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Analysis of arsenic(V) by cathodic stripping voltammetry
ANALYTICA CHIMICA
ACTA ELSEVIER
Analytica
Chimica Acta 306 (1995) 217-223
Analysis of arsenid, V) by cathodic stripping voltammetry U. Greulach, G. Henze Fachbereich
Chemie der Unirersitiit Kaiserslautern
Received 3 October
*
und Abteilungjiir anorganische und analytische Chemie der Uniwrsitiit Box 3825, 54228 Trier, Germany
1994; revised 6 December
1994; accepted
10 December
Trier, P.O.
lYY4
Abstract A novel electrochemical stripping approach is presented for the trace measurement of arsenate. It is based on the reduction of As(V) in a mannitol and perchlorate containing solution, the coprecipitation with copper and the voltammetric determination by further reduction to ASH, at the hanging mercury drop electrode. For an accumulation period of 60 s, the detection limit has been found to be 4.4 pg/l; the determination limit was calculated to be 11 pg/l. The method has been applied to the analysis of arsenic in a standard stream sediment and in water samples. Keywords:
Stripping
voltammetry;
Arsenic;
Environmental
samples; Waters
1. Introduction Arsenate is generally regarded to be electrochemitally inactive, and normally the determination of As(V) requires its prior reduction to the trivalent state [I]. However, in perchloric acid solutions and in the presence of phenols or aliphatic polyhydroxy compounds such as D-mannitol, As(V) becomes electroactive and can be determined by polarographic techniques [2-51. Cathodic stripping voltammetric procedures have been developed for the determination of arsenite. Accumulation prior to the scan is effected with copper or selenium [6-81, or by adsorption of a complex of arsenite-pyrrolidine dithiocarbamate [9] on the surface of the hanging mercury drop electrode
* Corresponding
author.
0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0003.2670(94)00681-4
(HMDE). The determination of arsenite by anodic stripping voltammetry using a gold or gold plated carbon electrode does not offer sufficient reproducibility [lo-l 41. Although arsenate is electrochemically inactive, it can be directly determined - without prior chemical reduction - by stripping analysis in strong hydrochloric acid solutions at a gold-coated platinum-fibre electrode by application of a high negative overpotential [ 151. However, such measurements suffer from problems caused by large hydrogen and chlorine generation at the electrodes and are therefore unsuitable for routine analysis. On the basis that arsenic(V) can be reduced in perchloric acid solutions containing D-mannitol combined with the accumulation of arsenic by coprecipitation with copper on an HMDE, a highly selective and sensitive procedure for the direct determination of the pentavalent arsenic by cathodic stripping voltammetry (CSV) has been developed. The estab-
218
U. Greulach, G. Henze /Analytica Chimica Acta 306 (1995) 217-223
lishment of the analytical protocol and the application of the procedure is described in this paper.
2. Experimental Differential pulse voltammograms were recorded using a Metrohm Polarecord 626 in conjunction with an E 608 timer and a 663 VA stand. The reference electrode was a double junction Ag/AgCl/3 M KC1 electrode. Because of the precipitation of potassium with perchlorate, the potassium chloride solution in the outer jacket was replaced by 3 M sodium chloride. For pH measurements an Orion 960 pH meter with a Metrohm glass electrode was used. Pretreatment of water samples was performed in a Metrohm Model 705 UV irradiator. Sediments were digested with HF-HNO, in a Berghof pressure-digestion system. All chemicals used were of analytical reagent grade and purchased from Merck (Darmstadt). Water was purified by a Millipore Mini-Q system. A stock solution with 1 g/l arsenate was prepared by dissolving 0.4223 g Na,HAsO, .7H,O in 1 1 water. Prior to voltammetric investigations, the solutions were deaerated with nitrogen.
3. Results and discussion The principle for the analytical procedure is to preconcentrate arsenic in the presence of copper on an HMDE by controlled potential electrolysis and subsequently to strip the arsenic by applying a cathodic potential scan, whereby the arsenic is further reduced to arsenic in the -3 oxidation state. The supporting electrolyte is 0.3 M NaCl and the test solution must also contain mannitol, sodium perchlorate, perchloric acid and copper011 sulfate. It is these additional components of the electrolyte solution, that makes the electrochemical reduction of arsenic(V) in the preconcentration step feasible. So the conditions for the two major processes influencing the efficiency of the procedure, the electrolytic reduction of arsenate and the accumulation of arsenic at the HMDE, have to be considered. Above all the problem is the reduction of arsenate to arsenite, which is only practicable in acidic per-
chloric solutions containing phenols, such as pyrocatechol, pyrogallol, or in the presence of polyhydroxy compounds, such as mannitol. Preliminary investigations have shown that the use of the phenols is unsuitable because of their strong adsorption on the surface of the mercury drop, which is of influence on the accumulation of arsenic. Best conditions are given using mannitol. This compound has not these strong adsorptive tendencies and it provided the best yield in the reduction of arsenate [4]. The other process affecting the procedure is the accumulation of arsenic at the mercury electrode. Because arsenic does not form amalgams it has to be coprecipitated with copper [6]. So the accumulation is influenced by the concentration of Cu(II1 in the solution and furthermore by the pH, just as pH influences the electrolytic reduction of arsenate. Fortunately the best pH values of the reduction of arsenate to arsenite and of the coprecipitation of arsenite as CuAs, on the HMDE [6,7] are in the same range. Hence it follows that various parameters have to be checked to optimise the whole procedure. 3.1. Optimisation
of the experimental
parameters
To achieve maximum sensitivity in the voltammetric response, first of all the influence of the copper(B) and the mannitol concentration in the supporting electrolyte solution have to be examined.
