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cation chromatography
Analytica Chimica Acta 386 (1999) 77±88
Aluminum speciation by means of anion chromatography and coupled anion/cation chromatography Guido Borrmann, Andreas Seubert* Institute of Inorganic Chemistry, University of Hannover, Callinstrasse 9, D-30167, Hannover, Germany Received 27 April 1998; received in revised form 14 December 1998; accepted 15 December 1998
Abstract Anion chromatography had been applied to the speciation of synthetic aluminum solutions containing medium to strong ligands. A gradient elution based on a chloride start eluent and a perchlorate ®nal eluent allowed the separation of neutral or cationic and of negatively charged Al species. The separation of Al species with a nominal charge of ÿ1, ÿ2 and ÿ3 or higher required a run time of less than 10 min. The ligands investigated were ¯uoride as monodentate, oxalate as bidentate and citrate as tridentate anion in binary, ternary or quaternary mixtures at different pH values. The results of anion chromatography are compared to those obtained by calculation methods and by cation chromatography. Anion chromatography turned out to be a useful tool for the speciation of Al solutions containing strong ligands such as citrate, whereas anionic Al±oxalate and Al± ¯uoride species are at least partially disintegrated. The favorite detection technique is on-line coupling with atomic spectrometry. An attempt to overcome the limitations of either anion or cation chromatography is the combination of both methods in a single chromatographic procedure. A slight modi®cation of the methods used for anion and cation separation allowed a complete speciation of Al solutions within 20 min. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Aluminum; Speciation; Anion chromatography; Cation chromatography
1. Introduction The speciation of aluminum in presence of organic or inorganic ligands in aqueous solutions is of interest because: 1. Al is an important pH buffer, 2. Al may influence the cycling of important elements like phosphorus and organic carbon, and 3. Al is potentially toxic to aquatic and terrestrial organisms. *Corresponding author. Tel.: +49-511-762-3174; fax: +49-511762-2923; e-mail: [email protected]
An understanding of the speciation of Al is essential for the evaluation of these processes. A clear comprehension of the forms of Al present in solutions is necessary to diagnose and predict Al toxicity in soil science, plant physiology, biology and medicine [1]. For example, plant physiological studies showed that root elongation is best correlated with the concentration or better with the activity of monomeric Al in solution [2±6]. The role of complexing ligands in attenuating Al toxicity and in in¯uencing Al transport in soils and waters has been recognized for some time, though the kinetics of such complexation reactions
0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0003-2670(99)00006-9
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within complicated multi-ligand aqueous solutions have not been successfully evaluated before. Numerous methods have been applied to Al speciation [7]. The ®rst is essentially computational, where the experimentally determined values of dissolved Al and other relevant components are used in combination with a thermodynamic data base with equilibrium approach such as GEOCHEM [8,9]. Several thermodynamic equilibrium models have been used for the calculation of Al speciation [10,11], but it is not expected that those methods will yield good results as long as the nature of some important ligands and equilibrium constants are unknown, as it is the case for most organic ligands in soils. The second kind of method used for the speciation of Al involves the analytical separation of various Al species based on reaction kinetics [12±14] and/or the physicochemical separation of Al species based on size or charge [15±17]. The third general approach uses direct spectrometric techniques such as nuclear magnetic resonance (NMR). The most widely applied technique is 27 Al-NMR for the determination of Al H2 O3 6 ; ÿ Al13 O4 OH7 24 ; Al OH4 and complexed aluminum [18±22]. Finally, a symbiosis of group 2 and 3 had been used for Al speciation. Those methods involve the combination of one or more analytical separation technique with a spectrometric detection [23±29]. A successful application is the use of ion chromatography (IC) for the speciation of Al in aqueous solutions. Previously, mostly cation chromatography has been applied to the speciation of aluminum [23±26,28], but a number of potential ligands for aluminum form more or less stable anionic species. The shortcoming of cation chromatography is the inability to resolve weakly or unretained species,
whereas the use of a recently developed cation exchanger [30,31] allowed the partial separation of anionic and neutral species by a combination of ion exclusion and ion exchange [23]. This paper describes the ability of anion chromatography for aluminum speciation in simple aluminum± ligand systems. The ligands were chosen with respect to earlier work performed by cation chromatography and to the chemical properties of the ligands. Fluoride was considered as an inorganic monodentate strong complexing agent for aluminum, oxalate as a bidentate ligand and ®nally citrate as a tricarbonic acid with high importance in natural systems. 2. Experimental 2.1. Apparatus and instrumentation All investigations were performed on a Dionex DX500 chromatography system built up of a gradient pump module GP40, a photometric detector AD20 and a post-column derivatization system. The eluent ¯ow rate was 1.0 ml minÿ1 throughout all experiments and the time programs are as given in Table 1 for anion chromatography and Table 2 for combined anion/ cation chromatography. Sample size was 50 ml for photometric detection and 200 ml for detection by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The post column reagent was made of 1 mol lÿ1 ammonia acetate and 310ÿ4 mol lÿ1 Tiron (sodium salt of 4,5-dihydroxybenzenedisulfonic acid) in deionized water, buffered at pH 6.5 with perchloric acid. It was added to the eluate stream at a ¯ow rate of 0.7 ml minÿ1 using a Dionex Reagent Delivery Module (RDM) and a PTFE mixing tee, followed by a
Table 1 Time schedule for the separation of anionic species by anion chromatography with gradient elution Time (min)
Eluent A (%)
Eluent B (%)
Gradient curve
Injection valve
Comment
0.0 3.0 8.0 10.0 10.1 13.0
100 100 90 90 100 100
0 0 10 10 0 0
5 5 6 5 5 5
Load Inject Inject Inject Inject Load
Start conditions Sample injection and start of data aquisition Concave gradient Hold step and column clean-up Return to start conditions End of run
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Table 2 Time schedule for the separation of aluminum species by coupled anion/cation chromatography with gradient elution Time (min)
Eluent A (%)
Eluent B (%)
Eluent C (%)
Gradient curve
Valve 1
Valve 2
Injection valve
Comment
0.0 3.0 4.5
100 100 100
0 0 0
0 0 0
5 5 5
On On Off
Ona On Offb
Load Injection Injection
Start conditions Sample injection Switch to anion chromatography and start of data aquisition
8.0 9.0 9.5
90 80 100
10 20 0
0 0 0
8 3 5
Off Off Off
Off Off Off
Injection Injection Injection
Concave gradient Convex gradient Flush of anion column and anion catcher
10.5 16.0 17.0 20.0 20.1 22.5
100 0 0 0 100 100
0 100 0 0 0 0
0 0 100 100 0 0
5 8 5 5 5 5
On On On On On On
Off Off Off Off Off On
Injection Injection Injection Injection Injection Load
Start cation chromatogram Concave gradient Decrease of eluent-pH Column clean-up Flush of cation column Back to start conditions and end of run
a
``On'' indicate ports 1±2 and 3±4 connected (bold line). ``Off'' indicate ports 1±4 and 2±3 connected (dotted line).
