Determination of traces of aluminium with chrome azurol S by solid-phase spectrophotometry

Tuhra, Vol. 40, No.7, pp. 1059-1066, 1993 Printed m Great Bntain. All rightsreserved

0039-9140/93 56.00 + 0.00

Copyright0 1993Pergamon Press Ltd

DETERMINATION OF TRACES OF ALUMINIUM CHROME AZUROL S BY SOLID-PHASE SPECTROPHOTOMETRY A. MOLINA-DIAZ,* J. M. H~~~R-MA~scAL

WITH

and M. I. PASCUAL-FWWERA

Department of Analytical Chemistry, Faculty of Experimental Sciences, University of Granada, 23071 Jaen, Spain L. F. CAPITAN-VALLVEY Department of Analytical Chemistry, Faculty of Sciences, University of Granada, 18071 Granada, Spain (Received 6 August 1992. Revised 7 December 1992. Accepted 7 December 1992)

Summary-A microdetermination method at sub-pg/l. level for aluminium by solid-phase spectrophotometry has been developed. Chrome axurol S was used as chromogenic reagent to form a blue complex which was easily and strongly sorbed and concentrated on a dextran-type anion-exchange resin. The resin-phase absorbances at 615 and 800 nm were measured directly. ~~~~ can be determined in the 0.640 rg/I. range with a RSD of 2.1%. The method is applied to the dete~ination of al~nium in micaschist, natural and tap water samples.

The potential toxicity of aluminium in man is universally recognized. A series of clinical disorders, including dialysis dementia, has been associated with its accumulation in several tissues (brain and bone principally) in renal-failure patients undergoing regular hem~a1ysis.l It has been suggested’ that alumini~ increases the pe~eability of the blood-brain barrier which may lead to dementia or other neuronal disorders. Aluminium is present at approximately 1.4 ppm or 100 mg Al/70 kg man and it is ingested with food; the approximate daily dietary intake is 36.4 mg2 varying with degree of dietary exposure. Aluminium ions in our diet are completely non-bioavailable from the small intestine because the aquated charged ions are not able to penetrate the lipid protein membranes of the duodenal mucosa and thus pass on into the bloodstream.3 On the other hand, it can be assumed that at least part of human dietary aluminium intake is in the form of chelates with natural food components such as citric, lactic and oxalic acids.4 In these chemical forms, Slanina et ~1.~showed (working with rats) that aluminium is bioavailable. In moderate amounts, aluminium does not exert prejudicial effects in man. It is used in the treatment of tap waters. On the other hand, it has been shown

*Author for correspondence.

that aluminium is toxic to fish’. Hence a great deal of attention to the determination of this element has been paid recently. Numerous binary complexes of Al(II1) with (a) azo reagents, (b) xantene reagents and (c) ~phenylme~e reagents [eriochrome cyanine R, pyrocatechol violet and chrome azurol S (CAS)] have been used for s~trophotometric determination of aluminium. They exhibit moderate sensitivity6 but it can be increased by addition of a catidnic surfactant such as cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, cetylpyridinium chloride, cetylpiridinium bromide, tetraphenylchloride and zephiramine.’ phosphonium However the spectrophotometric determination of aluminium at sub-kg/l. level requires a step of precon~ntration. Nevertheless, solid phase s~trophotome~ (SPS) enables dete~ining analyte concentrations at this level without requiring an expensive instrumentation combining the measurements of solid-surface absorbance with using a solid support to preconcentrate selectively the analyte and measuring the absorbance directly in the solid phase with the aid of a chromogenic reagent.R9J0 Detection limits as low as 0.09 c(g/l. have been reported.” The aim of this paper has been to develop a sensitive and selective method for the spectrophotomet~c dete~nation of alder

A. MOLINA-DIAZet al.

1060

with CAS by selective enrichment of the metal as Al(III)-CAS species on a dextrane type resin and measuring of the absorbance in resin phase, e.g. by using SPS. The proposed method has been satisfactorily applied to the determination of aluminium in natural and tap water samples, and other matrices, such as micaschist and enables determining of this element at sub-pug/l. level.

