Flow-injection analysis for aluminium with indirect amperometric detection

117

An&y&a Chrmca Acta, 256 (1992) 117-123 Elsevler Science Publishers B V , Amsterdam

Flow-injection

analysis for aluminium with indirect amperometric detection

Ahson J Downard, H Ktpton J Powell * and Shuanghua Xu Department of Chemrstry, Unruersrty of Canterbury, Pnvate Bag, Chmtchurch (Received

13th April 1991, revised manuscript

received

(New Zealand)

18th June 1991)

Abstract Alummmm was determined m a flow-llyection system mvolvmg the formation of the alummmmUII)-l,Zdihydroxyanthraqumone-3-sulphomc acid (DASA) complex at pH 9 0 and amperometrlc measurement of excess of DASA at +0 50 V on a gold electrode Electrode fouhng by adsorption of DASA oxldatlon products was mmumzed by use of a double pumpmg system to provide a reagent cycle and a wash cycle During the wash cycle the gold electrode surface was reactivated electrochemically by cathodic-anodic voltage cychng The hnear workmg range was governed by the DASA concentration For 2 x 10e5, 5 x 10m5 and 5 x 10e4 M DASA the hnear ranges were (0 05-l 0) X lo-‘, (0 l-2 2) x 10e5 and (0 l-l 2) X 10e4 M Al(III), respectwely The detectIon hnut when using 2 X 10m5 M DASA was 2 5 x 10e7 M Al(III) and the R S D for 9 x 10W6 M Al(W) was 4 3% (n = 6) Nitnlotnacetic aad, tannins and Fe(III) interfered, no interference was observed with F-, PO:-, citnc, oxalic and fulvrc acids, Mg2+ or Ca2+ The method was applied to the determmation of Al(II1) m soil extracts Keywords

Amperometry,

Flow system, Alummmm,

Soils

The phytotoxlc role of alummmm toward algae and fish m natural waters and toward plant rootmg systems m soils or hydroponic culture 1s well documented The incidence of Alzheuner’s dlsease and of dialysis encephalopathy has hlghlighted the toxic role of alummmm m human biology The determmatlon of alummmm at envu-onmentally or physlologlcally significant concentrations can be achieved by electrothermal atomic absorption spectrometry (AAS) [ll or by a variety of convenient spectrophotometnc methods 121 The sensitivity of spectrophotometrlc techniques may be enhanced by use of surfactants [3,41 Several colorlmetrlc reagents have been successfully adapted to flow-mjectlon analysis for alummmm(III), e g , Pyrocatechol Violet (PCV> 131, Erlochrome Cyanme R (ECR) 151, Xylenol Orange 161,Chrome Azurol S (CAS) [71 and Pyrogal101 Red 141 However, the wide mtrmslc workmg 0003-2670/92/$05

00 0 1992 - Elsevler

Science Pubhshers

range accessible by amperometrlc techniques and the low volume possible m amperometrlc cells makes the development of such methods attractive Both spectrophotometnc and amperometrlc techniques are susceptible to fouling, e g , adsorption of coloured matrix components or substrate on optical fibres or absorption cell wmdows, or adsorption of redox products on the workmg electrode surface, respectively However, the latter can often be rectified m a programmed manner by electrochemical cleamng As part of a project to develop and apply electrochemical sensors for the determmatlon of alummmm a method mvolvmg indirect amperometric detection m a micro flow cell 1s reported The reaction between alumimum ions and the redox active chelatmg agent l,Zdlhydroxyanthraqumone-3-sulphomc acid (DASA) was exploited Both DASA and its alurnmmm complex adsorb strongly on mercury, the shift m the DASA

