Modification of the solochrome violet RS method for the cathodic stripping voltammetric determination of aluminium

Analynca Chmuca Acta, 262 (1992) 339-343

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Elsevler Sctence Pubhsbers B V , Amsterdam

Modification of the Solochrome Violet RS method for the cathodic stripping voltammetric determination of aluminium Ahson J Downard *, H Upton J Powell and Shuanghua Xu Department of Chenustry, Unruers@ of Canterbury, Chnstchurch (New Zealand)

(Recemed 16th August 1991, revised manuscript recewed 13th January 1992)

Abstract A modlficatlon of the method for the determmatlon of alummmm by cathodic strlppmg voltammetry of Its Solochrome Violet RS (SVRS) complex 1s reported A rapld reactlon occurs between alummmm(III) and SVRS at room temperature at pH 8 8 Thus ehmmates the heating and coohng step reqmred for reaction at pH 4 5, the analysis time 1sconsIderably shortened as complex formatlon occurs durmg the solution degassmg step The reaction does not go to completion and ISstopped after 10 mm by lowenng the pH to 4 6 For 1 X 10m6M SVRS the detection limit was 4 5 x 10T9 M Al and the relative standard deviation at 2 x lo-’ M Al was 5% (n = 8) Keywords Voltammetry, Ahmunmm, Solochrome Violet RS

The use of Solochrome Violet RS (SVRS) for the voltannnetnc determmatlon of ahnnmmm has become well established smce it was first reported by Wdlard and Dean m 1950 [l] They found that m the presence of alummmm(III), the single polarographlc reduction wave of SVRS was spht mto two with the height of the wave at more negative potentials proportional to alummmm concentration At pH 4 6 equlhbratlon of the alummmm-SVRS solutions required over 4 h at room temperature, and complex formatlon was most conveniently achieved by heatmg at 5%70°C for 5 mm Subsequently detads of the solutron chemistry, the electrochemrcal response and the analytical utility of the method were exammed [2-61 The oxldatlon of Al-SVRS complexes at pyrolytic graphite [71 and carbon paste [81 electrodes has also been found to be useful for alummmm determination

More recently, a very sensitive method based on cathodic strlppmg voltammetry (CSV) of the Al-SVRS complex at a hangmg mercury drop electrode (HMDE) was reported by Wang et al [9] The detection hmlt was 0 15 pg I-’ (5 5 x 10e9 M Al) after 10 mm accumulation, with 1 mm accumulation the lmear workmg range was 5-30 pg I-’ (185 x lo-‘-11 1 x lo-’ M) However, the method has the dlstmct disadvantage that to effect reaction m the acetate buffer (pH 4 5), solutions must be heated for 10 mm at 90°C and cooled for 15 mm poor to analysis Van den Berg et al [lo] described a faster method mvolvmg the accumulation of the Al1,2-dlhydroxyanthraqumone-3-sulphomc acid (DASA) complex (which forms rapldly at pH 7 1) followed by dlfferentml pulse CSV The method has a lower hmlt of detectlon (10 x 10e9 M for 45 s accumulation from 1 X lo-’ M DASA) and has a longer linear working range than Wang et

0003-2670/92/$05 00 0 1992 - Elsevler Saence Pubhshers B V All nghts reserved

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al’s procedure However, the pH reqmred for this analysis (7 1) IS close to that for muumum solubdlty of hydrated alummmm oxide, It IS probable that the buffer leads to rapid mltlal polymerlzatlon of A13+ m a natural water, modlfymg the speclatlon, and leading to a “reactive alummmm” fraction which 1s poorly defined As part of a programme to develop new and improved methods for the indirect electrochemlcal determination of Al [11,12], a modlflcatlon of one of these methods, viz , the SVRS method of Wang et al, 1s reported here (91 The modlflcatlon was developed by optlmlzmg the reaction condltlons for formation of the Al-SVRS complex As noted by earlier workers [2,5,6], the reaction rate increases with increase m pH In the modified procedure the lo-mm heating step at pH 4 5 and the 15mm coolmg step are ehmlnated and replaced by a IO-mm reaction effected during degassmg m the electrochemical cell at room temperature and pH 8 8 An addltlonal advantage of working at pH 8 8 1s that the “reactive alummium” fraction will approximate to total dissolved alummmm, as established for alummmm determination using ahzarm [ll] and DASA [12] at this pH The combination of rapid SVRS complex formation at pH 8-9 and the high sensitivity of the CSV technique has not prevlously been exploited