Absorbance measurements The absorbance of the complex species sorbed on the resin was measured in a 1 mm cell at 615 (corresponding to the absorption maximum of the coloured species) and 800 nm (in a region where only the resin absorbs light). The net absorbance (A,) for the complex was calculated from:8 A, = As,s - Asoo,

EXPERIMENTAL

Reagents All reagents were of analytical-reagent grade and the water was doubly distilled. Sephadex QAE A-25 Zon exchanger, (Aldrich), anion-exchange resin was used in the chloride form in original dry state as obtained from the supplier and without pretreatment. Aluminium (ZZZ) standard solution 1.000 g/l., prepared by dissolving 17.5820 g of aluminium potassium sulfate dodecahydrate AlK(SO& . 12H,O (Merck) in doubly distilled water containing 5 ml of concentrate. H$O, and diluting the solution with doubly distilled water to 1000 ml. Aluminium (III) working solutions were made in situ by dilution with doubly distilled water. Bufer solution of pH 4.60, was made by dissolving 35.05 g of hexamethylenetetramine (HMTA) and 45 ml of HCl 4M in 1000 ml of doubly distilled water and adjusting the pH with a few drops of HCl 4M or NaOH 4M. This solution was stored under refrigeration. Chromazurol S solutidn of various concentrations, prepared by dissolving the necessary amount of the dye in doubly distilled water. This solution was stored under refrigeration. In this conditions the solution is stable for at least one month. Apparatus A GBC 911 microcomputer-controlled UVVIS spectrophotometer with glass cells (l-mm optical path length) was employed for all spectral measurements. The spectrophotometer was controlled by a Bravo AST/80286 microcomputer connected by means of a serial port. A Comx PL80 plotter was used for graphical representations. The pH measurements were made with a Crison Model 2002 pH-meter fitted with a glass-saturated calomel electrode assembly and a temperature probe. An Agitaser 2000 rotating agitator was also used and a centrifuge Selecta model S-240.

(1)

where: A6,5 = Adls -A,,, and Asoo= As8M)A b8ooand Asxxx and AbXXXare the absorbances of the sample and the blank (cell packed with resin equilibrated with blank solution), respectively at the indicated wavelength. In fact, the observed absorbance, A, at a given wavelength is obtained by:’ A = A, + &,n + AR + A,,,, where A, represents the absorbance of the complex species sorbed on the resin, AwInthat of the interstitial solution between the resin beads (it can be neglected), A, that of the resin N 1.000) and A,, that of the background (A RB,,,, reagent in the solid phase. The packing of the resin in the cell beads affects the values of A,, AR and ARL, but when the absorbance is measured at two different wavelengths, one corresponding to the absorption maximum of the coloured species (615 nm) and the other in a region where only the resin absorbs (800 nm), the absorbance difference, A,,, - Aaoo, can be assumed to be constant under the similar packing conditions. On the other hand, if the absorbance of a blank resin is measured at the same two wavelengths, the absorbance difference, AMI5- Absw, allows us to estimate the absorbance of the complex, A, (that is related with the concentration of analite in the solution as it was shown by Yoshimura)’ from the equation (1). Procedures (I) A 10 ml sample solution containing 20-200 pg/l. (0.74-7.4 pmole/l.) of aluminium was transferred into a 20-ml glass tube with stopper containing 4 ml of 1.5 x lo-’ M CAS solution and 1 ml of pH 4.60 HMTA buffer solution (total volume = 15 ml). After 10 min, 30 mg of Sephadex QAE A-25 resin were added. The mixture was shaken mechanically for 5 min after which the resin beads were collected by suction with the aid of a pipette and packed into a 1 mm cell together with a small volume of the aqueous solution. Then, the cell was centrifuged

