influence of column temperature on the ion chromatographic separation of aluminum species

Analytica Chimica Acta 399 (1999) 223–235

Influence of column temperature on the ion chromatographicseparation of aluminum species

Matthias Buscha, Andreas Seuberta,b,∗a University of Hannover, Institute of Inorganic Chemistry, Callinstraße 9, D-30167 Hannover, Germany

b University of Kassel, FB 19 Biology and Chemistry, Heinrich-Plett-Straße 40, D-34132 Kassel, Germany

Received 4 January 1999; received in revised form 26 May 1999; accepted 30 May 1999

Abstract

Ion chromatography using a combined size exclusion and cation exchange column was applied to the determination ofaluminum and its fluoro, oxalate and citrate species. For the verification of an assumed disintegration of Al species duringtheir ion chromatographic separation, the column temperature was varied from−10 to 55◦C. The species were detected bypost-column reaction with a Tiron based solution, followed by UV photometry as well as by on-line coupling to atomicspectrometry.

The results showed that weaker neutral and negatively charged Al species based on ligands such as fluoride and oxalate arepartly disintegrated during the chromatographic process. The disintegration of these species could be suppressed to a largeextent utilizing column temperatures below 10◦C. A significant effect of the column temperature on Al–citrate species wasnot observed.

The agreement between speciation data determined experimentally by ion chromatography and by calculations based onstability constants became even better with decreasing column temperature and decreasing Al to ligand molar ratio. ©1999Elsevier Science B.V. All rights reserved.

Keywords:Liquid chromatography; Aluminum; Speciation; Column temperature dependence

1. Introduction

Aluminum is the third most abundant element inthe earth’s crust, and the most abundant metal [1]. Inrocks, it occurs primarily in Al–silicate minerals, suchas feldspars, micas and clay minerals, and as hydroxideminerals, for example, in bauxite and laterites [2].

Aluminum exists in natural systems as a huge num-ber of different complex species. The [Al(H2O)6]3+-

∗ Corresponding author. Tel.: +49-511-762-3174;fax: +49-511-762-2923E-mail address:[email protected] (A. Seubert)

ion is acidic, and hydrolyses even below pH 3. Thisspecies, therefore, rarely dominates among Al speciesin natural waters. With increasing pH, monomerichydroxo species are formed. Additionally, there is astrong tendency for polymerization. The exact natureof these polymeric species is still under debate [3,4].A main inorganic ligand beside hydroxide is fluoride,which forms strong complex species with Al [5,6].The most frequently occurring organic ligands aredicarbonic acids (e.g. oxalic acid.), hydroxycarbonicacids (i.e. citric acid) and more complex moietiessuch as polyphenolic compounds, and fulvic and hu-mic acids [7–10]. A clear comprehension of these

0003-2670/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved.PII: S0003-2670(99)00465-1

224 M. Busch, A. Seubert / Analytica Chimica Acta 399 (1999) 223–235

different Al ligand systems present in aqueous solu-tions is necessary to diagnose and predict Al toxicity,bioavailability and transport mechanisms in soil sci-ence, plant physiology, biology and medicine [11].

Several methodologies for the determination of Alspecies in solution have been developed. The meth-ods can be classified into groups [12] according tothe principles used, e.g. kinetic or binding strengthdiscrimination, non-invasive methods or equilib-rium calculations. The computational methods, i.e.GEOCHEM-PC, combine experimentally determinedvalues of dissolved Al and other relevant componentswith a thermodynamic data base with equilibriumapproach [13,14]. Numerous thermodynamic equilib-rium models have been developed on the basis of themeasured stability constants of various Al species inaqueous solutions [15,16], but it is not expected thatthose methods will lead to good results as long asthe nature of some important ligands and equilibriumconstants are unknown, as it is the case for mostorganic ligands in soils.

Methods based on kinetic or binding strength dis-crimination involve the physicochemical separation ofAl species based on charge [17,18] or size and/or theanalytical separation of various Al species, based ondifferential reaction kinetics with complexating agentslike 8-hydroxyquinoline [19].

