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The determination of bromide, chloride and lead in airborne particulate matter by stripping voltammetry
Analytica Chimica Ada, 86 (1976) 27-34 @Elsevier Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
TI3E DE~RMINATION OF BROMIDE, CHLORIDE AND LEAD IN AIRBORNE PARTICULATE MATTER BY STRIPPING VOLTAMMETRY
B. L. DENNIS,
G. S. WILSON and J. L. MOYERS
Uniuersify Analytical Center, Department Arizona 85721 (U.S.A.)
of Chemistry, Uniuersity of Arizona, Tucson,
(Received 20th April 1976)
SUMMARY Cathodic and anodic stripping voltammetry are proposed for the simuItaneous determination of chloride, bromide and lead in atmospheric particulate matter, Particulate matter colfected on Nudeopore membrane filters with high-volume pumps is digested in a dilute nitric acid wash at elevated temperature and pressure for I h. The sample is then removed to the electrochemical cell and analyzed directly for chloride and bromide by cathodic stripping and for lead by anodic stripping. The total amount of halide is first determined and then, by changing the deposition potential, the amount of bromide only is determined. The chloride is found indirectly by difference. Analysis time for the three elements is approximately 20 min beginning with a prepared sample-
Reliable and sensitive methods
for the determination
of non-metals
are
essential in the course of many routine laboratory analyses, and especitily so for en~onment~ research. Presently, rapid sensitive methods for the simultaneous determination of halogens or halides are very much in demand. Although extensive work has been done on halogen determinations by neutron activation analysis (n-a-a.) [ 1-53 with excellent precision and sensitivity, this method does not lend itself well to routine analysis, because of the need for costly reactor facilities and the time required for activation and counting. Very many spectrophotometric methods have been described, but with very few exceptions [6], these are total halogen methods which do not distinguish between chlorine and bromine. Further, several of these methods are sensitive to the presence of various metals and in some cases depend on maintaining close pH and temperature control over the samples. X-ray fluorescence has been used [7] to determine bromine with relatively good precision; however, the method is unsuitable for chlorine determinations, simultaneous determinations of halogens are impossible, and this technique imposes some restrictions on the type of sample to be analyzed. The electrochemical procedure described here for the determination of halogen is essentially that of Colovos et al. [8] modified for atmospheric samples, and offers good reliability as well as a sensitivity similar to, or better than, that of most of the above-mentioned procedures. Furthermore,
28
the procedure is relatively simple and fast to use and has been routinely applied to the study of atmospheric particulate lead and halogens in this laboratory [ 10 J . EXPERIMENTAL
Electrochemical apparatus A Princeton Applied Research (PAR) Model 174 polarographic analyzer and a Hewlett-Packard 7005 x-y recorder were used. Peak areas were integrated by connecting an operational amplifier integrator (Burr-Brown Model 3062/15) with an RC time constant of 0.012 s and powered by a f 15-V power supply between the output of the Model 174 and the input of a Hewlett-Packard 1750 A strip chart recorder. The working electrode was a Brinkman Instruments Model E-410 hanging mercury drop electrode (HMDE). A saturated.calomel electrode (SCE) and a platinum counter electrode were used to complete the three-electrode system. Contamination of the cell solution by potassium chloride was eliminated by introducing a slow supply (approximately 0.2 ml min-‘) of 1 lb1KNOX into the double junction salt bridge and removing it at the top. The design and construction of the cell and electrode assembly have been previously described [9]. Reproducible stirring was maintained by means of a 600 r.p.m. synchronous magnetic rotor (E. H. Sargent Co.) positioned directly below the cell, with a 3 X 17 mm Teflon-covered stirring bar. Cell solutions were deaerated with nitrogen which h-ad been passed through a catalytic column (BASF Catalyst R3-11) at room temperature. The minimum cell volume used was 15 ml. Collection of samples Atmospheric samples were collected on Nucleopore membrane filters (G.E., 8 X 10 in. sheets, 1.0~pm pore diameter) with high-volume filtration pumps (e.g. Sierra Instruments, Model 305 with Model 310 flow controllers) operated at a flow rate of 50 m3 h-‘. Sample collection times were 12-24 h. Reagents and standards Dilute ultra-pure nitric acid (Suprapure, EM Laboratories) was used as supporting electrolyte. Distilled, doubly de-ionized water was used. All other chemicals were of analytical reagent-grade quality. Synthetic halide stock solutions were made by serial dilution of 1.0 M solutions of the potassium salts of the appropriate anion. Standard curves were prepared from 3: 1 molar mixtures of Cl- :Bf. Sample
preparation
and analytical
procedure
Cut half of an 8 X 10-m. Nucleopore filter into small pieces and place in
the Teflon cup (70-ml capacity) of an acid digestion bomb (Uniseal Decomposition Vessels Ltd.). Be careful during this procedure so that
30
the digestion procedure; and if the amounts of lead, bromide, and chloride observed in the eiectrochemical analysis compared favorably with the results observed by a referee method.