Determination of traces of aluminium with chrome azurol S

for 1 min at 5000 rpm. A blank solution containing all reagents except aluminium was prepared and treated in the same way as described for the sample. The absorbances (AS615, As@, Awls and Am) were measured as described under “Absorbance measurements” 30 min after collection of resin. The calibration graph was constructed in the same way using aluminium solutions of known concentration. (II) A 100 ml sample solution containing 2.0-26 pg/l. (0.074-0.96 pmole/l. of aluminium was transferred into a l-l. polyethylene bottle containing 10 ml of 1.5 x 10e4 M CAS solution, 5 ml of pH 4.60 HMTA buffer solution and, after 10 min, 30 mg of Sephadex QAE A-25 resin were added. The mixture was shaken mechanically for 15 min after which the resin beads were collected by filtration and with the aid of a pipette, packed into a l-mm cell as above procedure, and the absorbance measurement was carried out in the same way. (III) A 500-ml sample solution containing 0.7-5.0 pg/l. (0.026-0.185 pmole/l.) of aluminium was transferred into a l-l. polyethylene bottle containing 20 ml of 1.5 x 10m4 it4 CAS solution and 25 ml of pH 4.60 HMTA buffer solution. After 10 min, 30 mg of Sephadex QAE A-25 resin were added. The mixture was shaken mechanically for 110 min. The absorbance of the coloured species was measured as the above procedures. (IV) A 1000 ml sample solution containing 0.6-4.0 pg/l. (0.022-0.148 pmole/l.) of aluminium was placed in a 2-1. container of polyethylene containing 5 ml of 6.3 x 10m4 M CAS solution and 50 ml of pH 4.60 HMTA buffer solution. After 10 min, 30 mg of Sephadex QAE A-25 resin were added. The mixture was shaken mechanically for 125 min and the absorbance measured as described.

1061

Waters. Natural waters were filtered through a filter paper with a pore size of 0.45 pm (Millipore), preserved with concentrate HN03 (0.25 ml/l. of water) and stored in a polyethylene container. The samples were stored at 4°C until analysis. Analysis were performed with the least possible delay. The usual general precautions were taken to avoid contamination. The tap water was analysed without pretreatment. Distribution measurements

CAS, buffer solution and 30 mg of Sephadex QAE A-25 resin were added to an aqueous solution containing 740 nmole of Al(II1) and the solution (100 ml) was stirred for 60 min. The equilibrated solution was separated from the resin. Afterwards, the solution was treated in the same way with a further batch of resin (30 mg) and the aluminium left in that was determined as described under “Procedures”. The distribution ratio D @mole of aluminium sorbed per gram of resin/pmole of Al(II1) per ml of solution) was calculated from the initial and equilibrium concentration in the solution. An average value of D = 23,400 and S.D. = 600 was obtained from five replicates experiments. RESULTS AND DISCUSSION

Absorption spectra in resin phase

CAS reacts with Al(II1) to originate a blue complex in solution in the pH range 3.5-7.5. Between pH = 2.5 and 8.0 this complex is sorbed on an anion-exchange resin showing the absorption maximum at 615 nm and reaching the absorbance the highest value at pH about 4.6 (Fig. la-e). The spectrum of this complex at above pH in aqueous solution shows (Fig. If) much lower absorbance than in resin phase: the increasing of sensitivity in resin-phase is selfevident.

Treatment of samples Analysis of micaschists. A suitable weight of

sample (generally 50-100 mg) was treated in a nickel crucible with 2 g NaOH in the usual way. The melt was leached with distilled water with warming and poured into 250 ml HCl 0.44M and then distilled water added up to 500 ml (this solution was stored in a polyethylene container carefully cleaned with nitric acid and the samples were stored at 4°C until analysis). For the later process this solution was diluted 1: 100 with addition of 0.4 ml NaOH 0.4M per ml of initial solution.