!@ 464 6w 800 iw c fW
IX4l(w WI
I@3 MM ifM
s:
Q 0,l
Q,Z
0,J
nunnlld
Q,4
0,3
0,s
[mdll]
Fig. 1. Effect of varying Cu’+ ( a ) an d mannitol (b) concentration on the determination of 40 pg/I arsenate each. Supporting electrolyte: 2 M NaCIO,, 0.3 M NaCI, 0.5 M mannitol (a) and 2 mM Cu*+ (b) at pH 1.7; E,,, = -0.55 V and I,,, = 60 s (unstirred).
According to the results shown in Fig. I, additions of the both components are very effective. The peak current increases as the copper concentration is increased (Fig. la), For a 40 fig/l arsenic soiution at an a~~umuIation time of t,,, = 60 s and an accumulation potential of E,,, = -0.55 V, the optimum copper00 concentration in an unstirred 2 M NaClO, solution of pH 1.7 was found to be > 1600 ~mol/l. This is in a very good agreement with the value of 2 mM Cu*+ reported by Henze et al. [61 for the dete~ination of arsenite, accumulated as an intermetallic compound with copper in a hydrochloric acid containing solution. Furthermore the current depends on the concentration of mannital, which affects reduction of As(V) to As(ffI). At a mannitol &un~entration of 0.5 M, the maximum peak height is obtained for a sample solution containing 40 pg/l As(V) at a pH of 1.7 (Fig. lb). The pH of the sample solution significantly affects the determination process, as the peak current varies markediy over a narrow pH range as can be seen in Fig. 2. pH was detected after a caIibration with five HCl-citrate standards between pH 1 and 2, in spite of the problems detecting the true value of pH in this range with a glass electrode. The optimum pH with regard to the sensitivity is 1.6-1.8. At this pH range the peak current reaches a maximum value and below pH 1.6 the peak decreases significantly.
F-o]
---+----
-- *
L__. ! .“. II 191 I,3 I,3
peakpotenlinl_Y
-.. -.m-/r. .-._~_* __,__
_ I,4
1,;
I,6
--. -----I--
l,?
I,8
1,s
:-0,8
2
4
Fig. 2. InfTuence of the pH on the peak current and the peak potential (vs. Ag/AgC1/3 M KCI) of 40 pg/l As(V). Supporting electrolyte: 2 M NaClO,, 0.3 M N&I, 0.5 M mannitol and 2 mM CL?+; E,,, = -OS V and f,,, = 60 s (unstirred).
301
I *i..
0
.-._--
500
too0
1500
2000
2500
3000
stirrer speed [mitv] Fig. 3. Distortion of the arsenate signal by stirring the solutions. Content of As(V) = 40 &g/l, other conditions as in Fig. 2.
The peak potential shifts slightly towards more negative values with increasing pH. In addition to the electrolyte composition, working conditions such as accumufation potential and accumulation time have to be optimised. Furthermore it could be observed that the sensitivity of the voltammetric response in this procedure depends an the stirrer speed in an unusual way. There is an inverse relationship between the speed of stirring and the peak height as is shown in Fig, 3, A consideration of the various electrochemical processes occurring in the accumulation process may provide an explanation for this effect. During the accumulation arsenic(II1) is produced by the electrochemical reduction of arsenic(V). This a~eni~~II~ is then capable of either forming an inte~etallic compound with copper on the HMDE or diffusing to the bulk solution. Diffusion to the bulk solution is driven by the concentration gradient which is created by the electrochemical production on the electrode surface. fi.e., cs~~~~~~As~III~~X- c~~~~~As~III~= 01. Diffusion from the electrode surface to the bulk solution wiI1 be enhanced by stirring and this, of course, will decrease the amount of arsenic deposited as its intermetallic compound with copper on the HMDE. That is, maximum deposition of arsenic will occur in unstirred solutions, as shown in Fig. 3. The dependence of peak current on the accumufation time is presented in Fig. 4a. After 60 s of accumulation, in a solution containing 40 pg/l
220
U. Greulach, G. Henze/Analytica
If. nr
15
L
I\
I/
I
1’
,/
10'
i
lot:’
\
d
5
i
)L
,
20 25 Jo 35 40 45 IO 55 80 65 70 76 80 85 w t M
“0
IL--_,
d,7 d,B
086 035 d.5 0845 d,4=>3 L
M
Fig. 4. Effect of varying the accumulation time t,,, (a) and potential E,,, (b) in an unstirred test solution with 40 pg/l (a) and 60 fig/l (b) As(V); other conditions as in Fig. 2.