Sample dissolution
and analytical procedures
Complete dissolution of water-soluble material was checked by comparison between a portion of a filter that was worked up by the digestion procedure previously described and a portion of the same collected sample that was completely destroyed in a low-temperature asher (LTA) (Model LTA-600, LFE Corp., Trapelo Div.) at approximately 60 W r.f. and 0.1 mm Hg. Lead was used as the test ion, since destruction of the filter by lowtemperature ashing results in the loss of all halogen material through oxidation and volatilization. Atomic absorption spectrometry was used to obtain the data (see Table 2). The results indicate that, in general, only a few percent of the total lead remains on the-filter material after the wet digestion procedure, and it is assumed that lower or similar levels of halide may remain undissolved as well. Synthetic halide samples were tested for halide losses during the digestion by adding known amounts of chloride and bromide (in a Cl-:Bf molar ratio of 3:l) to the bomb and going through the digestion procedure in the same way as for atmospheric particulate samples. The results showed that essentially no bromide or chloride was lost.
Standard reference materials The ability of the as-v. and C.S.V.procedures to determine accurately small quantities of lead and chlorine (as chloride) was tested by analysis of NBS Orchard Leaves (SRM 1571). Samples analyzed for lead were ashed in a muffle fimace at 400 “C for 8 h. The residue was dissolved in 3 M HCl and digested on a hot plate for 3 h in a Teflon beaker_ The samples were taken to dryness and diluted with 0.15 M HN03. Insoluble silicates were removed by centrifugation, and an aliquot was taken for analysis. Chloride samples were prepared by burning a 50-mg sample of Orchard Leaves in a Schijniger combustion flask (A. H. Thomas Co.) in an oxygen atmosphere; combustion gases were absorbed in distilled, deionized water. After allowing TABLE
2
Comparison of lead found in atmospheric (LTA) and digestion procedures
particulate
samples
by low-temperature
Sample
Pb found (LTA) iJg
Pb found (digestion) fig
Difference %
1
240 600 80.0 472
238 586 71.3 451
0.9 2.3 10.9 4.4
2 3 4
ashing
31
15 min for absorption of the gaseous products, an aliquot was acidified with ultrapure nitric acid and analyzed. Three analyses for lead on sample weights of about 0.88 g of SRM 1571 gave an average value of 45.7 (k 1.7) pg Pb g-’ (certificate value 45.0 (+3) pg Pb g-‘). Four analyses for chlorine on sample weights of 0.050 g gave an average value of 738 (i. 38) pg Cl g-l (provisional NBS value 700 pg Cl g-l)_ Although the lead values are certified, the chlorine values are not (since they were based on the results of a single method of analysis), and are given for information only. Comparison of lead and halide data by stripping voltammetry with referee methods Lead values obtained by a.s.v. on particulate samples were compared to values obtained by atomic absorption spectrometry (see Table 3). Standard deviations for both methods are approximately 3 %. Bromide and chloride values obtained by C.S.V.were compared with values obtained by neutron activation for particulate samples (see Table 4).