Non-invasive methods use direct spectromet-ric techniques such as nuclear magnetic resonance(NMR). For the determination of [Al(H2O)6]3+,[Al 13O4(OH)24]7+ and [Al(OH)4]− at a higher con-centration level27Al-NMR is by far the best choice[3,4,9,20–22].

In this investigation, ion chromatography (IC) isused as an analytical separation technique and itsresults are compared with a geochemical speciationmodel (GEOCHEM-PC). For several years, IC withspectrometric detection has been, at least at first view,successfully used for the determination of cationicand anionic Al species in aqueous solutions [23–29].

One must keep in mind that chromatography isa slow method compared to the rate of reaction ofmost labile species. In the case of aluminum, whichshows, for water molecules at room temperature, aligand exchange rate in the inner coordination sphereof approximately 1 Hz [20], the unquestioned use ofa separation technique is very dangerous. A chro-matogram with sharp, well defined peaks could mean,

for instance, a perfect separation of all involvedspecies or just a good separation of all disintegrationproducts of the originally present species. Our attemptto gain information about the degree of degradationof aluminum species during a chromatographic sep-aration is based on the Arrhenius equation, whichpredicts the decrease in the rate constant of a chemi-cal reaction when lowering the temperature, e.g. thecolumn temperature of our chromatographic system.The parameter temperature is not used as a thermody-namic influence on complex equilibria as all sampleswere prepared at room temperature. Furthermore,model calculations using GEOCHEM showed that thespecies’ distribution of aluminum–fluoride speciesremains virtually constant over the temperature range.

The influence of the temperature of ion chromato-graphic separations is examined for well defined Alligand systems. The ligands selected are citrate as atricarbonic acid with high importance in natural sys-tems, oxalate as a bidentate ligand for Al, and finally,fluoride as an inorganic monodentate strong complex-ing agent. These ligands were chosen with referenceto earlier works performed by cation chromatography[26,30] and because of the well known chemical prop-erties of the ligands.

2. Experimental

2.1. Reagents

All experiments were carried out with syntheticinorganic and organic Al solutions. All reagents usedin this study were of pro-analysi-grade (E. Merck,Darmstadt, Germany), and deionized water was ob-tained from a Milli-Q water purification system (Mil-lipore, Eschborn, Germany). Stock solutions of Al,fluoride, citrate and oxalate had a concentration of3.7× 10−2 mol l−1 (equivalent to 1000mg ml−1 Al)and were prepared by dilution of aluminum chloridehexahydrate, ammonium fluoride, citric acid and ox-alic acid dihydrate. To prepare working standards,0.5 ml of the Al stock solution and different quantitiesof the ligand solutions were mixed and the pH wasadjusted with perchloric acid and ammonia. The con-centration of Al in all investigations was 10 mg l−1.

The eluents used for gradient elution consisted ofA: perchloric acid (10−3 or 10−4 mol l−1, depending

M. Busch, A. Seubert / Analytica Chimica Acta 399 (1999) 223–235 225

Table 1Time schedule of the gradient pump GP40 used for aluminum speciation

Time (min) Eluent A (%) Eluent B (%) Eluent C (%) Gradient curve Injection valve Comment

0.0 100 0 0 5 load start conditions3.0 100 0 0 5 injection sample injection and start of data collection5.0 97 3 0 3 injection concave gradient8.0 0 100 0 7 injection increase in eluent strength

10.0 0 0 100 4 injection decrease in eluent-pH12.0 0 0 100 5 injection column clean-up12.1 100 0 0 5 injection return to initial conditions15.0 100 0 0 5 load stop data collection

Table 2Physical properties of the cation exchange columns

Physical properties BioS7C270 BioS3C510

Basic substrate BioGel SEC 3 (BioRad Lab.) BioGel SEC 3 (BioRad Lab.)Functional group poly-(sulfopropionylphenyl)-ethylene poly-(sulfopropionylphenyl)-ethyleneParticle size 5mm 5mmMean pore size 50 dp 7 nm 3 nmMaximum pore size 10 dp − 11 nmSurface area 450 m2 g−1 500 m2 g−1