B V All rights reserved

118

A J DOWNARD

reduction potential on complexatlon w&h alummmm ions 1s the basis of a sensltlve analytlcal method using cathodic strlppmg voltammetry [81 On mert electrodes, such as gold or graphite, oxldatlon processes may also be observed, although only uncomplexed DASA gave a dlstmctlve oxldatlon wave by cychc voltammetry (CV) under the condltlons used For gold this 1s at + 0 32 V (pH 9 0) The decrease m current for this wave m the presence of Al(II1) 1s the basis of the method reported

PVC block housing electrodes

PVC block housing

Inlet&outlet tublngs

(Au) ae (Pt) r e (AgIAgCI) we

EXPERIMENTAL

Reagents DASA (Hopkm and Wllhams) was used as supplied The source and isolation of the podzol B, fulvlc acid [9] and condensed tannin [lo] have been described All other reagents were of analytIcal-reagent grade and water was doubly dlstllled Alummmm standards were prepared from an acldlfied (pH 3) stock solution of KAl(SO,), 12H,O (1 X 10m3 M) Sol1 samples were supphed by DSIR (NZ) Land Resources Equipment Amperometrlc and voltammetrlc measurements m the flow cell were obtamed using a PAR 173 potentlostat coupled to a PAR 175 universal programmer and a Graphtec WX 1200 recorder The flow system consisted of an Ahtea C4-XV perlstaltlc pump (A), an Ismatec IPN-12 penstaltlc pump (B) and the flow manifold shown m Fig 1 The flow-rates for the carrier (Rl = water), reagent (R2 = DASA m 0 5 M ammonium ac-

Fig 1 Manifold for pulsed flow mjectlon Rl, water, R2 DASA m 0 50 ammonium acetate-ammoma, pH 9 0, R3, 0 1 M KC1 (for reference electrode), R4, 0 50 M ammonmm acetate-ammoma, pH 9 0, S, sample (25 rl), F, flow cell, W, waste

ET AL

ABC Fig 2 Diagram of the flow cell, dlmenslons x 32 mm Large holes for clampmg screws

_

Carrier Waste Reference electrolyte each half 10X20

etate-0 5 M ammonia, pH 9 0) and reference electrode electrolyte (R3 = 0 1 M KC11 and wash solution (R4 = 0 5 M ammonrum acetate-ammonia, pH 9 0) were 0 76, 1 19, 0 16 and 1 19 ml mm-’ respectively The reaction cod conslsted of 100 cm X 0 51 mm 1d kmtted mlcrolme tubing (Cole-Palmer) The rotary lqectlon valve had a 25-~1 sample loop, the dispersion coefficient (D) for the inJected solution was 5 9 and the time elapsed from mJectlon to detection was 6 s The detector (F) consisted of a wall-jet mlcroflow cell (Fig 2) fabricated from two blocks of PVC (10 X 20 x 32 mm) separated by a 0 76 mm Teflon spacer with 11 mm X 2 mm reaction channel (volume 17 ~1) The electrodes consisted of 0 5 mm diameter platinum (auxdlary), 0 55 mm gold (working) and 0 6 mm sliver wire (Ag/AgCl reference electrode) Procedure Prior to use, the electrodes were hand polished with l-pm diamond paste The workmg electrode m the assembled cell, with pump B on (A off), was then actwated electrochemlcally for 10 mm by voltage cycling, 0 50 + 0 10 V --) 1 0 --) 0 5 V, at 100 mV s-l An mdlvldual measurement mvolved loadmg 25 ~1 of standard or test solution [AI(III)I, switching pump A on (B off) and mJectmg the sample When the DASA solution reached

FIA

FOR ALUMINIUM

WITH

INDIRECT

AMPEROMETRIC

119

DETECl-ION

Fig 3 Current at gold electrode detector vs time for one cycle of reagent pulse (pump A on) and sample mJectlon DASA, 5 x 1K4 M m pH 9 0 buffer, Al(III), 2x 1O-4 M, 25 PI

the detector after flow from point C, an anodlc response was observed, wth the current quickly reachmg a plateau value Complexatlon of DASA by Al0111 produced a spike m the current vs time trace After 50 s, pump A was switched off, pump B on and a smgle voltage cycle used to activate the electrode for the next sample mjectlon A typical current-time trace IS shown m detail m Fig 3 To mmlmlze contact of DASA with the workmg electrode, pump A was operated only for sample InJectIon Baseline noise arises here (and m Fig 5) because no pressure compensation was employed m the manifold Sod analyses