As(V) at a pH of 1.75 and an accumulation potential of -0.55 V, a maximum is reached. Fig. 4b shows the peak current of a 60 pg/l As(V) solution in relation to accumulation potential. A broad maximum is observed around a potential of E,,, = -0.55 V. This agrees with the observations made by Henze et al. [6] and Sadana 171, who detected an optimum accumulation potential of -0.55 V and -0.6 V, respectively.
Chimica Acta 306 (1995) 217-223
linear calibration plot extends up to a concentration of 100 pg/l. The detection limit calculated by the calibration curve procedure proposed in Refs. [16,17] was found to be 4.4 pg/l whilst the limit of determination was calculated to be 11 pg/l. Interestingly, the calibration graph does not intersect the origin, but rather crosses the x-axis at a value of 3.4 pg/l. An explanation for this behaviour is provided by Fig. 6. Fig. 6 presents a more detailed calibration curve obtained with a reduced copper concentration of 0.2 mM and a prolonged accumulation time of 140 s. It is apparent that below 10 pg/l, there is a second calibration line. This may well be a consequence of a different arsenic/copper intermetallic phase being formed on the electrode at low arsenic concentrations. However, when the added copper concentration was decreased and accumulation time increased
E,
=
-o.aav
5nA :
3.2. Procedure As the result of these investigations the following operational parameters for the determination of As(V) have been chosen. The supporting electrolyte solution containing 2 M NaClO,, 0.5 M mannitol, 0.3 M NaCl and 2 mM CuSO, at pH 1.7 has been deaerated by nitrogen for 5 min. The accumulation potential of E,,, = -0.55 V has been applied to a fresh mercury drop (size = 3) while the solution is unstirred. Following the preconcentration step the voltammogram has been recorded in the differential pulse (DP) mode (pulse amplitude 50 mV) by applying a cathodic linear potential scan with 10 mV/s scan rate. Voltammograms for various concentrations of As(V), recorded using these conditions and after a preconcentration time of t,,, = 60 s, are shown in Fig. 5. The corresponding calibration graph has the linear equation of y = 0.75x - 2.3 (r = 0.9994). The
-0.6
-0.8 E (vs. Ag/AgCI/KCI,
-1.0 v 3 M)
Fig. 5. Voltammograms of the As(V) determination in a supporting electrolyte of 2 M NaCIO,, 0.3 M NaCI, 0.5 M mannitol and 2 mM Cu*+ at pH 1.7 with a scan rate of 10 mV/s, E,, = -0.55 V and t,,, = 60 s (unstirred). (a) Blank, (b) 5, (c) 10, (d) 15, (e) 20, (f) 25, (g) 30 and (h) 35 Kg/l As(V).
U. Greulach, G. Henze/Analytica
221
Chimica Acta 306 (1995) 217-223
in an attempt to maintain the appropriate arsenic to copper ratio, the resulting voltammograms showed double peaks which could not be evaluated analytically.
using UV irradiation. Simultaneously interferences caused by organic matter were eliminated.
3.3. Interferences
Some mineral waters and a stream sediment were used to test the applicability of the proposed procedure for the CSV determination of arsenate. The water samples were acidified with perchloric acid (pH 1) and stored in a brown glass flask. 10 ~1 of 30% HZO, were added to 10 ml of the sample and it was irradiated for 1 h. The decomposed and oxidised sample was mixed with sodium perchlorate, mannitol, copper sulfate and adjusted to pH 1.7. Voltammograms were recorded after a preconcentration time of 60 s at a potential of -0.55 V (vs. Ag/AgC1/3 M KCl). The sediment sample (Chinese stream sediment GB7305) was digested by treating 0.5 g with 1 ml of 60% HNO, and 0.5 ml of 40% HF in a closed PTFE
3.4. Application
The influence of the metal ions Pb(lI), Bi(III1, Sn(IV), Sb(V) and Se(W) at a lOO-fold mass excess on the stripping peak for arsenic has been tested. Only antimony(V) and selenium(W) show a large suppression of the arsenic peak, but when present in equimolar amounts, these two elements do not interfere. However, significant interference occurs with mixtures of As(V) and A&II), the calibration curve becomes S-shaped and no correct result for As(V) can be obtained. This necessitates the prior oxidation of A~(1111to As(V) before analysis. Traces of A~(1111 in natural waters can be quantitatively oxidised by
to enrlironmental
samples
60 -
40 q
20 -
od 0
a)
q
//
IO
I
I
I
I
I
I
I
20
30
40
50
60
70
80
I 90
100
Fig. 6. Calibration plot of the CSV determination of As(V) between lo-100 pg/l (a) and I-10 pg/l As(V) (h) with a reduced Cu’+ concentration of 0.2 mM and a lengthened accumulation time of 140 s. Other conditions are the same as in Fig. 2.