EXPERIMENTAL

Equrpment and reagents

Linear-sweep and stnppmg voltammograms were obtained usmg a PAR Model 174A Polarographic Analyzer coupled to a PAR Model 303A static mercury drop electrode and a PAR Model RG0074 X-Y recorder A standard three-electrode cell was used, conslstmg of a medmm-sized mercury drop working electrode, a platinum wire counter electrode and an Ag/AgCl (saturated KC0 reference electrode The potential of the reference electrode was checked using the reduction (differential-pulse polarography) of Cd’+ m 0 1 M KNO, A PAR Model 305 magnetic stirrer was operated at 400 rpm during the accumulation step Spectrophotometrlc measurements were

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Chrm Acta 262 (1992) 339-343

made using a Varran Superscan 3 spectrophotometer All solutions were prepared using water purlfled with a M&-Q system (M&pore) SVRS (Aldrich) was used as received and a 2 x 10e4 M stock solution was prepared daly This concentration assumes a ca 60% dye content as stated by the supplier Al(II1) standards were prepared from a 1 x 10v3 M stock solution of AU, 6H,O (AnalaR, BDH) The 1 M ammoma-ammomum acetate buffer was prepared from lsoplestlcally distilled ammonia solution and acetic acid Aristar-grade sodium acetate and perchlonc acid (BDH) were used Procedures

All sample preparations and measurements were performed m a Class 100 clean room Solutions for analysis were prepared by successive addition of 0 5 ml of 1 M ammonia-ammonmm acetate buffer (pH 8 81, l-2 ml of stock or dlluted stock SVRS solution and 20-1000 ~1 of stock or diluted stock ANIII) solution to a lo-ml volumetric flask Timing was commenced on addition of the AN110 solution The total volume was made up to 10 ml with Mdh-Q-purified water and the contents of the flask were transferred to the electrochemical cell The solution was purged with nitrogen and after a total tune of 10 mm 35 ~1 of concentrated HCIO, were added The accumulation potential, -0 20 V, was applied to a fresh mercury drop for 60 s while the solution was stirred Followmg a 15-s rest period, the strlppmg voltammogram was recorded by scanning at 50 mV s-l from -020 to -070 V The blank was determined by the same procedure, wlthout the addition of Al stock solution

RESULTS AND DISCUSSION

Effect of pH on complex formation and electrochemwy

The rate of formation of the Al-SVRS complex as a function of pH was examined spectrophotometrlcally at room temperature Absorbance changes at 600 nm (where there IS the maxlmum difference m molar absorptlvlty for

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Chm Acta 262 (1992) 339-343

SVRS and rts Al complex) mdlcated that the reaction 1s muneasurably slow at pH 4 5 but the rate increased by a factor of 25 over the pH range 7 O-9 3 The reduction potent& of SVRS and Its Al complex at the HMDE are pH dependent Figure 1 shows linear scans of a SVRS (2 X 10e6 M) and Al(II1) (4 x lo-’ M) solution after reaction for 10 mm at room temperature and pH 8 8 Figure 1A was recorded at pH 8 8 and Fig 1B after lowering the pH to 4 6 In each scan, the large peak corresponds to reduction of SVRS At pH A

4 6 the small peak 1s due to SVRS complexed to Al After a 60-s accumulation at -0 47 V, the solution at pH 8 8 gave the strlppmg peak shown m Fig 1C (E,, = - 0 60 V), a 60-s accumulation at -0 20 V, after lowermg the pH to 4 6, gave the strlppmg peak shown m Fig 1D
B

7.r I

250

\, C

I

nA

250

~-IA

D

I

I’100

nA

I

I

-0

I

4

I

I

poTEkmIAL

1

I

-0.6

-0

(V)

a

PolmmIAL

(V)