Optimization of variables pH dependence. The optimum pH in the solution phase for the formation of the blue species on the resin falls in the range 3-5. At pH values below 3 and above 5 the absorbance value at 615 nm decreases significantly. We chose pH 4.60 as optimum pH value for the experiences. The pH can be following satisfactorily adjusted by addition of a 0.25MHMTAIHCl buffer solution. Other buffers (acetic acid/acetate, formic acid/ formiate) decreased the absorbance in respect to

Fig. 1. Net absorption spectra of Al(IKI)-CA§ species: (a)-(e) on the resin (resin as reference). [CAS] = 1.40 x low5 M, [Al(W)] = 1.48 x 10m6M, 50 mg of resin, 1 mm optical path length, sample volume 100 ml, stirring time 15 min: (a) pH=4.0; (b) pH -4,5; (c) pH= 5.0; (d) pH=5.5; (e) pH = 6.0; (r) in aqueous solution, [CAS] = 9.0 x lo-$ M, [AElI)] = 9.0 x W4 An, 10 mm optical path length, pH = 5.0.

HMTA/HCl buffer (80 and 26%, respectively for au aluminium concentration of 40 &g/i. using 50 mg of resin and a final volume of 100 mf), Reagent ~~n~~~~r~~~n_Absorbance increases with an increasing CAS concentration, a plateau use

occurring from 1.2 to 2.5 x lOIS (for 100 ml sample). It seems that the excess of free ligand competes with the complex for sorption on the resin phase and it originates these plateaux_ The optimum molar ~CAS~Al~III)] ratio necessary was 8, 13,40 and 53 {for 15, 100, 500 and 1000 ml sample volumes, respectively). 4.0 x 10-5, 1.5 x 10m5, 6.0 x 10W6 and 3.15 x 10W6A4 reagent concentration were chosen for 15, 100, 500 and 1000 ml sample volumes, respectively. O&r ~~~E~~rn~~~~~ ~~~~~r~~~. The optimum stirring times are 5, t 5, 110 and 125 min for 15, 100, 500 and 1000 ml sample volumes, respectively (Fig. 2). The fixed complex is stable for at least ‘7 hr after equilibration. The order of addition of the reagents affects signi~~antly the results obtained, the order used was: reagent, buffer, aluminium, resin. The use of a large amount of resin (m,, g) reduces as usual”” the absorbance values. Absorbance decreases according to the equation: (r = 0.3987) A c = -0.014 -i- 0.0224~~~ (Fig. 3). 30 mg is in the minimum amount of dry resin that gives the highest absorbance and greatest ease of handling. N&E

of

ffie Jixed

eompfex

The composition of the fixed Al(III)-CAS complex on Sephadex was established at the

A

Fig. 2. Stirring time dependence on color development in resin-phase. 30 mg of resin, pH = 4.4, 0.074 pmole of Al(II1). (a) MO ml of sample, [CAS] = 1.5 x IO- M. (b) 500 ml of sample, [CAS] = 6 x 10e6 M. (c) 1000 ml of sample, [CAS] = 3.15 x 10V6M.

1063

Determination of traces of aluminium with chrome azurol S

0

5

10

15

20

25

30

35

l/mass of resin (l/g) Fig. 3. Resin amount dependence. [CAS] = 1.5 x 10m5M, [Al(III)] = 3.7 x lo-‘, pH = 4.60, 1 mm optical path length, sample volume 100 ml, stirring time 40 min.

working pH of 4.6 using the Job’* and the equilibrium shiftI methods. Job method showed a ratio CAS/Al(III) = 2. Moreover, the graph of log (AI&, -A) us. log [CAS] (equilibrium shift method) gave a slope of 2.087 (r = 0.9969). Consequently, results indicated that a 2: 1 [CAS : Al(III)] anionic complex is fixed on the anionic resin. These results agree with those found in aqueous solution.‘4 Analytical data Calibration and precision. The calibration graphs are linear in the concentration ranges: 20-200 g, 2-26 g, 0.7-5.0 and 0.6-4.0 fig for 15, 100, 500 and 1000 ml samples, respectively. The analytical parameters are shown in Table 1. The reproducibility was established using 15, 100, 500 and 1000 ml sample solutions with a Al(II1) concentration of 107.0, 12.5,4.0 and 2.0 pg/l ., respectively. It was possible verifying that

the reproducibility is improved when the cells packed with the resin are centrifuged, during 1 min at 5000 rpm, previously the spectrophotometric measurements. The relative standard deviation (RSD) was 4.4% without centrifugation for 100 ml sample and 10 determinations. With centrifugation the absorbance values increase about a 12% and the RSD decreases at 3.0%. For 15 ml sample volume it was studied the effect of adding the resin in a suspension (5 ml) of 3.00 g in 500 ml of distilled water. The RSD increases at 5.3%, but it offers the advantage of a quicker and easier measurement of the amount of resin because it was not necessary weigh it. Sensitivity and detection limit. In Table 2 the sensitivity, expressed as apparent molar absorptivity, of the proposed procedures is compared with that of spectrophotometric procedures (including extractive procedures and formation

