Equilibrium modelling of interferences in the visible spectrophotometric determination of aluminium(III): Comparison of the chromophores chrome azurol S, eriochrome cyanine R and pyrocatechol violet, and stability constants for eriochrome cyanine R-aluminium complexes

Ar+bwTIcA CHIMICA

ACTA ELSEVIER

Analytica Chimica Acta 319 (1996) 305-314

Equilibrium modelling of interferences in the visible spectrophotometric determination of aluminiumc III) : comparison of the chromophores chrome azurol S, eriochrome cyanine R and pyrocatechol violet, and stability constants for eriochrome cyanine R-aluminium complexes David J. Hawke, H. Kipton J. Powell

*,

Stuart L. Simpson

Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand Received 24 July 1995; accepted 2 October 1995

Abstract Equilibria in the Al 3f-H+-eriochrome cyanine R (ECR) system were studied by potentiometry and spectrophotometry in 0.10 M KC1 at 25°C and p[H+] 2.5-6.5. Potentiometric titration data were interpreted in terms of a monomeric 1:l (log &2,2 = 13.44), and excess ligand complex AlH_,L (log &-1,1 = 1.75), an excess metal polymer Al,H_,L, polymers with stoichiometries Al,H_,L, and Al,H_,Ls (log /?-s,s = 29.07; log /3,,_4,5 = 25.30). For titrations involving more rapid addition of alkali there was evidence for an additional polymeric conjugate base, Al,H_,L, (log &,_5,5 = 20.67). The ECR-aluminium stability constants, along with literature values for those of chrome azurol S (CAS) and pyrocatechol violet (PCV), were used to calculate interference effects in the spectrophotometric determination of Al. Calculations established that interference by fluoride, citrate, oxalate and salicylate in aluminium assays was a minimum for PCV at pH 6.5 and maximum attainable chromophore concentration. CAS and ECR were more subject to interference and had pH values of minimum interference which varied with the interferent according to its acid-base properties. The use of mass action effects to suppress interferences was limited by the comparatively high ligand-only absorbances for CAS and ECR . Keywords:

Spectrophotometry;

Potentiometry;

Aluminium;

Chromophore

1. Introduction The toxicity of Al in acidified environmental systems is well known. This toxicity is masked by fluoride [l], silicon [2], organic ligands [3], and soil organic matter [4]. However, regulatory requirements

* Corresponding

author.

0003-2670/96/$15.00 6 1996 Elsevier Science B.V. All rights reserved SSDI 0003-2670(95)00497-l

complexes

(e.g. for drinking water) frequently specify analysis for total (soluble) Al, rather than free (labile) Al. Further, the calorimetric assays frequently used for Al in environmental samples all suffer interference to some degree from the naturally occurring ligands that mask Al toxicity. The choice of calorimetric reagent may therefore be infhrenced by susceptibility to interference. The

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D.J. Hawke et al./Analytica

interferents in calorimetric assays for Al have a wide range of acid-base behaviour and Al complex stabilities. This leads to possible pH differentials in interference effects. When combined with different acid-base and Al complexation behaviour among candidate calorimetric reagents, the determination of relative interference effects becomes difficult. The situation is further complicated by changes in analyte pH during reagent use. Interferents are usually evaluated experimentally under narrowly defined analytical conditions. These conditions are often those optimised for standard solutions of the analyte. A complementary approach to evaluating interferences is to carry out computer modelling studies using equilibrium constants. This approach is often not feasible due to lack of appropriate equilibrium data for either the interferent or the calorimetric reagent. We have recently published equilibrium constants for two commonly used Al chromophores: chrome azurol S (CAS) 151, and pyrocatechol violet (PCV) [6]. In this paper, we present equilibrium constant data (0.10 M KCl, 25°C) for a third Al reagent [7], eriochrome cyanine R (ECR). We use these constants to calculate pH and mass action effects of the representative interferents citrate, oxalate, salicylate, and fluoride on the reaction with the three 3 chromophores. The implications of these results for assays for total Al and toxic (free> Al are discussed.

