ABSTRACT
This chapter is of utmost importance since it introduces the concept of partition coefficient. All the methods of separation used in analytical chemistry which are investigated later in this book are founded on it. It is introduced in this chapter from a purely thermodynamic standpoint since it is presented as being the partial molal Gibbs energy or the chemical potential of each species belonging to a system. By taking the analogy with the part played by the voltage for the origin of an electric current, one immediately understands that a difference of chemical potential µi of a species present in two non-miscible phases α and β in contact leads to a spontaneous transfer of i from one phase to the other until the equality µi α = µi β is reached. This is the partition equilibrium. It is demonstrated that, at this time, the activities of I; ai in both phases are in the constant ratio P = ai β / ai α. P is the thermodynamic partition coefficient. In some conditions, the activities in both phases at equilibrium may be replaced by the concentrations. Then, the constant ratio is called the distribution ratio D.
