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Separation of dissolved alkyllead and inorganic lead species by coprecipitation with barium sulphate
Analytica Chimica Acta, 212 (1988) 349-353 Elsevier Science Publishers B.V., Amsterdam -
349 Printed in The Netherlands
Short Communication
SEPARATION OF DISSOLVED ALKYLLEAD AND INORGANIC LEAD SPECIES BY COPRECIPITATION WITH BARIUM SULPHATE
N. MIKAC* and M. BRANICA Centre for Manne Research, Ruder BoikoviC Institute,
41001 Zagreb (Yugoslavia)
(Received 17th March 1988)
Summary Coprecipitation with barium sulphate is shown to be an efficient and convenient method of preventing the interference of relatively large amounts of inorganic lead in the determination of alkyllead species by differential-pulse anodic stripping voltammetry. The coprecipitation method has several advantages over complexation of lead with EDTA, ionic alkyllead can be measured in solutions of low pH (pH < 2)) or at deposition potentials more negative than - 1.0 V.
Tetraalkyllead compounds are introduced into the environment mainly because of their usage in gasoline. A fraction of these alkyllead compounds (l2% ) is emitted unchanged from motor vehicles [ 1,2]. Together with other sources of alkyllead, however, these toxic compounds [ 3,4] form only a minor fraction of total lead present in the environment. Under natural conditions, the initial tetraalkyl compounds are subject to degradation processes [ 1,2,5], forming ionic alkyllead derivatives (tri- and di-alkyllead). The much higher concentrations of inorganic lead in natural samples compared to alkyllead species are a source of interference in almost every analytical method used for alkyllead determination [ 2 1. Accordingly, such procedures should include a step where inorganic lead is either removed from the sample, or masked with respect to the detection system applied. Complexation with EDTA (ethylenediaminetetraacetic acid) is widely used for the elimination of inorganic lead interference in various chromatographic [6-81 or electroanalytical methods [9,10 1. In some of these methods, EDTA is used in the extraction step [ 6-81, while in others it is applied during the measurement process [g-12]. In the course of developing an anodic stripping voltammetric method for the determination of ionic alkyllead compounds in natural waters based on selective extraction and electroanalytical detection, the applicability of EDTA complexation was investigated. The investigation showed that the use of EDTA is
0003-2670/88/$03.50
0 1988 Elsevier Science Publishers B.V.
350
efficient only under limited experimental conditions. In a search for an alternative method, coprecipitation of inorganic lead with barium sulphate was found to be a convenient way of overcoming the lead interference in various electrolyte solutions and under different experimental conditions. Experimental Apparatus.
For the voltammetric measurements, a Princeton Applied Research (PAR) 174A polarographic analyzer was used with an HP-7045A x-y recorder. The working electrode was a hanging mercury drop electrode (Metrohm E-290); the auxiliary electrode was a platinum wire and the reference electrode was Ag/AgCl. For differential-pulse anodic stripping voltammetry (DPASV), the conditions were: pulse amplitude 50 mV, pulse frequency 0.5 s, and scan rate 5 mV s-l. Chemicals. Trialkyllead chlorides were obtained from Alfa-Ventron. Dialkyllead compounds were prepared from the corresponding trialkyllead standards by reaction with iodine monochloride [ 131. Standard solutions were prepared in tetra-distilled water and stored in the dark. The inorganic lead standard was from Merck. Other chemicals (sodium sulphate, barium chloride and EDTA) were of analytical-reagent grade, except nitric acid which was tetradistilled. The solution in the electrochemical cell was deaerated with highpurity nitrogen. All glassware was washed with diluted nitric acid prior to use. Procedure. The solution in the electrochemical cell (25-50 ml) was deaerated and inorganic lead was removed by coprecipitation with barium sulphate. For this purpose, 2-4 x 50 ~1 of sodium sulphate solution (1.2 mol 1-l ) and the same volumes of barium chloride solution (1 mol 1-l) were added. In the case of seawater, only the barium chloride solution was added, because the natural level of sulphate (2.9 x 10m2 mol 1-l ) was high enough for the purpose. The alkyllead was measured in the presence of the barium sulphate precipitate by the usual standard addition method. Ionic alkyllead compounds were deposited at different potentials: - 1.5 V and - 1.0 V for trialkyl species and - 1.5, - 1.0, - 0.7 and - 0.6 V for dialkyl species. Under the conditions applied, the detection limit of the method was around 2 x lo-” mol Pb 1-l. Results and discussion