b
Dionex packed-bed mixing coil (3 m in length). The AD20 absorbance detector was set at 310 nm and 0.3 mV absorbance unit full scale. Data acquisition and peak integration were performed using the Dionex Peaknet software. For on-line coupling experiments the chromatography module was stripped from the post-column reaction detection and the outlet of the separation column was directly connected to the nebulizer system of the ICP-AES. Properties of the ICP-AES Spectro¯ame P (Spectro, Kleve, Germany) as well as installed emission lines are given in [32]. For the experiments of this paper the channels 396.152 nm for aluminum and 182.037 nm for sulfur were used. Data acquisition without any dead time was performed using the TRANSIEN.EXE program delivered by Spectro AI Kleve, Germany. The peak integration and generation of graphics was done by self-written software, other calculations were done by using a commercial spreadsheet program. 2.2. Separation columns The separation columns used for anion chromatography were all home made from a commercially available macroporous divinylbenzene polymer by bromoacetylation and subsequent SN-substitution
using 2-(dimethylamino)-ethanol amine. Advantages of home made columns are the well-known structure and properties of the basic polymer, the chemically de®ned type of functionalization as well as the exactly known capacity of the columns. Furthermore, the columns are all 100% solvent compatible for cleanup procedures and they are easily re®lled in case of damage. The physical properties are given in Table 3. The columns used differed in geometry and exchange capacity. Most of the experiments were performed using the AS1 column. The speciation of the Al±¯uoride±citrate system was investigated using the AS2 column, but the results obtained with AS1 and AS2 are equal within a small difference of retention times for single charged species. The cation exchange column used for the anion/cation chromatography differed from the one used in [23] only by the column geometry of 1204 mm ID. The anion catcher AC was used as an interface column between the cation and anion separation in the coupled anion/cation speciation. 2.3. Gradient elution program for the separation of anionic aluminum species In contrast to a previous gradient program [23] the starting point is given by a dilute hydrochloric acid
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Property
AS 1
AS 2
AC (anion catcher)
Substrate Functional group Particle size (mm) Q (mmol Clÿ) Column body, lengthID (mm)
BioGel SEC 3 (BioRad Lab.) Quaternized 2-(di-methylamino)-ethanol 5 0.21 Stainless steel, 1204
BioGel SEC 3 (BioRad Lab.) Quaternized 2-(di-methylamino)-ethanol 5 0.39 Stainless steel, 1204
BioGel SEC 3 (BioRad Lab.) Quaternized 2-(di-methylamino)-ethanol 5 0.1 Stainless steel, 304
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Table 3 Physical properties of the anion exchange columns used for the speciation of aluminum by anion chromatography and coupled anion/cation chromatography
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solution. This change is necessary because of the high af®nity of the perchlorate anion to common anion exchangers. Chloride is a much weaker eluent and is used at a concentration of 1 mmol lÿ1. The ®nal gradient eluent is a 40 mmol lÿ1 solution of ethylenediammonia perchlorate at pH 3 or 4 depending on sample pH. 2.4. Experimental set-up for combined anion/cation chromatography A scheme of the set-up used for combined anion/ cation chromatography is shown in Fig. 1. The arrangement of valves 1 and 2 allows the in-line switching of either the cation or the anion chromatography column. Valve 2 divides the ¯ow path into one path using the anion catcher as collector for the dead volume signal of cation chromatography and another path allowing the elution of the anion catcher onto the anion separation column. The separation starts with the injection of the sample and the valves 1 and 2 both in the ``on'' position (port 1±2 and 3±4 connected). The sample is eluted over the cation chromatography column and the species eluting in the dead volume are adsorbed on the anion catcher. Then the valves 1 and 2 switch to the ``off'' position and the anion catcher is eluted over the anion chromatography column to perform an anion separation. The cation chromatogram proceeds after switching valve 2 in the ``off'' position. 2.5. Gradient elution program for the separation of both anionic and cationic aluminum species The elution program is divided into three steps. The ®rst is the introduction of the sample on the cation exchange column, from which all anionic or neutral molecules are eluted using 0.001 mol lÿ1 HCl. The anionic compounds are collected on the AC-column. Second step is the development of the anion chromatogram by taking out the cation exchanger and insertion of the anion exchange column. The anion separation is achieved by an elution program similar to the one described in Table 1. The third step is the development of the cation chromatogram. Prerequisite is the change of the eluent back to the 0.001 mol lÿ1 HCl. The gradient for the cation separation is similar to the one used in [23].
Fig. 1. Flowpath of the coupled anion/cation chromatography for the separation of aluminum species. Detection was performed by a simultaneous inductively coupled plasma atomic emission spectrometer. Eluent 1: 1 mmol lÿ1 hydrochloric acid, pH 3; eluent 2: 1 mmol lÿ1 perchloric acid, pH 3; eluent 3: 500 mmol lÿ1 ethylendiamine perchlorate pH 3.
2.6. Reagents All experiments were performed with synthetic Al solutions. All reagents used in this study were of pro analysis grade (Merck, Darmstadt, Germany), deionized water was obtained from a Milli-Q water puri®cation system (Millipore, Eschborn, Germany). Stock solutions of aluminum, ¯uoride, citrate and oxalate were of the concentration 3.710ÿ2 mol lÿ1 (equivalent to 1000 mg lÿ1 Al), prepared by dilution of AlCl3, NaF, citric acid monohydrate and oxalic acid dihydrate. The eluents used for gradient elution consisted of A: hydrochloric acid (10ÿ3 mol lÿ1, pH 3), B: ethylenediamine perchlorate (0.4 mol lÿ1, adjusted to pH 3 with perchloric acid) and C: ethylenediamine perchlorate (0.4 mol lÿ1, adjusted to pH 1.5 with perchloric acid). To prepare the standards, 0.5 ml of the Al stock solution and different quantities of the ligand solutions were mixed and the pH was adjusted with perchloric acid and ammonia, respectively. The concentration of Al in all investigations was 10 mg lÿ1.
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2.7. Calculation of species distributions by GEOCHEM
Table 4 Stability constants used for the calculation of Al species distributions
All calculations of aluminum species distribution are performed by GEOCHEM PC 2.0 using stability constants [34±36] as listed in Table 4. The results are compared to experimentally achieved species distributions in Section 3. Some of the species are combined to ®t the resolution ability of the chromatographic method. For example, if Al3 and AlHCit are not separated, the sum of these species is used for the comparison with experimental data.
Abbreviation
log K
Reference
AlF2 AlF 2 AlF3 AlFÿ 4 AlF2ÿ 5 AlF3ÿ 6 AlHCit AlCit AlCitÿ AlCit3ÿ 2 AlCit4ÿ 2 AlOx AlOxÿ 2 AlOx3ÿ 3
7.0 12.7 16.8 19.4 20.6 20.6 10.9 7.8 6.4 8.7 11.75 6.1 11.1 15.1
[34] [34] [34] [34] [34] [34] [35] [35] [35] [35] [35] [36] [36] [36]
2.8. Determination of the charge of anionic species The determination of the effective charge of a species detected during a gradient ion chromatography run is usually a dif®cult task. We used methylsulfonic acid and sulfate as retention time monitor for single or double negatively charged species. The retention times are 5.9 min for methylsulfate and 7.8 for sulfate when using the time program of Table 2. Neutral or cationic species are expected to elute in the dead volume and all higher charged species are far behind the sulfate peak at the end of gradient program at a retention time of 10.6 min.