Table 1. Analytical parameters Volume sample (ml) Intercept Slope (l.lpg) Linear dynamic range (pg/l.) Correlation coefficient RSD (%) Detection limit (K = 3) (&I.) Quantification limit (K = 10) (pg/l.)

15

100

500

1000

-0.041 0.0108 20-200 0.9962 2.2 1.11 3.71

0.025 0.0529 2-26 0.9979 3.0 0.51 1.70

-0.015 0.1782 0.7-5.0 0.9947 3.7 0.20 0.67

0.025 0.2528 0.6-4.0 09984 2.1 0.14 0.47

1064

A. MOLINA-DIAZ et al. Table 2. Comparison of sensitivity of some aluminium methods Molar absorotivitv

Reaaent Chrome axurol S* Pyrocatechol violet? Eriochrome cyanine R* Eriochrome cyanine R/Cetylpiridinium chloride*# Chrome axurol S/Zephiramine*$ Chrome axurol S/Cetylpiridinium chloride*$ Eriochrome cyanine R/Zephiramine*# Chrome axurol S (15 mlH Chrome axurol S (100 ml)# Chrome axurol S (500 mix Chrome axurol S (1000 ml)!j

5.2 x 6.3 x 6.5 x 1.15 x 1.22 x 1.22 x 1.24 x 2.92 x 1.43 x 4.81 x 6.83 x

10’ 10’ 10’ l(r 105 1w 10’ lodll 10’11 1071] 10’11

Ref. 17.18

23 le52 l4 24 a, 24 This This This This

paper paper paper PaPer

*Solution methods. tExtractive procedures. $Ternary complexes. §Solid-phase spectrophotometry. l]Apparent molar absorptivity: absorbance value of the complex sorbed on the resin from a 1M aqueous solution of Al@) and measured in a 10 mm optical path length cell.

of ternary complexes with surfactants) described in the literature. It is shown that SPS methodology using CAS gives a very noticeable increase in sensibility in relation to the solution and extractive methods. The e$ect of volume on sensitivity. In SPS methodology the sensitivity can be enhanced by increasing the sample volume to be analysed. this effect may be calculated by measuring the absorbance of the resin equilibrated with different volumes of solutions containing the same concentrations of Al(II1) and optimum

established amounts of the other reagents. It is observed that a tendency of absorbance is to be independent of volume at higher volume values as usually in SPS”” (Fig. 4). The increase of sensitivity using a larger sample volume can be calculated, in practice, form the slope of the calibration graphs. The calculated values of the sensitivity ratio for the samples analysed here are: S,,/S, = 1.42, &mo/S,ao= 4.79 and S,,,,x,/S,s= 23.45, where the subscripts represent the sample volume (ml).

1.41,2l.O008OS6on4oa2-

00’ '0

260

600

760

1000

1250

1600

Volume of sample (ml) Fig. 4. Effect of volume on sensitivity. [CASj = 3 x 10-$ M-2.5 x 1O-6 M, [Al@)] = 1.48 x lo-’ M, pH x 4.60, amount of resin = 0.030 g, stirring time = U-150 min.