As is well known, many ions can coprecipitate with barium sulphate [14]. The extent of coprecipitation depends on the chemical similarity of these ions to the constituent ions of the precipitate and on the solubility of the compound formed between the foreign ion and the oppositely charged constituent ion [ 14,151. The ionic radii of lead and barium are very similar (1.2 nm and 1.4 nm, respectively) and the solubility product of lead sulphate is K, = 1.06 x lo-*. Thus the lead ion satisfies both requirements for efficient coprecipitation with barium sulphate. The concentrations of barium and sulphate ions necessary to remove ionic lead from solution (at least to the concentration detectable by DPASV) were
351
studied. The inorganic lead concentrations used were between 1 x lo-’ and 1 x 10m8mol l-l, which corresponds to the level of total lead in natural waters and polluted urban waters. The removal of ionic lead from seawater (pH 8) is shown in Fig. 1. Visible precipitation commenced at barium chloride concentrations exceeding 1 x 10m4 mol 1-l but a large excess of barium ions was of course needed to remove lead down to the detection limit. Several additions of a concentrated solution of barium enhanced the coprecipitation because a finely dispersed precipitate was formed; this is in contrast to traditional practice for precipitating barium sulphate. As barium sulphate particles are prone to adsorption and coprecipitation of other ions [ 14,151, competition from other components of the solution affects the efficiency of lead ion removal. This effect is evident from Fig. 2 which shows the removal of ionic lead from distilled water and seawater at different pH values. If enough barium and sulphate ions are present, lead can be removed efficiently even from solutions at pH 2. For comparison purposes, the coprecipitation of other metals (Cd, Cu and Zn) was also tested; cadmium and zinc were not coprecipitated and copper was affected only slightly. The lead was bound to the precipitate very strongly, so that ionic alkyllead could be determined by the standard addition method without removal of the precipitate from the cell. To check that ionic alkyllead compounds were not coprecipitated, experiments were done with methyl and ethyl species in seawater (pH 8) and in
log [BaCl2 1(
mol
I-‘)
IBaClpl
( 10‘3mol
I-’
)
Fig. 1. The coprecipitation of dissolved inorganic lead with barium sulphate in seawater (pH 8 ) . Fig. 2. The coprecipitation of inorganic lead (1 x lo-’ mol 1-l ) with barium sulphate in various solutions: (0 ) distilled water, pH 2; (0 ) distilled water, pH 5; (0 ) seawater, pH 2; (w) seawater, pH 8.
352
distilled water (pH 1.6). The effects of the precipitation and of the deposition potential on the peak heights of four alkyllead species are shown in Table 1. In seawater, the peak heights decrease only slightly, whereas at low pH values the decrease is more pronounced for dialkyllead compounds, probably because of the dissociation of dialkyl species at low pH values. As tri- and di-alkyllead species can be distinguished by deposition at different plating potentials [lo], further tests were done. As shown in Table 1, triand di-alkyllead species were deposited at - 1.5 and - 1.0 V, whereas at potentials of -0.6 or -0.7 V only dialkyllead was reduced. However, in acidic solution the dialkyllead peak current was significantly decreased for a deposition potential of -0.6 V, thus a potential of -0.7 V was chosen. For all the compounds investigated, the standard addition method made it possible to determine the organolead compounds accurately, despite the small decreases in the voltammetric response caused by the presence of the precipitate. In further experiments, it was found that addition of EDTA influences the re-oxidation current, especially at low deposition potentials (Table 2). The electroactive complex of inorganic lead with EDTA is reduced at potentials more negative than that for free lead ion [ 161. In acidic solutions (pH < 2 ), Pb-EDTA complexes are mainly dissociated [ 161 so that complexation with EDTA cannot be used to overcome the interference of inorganic lead. Colombini et al. [lo] also found that EDTA lowers the re-oxidation current of ionic organolead compounds. TABLE 1 Effect of the precipitation process on the voltammetric peak height of ionic alkyllead compounds at different deposition potentials in seawater (pH 8)* and in distilled water (pH 1.6)b Deposition potential (V) Seawater -0.6 -0.7 - 1.0 - 1.5 Distilled water -0.6 -0.1 - 1.0
Percentage of peak height found’ Et,PbCl
94 100
95
Me,PbCl
EtpPbClp
MezPbCI,
100 96
90 90 94 96
93 95 90 98
90
60 67 63
30 65 60
“Precipitate formed by three successive 50-/d additions of 1 mol 1-l BaCl,. bPrecipitate formed by three successive additions of 50 ,~l each of 1.2 mol 1-l Na2S0, and 1 mol 1-l BaCl,. The plating potential of - 1.5 V was not used because of hydrogen evolution in the acidic solution. “Relative to the peak heights found in the absence of interferences.