Sol1 samples (2 5 g) were extracted with 10 M KC1 (25 ml) by the rapid (5 mm) extraction method [ll], followed by membrane filtration (0 45 pm> and centrlfugatlon The extract was diluted with buffer (100-1000 ~1 to 10 ml) and the Al010 concentration was determmed relative to a standard cahbratlon graph and also by the method of addltlons

graphite electrodes On graphite, an oxldatlon wave is observed for DASA at + 0 35 V (curve b), this wave dimunshed m the presence of Al(II1) (not shown) The gold electrode m buffer only (curve c) shows an oxtdatlon wave at ca 0 80 V, and reduction of surface oxides at ca 0 30 V on the return scan In the presence of DASA an oxldatlon wave was observed at 0 35 V (curve d) On neither electrode was there evidence for a separate oxldatlon wave for Al(III>complexed DASA However, the attenuation of the DASA oxldatlon wave at ca 0 35 V m the presence of Al(II1) can be monitored (curve e) Gold was chosen as the workmg electrode because of the ease of reactivation (see below) Figure 5 shows a typical output for pulsed flow injection involving electrochemical cleaning of the

MAI

+d b

0

V(vs

Ag AgCI)

la

RESULTS 6

0

Figure 4 gives the cyclic voltammogram for DASA (2 X lop3 M), recorded m the flow cell under stop-flow condttlons, in the presence and absence of Al(II1) (1 x 10m3 M) m 0 5 M ammomum acetate-ammoma, pH 9 0, on gold and

.

.

V(vs

.

Ag AgCI)

.

0

K)

l+g 4 Cyclic voltammogram (50 mV s-l) for DASA (2X 10e3 M) and DASA+ AK1111 (1 X 10K3 M) m 0 40 M ammomum acetate-ammoma buffer (pH 8 6) on (a, b) graphite and (c-e) gold electrodes (a and c) Buffer only, (b and d, DASA, (e) DASA+Al(III)

120

A I DOWNARD

a

b

c

d

f

a

c

9

ET AL

I

-b -0 I

50

25 105x

75

4

100

[Allrnll

Fig 6 Lmear workmg ranges for DASA concentrations 2x lo-‘, (b) 5 x 10e5 and (cl 5 x 1O-4 M

of (a)

with various

l+ 5 Signals for pulsed flow mjectlon of 25 ~1 of Ai(III) mto 5 x 10m4 M DASA m ammonium acetate-ammoma buffer [Al(III)] (a) 005 (b), 0 1OX1O-4, (cl 0 20x10m4, (d) 050x 10~4,(D20~10~4, (g)50~10-~ M

working electrode between each reagent pulse/ mjectlon, Fig 5 refers to (0 5-50) x lo-’ M Al(III) standards with 5 X 10e4 M DASA (dllutlon factor = 164) The hnear workmg ranges (correlation coefficients R > 0 999) for pulsed flow inJection are shown dlagrammatlcally m Fig 6 For 2 X lop5 M DASA the detection limit, measured as 2a for eight replicate measurements for 1 X lop6 M Al(III), was 2 5 X lo-’ M For 2 X 10e5 M (or 5 X 10m4 M) DASA and 9 x lop6 M (or 1 X low4 M) Al(II1) the R S D (n = 6) was 4 0% (or < 1%) Interferences