2. Experimental

2.1. Reagents Chemicals were analytical grade and used as supplied except as indicated below. KOH solutions were prepared every 2-3 weeks in CO,-free H,O from washed pellets (BDH, AnalaR) and standardised against Tris-HCl (Fluka, puriss). HCl solutions were standardised against this KOH. A13+ solutions were prepared in dilute HCl from AlCl, . 6H,O (Alfa, 99.9995%). The Al content was determined gravimetrically using 8-quinolinol; acidity was determined by Gran potentiometry. ECR (3”-sulfa-3,3’-dimethyl-4-hydroxyfuchson-5,5’-dicarboxylic acid: I, H,ECR) (Sigma) was purified by double recrystalli-

Chimica Acta 319 (19%) 305-314

sation from 120 ml of a filtered 10% aqueous solution by addition of an equal volume of concentrated HCl. The product was vacuum dried over NaOH pellets then P,O, at room temperature. Microanalysis gave C 57.3%; H 4.2% (calculated for H,ECR . H,O: C 56.6%; H 4.2%).

CH3

CH3

A major impediment to the determination of accurate stability constants was the slow “loss” of ECR from solution, especially in the presence of A13+. The pH dependence of this process (most rapid at low pH), plus non-retention of ECR on membrane filters, indicated adsorption rather than oxidation or precipitation. Rather than use stock solutions, ECR was therefore weighed into the titration vessel immediately prior to each experiment.

2.2. Potentiometry Titrations were microprocessor controlled and carried out as previously described [8]. All spectrophotometric and potentiometric titrations were performed in 0.10 M KC1 in an airtight, thermostatted 120 mL reaction vessel held at 25 f O.l”C and continuously flushed with O,-free N,. The glasscalomel electrode system was calibrated as an [H+] probe prior to each experiment by titration of HCl with standard KOH to pH ca. 11. Plotting pH (observed) against p[H+] (calculated) over the p[H+] ranges 2.5-3.5 and 9.5-11.0 gave linear relationships (r2 2 0.995). Because of adsorption of ECR, titrations were effected with short equilibration times. Solution pH was monitored at 150 s intervals after addition of each aliquot until successive readings (each the mean of 30 measurements) agreed within a pre-set drift tolerance (0.003 pH). If drift was still continuing

D.J. Hawke et al./Analytica

after 3 readings, the datum point was flagged and the titration continued. Equilibrium constants were refined from the data using the program SUPERQUAD [9]. Six ligand-only titrations (ca. 55 data points each) were carried out at ca. 2 X lop3 M ECR. Total ligand concentration was fixed in the calculations according to the equivalence point volume for each titration. Equivalence volumes agreed closely (98.6100.3%) with those expected from the weighed amount of ECR. Seven titrations of A13+ and ECR mixtures (ca. 75 data points each) were carried out over the range 5.6 X 10e4 to 8.3 X 10e4 M A13+, and 6.7 X 1O-4 to 1.4 X low3 M ECR. Ligand-tometal ratios were in the range 1.2 to 2.05.

2.3. Spectrophotometry For spectrophotometry the titrant was added from a calibrated 1 mL micropipette, for which the measured uncertainty (p = 0.05) in the delivered volume was < 2 ~1. Solutions were pumped through a 1 cm flow cell mounted in an adjacent GBC UV/VIS 918 spectrophotometer using a small peristaltic pump. The use of small-bore tubing (0.51 mm i.d.) and a 70 pL flow cell minimised dead volumes. The electrode system was calibrated as an [H+] probe as described above. Titrations of ECR with A13+ (2.1 X 10e4 M) at fixed p[H+] (5.70 or 6.00) were carried out at 7 X 10m6 M and 5 X 10m5 M ECR to establish spectral characteristics and L:M (ligand-to-metal) stoichiometries of metal complexes. The required p[H+] was maintained by addition of small volumes of acid or base. Absorbance was scanned from 400 to 600 nm. Five competitive ligand titrations of malonate into solutions of A13+ plus ECR were carried out at p[H+] = 5.70. Concentrations were 5-30 X 10m6 M ECR, 3-7 X lop6 M A13+, and 11-240 X 10m6 M malonate. The ECR + A13+ solution was equilibrated for 6 min prior to beginning of each titration. Each titration consisted of 3 additions, each incrementing the malonate concentration by lo-60 PM. The system was equilibrated for 5 min between each malonate addition. Prior kinetic experiments had established these as adequate equilibration times. Absorbance was scanned from 450 to 550 nm.