TABLE 3 Comparison of analyses for lead in particulate samples by anodic stripping voltammetry and atomic absorption spectrometry Samplea
Pb found (as-v.) JJg
Pb found (a.a.s.) fig
1 2 3 4
240 f 294 f 722 456 +
227 + 7 316 +_9 72+ 2 464 + 14
7 9 2 14
%amples for the two methods were taken after dissolution step.
TABLE 4 Comparison of cathodic stripping voltammetry (c.s.v.) and instrumental neutron activation analysis (i.n.aa.) for bromine and chlorine in particulate samplesa Sample
C.S.V.
1.n.a.a.
Br (rg) 1 2 3 4
24.3 24.3 16.0 26.9
f t t +
1.7 1.7 1.1 1.9
CI (rg)
Br (pg)
20.2 24.1 33.0 31.1
26.2 25.1 15.6 26.5
f 1.4 2 1.7 it 2.3 + 2.2
+ * i f
Ci (fJg) 2.0 2.0 1.3 2.0
24 22 38 28
r_ 3.5 2 3.0 + 5.0 + 4.0
“Samples for the two methods were taken after dissolution step.
32
Construction of standard curves for total halide and bromide determination Based on preliminary findings with collected samples, standard curves for total halide and bromide analysis were constructed in a 3:l Cl- :Br- molar ratio. Earlier work showed that the Cl-:Br- concentration ratio varied from 2 to 5 in most of the samples tested. In some instances, higher Cl-:Brratios were observed, but only rarely did the ratio fall below 2. Since it was likely that ratios in future samples would continue to vary between 1 and 5, a study was undertaken to determine the accuracy with which samples could be analyzed when 3:l molar ratio standard curves were used. Known amounts of halide in ratios representative of above extremes were checked against standard curves prepared in 3:l Cl:Br concentration ratios. Samples taken in a 3:l molar ratio showed an overall accuracy of approximately 7 70 for the chloride determination, and nearly 5 % for the bromide determination over-the range investigated. These estimates are in good agreement with the results of Colovos et al. is]. At higher concentration ratios, the accuracy in the chloride determination improved markedly. This may be the result of increased efficiency of the deposition process at higher chloride concentrations, as well as a more efficient exchange reaction between chloride and bromide at the electrode surface under such conditions_ The error in the bromide determination remained nearly the same as was found for the 3:l molar ratio. The magnitude of the error in the chloride determination for the 1:l molar ratio samples was 15-20 %. Decreased rate of the exchange reaction between the two halides may be the reason for the observed results. Errors in the bromide determination were similar to those observed in samples of 3:l and 5:l (Cl-:Br-) molar ratios. Simultaneous determinations of chloride and bromide in atmospheric samples by the outlined procedure should not produce any serious problems in the accuracy of the determinations in this study since only two of the samples analyzed showed a Cl-:Br- molar ratio of less than 2:l. The influence of kinetic factors and film solubility on the detection limits for bromide and total halide determinations is very striking. In the procedure used, final solution concentrations of less than 2 - lo-’ M in total halide and 6 - 10e6 M bromide gave no stripping peak at all, yet concentrations of 2.5 - lo-’ M total halide and 6.2 * 10e6 M (ca. 0.5 p.p.m_) bromide showed small, but quantifiable peaks. Below these concentrations, apparently no film formation occurs; only a baseline response was observed in such cases. Instrumental limitations are not a factor in defining the detection limits. Peak areas, obtained by electronic integration, were used in the construction of standard curves, since the stripping peaks were sometimes distorted at lower concentrations of halide, so that the use of peak heights was undesirable. -4 second reason for the use of peak areas was that in some instances shoulders were observed on the stripping peaks, owing to the influence of the exchange reaction_ Although these shoulders represent halide, they do not contribute to the height of the main peak unless sufficient time is allowed for the exchange reaction to be completed at open circuit [S] . Integration of the
33
entire area under the stripping curve measures all halide present without increasing the analysis time. Interferences in the analysis procedure for halides by C.S.V. The effect of various ions on the stripping peaks of synthetic,samples was investigated. Halide samples in 3:l (Cl-:Br-) concentration ratios were analyzed in the presence of various elements in amounts approximating those found in Tucson atmospheric particulate samples on an entire filter. The amounts of the ions studied are listed in Table 5. Aluminum and iron were added in quantities approximately half as large as normally observed; however, a large portion of these two elements are present as soil components and are solubilized only by hydrofluoric acid. Copper was added in quantities almost
three
times
that normally
observed.
