U. A. Th. Brinkman ond G. d e Vries Free Reformed University Amsterdam, The Netherlands
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Liquid Anion-Exchangers in ReversedPhase Extraction Chromatography
In 1948, Smith and Page ( 1 ) introduced high-molecular-weight amines for the extraction of inorganic and organic acids and they compared this process with sorption by anion-exchange resins. Extensive investigation on a wide variety of technical-grade amines and related substituted quaternary ammonium salts commenced in the middle 195O's, and since that time extraction of acids and metal ions from acid solution with this class of extractants has received considerable attention. Over the years, these processes have become known as liquid anion-exchange, which can be defined for most purposes as follows: liquid anion-exchange refers to liquid-liquid extraction systems that operate, at least formally, by interchange of anions at the interface between an aqueous solution and an immiscible solvent (the diluent), with negligible distribution of the extractant to the aqueous phase. Interchange of anions is generally assumed in preference to the addition of neutral species. Although these alternatives may sound drastically different, they represent only an arbitrary choice of description for equilibrium extractions, as the alternatives are thermodynamically equivalent, and cannot he distinguished by any measurements made at equilibrium. As an example, metal extraction by a tertiary amine may be represented' by
About a decade ago, an interesting development arose, in which liquid anion-exchangers were used in reversedphase extraction chromatography ($, 3). In this technique, the support material (paper, cellulose powder, Silica Gel, Kieselguhr, etc.) impregnated with the liquid anion-exchanger, is used as the stationary phase and an aqueous solution of an acid or one of its salts is used as the mobile phase (eluant). The application of liquid anion-exchangers in reversed-phase extraction chromatography has a high potentiality. Many interesting qualitative and quantitative separations have been achieved, especially when employing exchanger-C1systems. In the present paper, the use of exchangerC1- systems in thin-layer and paper chromatography will be discussed. Attention will moreover he paid to the analogy between the processes of reversed-phase chromatography and liquid-liquid extraction. 'The formulas for the monomeric dkylammonium salts have been consistently written in this paper, though it is known that aggregation of these entities may occur.
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Characteristics of Some Liquid Anion-Exchangers Exehrtn~er Trade name or abbreviation Composition Irrtilq pi-^ Primene JM-T
trialkylmethyl-
Secondary Amberlite LA-1 N-dodecenyltrialkvlmethvladne " Amberlite LA-2 N-lauryltrialkylmethylamine Tertisry Alamine 336 tri-n-(octvl t TnOA TiOA Quaternary Aliqurtt 336
Mean mol. mt.
Manufacturer
311 Rohm & Haas 372 Rohm & Haas 374 Rohm & Haas 392 General Mills
tri-n-&tylarnine tri-i-oetylamine
353 e.g. K & K, Light 353 e.g. K & K, Light
methyltri-n-(oetyl decyl) am-
475 General Mills
+
450-
Liquid Anion-Exchangers
The use of liquid anion-exchangers in analytical chemistry has been greatly enhanced by the increasing availability of low-cost commercial products. The table collects the extractants which are most popular at the time, together with their main characteristics. Liquid anion-exchangers are normally used as supplied. They must he handled with some care: it is important to provide adequate ventilation to prevent inhalation of the vapors; moreover, contact with the eyes and prolonged contact with the skin should be avoided. Amine-HX Systems
Solutions of liquid anion-exchangers may be prepared by simply weighing out the appropriate amount and dissolving it in a suitable dilnent such as toluene, xylene, or chloroform. For accurate work, standardization by titration-e.g., in an ethanol-water medium, using 0.1 N HCIOa as a titrant-is recommended. An amine is converted into the appropriate salt form by shaking its solution in toluene with an excess of an aqueous solution of the acid in question. The uptake of acid by the amine as a function of the aqueous acid normality is an important aspect when interpreting the chromatographic data (see below). Figure 1illustrates data obtained for the extraction of HCl, HCIOP, and HN03 by a tertiary amine. It is evident that the extraction of perchloric acid involves the formation of R3NH+.C10n-only, whereas in the case of nitric acid, a considerable amount of excess acid is extracted; this is ascribed to the successive formation of R,NH+.NOs. Hydrochloric acid takes and R3NH+(N03...HN03)an intermediate position. Depending on the class of
-
equil. conc. HX aq.