Chimica Acta 319 (1996) 305-314

307

2.4. Equilibrium modelling of interferences Modelling calculations for the effect of interfering ligands on CAS, ECR and PCV assays for Al were performed using the equilibrium program SOLGASWATER [lo]. The chosen pH range (4.5-6.5) reflects the pH of natural systems (especially soil solutions), and encompasses the pH optima for chromophore complexation. The calculations used a pH scale based on concentration, p[H+], rather than activity, pa(H+) as used by most analytical chemists. However, the two scales are sufficiently similar for the purposes of this study. Choice of model ligands was constrained by the availability of stability constants. The ligands chosen were the strong organic complexant citrate [ll], the weaker complexants oxalate [12] and salicylate [13] and the strong inorganic complexant fluoride [14]. Stability constants for the chromophores (CAS [8]; PCV [15]; ECR, this work) and A13+ hydrolysis [16] were available at an ionic strength Z of 0.1 M. Constants were not available for the model ligands at 0.1 M, but were available at Z = 0.6 M (F-, Z = 0.53 M). While the difference in ionic strengths was not ideal, it was a consistent factor. Concentrations chosen for the modelling reflected analytical and regulatory requirements. The WHO guideline for total Al in drinking water (I 7.4 Z.LM = 20 ng ml-‘) was used. Chromophore concentrations were 20 PM (typical of kinetic flow injection methods [17]) and 100 Z.LM(typical of conventional calorimetry). This range also allowed the exploration of mass action effects. Ligand concentrations were 20 PM (citrate, fluoride, oxalate) or 500 PM (salicylate).

3. Results ECR has both salicylic and p-quinomethide-2carboxylic acid residues as potential coordination sites (called sites A and B respectively). ECR differs from CAS only in the presence of two chlorine atoms at the 2 and 6 positions on the sulfonic acid ring, and the location of the sulfonate group at the 3 position rather than the 2 position in CAS. ECR has 4 acidic groups in aqueous solution. However, the zero proton level was chosen as the

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Chimica Acta 319 (1996) 305-314

Table 1 Equilibrium constants &4,r for the system pA13+ + qH+ + rL3- at 25°C in 0.10 M KCl, determined from n titrations. The corresponding chrome azurol S (CAS) constants [8] are shown for comparison Constant

n

VALUE f SD.

Method =

CAS Constant

log log log log log log [log

5 6 5 4 3 4 2

4.42 + 0.03 6.93 f 0.01 1.75 * 0.07 13.44 f 0.15 29.07 f 0.06 25.30 f 0.7 20.67kO.2

P P S P P P PI b

4.64 6.93 2.01 12.29 31.35 26.57 19.53 c

P0,l.l PO,Z,I Pl,-

1.1

83,-2.2

&,-3.5 &-4.5 &,_5,5

The data were fitted according pA13++ qH++ *

rL3-

[~,HqL,]3p+q-3r log &_

H+ + L3- = HL*2H++ L3- = The equilibrium

H,L-

log PO,I,I log

constants

3.2. Metal complexation metric analysis

Po,2,1

are given in Table 1. equilibria:

spectrophoto-

Spectrophotometry at p[H+l = 5.70 (Fig. 1) showed an isosbestic point at 494 nm and A,,, at 533 nm on addition of A13+ to ECR. Unlike CAS [8], there was no change in A,,, under excess metal conditions. A plot of absorbance (corrected for uncomplexed ligand) against [A13+],, for the titration in Fig. 1 contained 2 linear segments corresponding to excess ligand and excess metal conditions, respectively. The provided a ligand-toM3+ I,,, at the intersection metal ratio, Y, for the terminal species under excess ligand conditions. The value for Y was 1.02 f 0.05, indicating a 1: 1 species under spectrophotometric conditions. By analogy with CAS [8], this species is given the stoichiometry AlH _ 1L. Experiments using malonate as a competitive ligand were used to determine log j3r _ r,r. We assumed that no polymeric Al-ECR species were present under the conditions used. Support for this assumption was found in the absence of systematic drift to higher apparent constants under conditions that

a S = Spectrophotometry; P = potentiometry. b Species formed at higher alkali addition rate. ’ I = 0.6 M KCl.

dicarboxylate L 3- because of the very weak acidity of the salicylate -OH group. The phenolic proton in such systems usually has pK, = 12 (but dissociates well into the acidic pH range upon metal coordination [ 131). 3.1. Ligand protonation

to the definitions:

equilibria

Deprotonation of the carboxylic group on the salicylate ring and the P-ketocarboxylic group on the p-quinomethide ring were studied potentiometrically. The strongly acidic sulfonic acid group and the weakly acidic phenolic group were not studied; the former is fully dissociated and the latter undissociated in the absence of A13+ in the pH range of A13+ complexation.

0.0 I

I

450

500 Wavelength

550 (nm)

Fig. 1. Selection of data from the spectrophotometric titration of 7.75 X 10m6 M ECR with A13+ at p[H+] = 5.70. [A13+], for curves (a) to (f) was 0, 2.0, 4.0, 5.9, 7.9 and 16 X 1O-6 M respectively.

D.J. Hawke et al./Andytica

should increasingly favour polymerisation (low malonate, and high Al and ECR concentrations). The concentration of the Al-ECR complex was calculated after each titrant increment using absorbance data (A) at 533 nm and the equation [complex] = (A -

EECR

. [ECRltot)/hxnp~ex

-

‘ECR) (1)

where [ECR],,, = total ECR concentration and E refers to molar absorptivity. Eq. (1) assumes no ternary malonate complex formation. Visible spectra

(A)

‘.’ /

309

Chimica Acta 319 (1996) 305-314

monitored during the experiments showed the expected isosbestic point at 494 nm and A,,, at 533 nm. Adsorptive loss of ECR was significant at the micromolar concentrations used in the experiments. was therefore calculated for each datum [ECRI,,, point from the absorbance at the 494 nm isosbestic point. Molar absorptivities were measured to be: At p[H+] = 5.70, cECR = 2.06 X lo4 1 mol- ’ cm- ’ (494 nm), 1.11 X lo4 (533 nm); and &,,,rlex = 9.50 (533 nm). Uncertainties X lo4 1 mol-’ cm-’ (kS.D.1 were 400 for cscR, and 1.8 X lo3 for %l,plex.

/----L-L--

___----

Polymers

P[H+l

/ /

Y-

T_

AIH.IL

/

4.5

P[H+l

5.5

6.5

4.5

P[H+l

Fig. 2. Speciation in the Al 3+-H+-ECR system at 1igand:metal = 2.0, and (a) 1 X Km3 M ECR; (b) 1 X 10m4 M ECR; (c) 1 X lo-’ ECR, calculated using the equilibrium program SOLGASWATER [lo].

M

D.J. Hawke et al./Analytica

310

An iterative procedure

was used to determine

Chimica Acta 319 (19%) 305314

libration led to increased pH drift during data collection, indicating incomplete equilibration. We used a 150 s equilibration time (see Experimental), which was found adequate for the Al-CAS potentiometric study [81. Endpoint stoichiometries gave a mean of 0.92 f 0.06 H+:Al (p = 0.05; 5 titrations). This is analogous to the result for CAS at I = 0.10 M KCl, a system for which the equilibrium model involves a polymer of stoichiometry AIXH1_XLX+l with one conjugate base, Al,H_,L,+ 1. In determining the ECR model we made an initial assumption that complexation would follow the pattern for CAS [S], calculations viz., x = 4 or x = 5. SUPERQUAD showed that the fit for x = 4 was substantially better than for x = 5. Thus, the final model was based on x = 4. The following Al 3+-ECR species were required:

the

value of log &,-I,1 using the chemical equilibrium program SOLGASWATER [lo]. Required inputs were the stoichiometric concentrations of ECR, Al and malonate, and the equilibrium constants for Almalonate [191, Al hydrolysis 1161 and ECR protonation (this study). Trial values of PI,_ 1,1 were used to calculate values of [AlH_,L] which were then compared with those derived spectrophotometrically. The PI,- 1,l value was adjusted to achieve agreement between calculated and observed results. The quoted PI - 1,l is the mean value over the range of data points. The resulting stability constant was log = 1.75 f 0.07. The composition of the soluPI,- 1,l tions ranged up to 92% Al-malonate complex, with [AlH_,L] between 3.8 X 1O-7 M and 4.9 X 10e6 M. 3.3. Metal complexation analysis

equilibria:

Potentiometric

A13+ + L3- = AlH_,L-+ 3A13+ + 2L3-

Unlike the spectrophotometric analyses, we found no simple method for correcting for ECR loss. Titrations over increasing equilibration times showed decreasing equilibrium constants, consistent with ECR loss. However, decreasing the time allowed for equi-

“’ f

H+

* Al,H_,L,++

and the acid-base

log /?1,-1,1 2H+

log P3,-2,2

conjugates:

4A13+ + 5L3-

+ Al,H_,L;-

+ 3H+

log p4_35 3 2

4A13+ + 5L3-

* Al,H_,L7,-

+ 4H+

log &_45 , 3

(A)

0.6

’ AIPCV

ai 0.6

0.6

t AIPCV,

I

’’ :

0.4

0.4

. ’ i

I

4.5

5.0

5.5

6.0

6.5

4.5

,AI-CAS .

5.0

5.5

6.0

6.5

Fig. 3. Calculated Al speciation in the presence of 20 /.L.Mcitrate, and (A) 20 PM chromophore and (B) 100 PM chromophore at 7.4 PM total Al. q represents the fraction of Al complexed by each chromophore. Also shown is the fraction of non-complexed Al in the absence of chromophore (0).

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D.J. Hawkeet al./Analytica ChimicaActa 319 (1996)305-314

100 PM, decreased citrate interference. At the optimum pH for each chromophore determined by the original investigators (CAS, pH 5.0 [5]; PCV, pH 6.2 [6]; ECR, pH 5.5 [7]), the amount of sequestered Al increased 3-fold (CAS, 7 + 23%; PCV, 30 + 90%), or 4-fold (ECR, 1.5 + 6%). For PCV at 100 PM, increasing the pH from 6.2 to 6.5 led to negligible citrate interference. The above calculations clearly demonstrate the importance of both pH and mass action effects in controlling interference. However, (20 ,uM)-citrate equilibration in the A13+ -H+-PCV system was found to require in excess of 300 min at pH 6.2. The significance of slow equilibration in the A13+-PCV system under spectrophotometric conditions has previously been demonstrated by us. In experiments carried out in a flow injection system [17], we showed that several hours were required for complete colour development with A13+ standards. For experimental studies, slow equilibration may result in interference assessments being operational in nature. For modelling studies, the result may be interference assessments which do not match the effects experienced in routine analysis. Oxalate is a significantly weaker ligand than citrate. Thus, at 20 PM oxalate and 20 PM PCV (Fig. 4a), oxalate showed negligible interference at pH 2