3. Results and discussion 3.1. Speciation of the Al±fluoride system by anion chromatography Independent from molar ratio and pH value the chromatograms of ¯uoride containing Al solutions showed only a single peak for not retained and therefore neutral or cationic species. At a molar ratio of Al:F of 1:8 a broad peak for an anionic species (tR3± 6 min) could be detected. Anion chromatography in its present form seems to be useless for the speciation of aluminum±¯uoride species. This conclusion is in good agreement with the disintegration of higher coordinated aluminum±¯uoride species on cation exchangers [23±26].
3.2. Speciation of the Al±citrate system by anion chromatography In contrast to the ¯uoride system, citrate forms stable anionic complexes more suitable for anion chromatography. Two factors seem to be important: 1. citrate forms more stable anionic complexes, and 2. citrate as tridentate ligand should show slower kinetics for the rearrangement of ligands in the inner coordination sphere of aluminum. For a more detailed view the pH and the molar ratio dependence of the species distribution in the Al± citrate system was studied. The results presented in Figs. 2 and 3 show a strong dependence of the species distribution on both pH and molar ratio. Solutions with an excess of citrate and pH values of 4 and 5 show anionic species with a charge of ÿ3 (Fig. 2). This assumes a complex stoichiometry Al:citrate of 1:2. Fig. 2 shows an example (Al:citrate 1:2 and pH 5) where Al Cit3ÿ is the only species present. This 2 behavior is in disagreement with results of cation chromatography [33], which showed also AlCitÿ, AlCit and AlHCit. Those species could be the result of the disintegration of Al Cit3ÿ 2 , but this is unlikely because of the negative charge of this species which prevents it from retention on cation exchangers. At a molar ratio of Al:Cit 1:2 at pH 2 no anionic species is present (Fig. 2). An increase of the pH value results in
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Fig. 2. Influence of the pH value of an Al±citrate solution with a molar ratio of 1:2 on the species distribution obtained by anion chromatography. 10 mg lÿ1 Al; I0.006 mol lÿ1; time program as given in Table 1; detection by ICP-AES.
Fig. 3. Influence of the pH value of an Al±citrate solution with a molar ratio of 1:1 on the species distribution obtained by anion chromatography. 10 mg lÿ1 Al; I0.006 mol lÿ1; time program as given in Table 1; detection by ICP-AES.
the formation of AlCit and ®nally of Al Cit3ÿ 2 . At a molar ratio of Al:citrate0.5 and a pH>4 only the species Al Cit3ÿ 2 species is observed. Despite its high retention this species does not disintegrate during the separation process. A different behavior is observed for equimolar solutions of Al and citrate. The degree of AlCitÿ increases with increasing pH until AlCitÿ is
the only species present. Even at pH 5 the species Al Cit3ÿ 2 could not be detected. The Fig. 4(a)±(d) show a comparison of calculated species distributions versus the anion chromatographic determined distribution for the pH range 2±5 and molar ratios of Al:citrate of 1:1, 1:2, 1:4 and 1:8. Lower ratios are not considered because of the
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Fig. 4. Comparison of Al species distribution determined by anion chromatography using the time program given in Table 1 and by calculation using the stability constants given in Table 4. The Al-concentration and the ionic strength of each sample was 10 mg lÿ1 and I0.006 mol lÿ1. The molar ratio of Al:citrate was varied from: (a) 1:1, (b) 1:2, (c) 1:4, and (d) 1:8. The filled symbols connected by bold lines indicate the species distribution determined by anion chromatography and the open symbols with thin lines represent calculated species values. 3 The figure captions are: ÐÐÐ*ÐÐÐ Al3; AlHCit; ÐÐÐ&Ð AlCit; - -~ - AlCitÿ; ÐÐÐ* Ð AlCit3ÿ 2 ; ÐÐÐ}ÐÐÐ Al ; ÿ 3ÿ AlHCit (calc.); ÐÐÐ&ÐÐÐ AlCit (calc.); ÐÐÐ~ÐÐÐ AlCit (calc.); ÐÐÐ*ÐÐÐ AlCit2 (calc.).
absence of anionic species. For the ratio Al:citrate of 1:1, Fig. 4(a) shows a good correlation between experimental and calculated values for AlCitÿ over the whole pH range. The experimentally determined amount of AlCit is somewhat lower and that of the cationic species higher than predicted by GEOCHEM. For the ratio 1:2 shown in Fig. 4(b) the behavior is reversed. The calculated amount of Al Cit3ÿ 2 for the ratio Al:citrate of 1:4 (Fig. 4(c)) is equal to the sum of the experimentally determined amounts of AlCit and Al Cit3ÿ 2 . It should be noted that the evaluation of the peak areas for the cationic aluminum species and
the AlCit species is rather dif®cult because of the strong overlap of these species. In general, the comparison of data obtained by anion chromatography or cation chromatography with the calculated values show a better agreement between calculation and anion chromatography for solutions with an excess of citrate. Nevertheless, it should be noted that the decision whether anion or cation chromatography gives the more correct data is impossible without an independent non-invasive technique. An attempt to achieve more information about the Al:citrate system is the application of directly coupled anion and cation chromatography.
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Fig. 5. Chromatogram obtained for coupled anion/cation chromatography using the setup shown in Fig. 1 and the time program given in Table 2. The samples are an Al±citrate solution with a molar ratio of 1:1 and different pH-values as indicated in the figure. 10 mg lÿ1 Al; I0.006 mol lÿ1; detection by ICP-AES.
3.3. Speciation of the Al±citrate system by coupled anion/cation chromatography The coupled anion/cation chromatography was realized by the insertion of two switching valves and the use of an anion catcher (AC) as interface column. The time schedule of the elution program is given in Table 2. The chromatographic run is divided into an anion and a cation separation. The ®rst column is the cation exchanger in series with the anion catcher. The AC retains the species eluting in or close to the dead volume of the cation exchanger. Some sample chromatograms of an equimolar solution of aluminum and
citrate at different pH values are shown in Fig. 5. The species distribution is comparable to those obtained by anion chromatography (Figs. 2 and 3). Advantage of the coupling anion and cation chromatography is the better resolution of equally charged species. A disadvantage could be the increased interaction between the species and the cation as well as the anion exchanger. The prolonged column clean-up with 0.4 mol lÿ1 ethylenediamine perchlorate at pH 1.5 used for this coupling application showed a previously undetected signal at the end of the run. The species eluting after Al3 increases with increasing sample pH and could
Fig. 6. Influence of the molar ratio of Al and oxalate at pH 4 on the species distribution obtained by anion chromatography. 10 mg lÿ1 Al; I0.006 mol lÿ1; time program as given in Table 1; detection by ICP-AES.