222
U. Greulach, G. Henze/Analytica
Table 1 CSV determination
of As(V) in water samples and in a stream sediment sample
Sample
Measured value (n = 4)
Mineral water (in pg/l) Taunus Spring water (in pg/l) Schwarzwald Tap water (in pg/l) Schwarzwald Well water (in pg/l) Schwarzwald Chinese stream sediment (in pg/g) References
Chimica Acla 306 (1995) 217-223
(GB 7305)
Certified a or reference value
75.1 f 2.3
75.4 f 3.1
58.3 f 4.2
60.0 + 2.5
24.5 + 2.8
23.5 + 1.0
104.8 k 3.5
94.1 + 4.5 75k5”
72+ 5.7
were made by hydride AAS.
digestion vessel for 12 h at 200” C. 5 ml of the solution of the digested sediment were diluted with 5 ml supporting electrolyte containing 4 M NaClO,, 1
M mannitol, 0.6 M sodium chloride, 4 mM copper sulfate, 60 mM EDTA (as masking agent for interfering metal ions) and the pH adjusted to 1.7. Under these conditions the poteniial of ;he CSV peak shifted to a less negative potential of - 0.78 V. The voltammograms were recorded after a shortened preconcentration time of 10 s at -0.55 V. The resulting voltammograms and calibration graph are given in Fig. 7. The arsenic concentration, calculated by standard addition, was found to 71.2 f 5.7 pg/g, in good agreement with the certified value of 75 k 5 lLg/g. In Table 1 the results of the analysed water samples and the standard sediment are summarized. Hydride atomic absorption spectrometry (AAS) has been used as reference technique for the As determination.
4. Conclusion
I 1 I - 0.6 - 0.6
8 I
b
- 1.0
E &‘I (vs. AglAgCll3M
KCU
I__
Fig. 7. Quantification of arsenate in Chinese stream sediment (tiB 7305) by standard addition. Supporting electrolyte as in Fig. 2 plus 60 mM EDTA and a reduced I,,., = 10 s at a potential of E,,, = - 0.55 V (unstirred). Standard addition was executed in 20 pg/l steps.
A new method of the electrochemical analysis of arsenic(V) by cathodic stripping voltammetry in a mannitol containing perchloric solution has been developed. It is based on the coprecipitation of arsenic with copper on the HMDE after the electrochemical reduction of arsenic(V) to arsenic(II1) during the accumulation step. The detection limit has been found to be 4.4 pg/l As(V) and the determination limit was calculated to be 11 pg/l, calculated with the calibration curve method proposed by the Deutsche Forschungsgemeinschaft. No significant interferences have been found in common ranges of environmental metal concentrations.
U. Greulach, C. Henze / Analytica Chimica Acta 306 (I 995) 217-223
Acknowledgements We gratefully thank the Deutsche Forschungsgemeinschaft (DFG) for financial support of this work. We also thank the Institute Heppeler (L5rrach) for leasing the water samples and the hydride AAS measurements.
References [I] G. Hcnze and R. Neeb, Elektrochemische Analytik, Springer-Vcrlag, Berlin, 1986, p. 185. [2] S.M. Cletus White and A.J. Bard, Anal. Chem., 38 (1966) 61. [3] C. McCrory-Joy and J.M. Rosamilla, Anal. Chim. Acta, 142 (lY82) 231. [4] D. Chakraborti, R.L. Nichols and K.J. Irgolic, Fresenius’ J. Anal. Chem., 319 (1984) 24X.