Figwe 1. Extraction of HCIO* HCI, and HNOo by a 0.1 M solution of Alamine 336 in toluene. The doto ore corrected for the uptoke of acid by toluene.
the exchanger and the diluent employed, extraction of excess acid starts in the region 2-8 N HC1; RaN: HC1 = 1:2 ratios may be reached with conc. HCl. Infrared spectra for organic phases containing an amine hydrochloride or a quaternary ammonium chloride and excess HCl always exhibit a characteristic broad absorption peak at 1110-1190 cm-', which has been assigned to the hydrogen dihalide ion HCL-. This suggests that the extraction of excess HC1 may be represented by the equation (R, alkyl; R', alkyl or H) RR'aN+C1-,.,
+ HCb-,.
e RR',NCHCb>,..
+ CI-,.
(3)
Procedure 10 ml of a 0.1 M solution of the liquid enion-exchanger in toluene are shaken for 10 min with an equal volume of an aqueous acid solution. After separation of the phases, an aliquot portion of the organic phase is pipetted, diluted with 70 ml of ethanol and titrated with aq. 0.1 N NaOH. The end-point is determined potentinmetrically or with phenolphthalein as an indicator,
The extraction of acids by liquid anion-exchangers is thoroughly discussed in references (4) and (5). Reversed-Phase Extraction Chromatography
It has repeatedly been stated that there are definite advantages to thin-layer chromatography over paper chromatography when the scale of the chromatogram is reduced to about the size of a microscope slide. In this section, therefore, emphasis is laid on the use of a smallscale thin-layer technique (6). Procedure The selected amine is converted into its HCI salt by equilibrating a 0.10 M solution in CHC5 for 10 min in a separatory funnel with an equal volume of 2 N HCL. The organic Solution is separated and filtered. Qusternary ammonium salts are equilibrated in the same way in order to convert any free amine present into its HCI salt. Thin law?. The organic solution is mixed thoroughly with Silica Gel (SiO%-CHC13,1:4, w/v; Kieselgel DC, Woelm, Eschwege, W. Germany'). The resulting suspension is either used immediately or stored overnight and agitated again before use. Chromatoplates are prepared by dipping ordinary microscope slides into the suspension. On leaving the glass plates for some minutes in the air, in order to evaporate offthe chloroform, a film of impregnated Silica.Gel adheres to the slides. Superfluous material is wiped off the back and a small margin is made along the edges. The plates are spotted (2-6 spots per
plate; diam. -1 mm) using a pointed paper wick partly impregnated with the solution to be analyzed. Samplesolutions contain 1-10 mgof ion per ml, and are acidified as far as necessary in order to prevent hydrolysis. Ascending chromatography is carried out for a. 30-mm run in suitable vessels, e.g., Hellendahl staining jars (approx. 2.5 ml of eluant, and 6 chromatoplates per vessel); chamber saturation is superfluous. With solutions of HC1 or other inorganic acids as eluants, development takes 10-15 min only. When, however, a solution of an acid is substituted by a solution of its (slightly acidified) lithium salt, the development time increases considerably, especially a t high molarities. The development is terminated when the eluant reaches a previously applied groove in the Silica Gel layer. After drying the plates in the air, detection is done using conventional visualization procedures, such as spraying with solutions of 4-(2-pyridylas0)-resorcinol (PAR), diphenylcarbaside, dithisone, bhydroxyquind i n e and alizarin. Paper. WhatmanNo. 1or S and S 2043a paper is impregnated with the liquld anion-exchanger by pulling the strips a t a rate of 2-3 cm/sec through a Hellendahl staining jar, partly filled with the solution of the exchanger. The impregnated paper is dried by hrtnging it in the air for some minutes. Spotting and ascendingdevelopment (10-15 cm; 2-3 hr) follow conventional procedures. Detection is done as described above.
Results
From numerous data in the literature (6-8) it is evident that the sorption strength/extraction efficiencyof liquid anion-exchangers in exchanger-monobasic strong acid systems increases in the sequence primary < secondary < tertiary = quaternary. The moderately sorbing secondary amine Amberlite LA-1 has been selected for the present discussion. RF versus IM C1curves for 28 ions are shown in Figure 2, and additional data are given in the legend to this figure. Silica Gel was used as support material and solutions of HC1 and slightly acidified (0.15 N) LiCl as eluants. Data on the latter system, which is rather unattractive because of the prolonged development times, have been included with a regard to the theoretical aspects involved, and will be discussed below. A survey of the R , spectra in Figure 2 and of similar data referring to tertiary amine-HC1 and primary amine-HC1 systems shows (6) that the ions may be divided into three groups as regards their sorption behavior. RF > 0.9 in All Amine-HC1 Systems. With the ions in thisgroup formation of negative metal-ehloro complexes is negligible and no anion-exchange occurs. Examples are reported in the legend to Figure 2. 0.9 > RP 2 0.0; Sorption Increasing in the Scpuence Primary < Secondary < Tertiary Amine System. Negatively charged eomplexes are formed and extracted by the liquid anion-exchangers a t least in part of the HCI concentration range investigated; the anion-exchange process may he represented by nRR'sN+C1>,..