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be the oligomeric ``Al7 13 species, which is the dominating species in the pH range 4±6. In previous attempts the total loss of aluminum during a species separation has been referred to this species. Obviously, the strong clean-up solution is able to destroy this extremely strongly adsorbed species to the more easily eluting Al3 cation. 3.4. Speciation of the Al±oxalate system by anion chromatography The chromatograms shown in Fig. 6 were measured at a constant sample pH of 4 and varying Al:oxalate ratios. The results are in good agreement with results obtained by cation chromatography [33]. At a molar ratio of 1:1 only cationic and therefore unretained species are present. A shoulder of the dead volume signal at tR2.5 min should refer to the AlOxÿ 2 species, whose concentration increases with decreasing Al:oxalate ratio. With an decreasing Al:oxalate ratio the cationic signal decrease and the baseline begins to increase at longer retention times (tR8±11 min). At a molar ratio of 1:4 a strongly fronted peak for AlOx3ÿ and a 3 strongly tailed peak for AlOxÿ 2 were observed. The peak distortion could be explained by disintegration of the AlOx3ÿ 3 species during the chromatographic run. After the loss of an oxalate ligand the retention is decreased and therefore the peak shows a fronting. The time scale of this disintegration is in the range of the chromatographic run time. A further disintegration is observed for the shoulder on the peak representing AlOxÿ 2 . This signal is fronted by a loss of an oxalate molecule and tailed by the newly generated AlOxÿ 2 resulting from the disintegration of AlOx3ÿ 3 . In addition the retention time for this single charged species is much shorter than for the single charged citrate species (tR [AlCitÿ]3.9 min, tR AlOxÿ 2 2:5 min). The triple charged oxalate species shows the same retention time than the AlCit3ÿ 2 species (tR10.6 min), but this part of the chromatogram is measured at the end of the gradient ramp. Al±oxalate solutions with a molar ratio of 1:4 show only a minor pH-dependence. At pH-values higher than 2, the detectable species distribution remains constant. A comparison of the results of anion chromatographic speciation and calculation (Fig. 7(a)±(c)) show a good agreement at low molar ratios of Al:ox-
Fig. 7. Comparison of Al-oxalate species distribution determined by anion chromatography using the time program given in Table 1 and by calculation using the stability constants given in Table 4. The Al-concentration and the ionic strength of each sample was 10 mg lÿ1 and I0.006 mol lÿ1. The molar ratio of Al:oxalate was varied from: (a) 1:2, (b) 1:4 to (c) 1:8. The filled symbols connected by bold lines indicate the species distribution determined by anion chromatography and the open symbols with thin lines represent calculated species values. The figure captions are: ÐÐÐ^ÐÐÐ Al3/AlOx; ÐÐÐ& Ð AlOxÿ 2 ; Ð Ð~Ð 3 ÿ AlOx3ÿ 3 ; ÐÐÐ}ÐÐÐ Al /AlOx ; ÐÐÐ&ÐÐÐ AlOx2 ; . ÐÐÐ~ÐÐÐ AlOx3ÿ 3
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Fig. 8. Influence of the pH value of Al±citrate±oxalate solutions with a molar ratio of 1:0.5:0.5 on the species distribution obtained by an anion chromatography. 10 mg lÿ1 Al; I0.006 mol lÿ1; time program as given in Table 1; detection by ICP-AES.
alate. At Al:Ox 1:2 (Fig. 7(a)) the amount of positively charged species is much higher for anion chromatography than calculated by GEOCHEM. At Al:Ox 1:4 (Fig. 7(b)) and 1:8 (Fig. 7(c)) the species distribution simpli®es and the agreement between both methods is better. Anion chromatography is suitable for the determination of all expected monomeric Al±oxalate species. The speed of disintegration of the species is within the time range of the chromatographic run. This causes peak distortion and surely some experimental error in the determined species distributions.
charged species could be observed. The sharp peak at tR5.2 min shown in Fig. 8 can be identi®ed as AlCitÿ and the broad peak directly after the dead volume equals in peak shape and retention time the AlOxÿ 2 species. Anion chromatography can distinguish between both equally charged species. The quantitative species distribution equals the results obtained by cation chromatography.
3.5. Speciation of the Al±fluoride±citrate system by anion chromatography
Anion chromatography allows the separation of stable anionic complexes aluminum. However, weaker anionic species such as ¯uoride and oxalate complexes are at least partly destroyed during the separation. In comparison to cation chromatography the amount of information given by an anion speciation is less signi®cant. The strength of an anion chromatography is their additional information, which allows to distinguish between dead volume signals occurring in the most widely used cation chromatography. Therefore, anion chromatography as a stand alone technique is an extension of the ion chromatographic tools for aluminum speciation. Consequently, the combination of anion and cation chromatography within a single run should give a more detailed view into the species distribution of aluminum containing solutions. A drawback of a
At pH 2 and 3 it was impossible to detect anionic species. At higher pH a signal for a anionic species with a charge of ÿ1 could be detected. This species should be AlCitÿ. The results for pH 4 and 5 are essentially the same. The amount of retained species is about 80% of the citrate present in the mixture and independent from the ¯uoride concentration, which is in good agreement which the assumed AlCitÿ species. 3.6. Speciation of the Al±citrate±oxalate system by anion chromatography The analysis of mixtures at pH 2 and 3 showed no anionic species. At higher pH values two single
4. Conclusions
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combined method is the time needed for two separation cycles, which is subject to further disintegration of aluminum species. Acknowledgements This research is supported by the Deutsche Forschungsgemeinschaft DFG (Project no. Se730/2-1 and Se730/2-2). The authors thank Dr. A. Klingenberg, Dr. G. Petzold and Dr. C. Seidel for the preparation of the resins. References [1] G. Sposito (Ed.), The Environmental Chemistry of Aluminum, 2nd ed., CRC Press, Boca Raton, 1996. [2] A.K. Alva, F.P.C. Blamey, D.G. Edwards, C.J. Asher, Commun. Soil Sci. Plant Anal. 12 (1986) 1271. [3] A.K. Alva, D.G. Edwards, C.J. Asher, F.P.C. Blamey, Soil Sci. Soc. Am. J. 50 (1986) 133. [4] A.K. Alva, D.G. Edwards, C.J. Asher, F.P.C. Blamey, Soil Sci. Soc. Am. J. 50 (1986) 959. [5] A.K. Alva, D.G. Edwards, C.J. Asher, S. Suthipradit, Agron. J. 79 (1987) 302. [6] F.P.C. Blamey, D.G. Edwards, C.J. Asher, Soil Sci. 136 (1983) 197. [7] N. Clarke, L.G. Danielsson, A. Sparen, Pure Appl. Chem. 68 (1996) 1597. [8] G. Sposito, S.V. Mattigod, GEOCHEM, Kearney Foundation of Soil Science, University of California, Riverside, CA, 1980. [9] D.R. Parker, L.W. Zelasny, T.B. Kinraide, Soil Sci. Soc. Am. J. 51 (1987) 488. [10] V.D. Nair, J. Prenzel, Z. Pflanzenernaehr. Bodenkd. 141 (1978) 741. [11] J. Prenzel, Geoderma 38 (1986) 31.