Pore volume − 0.67 ml g−1

Volume capacity for H+ 0.27 mmol ml−1 0.51 mmol ml−1

Column body stainless steel 120 mm×4 mm ID stainless steel 120 mm×4 mm ID

on the pH of the samples), B: ethylenediammonia per-chlorate (0.5 mol l−1, adjusted to pH 3 or 4 with per-chloric acid) and C: ethylenediammonia perchlorate(0.5 mol l−1, adjusted to pH 1.5 with perchloric acid).They were degassed by vacuum filtration through a0.45mm filter. For measurements below 0◦C, the elu-ents contained 10% w/w methanol. The perchloratewas chosen because of an extremely low complex-ation power. The driving cation was the di-positivecharged ion of ethylenediamine, which is the onlyspecies present at pH 4 and below. The driving ion wasselected because of the high purity of freshly distilledethylenediamine and the good compatibility with ourdetection devices, e.g. photometry using Tiron or in-ductively coupled plasma–atomic emission spectrom-eter (ICP–AES).

A solution of 3× 10−4 mol l−1 Tiron (sodium saltof 4,5-dihydroxybenzenedisulfonic acid monohy-drate) and 1 mol l−1 ammonia acetate, buffered atpH 6.7 with perchloric acid, was utilized to trans-fer the different Al species into the Al–Tiron com-plex for photometric detection via a post-columnreaction.

2.2. Apparatus and instrumentation

A Dionex DX500 ion chromatography system wasused in this investigation. The gradient pump mod-ule GP40 delivered the eluent with a flow rate of1 ml min−1, following the elution program as shownin Table 1.

Sample loops of 50ml for photometric detectionand 200ml for detection by ICP–AES were used. Thehomemade cation exchange columns BioS7C270 andBioS3C510 are based on a commercially available sizeexclusion gel and were modified by Friedel–Craftsacylation [31]. The reasons for using self-made cationexchanger are given elsewhere [26]. The physicalproperties are given in Table 2. The results of thespeciation do not depend significantly on the col-umn body material. The use of polymer instead ofstainless steel columns offered no significant advan-tage, whereas the cooling efficiency of metal is farsuperior to thick-walled and less thermal conductingpolymers. The BioS3C510 column was used onlyfor measurements below 0◦C. Column temperaturewas controlled by means of a self-made glass appa-


Fig. 1. Apparatus for controlling the temperature of the separation column.

ratus (Fig. 1) using an RC 3 MWG kryostat (Lauda,Lauda-Königshofen, Germany).

The post-column reagent was added to the eluatestream at a flow rate of 0.45 ml min−1 by means of aDionex Reagent Delivery Module (RDM) and a PTFEmixing tee, followed by a Dionex packed-bed mixingcoil (1.2 m in length). The AD20 absorbance detector,set at 310 nm, was used for photometric detection atroom temperature (UV lamp in low mode). Data ac-quisition and peak integration were performed usingthe Dionex Peaknet software.

For on-line coupling experiments, the chromatogra-phy module was stripped of the post-column reactiondetection. The outlet of the cation exchange columnwas directly connected to the nebulizer system ofa simultaneous working type ICP–AES instrument.Properties of the ICP–AES Spectroflame P (Spectro,Kleve, Germany) as well as the selected emissionlines are given in [32]. For the experiments of this

paper, the emission line 396.152 nm was used for thedetection of Al. Operating conditions of the ICP–AESwere chosen in conformity with the manufacturer’srecommendations. Data acquisition was performedwithout any delay using the TRANSIEN.EXE pro-gram delivered by Spectro A.I. Kleve, Germany.Peak integration and generation of graphics aredone by a self-written software. Further calculationswere carried out using a commercial spreadsheetprogram.

All calculations of Al species distributions wereperformed by GEOCHEM-PC 2.0 using the stabilityconstants [33–36] listed in Table 3. The results arecompared to experimentally achieved species distri-butions. To fit the resolution ability of the chromato-graphic method, some of the species had to be summedup. For example, if all AlFn species withn≥ 3 are notseparated, the sum AlF3...6 is used for the comparisonwith experimental data.