Interferences were studied by pre-equlhbratlon of an Al(II1) standard solution (2 X 10d5 M) TABLE

concentrations of mterferents, before mjectlon using 5 x 10e4 M DASA In all instances (except for Na+, K+, Mg’+, Ca’+, which showed no interference up to 5 X low3 M), the level of interference was decreased on Increasing the pH of the buffer solution (R2) from 7 0 to 9 0 Thus, at pH 8 2 (9 0) the mterferences from 2 X lo-’ M F- and citrate were -38% (0%) and - 20% CO%), respectively At pH 9 0, fulvlc aad, oxalic acid and PO:- did not interfere up to 25 mg kg-*, 2 x 10e4 M and 1 x 10e4 M, respectively The more strongly complexmg hgands mtrdotrlacetlc acid (NTA) and tannin (an eplcatechm polymer) gave posltlve mterferences at pH 9 0 (2 X lo-’ M, + 46%, 25 ppm, + 30%, respectlvely) For Fe(II1) (2 x 10e5 M) the interference was +63% at pH 90 Sol1 analyses Data are presented m Table 1 and compared with values reported for the ahzarm chemically modlfled electrode (CME) method [12] and for the 16-h extraction-AAS method

1

Exchangeable Sod type

(1 M KC1 extractable) Description

alumnuum

m sods

a

Depth (cm)

Alumnuum

(mg kg-‘)

DASA flow mjectlon Takahe Takahe Takahe Summit Walkarl Walklwl Bossu Mangatepopo Te Rapa Motomaoho

Sdt loam,yge Sdt loam,yge Sdt loam,yge Slit loam,ybe Clay,ybe Sdt clay loam,ybe Sdt loam,ybe Podzohzed yb pumice Peaty sdt loam Sdty peat

a yge = Yellow-grey earth, ybe = yellow-brown earth from method of addmon ’ Ref 12 d 16-h extraction

O-26 26-50 > 90 15-30 17-39 55-92 o- 75 10-20 o- 75 o- 75

69 101 150 162 1261 89 272 100 97 74

(169)

(92) (292) (103) (111) (100)

Ahzarm

bc

AASd

b (81) (118) (197) (145) (1210) (119)

b 5 mm extractIon [ill, values from cahbratlon Data supplied by DSIR(NZ) Land Resources

104 76 225 190 1215 117 297 81

_ graph

Values m parentheses

FIA FOR ALUMINIUM

WITH

INDIRECT

AMPEROMETRIC

DETECTION

DISCUSSION

A separate oxldatlon wave was not observed for Al(III)-complexed DASA on gold or graphite electrodes This IS m contrast to the Al(III)ahzarm [12] and Zr(IV)-DASA 1131 systems, for which the complexed hgand has an oxldatlon wave ca 0 35 V posltlve of that for the hgand (on graphite electrodes) However, the DASA oxldatlon current decreased linearly with increasing AKIII) concentration, and this was the basis of the method reported Application of this method requires the selection of a suitable DASA concentration to provide lmearlty of response for standards and test solution (Fig 6) Oxldatron of DASA leads to a rapld deactivation of the gold or graphite electrode surface, presumably by adsorption of polymerized oxldatlon products However, as reported by Johnson and co-workers [14-161, a gold (or platinum) electrode may be reactivated by voltage pulsmg mvolvmg an mltlal oxldatlve (or reductive) desorptlon of reaction products (and oxldatlon, or reduction, of the noble metal surface) followed by reduction (or oxldatlon) of the catalytic electrode surface In this study a single cathodlc-anodlccathodic cycle, (0 50 --) 0 10 + 10 + 0 5 V) at 100 mV s-l was most effective m retammg electrode activity The use of lower potentials (0 80 or 0 90 V) was slgmflcantly less effective An anodlccathodic-anodlc cycle was also less effective, and similar to the effect of chemical cleaning by mjection of NaOH (0 2 M) or flushing (5 mm) with ammonium acetate (pH 7 0) or ammonium acetate-ammonia (pH 9 0) at open circuit or with an applied potential of 0 50 V Importantly, this reactlvatlon is effected under pumping and m the absence of DASA (pump A off, B on) Followmg measurement at 0 50 V, the cathodlc-anodlc voltage scan traverses the potentials for reduction of the oxldlzed gold surface, then oxtdatron of the surface to Au0 m the alkaline buffer (Frg 4c) Desorption of orgamc oxldatlon products may be effected by substrate reduction at 0 1 V or by reduction of the electrode surface, after this electrode cleaning, actlvation is affected by oxldatlon of the clean gold surface This indicates that AuO, rather than