The calculated equilibrium constants are given in Table 1 and the metal speciation is illustrated in Fig. 2. Satisfactory fits to the titration data were obtained over the entire p[H+] range of data collection, through and beyond the endpoint (the least squares fitting by SUPERQUAD gave mean c = 8.2; mean x2 = 8.9, from 5 titrations). 3.4. Equilibrium

modelling of interferences

We modelled the determination of A13+ by the 3 chromophores under spectrophotometric conditions in the presence of the potential interferents oxalate, salicylate, fluoride or citrate using the equilibrium program SOLGASWATER [lo]. Citrate bound Al very strongly (Fig. 3). At 20 JLM citrate, > 90% of Al was complexed over the pH range evaluated in this work (4.5-6.5) and > 99% at pH > 5.7. Addition of chromophore data to the calculation showed that neither CAS nor ECR is effective in sequestering Al from citrate. At 20 PM chromophore concentration, CAS and ECR removed < 10% of Al from citrate at pH 4.5, decreasing to < 2% at pH 6.5. In contrast, PCV showed decreasing interference from citrate (increasing sequestration of Al) with increasing pH. Increasing each chromophore concentration fivefold, from 20 PM to

1.0.

//

0.8

_----’

ai 0.6

, ’ AI-PCV

I

Al4xa

0.01 --* 4.5

I

5.0

I

5.5

I

6.0

I

6.5

0.0’ - - * 4.5

I

I

I

5.0

5.5

6.0

I

6.5

P[H+l Fig. 4. Calculated Al speciation 3 for description of curves.

in the presence of 20 PM oxalate (A) or fluoride (B), and 20 PM chromophore

at 7.4 PM total Al. See Fig.

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4. Discussion AICAS _---- -

-AI-ECR

--

4.1. Equilibrium model for Al3 ‘-IS ‘-ECR

’ .I

.l --

I ,-

\

.

.

\ \

\ \

’ AIPCV

4.5

5.0

5.5

6.0

6.5

P[H+l

Fig. 5. Calculated Al(III) speciation in the presence of 500 /.LM salicylate and 20 PM chromophore at 7.4 /.LM total Al. See Fig. 3 for description of curves.

6.0. In contrast to citrate, oxalate interference in CAS and ECR determinations of Al decreased with increasing pH up to pH 6.2. This is a consequence of the different pH dependence of complexation, with Al complexation by oxalate being essentially complete at pH < 4.5. CAS and ECR showed least susceptibility to interference at pH 5.8-6.2. Fluoride (Fig. 4b) gave similar results to oxalate. Al complexation by both oxalate and fluoride decreased substantially at pH > 5.5, a consequence of the comparative strength of A13+ hydrolysis. Acid-base effects were clearly demonstrated by salicylate calculations (Fig. 5). The fraction of Al bound by 20 FM ligand in the absence of chromophore increased to a maximum at ca. pH 5.5, then decreased through competition from hydrolysis. The relatively high pH for maximum binding compared with oxalate and fluoride is due to the weakly acidic phenolic proton (p K, = 12) on salicylate. At 500 PM salicylate and 20 /.LM chromophore above pH 6.3, PCV sequestered virtually all of the Al. Increasing the CAS or ECR chromophore concentration to 100 PM essentially removed the salicylate interference shown in Fig. 5.

The model stoichiometry is closely analogous to that established for CAS [S]. The relevant 0.10 M KC1 CAS constants [8] are given alongside the ECR constants in Table 1. The ECR values for log PI,_ r,r; P4,- 3,5 and /34,_4,5 were all lower than the corresponding CAS constants; log /33,_2,2 was higher than for CAS. Protonation constants (Table 1) show that the ECR salicylate carboxylic group is a slightly weaker acid whereas the ECR P-ketocarboxylic group is a slightly stronger acid. The overall similarity of the ECR and CAS models and constants is consistent with the similar ligand structures. The results in the present study did not support the ML, complex proposed earlier for Al-ECR [18]. The discrepancy is almost certainly due to impure reagent; the ECR molar absorptivity quoted by Hill [18] was only 30% of that found in the present study. A full description of possible structures is given in our CAS paper [8]. Briefly, the species AlH_,L consists of A13+ bound to the fully deprotonated ligand (with loss of the phenolic proton). Al,H_ ,L (for CAS) represents the addition of a second A13+ to the remaining coordination site on AlH_, L. The excess metal species Al,H_ *L2 may be considered as the linear combination of AlH _ 1L and Al z H _ 1L, with Al bridging between ligands. The stoichiometry for the polymers can arise if the noncoordinated donors in the terminal ligands correspond to one site A and one site B function. The pH dependence of A13+-ECR speciation is shown in Fig. 2a-c for an L:M ratio of 2.0 at three ligand concentrations. For the purposes of displaying speciation, Al,H_,L, and its conjugate base were combined under the generic term “polymers”. At [ECR] = 1 mM (Fig. 2a; typical of potentiometric titrations), complexation of Al is complete above p[H+] = 4.0. While the excess metal species Al,H_ 2L2 is important at low p[H+], polymeric species increasingly dominate above p[H+] = 2.8. The monomer AlH_ 1L is a minor species throughout the p[H+] range. At [ECR] = 100 PM (typical of spectrophotometric determinations of A13+ >, speciation is complicated (Fig. 2b). The excess metal species Al,H_,L, dominates in the range p[H+l=