353 TABLE 2 Effect of the EDTA complexation on the voltammetric peak height of ionic alkyllead compounds at different deposition potentials in seawater (pH 8)” Deposition potential (V) -0.6 -0.7 - 1.0 -1.5
Percentage of peak height foundb Et,PbCl
80 10
Me,PbCl
EtpPbC1,
Me,PbCl,
93 32
40 44 50 58
43 44 70 56
“EDTA concentration 2.5 x 10m3mol 1-l. bRelative to the peak heights found in the absence of interferences.
In conclusion, the coprecipitation method has several advantages over the complexation of inorganic lead with EDTA. It is possible to measure ionic alkyllead compounds at low pH values (pH < 2 ) , or at very negative deposition potentials ( - 1.5 V). Greater sensitivity can thus be achieved because the reoxidation current of tri- and di-alkyllead species is significantly higher for a deposition potential of - 1.5 V, as well as in acidic solutions (by a factor of about two with respect to the pH values of seawater and acidified distilled water). This work was supported Research of Croatia.
by the Self-Management
Council
for Scientific
REFERENCES 1
C.N. Hewitt and R.M. Harrison, in P:J. Craig (Ed.), Organometallic Compounds in the Environment, Longmans, Harlow, 1986, p. 160. 2 M. Radojevic and R.M. Harrison, Sci. Total. Environ., 59 (1987) 157. 3 Y.K. Chau and P.T.S. Wong, in P. Grandjean and E.C. Grandjead (Eds.), Biological Effects of Organolead Compounds, CRC, Boca Raton, FL, 1984, p. 21. 4 G. Rodered, Environ. Exp. Bot., 24 (1984) 17. 5 M. RadojeviC and R.M. Harrison, Atmos. Environ., 21 (1987) 2403. 6 D. Chakraborti, W.R.A. De Jonghe, W.E. Van Mol, R.J.A. Van Clevenbergen and F.C. Adams, Anal. Chem., 56 (1984) 2692. 7 D. Chakraborti, R.J.A. Van Clevenbergen and F.C. Adams, J. Anal. At. Spectrosc., 1 (1986) 293. 8 M. Blaszkewicz, G. Baumhoer and B. Neidhart, Int. J. Environ. Anal. Chem., 28 (1987) 207. 9 D.J. Hodges and F.G. Noden, Manag. Control. Heavy Metal Environ. Int. Conf., London, 1979, p. 408. 10 M.P. Colombini, G.bCorbini, R. Fuoco and P. Papoff, Ann. Chim. (Rome), 71 (1981) 609. 11 J.P. Riley and J.V. Towner, Mar. Pollut. Bull., 15 (1984) 153. 12 M.P. Colombini, F. Fuoco and P. Papoff, Sci. Total. Environ., 37 (1984) 293. 13 R. Moss and E.V. Browett, Analyst, 91 (1966) 428. 14 See, e.g., L. Erdey, Gravimetric Analysis, Part 2, Pergamon, Oxford, 1965, pp. 668-683. 15 B. Teiak, Arh. Kern., 11 (1937) 58; 20 (1948) 16. 16 M. Shedula, Ph.D. Thesis, Prictina, 1987.