Table 3Stability constants used within GEOCHEM for the calculation of Al-species’ distributions

Al complex Formula Abbreviation LogK Reference

Monooxalatoaluminum [Al(C2O4)]+ AlOx+ 6.1 [33]Dioxalatoaluminum [Al(C2O4)2]− AlOx−

2 11.1 [33]Trioxalatoaluminum [Al(C2O4)3]3− AlOx3−

3 15.1 [33]Citratoaluminum I [Al(C6H6O7)]+ AlHCit+ 12.9 [34]Citratoaluminum II [Al(C6H5O7)] AlCit 9 .9 [34]Citratoaluminum III [Al(C6H4O7)]− AlCit− 4.5 [34]Dicitratoaluminum I [Al(C6H5O7)2]3− AlCit 3−

2 11.5 [35]

Dicitratoaluminum II [Al(C6H5O7)(C6H4O7)]4− AlCit 4−2 8.7 [35]

Monofluoroaluminum AlF2+ − 7.0 [36]Difluoroaluminum AlF+2 − 12.7 [36]Trifluoroaluminum AlF3 − 16.8 [36]Tetrafluoroaluminum AlF−4 − 19.4 [36]

Pentafluoroaluminum AlF2−5 − 20.6 [36]

Hexafluoroaluminum AlF3−6 − 20.6 [36]

3. Results and discussion

In previous investigations, IC was used as a methodfor the separation of single ligand systems to ob-tain information about equilibrium constants and tocompare the results with the data in analogous re-search [23–26,37]. In this investigation, the separationof the different Al ligand systems was carried out ona self-developed combined size exclusion and cationexchange column. As described in earlier work [24],we observed no peak for polymeric or hydroxy alu-minum species. Despite this fact, we noted a signifi-cant decrease in the total peak area of a chromatogramwhen polymeric aluminum species were supposed tobe present. We calculated the amount of ‘Al(OH)x’representing the non-detectable or non-eluting frac-tion of Al from the difference in the peak area be-tween the species’ chromatogram and a standard chro-matogramm obtained for a pure Al(H2O)3+

6 -solutionat pH 3.

3.1. Speciation of the Al–fluoride system

Standard solutions containing Al : fluoride in a mo-lar ratio of 8 : 1 (8), 4 : 1 (4), 2 : 1 (2), 1 : 1 (1), 1 : 2(0.5), 1 : 4 (0.25) and 1 : 8 (0.125) at pH 4 were usedto examine the complexation behavior of Al. A com-parison of experimentally determined species’ distri-butions versus calculated distributions is shown in Fig.2(a) and (b). The amount of AlF2+ gives a good cor-

relation between experimental and calculated valuesfor the molar ratio range of 8 to 1. In this range, theexperimentally determined amount of Al3+ is nearlyequal to the sum of the calculated amounts of Al3+and Al(OH)x because the labile hydroxo complexesdissociate to Al3+ during the chromatographic process[37]. The discrepancy between the measured and thecalculated Al distribution increases with decreasingAl : fluoride ratio. The predominant experimentally de-termined species is AlF2+, whereas GEOCHEM pre-dicted higher values of AlF+2 and AlF3...6. It is ques-tionable whether the chromatographic process influ-ences the relative concentration of Al species in thesamples.

The chromatograms shown in Fig. 3 were measuredby varying the column temperature from−10 to 55◦Cat a constant molar ratio of Al : fluoride of 1 : 8 at pH 4.The chromatogram measured at−10◦C shows a strongtailed peak of AlF3...6 (tR = 1.5. . . 6 min). At 0◦C, anadditional strongly fronted peak of AlF+2 is observed,whereas the appearance of the AlF+

2 peak attributes toa decreasing AlF3...6 peak. Also, the peak asymmetrycan be explained by disintegration of AlF3...6 speciesduring the chromatographic process. After the loss ofa fluoride ligand, the retention increases and the peakof the original species shows tailing, whereas the dis-integration product species shows fronting. At 10◦C,a fronted peak of AlF2...6 (tR ∼= 4 min) and a smallAlF2+ peak (tR ∼= 6 min) is detected. A further disin-tegration of AlF2...6 to AlF2+ appears at 25 and 35◦C,


