Acids, extracted with tri-n-octylamine, are extracted back to an aqueous phase. The back-extraction is based on a displacement reaction. Several displacing ions were used: chloride, bromide, iodide, nitrate and perchlorate. The efficiency increases in the order chloride < bromide < iodide < perchlorate. The back-extraction yield is directly proportional to the concentration of the displacing ion and inversely proportional to the tri-n-octylamine concentration.
The extraction of some organic acids by ion-pair formation with tri-noctylamine (TnOA) was discussed in the first part of this series [ 11. Quantitative recoveries can be obtained even with hydrophilic substances. The extracted species have to be identified and quantified by techniques such as thin-layer (t.1.c.) or high-performance liquid chromatography (h.p.1.c.). It was demonstrated earlier [2] that TnOA-dye ion-pairs can be chromatographed directly both in t.1.c. and h.p.1.c. However, in t.1.c. the mobile phase must be relatively polar and in h.p.1.c. the column must be rinsed regularly in order to elute the adsorbed TnOA which disturbs the elution of the solutes. Similar problems were also observed in liquid-liquid ion-pair chromatography on injection of solutes as ion-pairs with a hydrophobic counter-ion [ 31. Therefore, for h.p.l.c., it is preferable to extract the analytes back to an aqueous phase by means of a displacement reaction for which several anions have been tested [ 41. Such a back-extraction can also yield purer extracts, which may be of high importance in some cases.
This communication discusses the influence of the nature and concentration of the displacing ion on the back-extraction yield.
Experimental
Apparatus. Ultraviolet and visible photometric determinations were done with a Hitachi 200 Perk&Elmer spectrophotometer.
Ion concentrations were measured with an Orion 601 Ionalyzer, with a combined glass electrode, a combined bromide electrode and a combined iodide electrode (Orion).
Chemicals and reagents. Tri-n-octylamine
was from Aldrich Europe (Beerse, Belgium). Sodium sorbate (Fluka) and sodium benzoate (Merck)