Page 232 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANCE

An extensive review of ligand-exchange chromatography has been given by
Da~ankov.~'

7.7 LlQUlD ION EXCHANGERS
The ion exchange processes involving exchange resins occur between a solid
and a liquid phase whereas in the case of liquid ion exchangers the process
takes place between two immiscible solutions. Liquid ion exchangers consist of
high-molecular-weight acids and bases which possess low solubility in water
and high solubility in water-immiscible solvents. Thus, a solution of a base
insoluble in water. in a solvent which is water-immiscible. can be used as an
anion exchanger; similarly a solution of an acid insoluble in water can act as
a cation exchanger for ions in aqueous solution. A comprehensive list of liquid
.,
ion exchangers has been given bv Coleman et aL4'
"
The liquid anion exchangers at present available are based largely on primary,
secondary and tertiary aliphatic amines, e.g. the exchangers Amberlite LA.l
[N-dodecenyl(trialkylmethyl)amine] and Amberlite LA.2 [N-lauryl(trialky1-
methyl)amine], both secondary amines. These anion exchange liquids are best
employed as solutions (CU 2.5 to 12.5% v/v) in an inert organic solvent such
as benzene, toluene, kerosene, petroleum ether, cyclohexane, octane, etc.
The liquid exchangers ~mberlite LA.l and LA.2 may be used to remove
acids from solution

or in a salt form for various ion exchange processes

Examples of liquid cation exchangers are alkyl and dialkyl phosphoric acids,
alkyl sulphonic acids and carboxylic acids, although only two appear to have
been used to any extent, viz. di-(2-ethylhexyl)phosphoric(V) acid and
dinonylnaphthalene sulphonic acid.
The operation of liquid ion exchangers involves the selective transfer of a
solute between an aqueous phase and an immiscible organic phase containing
the liquid exchangers. Thus high-molecular-weight amines in acid solution yield
large cations capable of forming extractable species (e.g. ion pairs) with various
anions. The technique employed for separations using liquid ion exchangers is
thus identical to that used in solvent extraction separations and these exchangers
offer many of the advantages of both ion exchange and solvent extraction.
There are, however, certain difficulties and disadvantages associated with their
use which it is important to appreciate in order to make effective use of liquid
ion exchangers.
Probably the chief difficulty which arises is that due to the formation of
emulsions between the organic and aqueous phases. This makes separation of
the phases difficult and sometimes impossible. It is clearly important to select
liquid exchangers having low surface activity and to use conditions which will
minimise the formation of stable emulsions [see Section 6.7, consideration (3)].
Another disadvantage in the use ofliquid ion exchangers is that it is frequently
necessary to back-extract the required species from the organic phase into an
aqueous phase prior to completing the determination. The organic phase may,
however, sometimes be used directly for determination of the extracted species,