents used
Kpc is a named equilibrium concentration ratio between two solvents
The partition coefficient, Kpc, is the ratio of the equilibrium concentration of a solute in one solvent to its equilibrium concentration in a second immiscible solvent, at a fixed temperature.
Kpc(solvent 1/solvent 2)=[solute]solvent 2[solute]solvent 1
| Value of Kpc(solvent 1/solvent 2) | Equilibrium interpretation |
|---|---|
| > 1 | higher solute concentration in solvent 1 |
| = 1 | equal solute concentrations in the two phases |
| < 1 | higher solute concentration in solvent 2 |
Always state the solvent order. Reversing the order gives the reciprocal value, and Kpc describes equilibrium concentrations rather than the total mass in either layer.
Use Kpc with phase volumes to calculate how solute is distributed
Use the same solute physical state in both phases, allow the immiscible solvents to reach equilibrium, and express both concentrations in matching units.
A total of 1.00 g solute is shaken with 50.0 cm³ organic solvent and 100 cm³ water. At equilibrium Kpc(organic/aqueous) = 4.00. Let m be the mass, in g, in the organic phase; then 1.00 − m is in water.
4.00=(1.00−m)/100m/50.0=1.00−m2m
4.00(1.00−m)=2m⇒m=0.667 g in organic;0.333 g in water
The organic phase has twice the volume-normalised numerator factor and ends with twice the mass, giving concentrations 0.0133 and 0.00333 g cm⁻³; their ratio is 4.00.
Do not compare masses directly when the phase volumes differ. Kpc is a concentration ratio, so each phase amount must first be divided by its own volume.
Relative polarity controls which solvent better stabilises the solute
A solute reaches a higher equilibrium concentration in the solvent whose polarity and intermolecular attractions are more compatible with it. Favourable solute–solvent interactions compensate for separating particles of the pure solute and solvent.
| Solute character | Water versus a non-polar organic solvent | Expected Kpc(organic/aqueous) |
|---|---|---|
| non-polar | dispersion interactions are more compatible with the organic phase | relatively large |
| polar and able to interact strongly with water | dipole attractions or hydrogen bonding favour the aqueous phase | relatively small |
Changing either solvent changes the reference pair and therefore the numerical Kpc. A slightly polar organic solvent may stabilise a polar solute better than a hydrocarbon does, so Kpc is a property of the complete solute–two-solvent system at the stated temperature.
‘Like dissolves like’ is a polarity-based prediction, not a universal numerical rule. Also, a larger Kpc has no meaning until the numerator solvent is named.