D.J. Hawke et al./Analytica

3-4. Above p[H+] = 4.0 polymers and the AlH_,L monomer dominate. At [ECR] = 10 PM (Fig. 2c; typical of kinetic determinations of A13+ [17]), complexation is complete by p[H+] = 5.5. The dominant species is the monomer AlH_,L, while Al,H_,L, and polymers contribute only as minor species. A consequence of the adoption of sub-optimal equilibration times for the potentiometric titrations involving Al is that Al-ECR constants determined potentiometrically are possibly underestimates. However, the similarity between these constants and those for CAS indicates that the discrepancy should not be large. It is worth emphasising that the spectrophotometrically-derived constant ( &_ i,i) should be accurate, because corrections for ligand loss were able to be included in the calculations. Further, Al-ECR speciation at the Al concentrations used for modelling is dominated by the monomeric species &_ i,i (Fig. 2~). The conclusions involving ECR in the interference modelling portion of this study should therefore be valid. 4.2. Equilibrium

modelling of interferences

The equilibrium calculations showed that PCV is the chromophore least susceptible to interference from Al-binding ligands. Because of its low background absorbance at the wavelength and pH used for analysis, it is also the chromophore for which mass action effects can best be utilised. By using high PCV concentrations and a relatively high pH (= 6.5) interference from the ligands discussed in the present study should be minimal for common environmental samples (assuming equilibrium in the analyte-chromophore-interferent reaction). The results of the equilibrium interference calculations showed that PCV’s susceptibility to interference decreased with increasing pH, regardless of the competing ligand. In contrast, CAS and ECR reflected the pH dependence of Al complexing by the interferent ligand. With assays using CAS and ECR, the pH at which the competing ligands interfered least was in the order citrate (pH < 4.5) < salicylate (pH 5.2) < oxalate = fluoride (pH 6.1). Organic matter from soils and natural waters contains both salicylate and aliphatic carboxylate functional groups. Therefore, manipulating the pH to minimise interference when using CAS or ECR is a futile exercise in soil and water analysis.

Chimica Acta 319 (1996) 305-314

313

Using mass action effects to overcome interference is limited by the blank absorbance. At the respective h,,, and pH optima, the molar absorptivities for the pure chromophores are 5400 (CAS [S]), 11100 (ECR, this work), and 1500 1 mol-’ cm-’ (PCV [20]). If an upper limit on the blank absorbance of 0.7 was imposed, then the maximum chromophore concentrations would be 130 PM (CAS), 63 PM (ECR), and 470 PM (PCV). Therefore, PCV has the greatest potential for use of mass action effects. The focus of regulatory requirements is on the determination of total Al. However, much current research is aimed at measuring the toxic fraction. In the sense that the chromophore should not sequester Al bound to naturally occurring ligands, this requires that interference be maximised rather than minimised. Figs. 3-5 show that all chromophores (but particularly PCV) overestimate the free A13+ concentration, irrespective of the ligand. The results therefore support other work [17] showing that PCV is unsuitable for determinations of free (toxic) Al because of its aggressiveness towards complexed Al. While PCV causes minimal disruption of the primary equilibria at low pH (4.8-5.0), low sensitivity at this pH may cause such analyses to be of little practical value. The most promising use of chromophores for determination of free A13+ is therefore in kinetic, rather than equilibrium, assays [17]. 4.3. Evidence system

for metastability

in the Al3 ‘-ECR

Two metal-ligand titrations performed at twice the rate of base addition gave endpoints with an H+:Al stoichiometry of 1.17 f 0.06. Analysis of data from these titrations gave the same species (and stability constants within experimental error), plus the additional conjugate base Al, H _ 5Lt- : 4A13+ + 5L3-