121

gold, 1s the active surface for DASA oxldatlon An alternative procedure involved the use of double mjectlon, 250 ~1 of AUII) solution m line Rl (carrier = water) and 25 (~1 of DASA m hne R2 (carrier = buffer) Electrode activation was as described above, but pump B was not required The output for this procedure, current vs time, was m the form of anodlc signals arising from the oxldatlon of excess (non-complexed) DASA However, a narrower linear range was achieved and a systematic decrease m electrode sensltlvlty was observed across rephcate measurements Loss of electrode activity resulting from injection of DASA only was easily rectrfled by voltage pulsing However, when AlUII) solution was also mJetted to give a small zone of DASA nested m a larger zone of Al(III), there was a progresswe loss of electrode actlvlty (ca 2% per mjectlon) This was ascrlbed to a slow build-up of polymeric AKIII) hydroxo species on the electrode surface, resulting from hydrolysis m the alkahne carrier solution Further, a narrower linear range was achieved In contrast, when pulsed flow mjectlon was used [25 ~1 of Al(W) solution] the electrode activity was completely restored by a smgle voltage cycle Comparison with spectrophotometnc FL4 methods The detection hrmt of 2 5 X 10e7 M Al(II1) compares favourably with values reported for PCV (1 1 x 10e7 M [17l or 2 x 10m7 M [18]), CAS (3 x lop7 M) [7], qumohn-&ol-chloroform (5 X 10e7 M) 1191 and ECR (5 X 10m6 M) [20] However, the spectrophotometrlc detection lnmt can be lowered by a factor of ca 10 by use of surfactants [5,21] or by ca 100 by fluorescence measurement (of the qumolm-8-01 sulphonate) m the presence of surfactant [22] The hnear workmg range was snndar to that reported for spectrophotometrlc flow-mJectlon methods [5,17], but this 1s a result of the sample loop size and the concentration of complexmg agent The major drawbacks of the proposed method are the comparatively low sampling rate (ca 60 h-‘) and the mablhty to mask Fe(II1) (which 1s readily accomplished in spectrophotometrlc methods by use of hydroxylamme-l,lO-phenan-

A J DOWNARD

122

throhne [5,171 or ascorbic acid [7,20,231) An advantage of the method reported here accrues from use of a higher pH, thrs counters the mterference of some competing hgands that Interfere m spectrophotometrlc methods at lower pH [17l Further, sample turbidity and colour do not affect the analysis Znterferences The high affinity of alummum ions for OH-

and 1,Zdlhydroxyaryl hgands means that these donor groups may compete effectively against many other hgands [24,251 Thus, m the alkaline buffer, OH- counters possible interference from dl- and trlcarboxylate hgands, carboxylate polymers (fuhc acid), F- and PO:- Only the more strongly complexmg tannin and NTA hgands mterfere Then competition against OH- (and the 1,Zdlhydroxyaryl hgand DASA) was expected to effect a negative interference The observed posltlve interferences m each instance resulted from direct oxldatlon of the competing hgand (established from separate measurements) Interference from Fe(II1) arose from direct complexatlon mth DASA Attempts to mask this Interference by reduction of Fe(II1) (with ascorbic acid) and stablhzatlon of Fe(U) as the 2,2’-blpyrldyl or l,lO-phenanthrolme complex were not effective because of oxldatlon of the Fe(U) complex to Fe(III)-DASA at the measuring potential However, for sod extracts this interference will be mnumal (see below) Sol1 analyses The data presented m Table 1 indicate that the Al(III) concentration measured for sod extracts by the DASA flow-inJection method approxlmates to the “total” alummlum measured by AAS Interference from Fe0111 was not expected for an-dried soils (but may be problematic for field-moist anaerobic sods or for anoxlc sedlmerits) Measurement of Fe(II1) m the sod extracts by the ascorbic acid-2,2’-blpyrldyl method established [Fe(III)I/[Al0II)I d 0 015 Small dlfferences between the AAS and DASA values can arise from sample vanability, the contrlbutlon of colloidal species to the AAS value and mterference by strongly bmdmg organic hgands m the