349 Printed in The Netherlands
Short Communication
SEPARATION OF DISSOLVED ALKYLLEAD AND INORGANIC LEAD SPECIES BY COPRECIPITATION WITH BARIUM SULPHATE
N. MIKAC* and M. BRANICA Centre for Manne Research, Ruder BoikoviC Institute,
41001 Zagreb (Yugoslavia)
(Received 17th March 1988)
Summary Coprecipitation with barium sulphate is shown to be an efficient and convenient method of preventing the interference of relatively large amounts of inorganic lead in the determination of alkyllead species by differential-pulse anodic stripping voltammetry. The coprecipitation method has several advantages over complexation of lead with EDTA, ionic alkyllead can be measured in solutions of low pH (pH < 2)) or at deposition potentials more negative than - 1.0 V.
Tetraalkyllead compounds are introduced into the environment mainly because of their usage in gasoline. A fraction of these alkyllead compounds (l2% ) is emitted unchanged from motor vehicles [ 1,2]. Together with other sources of alkyllead, however, these toxic compounds [ 3,4] form only a minor fraction of total lead present in the environment. Under natural conditions, the initial tetraalkyl compounds are subject to degradation processes [ 1,2,5], forming ionic alkyllead derivatives (tri- and di-alkyllead). The much higher concentrations of inorganic lead in natural samples compared to alkyllead species are a source of interference in almost every analytical method used for alkyllead determination [ 2 1. Accordingly, such procedures should include a step where inorganic lead is either removed from the sample, or masked with respect to the detection system applied. Complexation with EDTA (ethylenediaminetetraacetic acid) is widely used for the elimination of inorganic lead interference in various chromatographic [6-81 or electroanalytical methods [9,10 1. In some of these methods, EDTA is used in the extraction step [ 6-81, while in others it is applied during the measurement process [g-12]. In the course of developing an anodic stripping voltammetric method for the determination of ionic alkyllead compounds in natural waters based on selective extraction and electroanalytical detection, the applicability of EDTA complexation was investigated. The investigation showed that the use of EDTA is
0003-2670/88/$03.50
0 1988 Elsevier Science Publishers B.V.
350
efficient only under limited experimental conditions. In a search for an alternative method, coprecipitation of inorganic lead with barium sulphate was found to be a convenient way of overcoming the lead interference in various electrolyte solutions and under different experimental conditions. Experimental Apparatus.
For the voltammetric measurements, a Princeton Applied Research (PAR) 174A polarographic analyzer was used with an HP-7045A x-y recorder. The working electrode was a hanging mercury drop electrode (Metrohm E-290); the auxiliary electrode was a platinum wire and the reference electrode was Ag/AgCl. For differential-pulse anodic stripping voltammetry (DPASV), the conditions were: pulse amplitude 50 mV, pulse frequency 0.5 s, and scan rate 5 mV s-l. Chemicals. Trialkyllead chlorides were obtained from Alfa-Ventron. Dialkyllead compounds were prepared from the corresponding trialkyllead standards by reaction with iodine monochloride [ 131. Standard solutions were prepared in tetra-distilled water and stored in the dark. The inorganic lead standard was from Merck. Other chemicals (sodium sulphate, barium chloride and EDTA) were of analytical-reagent grade, except nitric acid which was tetradistilled. The solution in the electrochemical cell was deaerated with highpurity nitrogen. All glassware was washed with diluted nitric acid prior to use. Procedure. The solution in the electrochemical cell (25-50 ml) was deaerated and inorganic lead was removed by coprecipitation with barium sulphate. For this purpose, 2-4 x 50 ~1 of sodium sulphate solution (1.2 mol 1-l ) and the same volumes of barium chloride solution (1 mol 1-l) were added. In the case of seawater, only the barium chloride solution was added, because the natural level of sulphate (2.9 x 10m2 mol 1-l ) was high enough for the purpose. The alkyllead was measured in the presence of the barium sulphate precipitate by the usual standard addition method. Ionic alkyllead compounds were deposited at different potentials: - 1.5 V and - 1.0 V for trialkyl species and - 1.5, - 1.0, - 0.7 and - 0.6 V for dialkyl species. Under the conditions applied, the detection limit of the method was around 2 x lo-” mol Pb 1-l. Results and discussion