Of all the metals
listed
in Table
5,
only copper presents any possibility of interference, because it is reduced at the HMDE during the potential scan; its presence can be observed as a small peak on the voltammogram at approximately 0.0 V vs. SCE. The bromide peak is observed from approximately + 0.12 to + 0.07 V vs. SCE. depending on the bromide concentration, and unless the copper concentration is very large, no problem will be encountered. No evidence of copper interference was found in samples analyzed during the sampling program. Although usually unimportant from the point of view of atmospheric samples, severe interferences from thiocyanate and sulfide were observed; these species react with mercury to form films of Hg2(SCN)2 and HgS, respectively. In both cases, the peak potentials of the reactions in acidic media are more negative than the deposition potential for either the bromide or total halide. At the halide deposition potentials, co-deposition occurs which apparently contributes to decreased halide film formation, leading to much smaller stripping currents. In the presence of thiocyanate (5.8 p-p-m-), the chloride peak was severely distorted and reduced significantly in size. In the presence of sulfide (4.3 p_p.m.), no stripping peak was observed. In order to avoid such interferences, these species must be removed from solution by precipitation or by selective oxidation to an electro-inactive species. The other interferent observed was oxygen. In acidic media oxygen is reduced at the HMDE, forming hydrogen peroxide, the EI~ value is approximately -0.05 V vs. SCE, so that interference from the rising portion of the reduction wave prevents the descending portion of the halide stripping peak TABLE
5
Composition
of synthetic
taken
Ion
pg
Al”
3000 3000 15
Ca”
Cd” cr”
15
samples
Ion Cu’ Fe”
used for evaluation
pg taken l
Mg**
375 1700 750
of halide
interferences
Ion
ug taken
Ion
pg taken
Mn”
225
SiO,‘-
9000
SO,‘_
2200
Ni”
Pb=’
Zn’ f
150
34
from returning to baseline_ This introduces an error into the area determination of the stripped peak if some correction is not applied. To avoid this problem, standards were run in the same manner as were the samples. As long as the oxygen content of the solution does not vary significantly from sample to sample, the error in the peak area will be small. The height of the oxygen wave was checked for several blanks and no significant change was noted from sample to sample. Deaeration of the sample in acidic solution was undesirable in the presence of trace amounts of halide. Peak areas for total halide analyses decreased by as much as 20 5%after sample deaeration, undoubtedly because of loss of hydrogen chloride during the deaeration step. The effect of deaeration on the bromide peak area was insignificant. We are grateful to L. E. Ranweiler and M. Seymour who assisted with the a.a.s. and i.n.a.a. This work was supported in part through a contract from the Electric Power Research Institute (RP-438-1). REFERENCES 1 J. F. Cosgrove, R. P. Bastram and G. H. Morrison, Anal. Chem., 30 (1958) 1872. 2 R. A. Duce and J. W. Winchester, Radiochim. Acta, 4 (1965) 100. 3 W. H. Zoller and G. E. Gordon, Anal. Chem., 42 (1970) 257. 4 R. Dams, J. A. Robbins, K. A. Rahn and J. W. Winchester, Anal. Chem., 42 (1970) 5 J. L. Moyers and R. A. Duce, Anal. Chim. Acta, 69 (1974) 177.
6 See, e.g. H. A. Laitinen and K. W. Boyer, Anal. Chem., 44 (1972) 920. 7 C!. S. Martens, J_ J. Wesolowski, R. Kaifer and W. John, Atmos. Environ., 7 (1973) 8 G. Colovos, G. S. Wilson and J. L. Moyers, Anal. Chem., 46 (1974) 1045, 1051. 9 G. Colovos, G. S. Wiison and J. L. Moyers, Anal. Chim. Acta, 64 (1973) 457. 10 B. L. Dennis and J. L. Moyers, Submitted to Atmos. Environ., (1976).