151 H. Gresconig
123
and K.J. Irgolic, Appl. Organomet. Chem.. 6 (1992) 565. [61 G. Henze, A.P. Joshi and R. Neeb, Fresenius’ J. Anal. Chem., 300 (1980) 267. [71 R.S. Sadana, Anal. Chem., 5.5 (1983) 304. [81 W. Holak, Anal. Chem., 52 (1980) 2189. [91 I. Zima and C.M.G. van den Berg, Anal. Chim. Acta, 28Y (1994) 291. [lOI G. Forsberg, J.W. O’Laughlin, R.G. Megargle and S.R. Koirtyohann, Anal. Chem., 47 (1975) 1586. [Ill P.H. Davis, G.R. Dulude, R.M. Griffin, W.Rn. Matson and E.W. Zink, Anal. Chem., 50 (1978) 137. 1121 T.W. Hamilton, J. Ellis and T.M. Florence, Anal. Chim. Acta, 119 (1980) 225. [131 F. Bodewig, P. Valenta and H.W. Niirnberg, Fresenius’ J. Anal. Chem., 31 I (1982) 187. [141 A.A. Ramadan and H.Mandil, Indian J. Chem., 28(A) (1989) 984. [I51 H. Huiliang, D. Jagner and L. Renman, Anal. Chim. Acta, 207 (1988) 37. [I61 H. Frehse and H.P. Thier, GIT Fachz. Lab., 4 (1991) 285. [I71 Entwurf zur DIN 38402. Beuth-Verlag, Berlin, 1993.
ACTA ELSEVIER
Analytica
Chimica Acta 306 (1995) 217-223
Analysis of arsenid, V) by cathodic stripping voltammetry U. Greulach, G. Henze Fachbereich
Chemie der Unirersitiit Kaiserslautern
Received 3 October
*
und Abteilungjiir anorganische und analytische Chemie der Uniwrsitiit Box 3825, 54228 Trier, Germany
1994; revised 6 December
1994; accepted
10 December
Trier, P.O.
lYY4
Abstract A novel electrochemical stripping approach is presented for the trace measurement of arsenate. It is based on the reduction of As(V) in a mannitol and perchlorate containing solution, the coprecipitation with copper and the voltammetric determination by further reduction to ASH, at the hanging mercury drop electrode. For an accumulation period of 60 s, the detection limit has been found to be 4.4 pg/l; the determination limit was calculated to be 11 pg/l. The method has been applied to the analysis of arsenic in a standard stream sediment and in water samples. Keywords:
Stripping
voltammetry;
Arsenic;
Environmental
samples; Waters
1. Introduction Arsenate is generally regarded to be electrochemitally inactive, and normally the determination of As(V) requires its prior reduction to the trivalent state [I]. However, in perchloric acid solutions and in the presence of phenols or aliphatic polyhydroxy compounds such as D-mannitol, As(V) becomes electroactive and can be determined by polarographic techniques [2-51. Cathodic stripping voltammetric procedures have been developed for the determination of arsenite. Accumulation prior to the scan is effected with copper or selenium [6-81, or by adsorption of a complex of arsenite-pyrrolidine dithiocarbamate [9] on the surface of the hanging mercury drop electrode
* Corresponding
author.
0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0003.2670(94)00681-4
(HMDE). The determination of arsenite by anodic stripping voltammetry using a gold or gold plated carbon electrode does not offer sufficient reproducibility [lo-l 41. Although arsenate is electrochemically inactive, it can be directly determined - without prior chemical reduction - by stripping analysis in strong hydrochloric acid solutions at a gold-coated platinum-fibre electrode by application of a high negative overpotential [ 151. However, such measurements suffer from problems caused by large hydrogen and chlorine generation at the electrodes and are therefore unsuitable for routine analysis. On the basis that arsenic(V) can be reduced in perchloric acid solutions containing D-mannitol combined with the accumulation of arsenic by coprecipitation with copper on an HMDE, a highly selective and sensitive procedure for the direct determination of the pentavalent arsenic by cathodic stripping voltammetry (CSV) has been developed. The estab-
218
U. Greulach, G. Henze /Analytica Chimica Acta 306 (1995) 217-223
lishment of the analytical protocol and the application of the procedure is described in this paper.
2. Experimental Differential pulse voltammograms were recorded using a Metrohm Polarecord 626 in conjunction with an E 608 timer and a 663 VA stand. The reference electrode was a double junction Ag/AgCl/3 M KC1 electrode. Because of the precipitation of potassium with perchlorate, the potassium chloride solution in the outer jacket was replaced by 3 M sodium chloride. For pH measurements an Orion 960 pH meter with a Metrohm glass electrode was used. Pretreatment of water samples was performed in a Metrohm Model 705 UV irradiator. Sediments were digested with HF-HNO, in a Berghof pressure-digestion system. All chemicals used were of analytical reagent grade and purchased from Merck (Darmstadt). Water was purified by a Millipore Mini-Q system. A stock solution with 1 g/l arsenate was prepared by dissolving 0.4223 g Na,HAsO, .7H,O in 1 1 water. Prior to voltammetric investigations, the solutions were deaerated with nitrogen.