[12] R.C. Turner, Can. J. Chem. 47 (1969) 2521. [13] C.T. Driscoll, Int. J. Environ. Anal. Chem. 16 (1984) 267. [14] S.L. Simpson, K.J. Powell, N.H.S. Nilsson, S. SjoÈberg, Anal. Chim. Acta (1998) 329. [15] B.D. LaZerte, Can. J. Fish. Aquat. Sci. 41 (1984) 766. [16] P.G.C. Campbell, M. Bisson, R. Bougie, A. Tessier, J.P. Villeneuve, Anal. Chem. 55 (1983) 2246. [17] S.L. Simpson, K.J. Powell, N.H.S. Nilsson, Anal. Chim. Acta (1997) 19. [18] J.W. Akitt, Prog. NMR Spectr. 21 (1989) 1. [19] S.L. Heath, P.A. Jordan, I.D. Johnson, G.R. Moore, A.K. Powell, M. Helliwell, J. Inorg. Biochem. 59 (1995) 785. [20] F. Thomas, A. Masion, J.Y. Bottero, J. Rouiller, F. Montigny, F. GeneÂvrier, Environ. Sci. Technol. 27 (1993) 2511. [21] J.F. Ma, S. Hiradate, K. Nomoto, T. Iwashita, H. Matsumoto, Plant. Physiol. 113 (1997) 1033. [22] B.C. Faust, W.B. Labiosa, K.H. Dai, J.S. MacFall, B.A. Browne, A.A. Ribeiro, D.D. Richter, Geochim. Cosmochim. Acta 61 (1997) 2651. [23] G. Borrmann, A. Seubert, Anal. Chim. Acta 332 (1996) 233. [24] M.A. Anderson, P.M. Bertsch, Soil Sci. Soc. Am. J. 52 (1988) 1597. [25] P.M. Bertsch, M.A. Anderson, Anal. Chem. 61 (1989) 535. [26] S.H. Sutheimer, S.E. Cabaniss, Anal. Chem. 67 (1995) 2342. [27] K. Tsunoda, T. Yagasaki, S. Aizawa, H. Akaiwa, K. Satake, Anal. Sci. 13 (1997) 757. [28] S.H. Sutheimer, S.E. Cabaniss, Geochim. Cosmochim. Acta 61 (1997) 1. [29] N. Wu, W.J. Horvath, P. Sung, C.W. Huie, J. Chromatogr. 635 (1993) 307. [30] A. Klingenberg, A. Seubert, J. Chromatogr. 782 (1997) 149. [31] A. Klingenberg, A. Seubert, J. Chromatogr. 804 (1998) 63. [32] A. Seubert, Fresenius' J. Anal. Chem. 345 (1993) 547. [33] G. Borrmann, Ph.D. Thesis, University of Hannover, Germany, 1997. [34] W.L. Lindsay, Chemical Equilibria in Soils, Wiley, New York, 1979. [35] R.J. Motekaitis, A.E. Martell, Inorg. Chem. 23 (1984) 18. [36] A.E. Martell, R.M. Smith, Critical Stability Constants, vol 5, Plenum Press, New York, 1982.
Aluminum speciation by means of anion chromatography and coupled anion/cation chromatography Guido Borrmann, Andreas Seubert* Institute of Inorganic Chemistry, University of Hannover, Callinstrasse 9, D-30167, Hannover, Germany Received 27 April 1998; received in revised form 14 December 1998; accepted 15 December 1998
Abstract Anion chromatography had been applied to the speciation of synthetic aluminum solutions containing medium to strong ligands. A gradient elution based on a chloride start eluent and a perchlorate ®nal eluent allowed the separation of neutral or cationic and of negatively charged Al species. The separation of Al species with a nominal charge of ÿ1, ÿ2 and ÿ3 or higher required a run time of less than 10 min. The ligands investigated were ¯uoride as monodentate, oxalate as bidentate and citrate as tridentate anion in binary, ternary or quaternary mixtures at different pH values. The results of anion chromatography are compared to those obtained by calculation methods and by cation chromatography. Anion chromatography turned out to be a useful tool for the speciation of Al solutions containing strong ligands such as citrate, whereas anionic Al±oxalate and Al± ¯uoride species are at least partially disintegrated. The favorite detection technique is on-line coupling with atomic spectrometry. An attempt to overcome the limitations of either anion or cation chromatography is the combination of both methods in a single chromatographic procedure. A slight modi®cation of the methods used for anion and cation separation allowed a complete speciation of Al solutions within 20 min. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Aluminum; Speciation; Anion chromatography; Cation chromatography
1. Introduction The speciation of aluminum in presence of organic or inorganic ligands in aqueous solutions is of interest because: 1. Al is an important pH buffer, 2. Al may influence the cycling of important elements like phosphorus and organic carbon, and 3. Al is potentially toxic to aquatic and terrestrial organisms. *Corresponding author. Tel.: +49-511-762-3174; fax: +49-511762-2923; e-mail: [email protected]
An understanding of the speciation of Al is essential for the evaluation of these processes. A clear comprehension of the forms of Al present in solutions is necessary to diagnose and predict Al toxicity in soil science, plant physiology, biology and medicine [1]. For example, plant physiological studies showed that root elongation is best correlated with the concentration or better with the activity of monomeric Al in solution [2±6]. The role of complexing ligands in attenuating Al toxicity and in in¯uencing Al transport in soils and waters has been recognized for some time, though the kinetics of such complexation reactions
0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0003-2670(99)00006-9
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within complicated multi-ligand aqueous solutions have not been successfully evaluated before. Numerous methods have been applied to Al speciation [7]. The ®rst is essentially computational, where the experimentally determined values of dissolved Al and other relevant components are used in combination with a thermodynamic data base with equilibrium approach such as GEOCHEM [8,9]. Several thermodynamic equilibrium models have been used for the calculation of Al speciation [10,11], but it is not expected that those methods will yield good results as long as the nature of some important ligands and equilibrium constants are unknown, as it is the case for most organic ligands in soils. The second kind of method used for the speciation of Al involves the analytical separation of various Al species based on reaction kinetics [12±14] and/or the physicochemical separation of Al species based on size or charge [15±17]. The third general approach uses direct spectrometric techniques such as nuclear magnetic resonance (NMR). The most widely applied technique is 27 Al-NMR for the determination of Al H2 O3 6 ; ÿ Al13 O4 OH7 24 ; Al OH4 and complexed aluminum [18±22]. Finally, a symbiosis of group 2 and 3 had been used for Al speciation. Those methods involve the combination of one or more analytical separation technique with a spectrometric detection [23±29]. A successful application is the use of ion chromatography (IC) for the speciation of Al in aqueous solutions. Previously, mostly cation chromatography has been applied to the speciation of aluminum [23±26,28], but a number of potential ligands for aluminum form more or less stable anionic species. The shortcoming of cation chromatography is the inability to resolve weakly or unretained species,
whereas the use of a recently developed cation exchanger [30,31] allowed the partial separation of anionic and neutral species by a combination of ion exclusion and ion exchange [23]. This paper describes the ability of anion chromatography for aluminum speciation in simple aluminum± ligand systems. The ligands were chosen with respect to earlier work performed by cation chromatography and to the chemical properties of the ligands. Fluoride was considered as an inorganic monodentate strong complexing agent for aluminum, oxalate as a bidentate ligand and ®nally citrate as a tricarbonic acid with high importance in natural systems. 2. Experimental 2.1. Apparatus and instrumentation All investigations were performed on a Dionex DX500 chromatography system built up of a gradient pump module GP40, a photometric detector AD20 and a post-column derivatization system. The eluent ¯ow rate was 1.0 ml minÿ1 throughout all experiments and the time programs are as given in Table 1 for anion chromatography and Table 2 for combined anion/ cation chromatography. Sample size was 50 ml for photometric detection and 200 ml for detection by inductively coupled plasma-atomic emission spectrometry (ICP-AES). The post column reagent was made of 1 mol lÿ1 ammonia acetate and 310ÿ4 mol lÿ1 Tiron (sodium salt of 4,5-dihydroxybenzenedisulfonic acid) in deionized water, buffered at pH 6.5 with perchloric acid. It was added to the eluate stream at a ¯ow rate of 0.7 ml minÿ1 using a Dionex Reagent Delivery Module (RDM) and a PTFE mixing tee, followed by a
Table 1 Time schedule for the separation of anionic species by anion chromatography with gradient elution Time (min)
Eluent A (%)
Eluent B (%)
Gradient curve
Injection valve
Comment
0.0 3.0 8.0 10.0 10.1 13.0
100 100 90 90 100 100
0 0 10 10 0 0
5 5 6 5 5 5
Load Inject Inject Inject Inject Load
Start conditions Sample injection and start of data aquisition Concave gradient Hold step and column clean-up Return to start conditions End of run
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Table 2 Time schedule for the separation of aluminum species by coupled anion/cation chromatography with gradient elution Time (min)
Eluent A (%)
Eluent B (%)
Eluent C (%)
Gradient curve
Valve 1
Valve 2
Injection valve
Comment
0.0 3.0 4.5
100 100 100
0 0 0
0 0 0
5 5 5
On On Off
Ona On Offb
Load Injection Injection
Start conditions Sample injection Switch to anion chromatography and start of data aquisition
8.0 9.0 9.5
90 80 100
10 20 0
0 0 0
8 3 5
Off Off Off
Off Off Off
Injection Injection Injection
Concave gradient Convex gradient Flush of anion column and anion catcher
10.5 16.0 17.0 20.0 20.1 22.5
100 0 0 0 100 100
0 100 0 0 0 0
0 0 100 100 0 0
5 8 5 5 5 5
On On On On On On
Off Off Off Off Off On
Injection Injection Injection Injection Injection Load
Start cation chromatogram Concave gradient Decrease of eluent-pH Column clean-up Flush of cation column Back to start conditions and end of run
a
``On'' indicate ports 1±2 and 3±4 connected (bold line). ``Off'' indicate ports 1±4 and 2±3 connected (dotted line).