861.
905.
Printed in The Netherlands
TI3E DE~RMINATION OF BROMIDE, CHLORIDE AND LEAD IN AIRBORNE PARTICULATE MATTER BY STRIPPING VOLTAMMETRY
B. L. DENNIS,
G. S. WILSON and J. L. MOYERS
Uniuersify Analytical Center, Department Arizona 85721 (U.S.A.)
of Chemistry, Uniuersity of Arizona, Tucson,
(Received 20th April 1976)
SUMMARY Cathodic and anodic stripping voltammetry are proposed for the simuItaneous determination of chloride, bromide and lead in atmospheric particulate matter, Particulate matter colfected on Nudeopore membrane filters with high-volume pumps is digested in a dilute nitric acid wash at elevated temperature and pressure for I h. The sample is then removed to the electrochemical cell and analyzed directly for chloride and bromide by cathodic stripping and for lead by anodic stripping. The total amount of halide is first determined and then, by changing the deposition potential, the amount of bromide only is determined. The chloride is found indirectly by difference. Analysis time for the three elements is approximately 20 min beginning with a prepared sample-
Reliable and sensitive methods
for the determination
of non-metals
are
essential in the course of many routine laboratory analyses, and especitily so for en~onment~ research. Presently, rapid sensitive methods for the simultaneous determination of halogens or halides are very much in demand. Although extensive work has been done on halogen determinations by neutron activation analysis (n-a-a.) [ 1-53 with excellent precision and sensitivity, this method does not lend itself well to routine analysis, because of the need for costly reactor facilities and the time required for activation and counting. Very many spectrophotometric methods have been described, but with very few exceptions [6], these are total halogen methods which do not distinguish between chlorine and bromine. Further, several of these methods are sensitive to the presence of various metals and in some cases depend on maintaining close pH and temperature control over the samples. X-ray fluorescence has been used [7] to determine bromine with relatively good precision; however, the method is unsuitable for chlorine determinations, simultaneous determinations of halogens are impossible, and this technique imposes some restrictions on the type of sample to be analyzed. The electrochemical procedure described here for the determination of halogen is essentially that of Colovos et al. [8] modified for atmospheric samples, and offers good reliability as well as a sensitivity similar to, or better than, that of most of the above-mentioned procedures. Furthermore,
28
the procedure is relatively simple and fast to use and has been routinely applied to the study of atmospheric particulate lead and halogens in this laboratory [ 10 J . EXPERIMENTAL
Electrochemical apparatus A Princeton Applied Research (PAR) Model 174 polarographic analyzer and a Hewlett-Packard 7005 x-y recorder were used. Peak areas were integrated by connecting an operational amplifier integrator (Burr-Brown Model 3062/15) with an RC time constant of 0.012 s and powered by a f 15-V power supply between the output of the Model 174 and the input of a Hewlett-Packard 1750 A strip chart recorder. The working electrode was a Brinkman Instruments Model E-410 hanging mercury drop electrode (HMDE). A saturated.calomel electrode (SCE) and a platinum counter electrode were used to complete the three-electrode system. Contamination of the cell solution by potassium chloride was eliminated by introducing a slow supply (approximately 0.2 ml min-‘) of 1 lb1KNOX into the double junction salt bridge and removing it at the top. The design and construction of the cell and electrode assembly have been previously described [9]. Reproducible stirring was maintained by means of a 600 r.p.m. synchronous magnetic rotor (E. H. Sargent Co.) positioned directly below the cell, with a 3 X 17 mm Teflon-covered stirring bar. Cell solutions were deaerated with nitrogen which h-ad been passed through a catalytic column (BASF Catalyst R3-11) at room temperature. The minimum cell volume used was 15 ml. Collection of samples Atmospheric samples were collected on Nucleopore membrane filters (G.E., 8 X 10 in. sheets, 1.0~pm pore diameter) with high-volume filtration pumps (e.g. Sierra Instruments, Model 305 with Model 310 flow controllers) operated at a flow rate of 50 m3 h-‘. Sample collection times were 12-24 h. Reagents and standards Dilute ultra-pure nitric acid (Suprapure, EM Laboratories) was used as supporting electrolyte. Distilled, doubly de-ionized water was used. All other chemicals were of analytical reagent-grade quality. Synthetic halide stock solutions were made by serial dilution of 1.0 M solutions of the potassium salts of the appropriate anion. Standard curves were prepared from 3: 1 molar mixtures of Cl- :Bf. Sample
preparation
and analytical
procedure
Cut half of an 8 X 10-m. Nucleopore filter into small pieces and place in
the Teflon cup (70-ml capacity) of an acid digestion bomb (Uniseal Decomposition Vessels Ltd.). Be careful during this procedure so that
30
the digestion procedure; and if the amounts of lead, bromide, and chloride observed in the eiectrochemical analysis compared favorably with the results observed by a referee method.