Fig. 2. Comparison of Al–fluoride species’ distributions at pH 4 and 25◦C: (a) determined by cation chromatography; gradient elution withperchloric acid (10−4 mol l−1, pH 4), ethylenediammonia perchlorate (0.5 mol l−1, pH 4) and ethylenediammonia perchlorate (0.5 mol l−1,pH 1.5) as shown in Table 1; UV detection (310 nm) following post-column reaction with Tiron; (b) calculated with GEOCHEM PC usingthe stability constants given in Table 3. The Al concentration and the ionic strength (I) of each sample were 10 mg l−1 andI ∼= 0.005 mol l−1,respectively. The molar ratios of Al : fluoride were 8 : 1 (8), 4 : 1 (4), 2 : 1 (2), 1 : 1 (1), 1 : 2 (0.5), 1 : 4 (0.25) and 1 : 8 (0.125). The figurecaptions are: —r— Al3+, – –j– – AlF2+, ---N--- AlF+

2 , –-×-– AlF3...6, –-d–- Al(OH)x.


Fig. 3. Chromatograms obtained at column temperatures from –10 to 55◦C for an Al–fluoride solution prepared and stored at roomtemperature with a molar ratio of 1 : 8 at pH 4 using cation chromatography: gradient elution with perchloric acid (10−4 mol l−1, pH 4),ethylenediammonia perchlorate (0.5 mol l−1, pH 4) and ethylenediammonia perchlorate (0.5 mol l−1, pH 1,5) as shown in Table 1; UVdetection (310 nm) following post-column reaction with Tiron; 10 mg l−1 Al; I ∼= 0.005 mol l−1.

which could be recognized since both peaks were notbaseline separated (tR = 3.5. . . 5.5 min). By increasingthe temperature from 45 to 55◦C, only a strong tailedpeak (tR = 7. . . 10 min) of Al3+ is observed. Hence,we may infer that, above 45◦C, all Al–fluoride com-plexes dissociate to Al3+ within the chromatographictime scale.

Finally, Fig. 4 shows the influence of temperatureon the observed distribution of Al–fluoride species, us-ing cation chromatography. The interactions betweenAl–fluoride species and exchange materials are low-est at temperatures below 0◦C so that a good agree-ment between experimental and calculated results isachieved.


Fig. 4. Influence of the column temperature on the species’ distribution obtained by cation chromatography for an Al–fluoride solution witha molar ratio of 1 : 8 at pH 4: gradient elution with perchloric acid (10−4 mol l−1, pH 4), ethylenediammonia perchlorate (0.5 mol l−1, pH 4)and ethylenediammonia perchlorate (0.5 mol l−1, pH 1,5) as shown in Table 1; UV detection (310 nm) following post-column reaction withTiron; 10 mg l−1 Al; I ∼= 0,005 mol l−1. The figure captions are: —r— Al3+, – –j– – AlF2+, ---N--- AlF+

2 , –-×-– AlF3...6, –-d–- Al(OH)x.

3.2. Speciation of the Al–oxalate system

Fig. 5(a) and (b) shows a comparison of the cationchromatographic determined species’ distributionsversus the calculated distributions for a molar ratioof Al : oxalate of 8 to 0.125 at pH 3. The amountsof Al3+ and AlOx+ give a very good correlationbetween experimental and calculated values for themolar ratio range of 8 to 1. Below a molar ratio of1 : 1, the AlOx+ and the AlOx2;3 species are ob-served. The concentration of AlOx2;3 increases andthe AlOx+ concentration decreases with decreasingAl : oxalate ratio. The concentrations of the AlOx2;3and the AlOx+ species predicted by GEOCHEM takea similar course, but the calculated value of AlOx2;3is higher and that of AlOx+ lower.

