The selectivity of one metal over the other(s) is the ease of
isolating that metal from the mixed electrolyte solution and is
expressed in terms of the separation factor, SM1=M2:
SM1=M2 ¼
DM1
DM2
where
Dnþ
M ¼
Mnþ
org
Mnþ
aq
Dnþ
M is the distribution coefficient of the metal, Mnþ, between the
organic and aqueous phases. The distribution coefficient per se is
not useful in industrial applications since its value depends on the
operating conditions such as pH, temperature, extractant concentration,
aqueous concentrations, phase ratio, etc. The separation
factor, on the other hand, is a very useful variable in the
commercial applications, though its value also depends on the
operating conditions. It is an indication of the ease of separating
mixed electrolyte systems under a given set of operating
conditions. The higher the separation factor, the easier it is to
perform selective extraction with little or no co-extraction