Sample dissolution
and analytical procedures
Complete dissolution of water-soluble material was checked by comparison between a portion of a filter that was worked up by the digestion procedure previously described and a portion of the same collected sample that was completely destroyed in a low-temperature asher (LTA) (Model LTA-600, LFE Corp., Trapelo Div.) at approximately 60 W r.f. and 0.1 mm Hg. Lead was used as the test ion, since destruction of the filter by lowtemperature ashing results in the loss of all halogen material through oxidation and volatilization. Atomic absorption spectrometry was used to obtain the data (see Table 2). The results indicate that, in general, only a few percent of the total lead remains on the-filter material after the wet digestion procedure, and it is assumed that lower or similar levels of halide may remain undissolved as well. Synthetic halide samples were tested for halide losses during the digestion by adding known amounts of chloride and bromide (in a Cl-:Bf molar ratio of 3:l) to the bomb and going through the digestion procedure in the same way as for atmospheric particulate samples. The results showed that essentially no bromide or chloride was lost.
Standard reference materials The ability of the as-v. and C.S.V.procedures to determine accurately small quantities of lead and chlorine (as chloride) was tested by analysis of NBS Orchard Leaves (SRM 1571). Samples analyzed for lead were ashed in a muffle fimace at 400 “C for 8 h. The residue was dissolved in 3 M HCl and digested on a hot plate for 3 h in a Teflon beaker_ The samples were taken to dryness and diluted with 0.15 M HN03. Insoluble silicates were removed by centrifugation, and an aliquot was taken for analysis. Chloride samples were prepared by burning a 50-mg sample of Orchard Leaves in a Schijniger combustion flask (A. H. Thomas Co.) in an oxygen atmosphere; combustion gases were absorbed in distilled, deionized water. After allowing TABLE
2
Comparison of lead found in atmospheric (LTA) and digestion procedures
particulate
samples
by low-temperature
Sample
Pb found (LTA) iJg
Pb found (digestion) fig
Difference %
1
240 600 80.0 472
238 586 71.3 451
0.9 2.3 10.9 4.4
2 3 4
ashing
31
15 min for absorption of the gaseous products, an aliquot was acidified with ultrapure nitric acid and analyzed. Three analyses for lead on sample weights of about 0.88 g of SRM 1571 gave an average value of 45.7 (k 1.7) pg Pb g-’ (certificate value 45.0 (+3) pg Pb g-‘). Four analyses for chlorine on sample weights of 0.050 g gave an average value of 738 (i. 38) pg Cl g-l (provisional NBS value 700 pg Cl g-l)_ Although the lead values are certified, the chlorine values are not (since they were based on the results of a single method of analysis), and are given for information only. Comparison of lead and halide data by stripping voltammetry with referee methods Lead values obtained by a.s.v. on particulate samples were compared to values obtained by atomic absorption spectrometry (see Table 3). Standard deviations for both methods are approximately 3 %. Bromide and chloride values obtained by C.S.V.were compared with values obtained by neutron activation for particulate samples (see Table 4).
TABLE 3 Comparison of analyses for lead in particulate samples by anodic stripping voltammetry and atomic absorption spectrometry Samplea
Pb found (as-v.) JJg
Pb found (a.a.s.) fig
1 2 3 4
240 f 294 f 722 456 +
227 + 7 316 +_9 72+ 2 464 + 14
7 9 2 14
%amples for the two methods were taken after dissolution step.