3. Results and discussion The principle for the analytical procedure is to preconcentrate arsenic in the presence of copper on an HMDE by controlled potential electrolysis and subsequently to strip the arsenic by applying a cathodic potential scan, whereby the arsenic is further reduced to arsenic in the -3 oxidation state. The supporting electrolyte is 0.3 M NaCl and the test solution must also contain mannitol, sodium perchlorate, perchloric acid and copper011 sulfate. It is these additional components of the electrolyte solution, that makes the electrochemical reduction of arsenic(V) in the preconcentration step feasible. So the conditions for the two major processes influencing the efficiency of the procedure, the electrolytic reduction of arsenate and the accumulation of arsenic at the HMDE, have to be considered. Above all the problem is the reduction of arsenate to arsenite, which is only practicable in acidic per-
chloric solutions containing phenols, such as pyrocatechol, pyrogallol, or in the presence of polyhydroxy compounds, such as mannitol. Preliminary investigations have shown that the use of the phenols is unsuitable because of their strong adsorption on the surface of the mercury drop, which is of influence on the accumulation of arsenic. Best conditions are given using mannitol. This compound has not these strong adsorptive tendencies and it provided the best yield in the reduction of arsenate [4]. The other process affecting the procedure is the accumulation of arsenic at the mercury electrode. Because arsenic does not form amalgams it has to be coprecipitated with copper [6]. So the accumulation is influenced by the concentration of Cu(II1 in the solution and furthermore by the pH, just as pH influences the electrolytic reduction of arsenate. Fortunately the best pH values of the reduction of arsenate to arsenite and of the coprecipitation of arsenite as CuAs, on the HMDE [6,7] are in the same range. Hence it follows that various parameters have to be checked to optimise the whole procedure. 3.1. Optimisation
of the experimental
parameters
To achieve maximum sensitivity in the voltammetric response, first of all the influence of the copper(B) and the mannitol concentration in the supporting electrolyte solution have to be examined.
!@ 464 6w 800 iw c fW
IX4l(w WI
I@3 MM ifM
s:
Q 0,l
Q,Z
0,J
nunnlld
Q,4
0,3
0,s
[mdll]
Fig. 1. Effect of varying Cu’+ ( a ) an d mannitol (b) concentration on the determination of 40 pg/I arsenate each. Supporting electrolyte: 2 M NaCIO,, 0.3 M NaCI, 0.5 M mannitol (a) and 2 mM Cu*+ (b) at pH 1.7; E,,, = -0.55 V and I,,, = 60 s (unstirred).
According to the results shown in Fig. I, additions of the both components are very effective. The peak current increases as the copper concentration is increased (Fig. la), For a 40 fig/l arsenic soiution at an a~~umuIation time of t,,, = 60 s and an accumulation potential of E,,, = -0.55 V, the optimum copper00 concentration in an unstirred 2 M NaClO, solution of pH 1.7 was found to be > 1600 ~mol/l. This is in a very good agreement with the value of 2 mM Cu*+ reported by Henze et al. [61 for the dete~ination of arsenite, accumulated as an intermetallic compound with copper in a hydrochloric acid containing solution. Furthermore the current depends on the concentration of mannital, which affects reduction of As(V) to As(ffI). At a mannitol &un~entration of 0.5 M, the maximum peak height is obtained for a sample solution containing 40 pg/l As(V) at a pH of 1.7 (Fig. lb). The pH of the sample solution significantly affects the determination process, as the peak current varies markediy over a narrow pH range as can be seen in Fig. 2. pH was detected after a caIibration with five HCl-citrate standards between pH 1 and 2, in spite of the problems detecting the true value of pH in this range with a glass electrode. The optimum pH with regard to the sensitivity is 1.6-1.8. At this pH range the peak current reaches a maximum value and below pH 1.6 the peak decreases significantly.
F-o]
---+----
-- *
L__. ! .“. II 191 I,3 I,3
peakpotenlinl_Y
-.. -.m-/r. .-._~_* __,__
_ I,4
1,;
I,6
--. -----I--
l,?
I,8
1,s
:-0,8
2
4
Fig. 2. InfTuence of the pH on the peak current and the peak potential (vs. Ag/AgC1/3 M KCI) of 40 pg/l As(V). Supporting electrolyte: 2 M NaClO,, 0.3 M N&I, 0.5 M mannitol and 2 mM CL?+; E,,, = -OS V and f,,, = 60 s (unstirred).
301
I *i..
0
.-._--
500
too0
1500
2000
2500
3000
stirrer speed [mitv] Fig. 3. Distortion of the arsenate signal by stirring the solutions. Content of As(V) = 40 &g/l, other conditions as in Fig. 2.