Determination of traces of aluminium with chrome azurol S

The standard deviation of the absorbance of the blank measured for 10 determinations (average values: 0.129, 0.373, 0.761, 0.869) were 0.004, 0.009, 0.012, 0.012 absorbance units for 15, 100, 500 and 1000 ml sample volumes, respectively. The IUPAC detection limit (K = 3)15and the quantification limit (K = 10)16 were calculated for 15, 100, 500 and 1000 ml sample volumes (Table 1). On the other hand, the interference level, can be reduced by diluting the samples, taking into account the sensitivity of the method proposed and the dependence of the sensitivity on the sample volume. Eflect of foreign ions In Table 3 the effect of various potentials interferent species, commonly found in the water, on the determination of 12.5 pg/l. of Al(II1) is listed. This effect was investigated by adding a known amount of the test ion to the aluminium solution. First a 20,000 ,ug/l. level of potentially interfering ions were tested and if the interference occurred, the concentration of interferent was reduced progressively until interference ceased. Higher concentrations were not tested. Tolerance level is defined as the foreign ion concentration that produces not more than &5% spectrophotometric error in the recovery of Al(II1). The most severe interference is caused by iron. It causes positive error because the iron forms a complex with CAS and this complex absorbs at the wavelength used in the procedures. The use of 1, IO-phenanthroline, after reduction with hydrogen in acidic medium, suppresses the iron interference up to 1500 pg/l. [for total Iron: Fe(I1) + Fe(III)]. The cationic complex Fe(II)1,lo-phenantroline remained in solution and it was not sorbed on the resin. On the other hand, the high tolerance level for Cu(I1) is probably due to the presence of HMTA as buffer. With Table 3. Effect of foreign ions on the determination 12.5 pg/l. of aluminium Foreign ion or species

of

Tolerance level (pgcgll.)

Cu(II), Mg(II), COTS, NO, Ca(II), Ni(II), Pb(II), Zn(I1) S*O,-2 H&Ji Fe(II), Fe(III)+ SOi* F-, EDTA Fe(II), Fe(II1) *In the presence of l,lO-phenanthroline, with hydrogen in acidic medium.

20,000 10,000 5000 2000 1500 1000 40 < 10 after reduction

1065

Table 4. Analytical applications

Watefl Rio Frio Jakn

Ubeda

Added (kg/l.)

Found* (&I.)

0.00 2.22 4.44

3.51 5.69 7.94

0.00 4.00 8.00

6.86 10.66 14.12

0.00 4.00 8.00

8.96 13.10 16.18

Average recovery (%) 98.3 99.1

Found by ICP k/l.) 3.3 6.0 8.5 6.5 11.6 13.1

9;2 95.0 101.1 95.4

9.5 12.3 17.3

Micaschist$

Found* (%)

Found by MS (%)

15 ml method

12.59 f 0.32

12.50

*Average values of three determinations. tstandard addition calibration graph method, 500 ml sample. $Qandard calibration graph method. 15 ml sample. Composition of micaschist: 12.50% Al, 4.19% Fe(II), 1.36% Fe(II1) and 0.65% Ti.

respect to the interference from anions F- and EDTA, it can be attributed to the complexation of aluminium. Moreover, in the case of the anions in general, the existence of competition for the anionic sites of the resin is probable. Analytical applications The proposed method has been applied to the determination of aluminium content in natural waters (tap waters and raw waters) and a mineral (micaschist). Natural waters. The method was applied to the determination of aluminium in water samples by standard addition calibration graph method (Table 4). (a) Tap water from Jatn city and Ubeda city (JaCn province); (b) raw water from the spring of Rio Frio (Los Villares city, Jdn province). The loss of sensitivity caused by the matrix effect could be evaluated from the ratio of slopes of the standard addition calibration graph and the standard calibration graph: the ratios were 0.476 (from Jdn), 0.565 (from Ubeda) and 0.555 (from Rio Frio). It is seen that the matrix effect in these waters is noticeable. We attribute this strong matrix effect to the high content of, principally, Ca(II), Mg(I1) and HCO; in these waters, because they are very hard waters. Mineral. The method was applied to the determination of aluminium in mineral samples (micaschist) by standard calibration graph method (Table 4).

to66

A. Mom-~

Ackmwledgements-The authors express their thanks to DiPutacibnn Provincial de J&n and to Dira&bn Gancral da Univtx&ladcs e Investigaci6n de la Junta de Andalucia (Spain) (Anmral Grant No. 1066) for lhuurcial support.

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