As is well known, many ions can coprecipitate with barium sulphate [14]. The extent of coprecipitation depends on the chemical similarity of these ions to the constituent ions of the precipitate and on the solubility of the compound formed between the foreign ion and the oppositely charged constituent ion [ 14,151. The ionic radii of lead and barium are very similar (1.2 nm and 1.4 nm, respectively) and the solubility product of lead sulphate is K, = 1.06 x lo-*. Thus the lead ion satisfies both requirements for efficient coprecipitation with barium sulphate. The concentrations of barium and sulphate ions necessary to remove ionic lead from solution (at least to the concentration detectable by DPASV) were
351
studied. The inorganic lead concentrations used were between 1 x lo-’ and 1 x 10m8mol l-l, which corresponds to the level of total lead in natural waters and polluted urban waters. The removal of ionic lead from seawater (pH 8) is shown in Fig. 1. Visible precipitation commenced at barium chloride concentrations exceeding 1 x 10m4 mol 1-l but a large excess of barium ions was of course needed to remove lead down to the detection limit. Several additions of a concentrated solution of barium enhanced the coprecipitation because a finely dispersed precipitate was formed; this is in contrast to traditional practice for precipitating barium sulphate. As barium sulphate particles are prone to adsorption and coprecipitation of other ions [ 14,151, competition from other components of the solution affects the efficiency of lead ion removal. This effect is evident from Fig. 2 which shows the removal of ionic lead from distilled water and seawater at different pH values. If enough barium and sulphate ions are present, lead can be removed efficiently even from solutions at pH 2. For comparison purposes, the coprecipitation of other metals (Cd, Cu and Zn) was also tested; cadmium and zinc were not coprecipitated and copper was affected only slightly. The lead was bound to the precipitate very strongly, so that ionic alkyllead could be determined by the standard addition method without removal of the precipitate from the cell. To check that ionic alkyllead compounds were not coprecipitated, experiments were done with methyl and ethyl species in seawater (pH 8) and in
log [BaCl2 1(
mol
I-‘)
IBaClpl
( 10‘3mol
I-’
)
Fig. 1. The coprecipitation of dissolved inorganic lead with barium sulphate in seawater (pH 8 ) . Fig. 2. The coprecipitation of inorganic lead (1 x lo-’ mol 1-l ) with barium sulphate in various solutions: (0 ) distilled water, pH 2; (0 ) distilled water, pH 5; (0 ) seawater, pH 2; (w) seawater, pH 8.
352
distilled water (pH 1.6). The effects of the precipitation and of the deposition potential on the peak heights of four alkyllead species are shown in Table 1. In seawater, the peak heights decrease only slightly, whereas at low pH values the decrease is more pronounced for dialkyllead compounds, probably because of the dissociation of dialkyl species at low pH values. As tri- and di-alkyllead species can be distinguished by deposition at different plating potentials [lo], further tests were done. As shown in Table 1, triand di-alkyllead species were deposited at - 1.5 and - 1.0 V, whereas at potentials of -0.6 or -0.7 V only dialkyllead was reduced. However, in acidic solution the dialkyllead peak current was significantly decreased for a deposition potential of -0.6 V, thus a potential of -0.7 V was chosen. For all the compounds investigated, the standard addition method made it possible to determine the organolead compounds accurately, despite the small decreases in the voltammetric response caused by the presence of the precipitate. In further experiments, it was found that addition of EDTA influences the re-oxidation current, especially at low deposition potentials (Table 2). The electroactive complex of inorganic lead with EDTA is reduced at potentials more negative than that for free lead ion [ 161. In acidic solutions (pH < 2 ), Pb-EDTA complexes are mainly dissociated [ 161 so that complexation with EDTA cannot be used to overcome the interference of inorganic lead. Colombini et al. [lo] also found that EDTA lowers the re-oxidation current of ionic organolead compounds. TABLE 1 Effect of the precipitation process on the voltammetric peak height of ionic alkyllead compounds at different deposition potentials in seawater (pH 8)* and in distilled water (pH 1.6)b Deposition potential (V) Seawater -0.6 -0.7 - 1.0 - 1.5 Distilled water -0.6 -0.1 - 1.0
Percentage of peak height found’ Et,PbCl
94 100
95
Me,PbCl
EtpPbClp
MezPbCI,
100 96
90 90 94 96
93 95 90 98
90
60 67 63
30 65 60
“Precipitate formed by three successive 50-/d additions of 1 mol 1-l BaCl,. bPrecipitate formed by three successive additions of 50 ,~l each of 1.2 mol 1-l Na2S0, and 1 mol 1-l BaCl,. The plating potential of - 1.5 V was not used because of hydrogen evolution in the acidic solution. “Relative to the peak heights found in the absence of interferences.