The peak potential shifts slightly towards more negative values with increasing pH. In addition to the electrolyte composition, working conditions such as accumufation potential and accumulation time have to be optimised. Furthermore it could be observed that the sensitivity of the voltammetric response in this procedure depends an the stirrer speed in an unusual way. There is an inverse relationship between the speed of stirring and the peak height as is shown in Fig, 3, A consideration of the various electrochemical processes occurring in the accumulation process may provide an explanation for this effect. During the accumulation arsenic(II1) is produced by the electrochemical reduction of arsenic(V). This a~eni~~II~ is then capable of either forming an inte~etallic compound with copper on the HMDE or diffusing to the bulk solution. Diffusion to the bulk solution is driven by the concentration gradient which is created by the electrochemical production on the electrode surface. fi.e., cs~~~~~~As~III~~X- c~~~~~As~III~= 01. Diffusion from the electrode surface to the bulk solution wiI1 be enhanced by stirring and this, of course, will decrease the amount of arsenic deposited as its intermetallic compound with copper on the HMDE. That is, maximum deposition of arsenic will occur in unstirred solutions, as shown in Fig. 3. The dependence of peak current on the accumufation time is presented in Fig. 4a. After 60 s of accumulation, in a solution containing 40 pg/l
220
U. Greulach, G. Henze/Analytica
If. nr
15
L
I\
I/
I
1’
,/
10'
i
lot:’
\
d
5
i
)L
,
20 25 Jo 35 40 45 IO 55 80 65 70 76 80 85 w t M
“0
IL--_,
d,7 d,B
086 035 d.5 0845 d,4=>3 L
M
Fig. 4. Effect of varying the accumulation time t,,, (a) and potential E,,, (b) in an unstirred test solution with 40 pg/l (a) and 60 fig/l (b) As(V); other conditions as in Fig. 2.
As(V) at a pH of 1.75 and an accumulation potential of -0.55 V, a maximum is reached. Fig. 4b shows the peak current of a 60 pg/l As(V) solution in relation to accumulation potential. A broad maximum is observed around a potential of E,,, = -0.55 V. This agrees with the observations made by Henze et al. [6] and Sadana 171, who detected an optimum accumulation potential of -0.55 V and -0.6 V, respectively.
Chimica Acta 306 (1995) 217-223
linear calibration plot extends up to a concentration of 100 pg/l. The detection limit calculated by the calibration curve procedure proposed in Refs. [16,17] was found to be 4.4 pg/l whilst the limit of determination was calculated to be 11 pg/l. Interestingly, the calibration graph does not intersect the origin, but rather crosses the x-axis at a value of 3.4 pg/l. An explanation for this behaviour is provided by Fig. 6. Fig. 6 presents a more detailed calibration curve obtained with a reduced copper concentration of 0.2 mM and a prolonged accumulation time of 140 s. It is apparent that below 10 pg/l, there is a second calibration line. This may well be a consequence of a different arsenic/copper intermetallic phase being formed on the electrode at low arsenic concentrations. However, when the added copper concentration was decreased and accumulation time increased
E,
=
-o.aav
5nA :
3.2. Procedure As the result of these investigations the following operational parameters for the determination of As(V) have been chosen. The supporting electrolyte solution containing 2 M NaClO,, 0.5 M mannitol, 0.3 M NaCl and 2 mM CuSO, at pH 1.7 has been deaerated by nitrogen for 5 min. The accumulation potential of E,,, = -0.55 V has been applied to a fresh mercury drop (size = 3) while the solution is unstirred. Following the preconcentration step the voltammogram has been recorded in the differential pulse (DP) mode (pulse amplitude 50 mV) by applying a cathodic linear potential scan with 10 mV/s scan rate. Voltammograms for various concentrations of As(V), recorded using these conditions and after a preconcentration time of t,,, = 60 s, are shown in Fig. 5. The corresponding calibration graph has the linear equation of y = 0.75x - 2.3 (r = 0.9994). The
-0.6
-0.8 E (vs. Ag/AgCI/KCI,
-1.0 v 3 M)
Fig. 5. Voltammograms of the As(V) determination in a supporting electrolyte of 2 M NaCIO,, 0.3 M NaCI, 0.5 M mannitol and 2 mM Cu*+ at pH 1.7 with a scan rate of 10 mV/s, E,, = -0.55 V and t,,, = 60 s (unstirred). (a) Blank, (b) 5, (c) 10, (d) 15, (e) 20, (f) 25, (g) 30 and (h) 35 Kg/l As(V).
U. Greulach, G. Henze/Analytica
221
Chimica Acta 306 (1995) 217-223
in an attempt to maintain the appropriate arsenic to copper ratio, the resulting voltammograms showed double peaks which could not be evaluated analytically.
using UV irradiation. Simultaneously interferences caused by organic matter were eliminated.