= Al,H_&-

+ 5H+

log & ,_5 35

This additional conjugate base can arise if the noncoordinated donors in the terminal ligands are both site B functions. In the A13+-CAS system a species with this stoichiometry has been postulated for the 0.6 M KC1 medium [8]. The effect of this additional equilibrium on the speciation of Al (Fig. 2) is negligible at micromolar concentrations of metal and ligand.

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DJ. Hawke et al. /Analytica

The additional species also exerts a negligible effect for millimolar concentrations below p[H+] = 4.5, above which its contribution increases to reach ca. 20% of total A13+ at p[H+l = 6.0. The observed dependence of reaction stoichiometry may stem in part from the necessarily short delay times used in the potentiometric titrations. It may also indicate that the A13+-ECR system is metastable. It is noted that the value of &,_4,5 calculated for the titrations having a 1:l endpoint stoichiometry has a large standard deviation; this may indicate a partial breakdown of the model in this part of the titration curve. Acknowledgements This paper is a partial fulfilment of Contract UOC 315 from the New Zealand Foundation for Research, Science and Technology to the University of Canterbury. Financial support for SLS was via a University of Canterbury Doctoral Scholarship. We thank Professor S. Sjiiberg for helpful discussions. References [l] R.S. Cameron, G.S.P. Ritchie and A.D. Robson, J. Soil Sci. Sot. Am., 50 (1986) 1231.

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121J.D. Birchall,

C. Exley, J.S. Chappell and M.J. Phillips, Nature, 338 (1989) 146. [31 N.V. Hue, G.R. Craddock and F. Adams, J. Soil Sci. Sot. Am., 50 (1986) 28. 141 R.J. Bartlett and D.C. Riego, Plant & Soil, 37 (1972) 419. [51 P. Pakalns, Anal. Chim. Acta, 32 (1965) 57. 161W.K. Dougan and A.L. Wilson, Analyst, 99 (1974) 413. 171 U.T. Hill, Anal. Chem., 38 (1966) 654. kJ1 D.J. Hawke, H.K.J. Powell and S. Sjiiberg, Polyhedron, 14 (1995) 377. Dl P. Gans, A. Sabatini and A. Vacca, J. Chem. Sot., Dalton Trans., (1985) 1195. [lOI G. Eriksson, Anal. Chim. Acta, 112 (1979) 375. Dll L.-O. Ghman, Inorg. Chem., 27 (1988) 2565. D21 S. Sjijberg and LO. Ghman, J. Chem. Sot., Dalton Trans., (1985) 2665. 1131L.-O. Ghman and S. Sjoberg, Acta Chem. Stand., 37A (1983) 875. [141 D.K. Nordstrom and H.M. May, in G. Sposito (Ed.), The Environmental Chemistry of Aluminium, CRC Press, Boca Raton, FL, 1989. ml S.L. Simpson, S. Sjiiberg and H.K.J. Powell, J. Chem. Sot., Dalton Trans., (1995) 1799. [I61 P.L. Brown, R.N. Sylva, G.E. Batley and J. Ellis, J. Chem. Sot., Dalton Trans., (1985) 1967. Hawke and H.K.J. Powell, Anal. Chim. Acta, 299 [171 D.J. , . (19941 257. [181 UT. Hill, Anal. Chem., 28 (1956) 1419. [19] H.K.J. Powell and R.M. Town, Aust. J. Chem., 46 (1993) 721. 1201 S.L. Simpson and H.K.J. Powell, unpublished results.