b
Dionex packed-bed mixing coil (3 m in length). The AD20 absorbance detector was set at 310 nm and 0.3 mV absorbance unit full scale. Data acquisition and peak integration were performed using the Dionex Peaknet software. For on-line coupling experiments the chromatography module was stripped from the post-column reaction detection and the outlet of the separation column was directly connected to the nebulizer system of the ICP-AES. Properties of the ICP-AES Spectro¯ame P (Spectro, Kleve, Germany) as well as installed emission lines are given in [32]. For the experiments of this paper the channels 396.152 nm for aluminum and 182.037 nm for sulfur were used. Data acquisition without any dead time was performed using the TRANSIEN.EXE program delivered by Spectro AI Kleve, Germany. The peak integration and generation of graphics was done by self-written software, other calculations were done by using a commercial spreadsheet program. 2.2. Separation columns The separation columns used for anion chromatography were all home made from a commercially available macroporous divinylbenzene polymer by bromoacetylation and subsequent SN-substitution
using 2-(dimethylamino)-ethanol amine. Advantages of home made columns are the well-known structure and properties of the basic polymer, the chemically de®ned type of functionalization as well as the exactly known capacity of the columns. Furthermore, the columns are all 100% solvent compatible for cleanup procedures and they are easily re®lled in case of damage. The physical properties are given in Table 3. The columns used differed in geometry and exchange capacity. Most of the experiments were performed using the AS1 column. The speciation of the Al±¯uoride±citrate system was investigated using the AS2 column, but the results obtained with AS1 and AS2 are equal within a small difference of retention times for single charged species. The cation exchange column used for the anion/cation chromatography differed from the one used in [23] only by the column geometry of 1204 mm ID. The anion catcher AC was used as an interface column between the cation and anion separation in the coupled anion/cation speciation. 2.3. Gradient elution program for the separation of anionic aluminum species In contrast to a previous gradient program [23] the starting point is given by a dilute hydrochloric acid
80
Property
AS 1
AS 2
AC (anion catcher)
Substrate Functional group Particle size (mm) Q (mmol Clÿ) Column body, lengthID (mm)
BioGel SEC 3 (BioRad Lab.) Quaternized 2-(di-methylamino)-ethanol 5 0.21 Stainless steel, 1204
BioGel SEC 3 (BioRad Lab.) Quaternized 2-(di-methylamino)-ethanol 5 0.39 Stainless steel, 1204
BioGel SEC 3 (BioRad Lab.) Quaternized 2-(di-methylamino)-ethanol 5 0.1 Stainless steel, 304
G. Borrmann, A. Seubert / Analytica Chimica Acta 386 (1999) 77±88
Table 3 Physical properties of the anion exchange columns used for the speciation of aluminum by anion chromatography and coupled anion/cation chromatography
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81
solution. This change is necessary because of the high af®nity of the perchlorate anion to common anion exchangers. Chloride is a much weaker eluent and is used at a concentration of 1 mmol lÿ1. The ®nal gradient eluent is a 40 mmol lÿ1 solution of ethylenediammonia perchlorate at pH 3 or 4 depending on sample pH. 2.4. Experimental set-up for combined anion/cation chromatography A scheme of the set-up used for combined anion/ cation chromatography is shown in Fig. 1. The arrangement of valves 1 and 2 allows the in-line switching of either the cation or the anion chromatography column. Valve 2 divides the ¯ow path into one path using the anion catcher as collector for the dead volume signal of cation chromatography and another path allowing the elution of the anion catcher onto the anion separation column. The separation starts with the injection of the sample and the valves 1 and 2 both in the ``on'' position (port 1±2 and 3±4 connected). The sample is eluted over the cation chromatography column and the species eluting in the dead volume are adsorbed on the anion catcher. Then the valves 1 and 2 switch to the ``off'' position and the anion catcher is eluted over the anion chromatography column to perform an anion separation. The cation chromatogram proceeds after switching valve 2 in the ``off'' position. 2.5. Gradient elution program for the separation of both anionic and cationic aluminum species The elution program is divided into three steps. The ®rst is the introduction of the sample on the cation exchange column, from which all anionic or neutral molecules are eluted using 0.001 mol lÿ1 HCl. The anionic compounds are collected on the AC-column. Second step is the development of the anion chromatogram by taking out the cation exchanger and insertion of the anion exchange column. The anion separation is achieved by an elution program similar to the one described in Table 1. The third step is the development of the cation chromatogram. Prerequisite is the change of the eluent back to the 0.001 mol lÿ1 HCl. The gradient for the cation separation is similar to the one used in [23].
Fig. 1. Flowpath of the coupled anion/cation chromatography for the separation of aluminum species. Detection was performed by a simultaneous inductively coupled plasma atomic emission spectrometer. Eluent 1: 1 mmol lÿ1 hydrochloric acid, pH 3; eluent 2: 1 mmol lÿ1 perchloric acid, pH 3; eluent 3: 500 mmol lÿ1 ethylendiamine perchlorate pH 3.