+ MC1,"-,.
= (RRa'N+).MC1,">,.
+ nC1-.,.
(4)
For examples, we point to the ions, for which RF spectra have
Siliea GelType DO fromFluka (Buehs, S. G., Switzerland) has been used in many investigations (6-10). However, its production seems to have been stopped, and it has been replaced with a Silica Gel, which-like many other types not mentioned here- is not suitable at all for reversed-phase work. Microcrystalline cellulose [Avieel TG104, F.M.C., Marcus Hooke (Pa.), U.S.A.] has been successfully used insbead of Silica Gel in, e.g., H F and NRSCN systems. As regards HCI systems, good results zre obtained wit,h Ahmine-impregnated Avieel; however, with the Amberlites LA-1 and LA-2, most elements show rather elongated spots.
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Figure 2.
RF rpedra for 28 ions using Silica Go1 IWoelm) impregnoted with Amberlite LA-I .HCI or stationary phore. Eluontr: solid line, HCli broken line, slightly acidified LiCI. Tho ions Mgllll, AilIIII, CaIIIl, SElllll, VIIVI, CrIIIIl, Nilll), YIIIII, the trivalent lanthanide ions and ThllV) have Rr 0.9 ot dl HCI concentrotionr
>
been drawn in Figure 2, excepting Ti(IV), Zr(IV), Ba(II), and Hf(IV) for reasons given below. 0.9 > RI- 0.0; Equal Sorplion in All Thwe Amine-HClSysterns. The role of anion-exchange - is neelieible and the shaoe of the RI- spectra is mainly determined by phenomena such adprecipitation of insoluble salts (Ba),adsorption to the support andlor hydrolysis (Ti, Zr, and Hf). This hypothesis is confirmed by the tact that the shape of the RP spectra does not alter when chromatography is carried out on Silica Gel which has not been impregnated with a liquid anion-exchanger.
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Separations
R, spectra such as those presented in Figure 2 can be used as guides for selecting favorable conditions for qualitative separations. Some examples are pictured in Figure 3. We may add that the Amberlite LA-1HC~ systemhas heen successfully for the analysis a large number sulfidic and non-sulfidic Ores (9,10). Silica Gel is used as support material, and
not fulfilled in practice. Only a semi-quantitative correspondence may therefore be expected between the changes in the values of l/Rp - 1 and of D with varying molarity of the aqueous C1- solution. The analogy is illustrated for a few examples in Figure 4. For the sake of simplicity, the experimentally determined quantities y0E and RF have been plotted as ordinates; consequently, the curves show an approximately opposite instead of an identical pattern. 15 ml of a 0.04 M solution of the cation to be investigated in aqueous HC1 or slightly acidified LiCl of varying molarity are shzken in a. separatory funnel with an equal volume of 0.10 M Amberlite LA-1 .HCI in toluene. Shaking for 10 mingives ample time for equilibrium to be reached. The metal ions are determined by stripping 10 ml of the organic phase with 0.1 N HNO. or HCI, and subsequent analysis of the aqueous extract. As a rule, titrations with EDTA are used. I t is recommended to evaporate the aqueous extract to dryness before analysis.
-
8
10
N
Figure 3. Exampler of quolit~tive~epardionscarried out on Silico Ge impregn~tedwith Amberlite LA-1 .HCI, and using rolutianr of HCI of various normality os eluant$.
only three eluants are employed, uiz. 2, 6, and 10 N HCl, this choice offering a favorable combination of separations for the elements of the minerals concerned. A few mg of powdered material are dissolved in moderately strong HCI and/or HNO. and the resulting solution-the metal content of which is not allowed t o exceed lO7,-is directly applied t o the chromatoplate. Subsequent,ly, the technique described above is followed. The I