TABLE 4 Comparison of cathodic stripping voltammetry (c.s.v.) and instrumental neutron activation analysis (i.n.aa.) for bromine and chlorine in particulate samplesa Sample
C.S.V.
1.n.a.a.
Br (rg) 1 2 3 4
24.3 24.3 16.0 26.9
f t t +
1.7 1.7 1.1 1.9
CI (rg)
Br (pg)
20.2 24.1 33.0 31.1
26.2 25.1 15.6 26.5
f 1.4 2 1.7 it 2.3 + 2.2
+ * i f
Ci (fJg) 2.0 2.0 1.3 2.0
24 22 38 28
r_ 3.5 2 3.0 + 5.0 + 4.0
“Samples for the two methods were taken after dissolution step.
32
Construction of standard curves for total halide and bromide determination Based on preliminary findings with collected samples, standard curves for total halide and bromide analysis were constructed in a 3:l Cl- :Br- molar ratio. Earlier work showed that the Cl-:Br- concentration ratio varied from 2 to 5 in most of the samples tested. In some instances, higher Cl-:Brratios were observed, but only rarely did the ratio fall below 2. Since it was likely that ratios in future samples would continue to vary between 1 and 5, a study was undertaken to determine the accuracy with which samples could be analyzed when 3:l molar ratio standard curves were used. Known amounts of halide in ratios representative of above extremes were checked against standard curves prepared in 3:l Cl:Br concentration ratios. Samples taken in a 3:l molar ratio showed an overall accuracy of approximately 7 70 for the chloride determination, and nearly 5 % for the bromide determination over-the range investigated. These estimates are in good agreement with the results of Colovos et al. is]. At higher concentration ratios, the accuracy in the chloride determination improved markedly. This may be the result of increased efficiency of the deposition process at higher chloride concentrations, as well as a more efficient exchange reaction between chloride and bromide at the electrode surface under such conditions_ The error in the bromide determination remained nearly the same as was found for the 3:l molar ratio. The magnitude of the error in the chloride determination for the 1:l molar ratio samples was 15-20 %. Decreased rate of the exchange reaction between the two halides may be the reason for the observed results. Errors in the bromide determination were similar to those observed in samples of 3:l and 5:l (Cl-:Br-) molar ratios. Simultaneous determinations of chloride and bromide in atmospheric samples by the outlined procedure should not produce any serious problems in the accuracy of the determinations in this study since only two of the samples analyzed showed a Cl-:Br- molar ratio of less than 2:l. The influence of kinetic factors and film solubility on the detection limits for bromide and total halide determinations is very striking. In the procedure used, final solution concentrations of less than 2 - lo-’ M in total halide and 6 - 10e6 M bromide gave no stripping peak at all, yet concentrations of 2.5 - lo-’ M total halide and 6.2 * 10e6 M (ca. 0.5 p.p.m_) bromide showed small, but quantifiable peaks. Below these concentrations, apparently no film formation occurs; only a baseline response was observed in such cases. Instrumental limitations are not a factor in defining the detection limits. Peak areas, obtained by electronic integration, were used in the construction of standard curves, since the stripping peaks were sometimes distorted at lower concentrations of halide, so that the use of peak heights was undesirable. -4 second reason for the use of peak areas was that in some instances shoulders were observed on the stripping peaks, owing to the influence of the exchange reaction_ Although these shoulders represent halide, they do not contribute to the height of the main peak unless sufficient time is allowed for the exchange reaction to be completed at open circuit [S] . Integration of the
33
entire area under the stripping curve measures all halide present without increasing the analysis time. Interferences in the analysis procedure for halides by C.S.V. The effect of various ions on the stripping peaks of synthetic,samples was investigated. Halide samples in 3:l (Cl-:Br-) concentration ratios were analyzed in the presence of various elements in amounts approximating those found in Tucson atmospheric particulate samples on an entire filter. The amounts of the ions studied are listed in Table 5. Aluminum and iron were added in quantities approximately half as large as normally observed; however, a large portion of these two elements are present as soil components and are solubilized only by hydrofluoric acid. Copper was added in quantities almost
three
times
that normally
observed.