ET AL

DASA method The last effect was expected to be small because of the low solublllty of sod organic polymers m the KC1 extractant and the low incidence of 1,Zdlhydroxyaryl moieties m these polymers [26] To test this point, a series of high-carbon soils was exammed BOSSU, Mangatepopo, Te Rapa and Motomaoho, contalmng 5 9, 9 2, 21 and 24% carbon, respectively Only for the last sod was the AN110 concentration measured agamst the cahbratlon graph slgmflcantly different from that obtamed by the methods of additions (Table 1)

REFERENCES

1 D C Mannmg, W Slavm and G R Carnrtck, Spectrochun Acta, Part B, 37 (1982) 331 2 KL Cheng, K Ueno and T Imamura, Handbook of Orgamc AnalytIcal Reagents, CRC, Boca Raton, FL, 1982 0 Royset, Anal Chum Acta, 178 (1985) 223 C Wyganowslo, Mlcrochem .I, 26 (1981) 45 0 Royset, Anal Chem , 59 (1987) 899 M TroJanowlcz and J Szpunar-Lobmska, Anal Chum Acta, 230 (1990) 125 7 D Zoltzer and G Schwedt, Fresemus Z Anal Chem, 317 (1984) 422 8 CM G van den Berg, K. Murphy and J P Riley, Anal Chum Acta, 188 (1986) 177 9 J E Gregor and H KJ Powell, J So11 SCI , 37 (1986) 577 10 H K J Powell and A W Rate, Aust J Chem , 40 (1987) 2015 11 E A Close and H K J Powell, Aust J Sod Res , 27 (1989) 681 12 A J Downard, H KJ Powell and Xu S, Anal Chum Acta, 251 (1991) 157 13 H E Zlttel and TM Florence, Anal Chem , 39 (1967) 320 14 S Hughes, P L Meschl and DC Johnson, Anal Chun Acta, 132 (1981) 1 15 D S Austm-Harnson and D C Johnson, Electroanalysa, 1 (1989) 189 16 DC Johnson and W R La Course, Anal Chem, 62 (1990) 589A 17 0 Royset, Anal Chum Acta, 185 (1986) 75 18 R L Benson, P J Worsfold and F W Sweetmg, Anal Chum Acta, 238 (1990) 177 19 H K.J Powell and K L Shearman, unpubhshed results 20 P W Alexander and A Hldayat, paper presented at the 9th Austrahan Symposmm on AnalytIcal Chemistry, Sydney, 1987 21 C Wyganowslu, S Motomlzu and K Toel, Anal Chum Acta, 140 (1982) 313

FIA FOR ALVMINIUM WITH INDIRECT AM~ROME~I~

DETECTION

22 J I G Alonso, A I. Garcta, A Sanz-Medel and E B Gonzales, Anal Chum Acta, 225 (1989) 339 23 B F Rels, H Bergamm, E AG Zagatto and FJ Krug, Anal Chum Acta, 107 (1979) 309 24 A E Martell, R J Motekaltls and R M Smith, Polyhedron, 9 (1990) 171

123 25 D K Nordstrom and H M May, m G Sposito (Ed ), The Environmental Chemistry of Alummmm, CRC, Boca Raton, FL, 1989 26 H K J Powefl, unpubll~ed results