These differences are attributed to interactions be-tween AlOx2;3 and the eluent as well as to the cationexchanger, which will cause disintegration to AlOx+.To prove this assumption, the temperature of the col-umn was varied within a range between−10 and 55◦C.Fig. 6 shows the chromatograms of Al : oxalate 1 : 2

at pH 3. At −10◦C, a strong tailed peak of AlOx2;3and a fronted peak of AlOx+, which were not base-line separated (tR = 1.5. . . 5.5 min), is observed. In ac-cordance with the results of the Al–fluoride system,the peak distortion could be explained by the disinte-gration of the AlOx2;3 species during the chromato-graphic run. After the loss of an oxalate ligand, theretention increases and the peak shows fronting. Thespecies distributions at 0 and 10◦C are in substan-tial agreement. Three baseline separated peaks are de-tected for AlOx2;3 (tR ∼= 1.5 min), AlOx+ (tR ∼= 4 min),Al3+ (tR ∼= 7.5 min). A further disintegration is ob-served at 25◦C, concerning the peaks of AlOx2;3 andAlOx+ (tR = 1.5. . . 4 min). The signal of AlOx2;3 istailed by a loss of an oxalate molecule and the newlygenerated AlOx+ resulting from the disintegration ofAlOx2;3 is fronted. The same effect appears in thechromatogram measured at 35◦C when the amount ofAlOx2;3 is lower and that of AlOx+ is higher thanthat determined at 25◦C. By increasing the temper-ature to above 35◦C, two strong fronted peaks (tR[AlOx+] ∼= 6 min, tR [Al 3+] ∼= 7.5 min) are observed.


Fig. 5. Comparison of Al–oxalate species’ distributions at pH 3 and 25◦C: (a) determined by cation chromatography; gradient elution withperchloric acid (10−3 mol l−1, pH 3), ethylenediammonia perchlorate (0.5 mol l−1, pH 3) and ethylenediammonia perchlorate (0.5 mol l−1,pH 1.5) as shown in Table 1; UV detection (310 nm) following post-column reaction with Tiron; (b) calculated with GEOCHEM PC usingthe stability constants given in Table 3. The Al concentration and the ionic strength (I) of each sample were 10 mg l−1 andI ∼= 0.006 mol l−1,respectively. The molar ratio of Al : oxalate was varied from 8 : 1 (8) to 1 : 8 (0.125). The figure captions are: —r— Al3+, – –j– –AlOx+, ---N--- AlOx2;3, –-d–- Al(OH)x.


Fig. 6. Chromatograms obtained at column temperatures from –10 to 55◦C for an Al–oxalate solution prepared and stored at roomtemperature with a molar ratio of 1 : 2 at pH 3 using cation chromatography: gradient elution with perchloric acid (10−3 mol l−1, pH 3),ethylenediammonia perchlorate (0.5 mol l−1, pH 3) and ethylenediammonia perchlorate (0.5 mol l−1, pH 1.5) as shown in Table 1; UVdetection (310 nm) following post-column reaction with Tiron; 10 mg l−1 Al; I ∼= 0.006 mol l−1.

The fronting of both peaks results from the disinte-gration of AlOx+ to Al3+ during the chromatographicrun.

The influence of temperature on Al–oxalate speciesusing cation chromatographic separation is presented

in Fig. 7. Below 10◦C, the species distributionsare nearly unaffected by interactions between theAl–oxalate species and the exchange material. There-fore, a good agreement between experimental andcalculated results is given.


Fig. 7. Influence of the column temperature on the species’ distribution obtained by cation chromatography for an Al–oxalate solution witha molar ratio of 1 : 2 at pH 3: gradient elution with perchloric acid (10−3 mol l−1, pH 3), ethylenediammonia perchlorate (0.5 mol l−1, pH3) and ethylenediammonia perchlorate (0.5 mol l−1, pH 1.5) as shown in Table 1; UV detection (310 nm) following post-column reactionwith Tiron; 10 mg l−1 Al; I ∼= 0.006 mol l−1. The figure captions are: —r—Al3+, – –j– – AlOx+, ---N--- AlOx2;3, –-d–- Al(OH)x.