Of all the metals
listed
in Table
5,
only copper presents any possibility of interference, because it is reduced at the HMDE during the potential scan; its presence can be observed as a small peak on the voltammogram at approximately 0.0 V vs. SCE. The bromide peak is observed from approximately + 0.12 to + 0.07 V vs. SCE. depending on the bromide concentration, and unless the copper concentration is very large, no problem will be encountered. No evidence of copper interference was found in samples analyzed during the sampling program. Although usually unimportant from the point of view of atmospheric samples, severe interferences from thiocyanate and sulfide were observed; these species react with mercury to form films of Hg2(SCN)2 and HgS, respectively. In both cases, the peak potentials of the reactions in acidic media are more negative than the deposition potential for either the bromide or total halide. At the halide deposition potentials, co-deposition occurs which apparently contributes to decreased halide film formation, leading to much smaller stripping currents. In the presence of thiocyanate (5.8 p-p-m-), the chloride peak was severely distorted and reduced significantly in size. In the presence of sulfide (4.3 p_p.m.), no stripping peak was observed. In order to avoid such interferences, these species must be removed from solution by precipitation or by selective oxidation to an electro-inactive species. The other interferent observed was oxygen. In acidic media oxygen is reduced at the HMDE, forming hydrogen peroxide, the EI~ value is approximately -0.05 V vs. SCE, so that interference from the rising portion of the reduction wave prevents the descending portion of the halide stripping peak TABLE
5
Composition
of synthetic
taken
Ion
pg
Al”
3000 3000 15
Ca”
Cd” cr”
15
samples
Ion Cu’ Fe”
used for evaluation
pg taken l
Mg**
375 1700 750
of halide
interferences
Ion
ug taken
Ion
pg taken
Mn”
225
SiO,‘-
9000
SO,‘_
2200
Ni”
Pb=’
Zn’ f
150
34
from returning to baseline_ This introduces an error into the area determination of the stripped peak if some correction is not applied. To avoid this problem, standards were run in the same manner as were the samples. As long as the oxygen content of the solution does not vary significantly from sample to sample, the error in the peak area will be small. The height of the oxygen wave was checked for several blanks and no significant change was noted from sample to sample. Deaeration of the sample in acidic solution was undesirable in the presence of trace amounts of halide. Peak areas for total halide analyses decreased by as much as 20 5%after sample deaeration, undoubtedly because of loss of hydrogen chloride during the deaeration step. The effect of deaeration on the bromide peak area was insignificant. We are grateful to L. E. Ranweiler and M. Seymour who assisted with the a.a.s. and i.n.a.a. This work was supported in part through a contract from the Electric Power Research Institute (RP-438-1). REFERENCES 1 J. F. Cosgrove, R. P. Bastram and G. H. Morrison, Anal. Chem., 30 (1958) 1872. 2 R. A. Duce and J. W. Winchester, Radiochim. Acta, 4 (1965) 100. 3 W. H. Zoller and G. E. Gordon, Anal. Chem., 42 (1970) 257. 4 R. Dams, J. A. Robbins, K. A. Rahn and J. W. Winchester, Anal. Chem., 42 (1970) 5 J. L. Moyers and R. A. Duce, Anal. Chim. Acta, 69 (1974) 177.
6 See, e.g. H. A. Laitinen and K. W. Boyer, Anal. Chem., 44 (1972) 920. 7 C!. S. Martens, J_ J. Wesolowski, R. Kaifer and W. John, Atmos. Environ., 7 (1973) 8 G. Colovos, G. S. Wilson and J. L. Moyers, Anal. Chem., 46 (1974) 1045, 1051. 9 G. Colovos, G. S. Wiison and J. L. Moyers, Anal. Chim. Acta, 64 (1973) 457. 10 B. L. Dennis and J. L. Moyers, Submitted to Atmos. Environ., (1976).
861.
905.