Liquid ion exchangers

The liquid anion exchangers at present available are based largely on primary, secondary and tertiary aliphatic amines, e.g. the exchangers Amberlite LA.l [A/-dodecenyl(trialkylmethyl)amine] and Amberlite LA.2 [A/-lauryl(trialkyl-methyl)amine], both secondary amines. These anion exchange liquids are best employed as solutions (ca 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 Amberlite LA.l and LA.2 may be used to remove acids from solution 

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. 

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 of liquid 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, 

---

The following set of suggested experiments describes the preparation of solid-state and liquid ion-exchange ion-selective electrodes, as well as potentiometric biosensors. 

This experiment describes the preparation of liquid ion-exchange electrodes for Gk and Ga +. The liquid ion-exchange solutions are incorporated into PVG membranes and fixed to the end of glass tubing. The internal solutions are either NaGl or GaGk, and a Ag/AgGl reference electrode is situated in the internal solution. 

Exhausted liquid ion exchangers may be regenerated in an analogous manner to ion exchange resins, e.g. Amberlite LA.l saturated with nitrate ions can be converted to the chloride form by treatment with excess sodium chloride solution. 

The properties and applications of liquid ion exchangers have been reviewed.44... 

H Green, Recent uses of liquid ion exchanges in inorganic analysis Talanta, 1964,... 

Light filters for colorimeters, see Filters, optical Limiting cathode potential 509 see also Controlled potential electro-analysis Linear regression 145 Lion intoximeter 747 Liquid amalgams applications of, 412 apparatus for reductions, 413 general discussion, 412 reductions with, (T) 413 zinc amalgam, 413 Liquid ion exchangers structure, 204 uses, 204, 560... 

S-2.2.2 Neutral Carrier Electrodes hi addition to charged liquid ion exchangers, liquid-membrane electrodes often rely on the use of complex-forming neutral carriers. Much effort has been devoted to the isolation or synthesis of compounds containing cavities of molecular dimensions. Such use of chemical recognition principles has made an enormous impact upon widespread acceptance of ISEs. The resulting neutral carriers can be natural macrocyclic molecules or synthetic crown... 

Schmuckler, G., High-performance liquid ion-exchange chromatography /. Liq. Chromatogr., 10, 1887, 1987. 

Heterogeneous liquid membrane electrodes. This type, which has become of considerable practical importance, consists of a liquid ion-exchange layer or a complex-forming layer within a hydrophobic porous membrane of plastic (PTFE, PVC, etc.), sintered glass or filtering textile (glass-fibre, etc.). The construction of such an electrode is depicted in Fig. 2.12. 

Ion-selective electrodes are membrane systems used as potentiometric sensors for various ions. In contrast to ion-exchanger membranes, they contain a compact (homogeneous or heterogeneous) membrane with either fixed (solid or glassy) or mobile (liquid) ion-exchanger sites. 

Virnig, M. J. Wolfe, G. A. LIX 79—a new liquid ion exchange reagent for gold and silver. Value Adding through Solvent Extraction, [Papers presented at ISEC 96], Melbourne, Mar. 19-23, 1996, 1, 311-316. 

Liquid ion-exchangers have been discussed in the section on solvent extraction (p. 65). They can be used in column form by coating them on to a solid support such as cellulose powder or Kel-F (polytrifluorochloroethylene). Tris-n-octylamine (TNOA) and bis(2-ethylhexyl)phosphoric acid (HDEHP) behave as strong-base and strong-acid exchangers for anions and cations respectively. 

Electrodes with liquid ion-exchange membranes are typified by a calcium-sensitive electrode (Figure 6.4). The membrane-liquid consists of the calcium form of a di-alkyl phosphoric acid, [(RO)2POO ] 2Ca2+, which is prepared by repeated treatment of the acid with a calcium salt. The internal solution is calcium chloride and the membrane potential, which is determined by the extent of ion-exchange reactions at the interfaces between the membrane and the internal and sample solutions, is given by... 

Liquid injection molding (LIM), 18 792 phenolic resins in, 13 794-795 Liquid inks, 14 315 Liquid iodine, viscosity of, 14 354 Liquid ion-exchange rhenium recovery process, 21 689... 

Porous membrane saturated with liquid ion-exchanger... 

Liquid membrane electrodes utilize porous polymer materials, such as PVC or other plastics. An organic liquid ion exchanger immiscible with water contacts and saturates the membrane from a reservoir around the outside of the tube containing the water solution of the analyte and the silver-silver chloride wire. See Figure 14.10. Important electrodes with this design are the calcium and nitrate ion-selective electrodes. 

K. Covington and P. Davison, Liquid ion exchanger types, in Ion-Selective Electrode Methodology, Vol. I, CRC Press, Boca Raton (1979), p. 85. 

Eisenman, Theory of membrane electrode potentials an examination of the parameters determining the selectivity of solid and liquid ion exchangers and of neutral sequestering molecules. Chapter 1 of Ion-Selective Electrodes (ed. R A Durst), National Bureau of Standards, Washington (1969). 

Fig. 5.6. A flow-through electrode system for liquid-membrane ISEs [111] 1 - reference electrode 2 - hole through which the sample solution flows 3 - liquid ion-exchanger reservoir, 4 - triangular piece of a frit soaked with the liquid ion-exchanger.

Before the appearance of analytically useful ISEs with liquid membranes, Sollner and Shean [200] obtained a liquid ion-exchange membrane with marked selectivity for anions. The first ISE with a liquid membrane was the calcium electrode described by Ross [179] with Ca " -dialkylphenylphosphate in dioctylphenylphosphonate. 

The object of equilibration is to provide a solvent that will effectively extract the required species, either because of the form of the active constituent of the solvent or by maintaining the necessary extraction pH. As an example of the former condition, consider the extraction of uranium using a tertiary aliphatic amine as extractant. Extraction of metal species by such amines is considered to occur by liquid ion exchange (see Chapters 3 and 4). For a tertiary amine to act in this manner, it must be first converted to an amine salt ... 

Tunley, T. H. Birch, C. P. The recovery of copper from sulphate leach liquors by liquid ion exchange with LIX64N, National Institute of Metallurgy, Johannesburg, South Africa, Report No. 140904. 

Although there is no direct connection between the rules discussed in Chap. 4 for the monovalent-divalent ion selectivity of ligands in liquid membranes and the electrochemical selectivity behavior of these membranes (cf. Eqs. (22) and (31)), the effect of solvent on Na+/Ca2+ selectivity (Fig. 22) is remarkably similar to the calculated effect shown in Fig. 18 (actually for K+/Ba2+ preference). The discrimination against Mg2+ and especially HaO+ is considerably better for the electrode discussed here 117) than for liquid ion-exchange membranes responsive to Ca2+ (123). 

---