353 TABLE 2 Effect of the EDTA complexation on the voltammetric peak height of ionic alkyllead compounds at different deposition potentials in seawater (pH 8)” Deposition potential (V) -0.6 -0.7 - 1.0 -1.5
Percentage of peak height foundb Et,PbCl
80 10
Me,PbCl
EtpPbC1,
Me,PbCl,
93 32
40 44 50 58
43 44 70 56
“EDTA concentration 2.5 x 10m3mol 1-l. bRelative to the peak heights found in the absence of interferences.
In conclusion, the coprecipitation method has several advantages over the complexation of inorganic lead with EDTA. It is possible to measure ionic alkyllead compounds at low pH values (pH < 2 ) , or at very negative deposition potentials ( - 1.5 V). Greater sensitivity can thus be achieved because the reoxidation current of tri- and di-alkyllead species is significantly higher for a deposition potential of - 1.5 V, as well as in acidic solutions (by a factor of about two with respect to the pH values of seawater and acidified distilled water). This work was supported Research of Croatia.
by the Self-Management
Council
for Scientific
REFERENCES 1
C.N. Hewitt and R.M. Harrison, in P:J. Craig (Ed.), Organometallic Compounds in the Environment, Longmans, Harlow, 1986, p. 160. 2 M. Radojevic and R.M. Harrison, Sci. Total. Environ., 59 (1987) 157. 3 Y.K. Chau and P.T.S. Wong, in P. Grandjean and E.C. Grandjead (Eds.), Biological Effects of Organolead Compounds, CRC, Boca Raton, FL, 1984, p. 21. 4 G. Rodered, Environ. Exp. Bot., 24 (1984) 17. 5 M. RadojeviC and R.M. Harrison, Atmos. Environ., 21 (1987) 2403. 6 D. Chakraborti, W.R.A. De Jonghe, W.E. Van Mol, R.J.A. Van Clevenbergen and F.C. Adams, Anal. Chem., 56 (1984) 2692. 7 D. Chakraborti, R.J.A. Van Clevenbergen and F.C. Adams, J. Anal. At. Spectrosc., 1 (1986) 293. 8 M. Blaszkewicz, G. Baumhoer and B. Neidhart, Int. J. Environ. Anal. Chem., 28 (1987) 207. 9 D.J. Hodges and F.G. Noden, Manag. Control. Heavy Metal Environ. Int. Conf., London, 1979, p. 408. 10 M.P. Colombini, G.bCorbini, R. Fuoco and P. Papoff, Ann. Chim. (Rome), 71 (1981) 609. 11 J.P. Riley and J.V. Towner, Mar. Pollut. Bull., 15 (1984) 153. 12 M.P. Colombini, F. Fuoco and P. Papoff, Sci. Total. Environ., 37 (1984) 293. 13 R. Moss and E.V. Browett, Analyst, 91 (1966) 428. 14 See, e.g., L. Erdey, Gravimetric Analysis, Part 2, Pergamon, Oxford, 1965, pp. 668-683. 15 B. Teiak, Arh. Kern., 11 (1937) 58; 20 (1948) 16. 16 M. Shedula, Ph.D. Thesis, Prictina, 1987.