3.3. Interferences
Some mineral waters and a stream sediment were used to test the applicability of the proposed procedure for the CSV determination of arsenate. The water samples were acidified with perchloric acid (pH 1) and stored in a brown glass flask. 10 ~1 of 30% HZO, were added to 10 ml of the sample and it was irradiated for 1 h. The decomposed and oxidised sample was mixed with sodium perchlorate, mannitol, copper sulfate and adjusted to pH 1.7. Voltammograms were recorded after a preconcentration time of 60 s at a potential of -0.55 V (vs. Ag/AgC1/3 M KCl). The sediment sample (Chinese stream sediment GB7305) was digested by treating 0.5 g with 1 ml of 60% HNO, and 0.5 ml of 40% HF in a closed PTFE
3.4. Application
The influence of the metal ions Pb(lI), Bi(III1, Sn(IV), Sb(V) and Se(W) at a lOO-fold mass excess on the stripping peak for arsenic has been tested. Only antimony(V) and selenium(W) show a large suppression of the arsenic peak, but when present in equimolar amounts, these two elements do not interfere. However, significant interference occurs with mixtures of As(V) and A&II), the calibration curve becomes S-shaped and no correct result for As(V) can be obtained. This necessitates the prior oxidation of A~(1111to As(V) before analysis. Traces of A~(1111 in natural waters can be quantitatively oxidised by
to enrlironmental
samples
60 -
40 q
20 -
od 0
a)
q
//
IO
I
I
I
I
I
I
I
20
30
40
50
60
70
80
I 90
100
Fig. 6. Calibration plot of the CSV determination of As(V) between lo-100 pg/l (a) and I-10 pg/l As(V) (h) with a reduced Cu’+ concentration of 0.2 mM and a lengthened accumulation time of 140 s. Other conditions are the same as in Fig. 2.
222
U. Greulach, G. Henze/Analytica
Table 1 CSV determination
of As(V) in water samples and in a stream sediment sample
Sample
Measured value (n = 4)
Mineral water (in pg/l) Taunus Spring water (in pg/l) Schwarzwald Tap water (in pg/l) Schwarzwald Well water (in pg/l) Schwarzwald Chinese stream sediment (in pg/g) References
Chimica Acla 306 (1995) 217-223
(GB 7305)
Certified a or reference value
75.1 f 2.3
75.4 f 3.1
58.3 f 4.2
60.0 + 2.5
24.5 + 2.8
23.5 + 1.0
104.8 k 3.5
94.1 + 4.5 75k5”
72+ 5.7
were made by hydride AAS.
digestion vessel for 12 h at 200” C. 5 ml of the solution of the digested sediment were diluted with 5 ml supporting electrolyte containing 4 M NaClO,, 1
M mannitol, 0.6 M sodium chloride, 4 mM copper sulfate, 60 mM EDTA (as masking agent for interfering metal ions) and the pH adjusted to 1.7. Under these conditions the poteniial of ;he CSV peak shifted to a less negative potential of - 0.78 V. The voltammograms were recorded after a shortened preconcentration time of 10 s at -0.55 V. The resulting voltammograms and calibration graph are given in Fig. 7. The arsenic concentration, calculated by standard addition, was found to 71.2 f 5.7 pg/g, in good agreement with the certified value of 75 k 5 lLg/g. In Table 1 the results of the analysed water samples and the standard sediment are summarized. Hydride atomic absorption spectrometry (AAS) has been used as reference technique for the As determination.
4. Conclusion
I 1 I - 0.6 - 0.6
8 I
b
- 1.0
E &‘I (vs. AglAgCll3M
KCU
I__
Fig. 7. Quantification of arsenate in Chinese stream sediment (tiB 7305) by standard addition. Supporting electrolyte as in Fig. 2 plus 60 mM EDTA and a reduced I,,., = 10 s at a potential of E,,, = - 0.55 V (unstirred). Standard addition was executed in 20 pg/l steps.
A new method of the electrochemical analysis of arsenic(V) by cathodic stripping voltammetry in a mannitol containing perchloric solution has been developed. It is based on the coprecipitation of arsenic with copper on the HMDE after the electrochemical reduction of arsenic(V) to arsenic(II1) during the accumulation step. The detection limit has been found to be 4.4 pg/l As(V) and the determination limit was calculated to be 11 pg/l, calculated with the calibration curve method proposed by the Deutsche Forschungsgemeinschaft. No significant interferences have been found in common ranges of environmental metal concentrations.
U. Greulach, C. Henze / Analytica Chimica Acta 306 (I 995) 217-223
Acknowledgements We gratefully thank the Deutsche Forschungsgemeinschaft (DFG) for financial support of this work. We also thank the Institute Heppeler (L5rrach) for leasing the water samples and the hydride AAS measurements.
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