2.6. Reagents All experiments were performed with synthetic Al solutions. All reagents used in this study were of pro analysis grade (Merck, Darmstadt, Germany), deionized water was obtained from a Milli-Q water puri®cation system (Millipore, Eschborn, Germany). Stock solutions of aluminum, ¯uoride, citrate and oxalate were of the concentration 3.710ÿ2 mol lÿ1 (equivalent to 1000 mg lÿ1 Al), prepared by dilution of AlCl3, NaF, citric acid monohydrate and oxalic acid dihydrate. The eluents used for gradient elution consisted of A: hydrochloric acid (10ÿ3 mol lÿ1, pH 3), B: ethylenediamine perchlorate (0.4 mol lÿ1, adjusted to pH 3 with perchloric acid) and C: ethylenediamine perchlorate (0.4 mol lÿ1, adjusted to pH 1.5 with perchloric acid). To prepare the standards, 0.5 ml of the Al stock solution and different quantities of the ligand solutions were mixed and the pH was adjusted with perchloric acid and ammonia, respectively. The concentration of Al in all investigations was 10 mg lÿ1.
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2.7. Calculation of species distributions by GEOCHEM
Table 4 Stability constants used for the calculation of Al species distributions
All calculations of aluminum species distribution are performed by GEOCHEM PC 2.0 using stability constants [34±36] as listed in Table 4. The results are compared to experimentally achieved species distributions in Section 3. Some of the species are combined to ®t the resolution ability of the chromatographic method. For example, if Al3 and AlHCit are not separated, the sum of these species is used for the comparison with experimental data.
Abbreviation
log K
Reference
AlF2 AlF 2 AlF3 AlFÿ 4 AlF2ÿ 5 AlF3ÿ 6 AlHCit AlCit AlCitÿ AlCit3ÿ 2 AlCit4ÿ 2 AlOx AlOxÿ 2 AlOx3ÿ 3
7.0 12.7 16.8 19.4 20.6 20.6 10.9 7.8 6.4 8.7 11.75 6.1 11.1 15.1
[34] [34] [34] [34] [34] [34] [35] [35] [35] [35] [35] [36] [36] [36]
2.8. Determination of the charge of anionic species The determination of the effective charge of a species detected during a gradient ion chromatography run is usually a dif®cult task. We used methylsulfonic acid and sulfate as retention time monitor for single or double negatively charged species. The retention times are 5.9 min for methylsulfate and 7.8 for sulfate when using the time program of Table 2. Neutral or cationic species are expected to elute in the dead volume and all higher charged species are far behind the sulfate peak at the end of gradient program at a retention time of 10.6 min.
3. Results and discussion 3.1. Speciation of the Al±fluoride system by anion chromatography Independent from molar ratio and pH value the chromatograms of ¯uoride containing Al solutions showed only a single peak for not retained and therefore neutral or cationic species. At a molar ratio of Al:F of 1:8 a broad peak for an anionic species (tR3± 6 min) could be detected. Anion chromatography in its present form seems to be useless for the speciation of aluminum±¯uoride species. This conclusion is in good agreement with the disintegration of higher coordinated aluminum±¯uoride species on cation exchangers [23±26].
3.2. Speciation of the Al±citrate system by anion chromatography In contrast to the ¯uoride system, citrate forms stable anionic complexes more suitable for anion chromatography. Two factors seem to be important: 1. citrate forms more stable anionic complexes, and 2. citrate as tridentate ligand should show slower kinetics for the rearrangement of ligands in the inner coordination sphere of aluminum. For a more detailed view the pH and the molar ratio dependence of the species distribution in the Al± citrate system was studied. The results presented in Figs. 2 and 3 show a strong dependence of the species distribution on both pH and molar ratio. Solutions with an excess of citrate and pH values of 4 and 5 show anionic species with a charge of ÿ3 (Fig. 2). This assumes a complex stoichiometry Al:citrate of 1:2. Fig. 2 shows an example (Al:citrate 1:2 and pH 5) where Al Cit3ÿ is the only species present. This 2 behavior is in disagreement with results of cation chromatography [33], which showed also AlCitÿ, AlCit and AlHCit. Those species could be the result of the disintegration of Al Cit3ÿ 2 , but this is unlikely because of the negative charge of this species which prevents it from retention on cation exchangers. At a molar ratio of Al:Cit 1:2 at pH 2 no anionic species is present (Fig. 2). An increase of the pH value results in
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Fig. 2. Influence of the pH value of an Al±citrate solution with a molar ratio of 1:2 on the species distribution obtained by anion chromatography. 10 mg lÿ1 Al; I0.006 mol lÿ1; time program as given in Table 1; detection by ICP-AES.
Fig. 3. Influence of the pH value of an Al±citrate solution with a molar ratio of 1:1 on the species distribution obtained by anion chromatography. 10 mg lÿ1 Al; I0.006 mol lÿ1; time program as given in Table 1; detection by ICP-AES.
the formation of AlCit and ®nally of Al Cit3ÿ 2 . At a molar ratio of Al:citrate0.5 and a pH>4 only the species Al Cit3ÿ 2 species is observed. Despite its high retention this species does not disintegrate during the separation process. A different behavior is observed for equimolar solutions of Al and citrate. The degree of AlCitÿ increases with increasing pH until AlCitÿ is
the only species present. Even at pH 5 the species Al Cit3ÿ 2 could not be detected. The Fig. 4(a)±(d) show a comparison of calculated species distributions versus the anion chromatographic determined distribution for the pH range 2±5 and molar ratios of Al:citrate of 1:1, 1:2, 1:4 and 1:8. Lower ratios are not considered because of the
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Fig. 4. Comparison of Al species distribution determined by anion chromatography using the time program given in Table 1 and by calculation using the stability constants given in Table 4. The Al-concentration and the ionic strength of each sample was 10 mg lÿ1 and I0.006 mol lÿ1. The molar ratio of Al:citrate was varied from: (a) 1:1, (b) 1:2, (c) 1:4, and (d) 1:8. The filled symbols connected by bold lines indicate the species distribution determined by anion chromatography and the open symbols with thin lines represent calculated species values. 3 The figure captions are: ÐÐÐ*ÐÐÐ Al3; AlHCit; ÐÐÐ&Ð AlCit; - -~ - AlCitÿ; ÐÐÐ* Ð AlCit3ÿ 2 ; ÐÐÐ}ÐÐÐ Al ; ÿ 3ÿ AlHCit (calc.); ÐÐÐ&ÐÐÐ AlCit (calc.); ÐÐÐ~ÐÐÐ AlCit (calc.); ÐÐÐ*ÐÐÐ AlCit2 (calc.).
absence of anionic species. For the ratio Al:citrate of 1:1, Fig. 4(a) shows a good correlation between experimental and calculated values for AlCitÿ over the whole pH range. The experimentally determined amount of AlCit is somewhat lower and that of the cationic species higher than predicted by GEOCHEM. For the ratio 1:2 shown in Fig. 4(b) the behavior is reversed. The calculated amount of Al Cit3ÿ 2 for the ratio Al:citrate of 1:4 (Fig. 4(c)) is equal to the sum of the experimentally determined amounts of AlCit and Al Cit3ÿ 2 . It should be noted that the evaluation of the peak areas for the cationic aluminum species and
the AlCit species is rather dif®cult because of the strong overlap of these species. In general, the comparison of data obtained by anion chromatography or cation chromatography with the calculated values show a better agreement between calculation and anion chromatography for solutions with an excess of citrate. Nevertheless, it should be noted that the decision whether anion or cation chromatography gives the more correct data is impossible without an independent non-invasive technique. An attempt to achieve more information about the Al:citrate system is the application of directly coupled anion and cation chromatography.