Fig 1 Lmear scans (50 mV s-l) of SVRS (2 X 10W6M) and Al(III) (4 X lo-’ M) after a IO-mm degassmg and reactlon tune at room temperature at pH 8 8 (A) pH 8 8, no accumulation, (B) solutton pH lowered to 4 6, no accumulation, (C) pH 8 8 after a 60-s accumulation at - 0 47 V, (D) pH 4 6 atIer a 60-s accumulation at - 0 20 V

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at pH 8 8 can be used as the basis for cathodic stripping analysis for Al optlmum condltwns The method described requires buffering of

solutions at pH > 8 and also at pH 4-5 (see below) Ammoma-ammonium acetate was chosen as the buffer because It has good buffer capacity at pH > 8 and, after addmon of HClO,, at pH 4-5 A solution pH of 8 8 f 0 1 was most convement for the standard procedure At the concentrations used for analysis, the reactlon between Al(II1) and SVRS 1s ca 30% complete after 10 mm at pH 8 8 Hence there is a contmued slow mcrease m strlppmg peak currents for solutions allowed longer reaction times Careful control of the reactlon time 1s requn-ed This IS achieved by lowermg the pH to 4 6 by addlhon of 35 ~1 of concentrated HClO, after a lo-mm degassmg and reaction tnne This stops the reactlon and the Al-SVRS complex IS then accumulated from the pH 4 6 solution Figure 2 shows the varlatlon of stripping peak current with accumulation time at pH 4 6 after room temperature reactlon of Al0111 with SVRS at pH 8 8 An accumulation time of 60 s was the best compromise between sensltwlty and speed of analysis The strlppmg peak current was highly

30

60

90

120

Aoolmulatxon tire (8) Fig 2 Dependence of the strlppmg peak ‘current bear scan, 50 mV s-l) on accumulatton tune for the Al-SVRS complex 2 x lop6 M SVRS, 7x lo-’ M ANIII), accumulation at - 0 20 VatpH46

Chm Acta 262 (1992) 339-343

sensltlve to the accumulation potential, accumulation at -0 20 V gave the largest current The lmear-sweep strlppmg mode afforded greater sensltlvlty than dlfferentlal-pulse voltammetry It may be noted that the optrmum accumulatlon potential and accumulation time determined m this study differ from those reported by Wang et al [9] We attnbute this to dtierent reference electrode potentials and SVRS samples

Detectwn llmlt and workzng range

Accumulation m the absence of Al estabhshed a measurable response for the blank Expenments m which the concentrations of the buffer and SVRS solutions were varied and polypropylene reaction cells were substituted for glass cells indicated that the response arose from the SVRS sample For 1 x 10e6 M SVRS, the blank was equivalent to 7 X 10m8MAl Hence the detection hmlt (DL) for Al determination depended on the SVRS concentration For 1 X 10e6 M SVRS acceptable hnearlty IS shown up to 2 5 x lo-’ M Al and the DL, calculated as 3~~ (n = 9) was 4 5 X 10m9 M Al, glvmg a workmg range of ca 2 X lope-2 5 X lop7 M Al (slope of the cahbratlon graph = 0 452 A 1 mol-‘, r2 = 0 9999, n = 6) The relative standard deviation (R S D) at 2 x lo-* M Al(III) was 5% (n = 8) Use of a higher concentratlon of SVRS extends the upper limit of the linear range The DL and R S D reported here are similar to those determmed by Wang et al [9], namely 55~10~~ M and 2% at 37~10~~ M Al, respectively, but apply to a l-mm rather than a lo-mm accumulation time Hence the modified procedure 1s suitable, and mdeed preferred, for sumlar apphcatlons In concluaon, for the CSV determmatlon of Al as Its SVRS complex, the modlflcatlons described here afford a simpler and faster method of analysis The heatmg and coohng steps (total time 25 mm), necessary when workmg at pH 4-5, are replaced with a lo-mm reactlon (effected during degassmg) at room temperature at pH 8 8 Control of the reaction time by lowermg the solution pH to stop the reactlon achieved excellent reproduclblhty

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Chrm Acta 262 (1992) 339-343

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