3.3. Speciation of the Al–citrate system

For the examination of the Al–citrate system, themolar ratio range of Al : citrate of 8 to 0.125 at pH 4was investigated. A comparison of calculated speciesdistributions versus experimentally determined distri-butions is shown in Fig. 8(a) and (b). It can be no-ticed that there is hardly any accordance between bothmethods.

Within the range of Al : citrate from 8 : 1 to 1 : 1,a decreasing ratio leads to decreasing concentrationsof Al3+ and Al(OH)x observed experimentally as wellas upon calculation. At lower ratios, neither increas-ing amounts of free Al nor the presence of Al(OH)x

conform to the GEOCHEM calculations.Considering the AlCit and the AlHCit+ species, an

enhancement can be observed for the range of ratiofrom 2 to 0.125. This contradicts the calculated resultswhich exhibit a lower concentration of AlCit.

Regarding the AlCit1;2 species, a maximum con-centration is detected at a molar ratio of Al : citrate of2 : 1, whereas GEOCHEM predicted a continuous in-crease in AlCit1:2 with a decreasing amount of Al.

The large deviations between both speciation meth-ods are presumably due to the following effects: sta-bility constants, which were used for the calculation ofAl–citrate species’ distributions, were obviously notappropriate for this investigation and/or Al–citrate so-lutions may not have reached a thermodynamic equi-librium.

Our attempt to reveal the main cause by varying thetemperature has failed as no influence of temperatureon the ion chromatographic separation was observedfor the Al–citrate system. This behavior implies thatthe distribution of the species is not essentially influ-enced by interactions between the Al–citrate speciesand the exchange material.

4. Conclusions

Cation chromatography can be applied as an ade-quate method for the separation of stable cationic Alcomplexes in aqueous systems. Weaker neutral andnegative Al species, such as fluoride and oxalate com-plexes, are partly disintegrated during the chromato-


Fig. 8. Comparison of Al–citrate species’ distributions at pH 4 and 25◦C: (a) determined by cation chromatography; gradient elution withperchloric acid (10−4 mol l−1, pH 4), ethylenediammonia perchlorate (0.5 mol l−1, pH 4) and ethylenediammonia perchlorate (0.5 mol l−1,pH 1.5) as shown in Table 1; detection by ICP–AES; (b) calculated with GEOCHEM PC using the stability constants given in Table 3.The Al concentration and the ionic strength (I) of each sample were 10 mg l−1 and I ∼= 0.006 mol l−1, respectively. The molar ratio ofAl–citrate was varied from 8 : 1 (8) to 1 : 8 (0.125). The figure captions are: —r— Al3+, – –j– – AlHCit+, ---N--- AlCit, –-×-– AlCit1;2–-d–- Al(OH)x.


graphic run. The reasons are presumably a combi-nation of decomposition by time and interaction be-tween the species and the ion exchanger material. Bothfactors, and therewith, the species’ distributions arestrongly influenced by temperature during the chro-matographic separation.

Minimal disintegration of the higher coordinatedAl–fluoride species (AlFn with n> 2) is obtained bymeasurements below 0◦C. For anionic oxalate com-plexes, this state is reached at temperatures below10◦C. Regarding Al–citrate complexes, no easily ex-plainable temperature dependence can be determined.The kinetic inertness of Al ligand species increaseswith increasing numbers of coordination sites at theligand, e.g. monovalent ligands, such as fluoride or hy-droxide, that are most critical when using separationtechniques for aluminum, whereas citrate species arealmost kinetically inert, and therefore, perfectly suitedto chromatographic separations.

In general, the comparison of species’ distribu-tions obtained by cation chromatography and byGEOCHEM PC shows good agreement for tempera-tures below 10◦C because of the reduced decomposi-tion rate of the species and for solutions with an excessof Al because of lower thermodynamic driving force.

Acknowledgements

This research is supported by the DeutscheForschungsgemeinschaft DFG (Project No. Se730/2-1and Se730/2-2).

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influence of column temperature on the ion chromatographic separation of aluminum species

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