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Fig. 5. Chromatogram obtained for coupled anion/cation chromatography using the setup shown in Fig. 1 and the time program given in Table 2. The samples are an Al±citrate solution with a molar ratio of 1:1 and different pH-values as indicated in the figure. 10 mg lÿ1 Al; I0.006 mol lÿ1; detection by ICP-AES.
3.3. Speciation of the Al±citrate system by coupled anion/cation chromatography The coupled anion/cation chromatography was realized by the insertion of two switching valves and the use of an anion catcher (AC) as interface column. The time schedule of the elution program is given in Table 2. The chromatographic run is divided into an anion and a cation separation. The ®rst column is the cation exchanger in series with the anion catcher. The AC retains the species eluting in or close to the dead volume of the cation exchanger. Some sample chromatograms of an equimolar solution of aluminum and
citrate at different pH values are shown in Fig. 5. The species distribution is comparable to those obtained by anion chromatography (Figs. 2 and 3). Advantage of the coupling anion and cation chromatography is the better resolution of equally charged species. A disadvantage could be the increased interaction between the species and the cation as well as the anion exchanger. The prolonged column clean-up with 0.4 mol lÿ1 ethylenediamine perchlorate at pH 1.5 used for this coupling application showed a previously undetected signal at the end of the run. The species eluting after Al3 increases with increasing sample pH and could
Fig. 6. Influence of the molar ratio of Al and oxalate at pH 4 on the species distribution obtained by anion chromatography. 10 mg lÿ1 Al; I0.006 mol lÿ1; time program as given in Table 1; detection by ICP-AES.
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be the oligomeric ``Al7 13 species, which is the dominating species in the pH range 4±6. In previous attempts the total loss of aluminum during a species separation has been referred to this species. Obviously, the strong clean-up solution is able to destroy this extremely strongly adsorbed species to the more easily eluting Al3 cation. 3.4. Speciation of the Al±oxalate system by anion chromatography The chromatograms shown in Fig. 6 were measured at a constant sample pH of 4 and varying Al:oxalate ratios. The results are in good agreement with results obtained by cation chromatography [33]. At a molar ratio of 1:1 only cationic and therefore unretained species are present. A shoulder of the dead volume signal at tR2.5 min should refer to the AlOxÿ 2 species, whose concentration increases with decreasing Al:oxalate ratio. With an decreasing Al:oxalate ratio the cationic signal decrease and the baseline begins to increase at longer retention times (tR8±11 min). At a molar ratio of 1:4 a strongly fronted peak for AlOx3ÿ and a 3 strongly tailed peak for AlOxÿ 2 were observed. The peak distortion could be explained by disintegration of the AlOx3ÿ 3 species during the chromatographic run. After the loss of an oxalate ligand the retention is decreased and therefore the peak shows a fronting. The time scale of this disintegration is in the range of the chromatographic run time. A further disintegration is observed for the shoulder on the peak representing AlOxÿ 2 . This signal is fronted by a loss of an oxalate molecule and tailed by the newly generated AlOxÿ 2 resulting from the disintegration of AlOx3ÿ 3 . In addition the retention time for this single charged species is much shorter than for the single charged citrate species (tR [AlCitÿ]3.9 min, tR AlOxÿ 2 2:5 min). The triple charged oxalate species shows the same retention time than the AlCit3ÿ 2 species (tR10.6 min), but this part of the chromatogram is measured at the end of the gradient ramp. Al±oxalate solutions with a molar ratio of 1:4 show only a minor pH-dependence. At pH-values higher than 2, the detectable species distribution remains constant. A comparison of the results of anion chromatographic speciation and calculation (Fig. 7(a)±(c)) show a good agreement at low molar ratios of Al:ox-
Fig. 7. Comparison of Al-oxalate species distribution determined by anion chromatography using the time program given in Table 1 and by calculation using the stability constants given in Table 4. The Al-concentration and the ionic strength of each sample was 10 mg lÿ1 and I0.006 mol lÿ1. The molar ratio of Al:oxalate was varied from: (a) 1:2, (b) 1:4 to (c) 1:8. The filled symbols connected by bold lines indicate the species distribution determined by anion chromatography and the open symbols with thin lines represent calculated species values. The figure captions are: ÐÐÐ^ÐÐÐ Al3/AlOx; ÐÐÐ& Ð AlOxÿ 2 ; Ð Ð~Ð 3 ÿ AlOx3ÿ 3 ; ÐÐÐ}ÐÐÐ Al /AlOx ; ÐÐÐ&ÐÐÐ AlOx2 ; . ÐÐÐ~ÐÐÐ AlOx3ÿ 3
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87
Fig. 8. Influence of the pH value of Al±citrate±oxalate solutions with a molar ratio of 1:0.5:0.5 on the species distribution obtained by an anion chromatography. 10 mg lÿ1 Al; I0.006 mol lÿ1; time program as given in Table 1; detection by ICP-AES.
alate. At Al:Ox 1:2 (Fig. 7(a)) the amount of positively charged species is much higher for anion chromatography than calculated by GEOCHEM. At Al:Ox 1:4 (Fig. 7(b)) and 1:8 (Fig. 7(c)) the species distribution simpli®es and the agreement between both methods is better. Anion chromatography is suitable for the determination of all expected monomeric Al±oxalate species. The speed of disintegration of the species is within the time range of the chromatographic run. This causes peak distortion and surely some experimental error in the determined species distributions.
charged species could be observed. The sharp peak at tR5.2 min shown in Fig. 8 can be identi®ed as AlCitÿ and the broad peak directly after the dead volume equals in peak shape and retention time the AlOxÿ 2 species. Anion chromatography can distinguish between both equally charged species. The quantitative species distribution equals the results obtained by cation chromatography.
3.5. Speciation of the Al±fluoride±citrate system by anion chromatography
Anion chromatography allows the separation of stable anionic complexes aluminum. However, weaker anionic species such as ¯uoride and oxalate complexes are at least partly destroyed during the separation. In comparison to cation chromatography the amount of information given by an anion speciation is less signi®cant. The strength of an anion chromatography is their additional information, which allows to distinguish between dead volume signals occurring in the most widely used cation chromatography. Therefore, anion chromatography as a stand alone technique is an extension of the ion chromatographic tools for aluminum speciation. Consequently, the combination of anion and cation chromatography within a single run should give a more detailed view into the species distribution of aluminum containing solutions. A drawback of a
At pH 2 and 3 it was impossible to detect anionic species. At higher pH a signal for a anionic species with a charge of ÿ1 could be detected. This species should be AlCitÿ. The results for pH 4 and 5 are essentially the same. The amount of retained species is about 80% of the citrate present in the mixture and independent from the ¯uoride concentration, which is in good agreement which the assumed AlCitÿ species. 3.6. Speciation of the Al±citrate±oxalate system by anion chromatography The analysis of mixtures at pH 2 and 3 showed no anionic species. At higher pH values two single
4. Conclusions
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