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Pheumatoamperometric determination of cyanide, sulfide and their mixtures
Analytica Chimica Acta, 169 (1985) 407-412 Elsevier Science Publishers B.V., Amsterdam -Printed
in The Netherlands
Short Communication
PNEUMATOAMPEROMETRIC DETERMINATION SULFIDE AND THEIR MIXTURES
FRANTISEK
OPEKAR
J. Heyrovsky Institute Academy of Sciences, STANLEY
OF CYANIDE,
of Physical Chemistry and Electrochemistry, Jilska 16,110 00 Prague 1 (Czechoslovakia)
Czechoslovak
BRUCKENSTEIN*
Chemistry Department, State New York 14214 (U.S.A.)
University
of New York at Buffalo,
Buffalo,
(Received 22nd August 1984)
Summary. A rapid pneumatoamperometric method for quantifying cyanide in the presence or absence of sulfide is described. The gaseous mixture of hydrogen cyanide and hydrogen sulfide is separated by inserting a short chromatographic column packed with silica gel between the reaction vessel used to generate the volatile acids and the porous gold electrode that detects them. The detection limit is ca. 5 ng cyanide in 2 ml of solution regardless of sulfide content. The detection limit for sulfide is ca. 1.0 ng in 2 ml of solution when cyanide is present and ca. 0.7 ng in absence of cyanide. Both sulfite and nitrite interfere.
The determination of cyanide described by Beran and Bruckenstein [l] is based on quantitative conversion of cyanide to iodine monocyanide and the pneumatoamperometric measurement of the equivalent amount of iodine liberated in the decomposition of ICN by a strong acid. The determination is sensitive and has a detection limit of 1.25 ng CN- ml-‘. If such sensitivity is necessary, the fact that the procedure requires eleven steps and the time is tens of minutes is unimportant. When such sensitivity is not required, the simpler and shorter pneumatoamperometric method described below is preferable. Another advantage of this pneumatoamperometric method is that sulfide, the main interference in most determinations of cyanide, does not interfere and can in fact be determined simultaneously. A pneumatoamperometric determination of sulfide having a detection limit of 2.73 ng S*- ml-’ has been developed by Nygaard [ 21. The present communication describes two pneumatoamperometric determinations of cyanide at a gold porous-teflon membrane electrode (AuPME), one in the absence of sulfide and the other in its presence. In both determinations, the sample is injected into a reaction vessel containing acid. If sulfide is absent, the hydrogen cyanide liberated is led to the AuPME where an anodic current corresponding to the formation of the Au(CN); complex [3] is measured. Sulfide ions present in the sample produce hydrogen sulfide 0003-2670/85/$03.30
o 1985 Elsevier Science Publishers B.V.
408
which leads to an anodic current that interferes seriously. However, inserting a simple chromatographic column between the reaction vessel and the AuPME separates the hydrogen cyanide and sulfide so that both can be quantified from the i/t response curve of the electrode. Compared with other electrochemical methods for simultaneous determination of these ions (e.g., rapid d.c. polarography [4] or ion chromatography with electrochemical detection [5], this pneumatoamperometric procedure offers a lower detection limit. Experimental Reagents. All the solutions were prepared from analytical-grade chemicals and water obtained from a Milli-Q reagentgrade water system (Millipore). A 0.1 M stock solution of potassium cyanide in lo-’ M sodium hydroxide was standardized by titration with silver nitrate. A 0.1 M stock solution of sodium sulfide in lo-? M sodium hydroxide was standardized iodimetritally. Solutions with lower concentrations were obtained by dilution of the stock solutions with 10m2M sodium hydroxide. Electrode and apparatus. The preparation of the AuPME and the pneumatoamperometric apparatus have been described [6-81. The reaction vessel was filled with 2 ml of 6 M sulfuric acid and 10e2 M sodium hydroxide was used as the supporting electrolyte in the voltammetric vessel. The electrode response was measured at 0.1 V vs. Ag/AgCl electrode. Nitrogen carrier gas was used at a flow rate of 9.4 ml s-’ to purge the solution in the reaction vessel. To separate the analytes, a glass chromatographic column (4-mm i.d., llO-mm long) packed with silica gel (Baker A. R.; 40-140 mesh) was used without any pretreatment. The silica gel was held in the column by glass wool plugs. The column was connected between the reaction vessel and the electrode [8]. All measurements were made at laboratory temperature, 22 + 3°C. Procedure. A constant flow rate of nitrogen through the pneumatoamperometric apparatus was established. After the residual current became constant, 10 or 100 ~1 of a solution of potassium cyanide, sodium sulfide or their mixture was injected (microliter syringe) into the reaction vessel containing the acid through a rubber septum. The i/t response curve of the AuPME was recorded. Results and Discussion Determination of cyanide. The dependence of the peak current, i,, on the amount of cyanide was linear over the studied range, 13-1560 ng. The regression equation for this dependence was i, = (8.72 f 0.09) nA ng-’ + (0.5 & 18.9) nA. The relative standard deviation was 1.27% for 10 replicate samples containing 52 ng of cyanide. The detection limit, defined as twice the peakto-peak background noise, was ca. 5 ng of cyanide in 2 ml of solution (9.6 X lO* M). The time needed was ca. 150 s. Characteristic AuPME responses are shown in Fig. 1. The greater peak
I
IJJA
( 100 5
,
Fig. 1. The AuPME responses: (a) 450 ng CN-; (b) 45 ng W. Fig. 2. Dependence of i, on the concentration 260 ng CN-).
of sulfuric acid in the reaction vessel (for
width for cyanide is caused by the lower rate at which hydrogen cyanide is transferred from the solution in the reaction vessel into the carrier gas stream; hydrogen cyanide is much more soluble in water than hydrogen sulfide. The differing rates of transfer to the gaseous phase is also reflected in their respective i, values (see pneumatoamperometric theory [ 91). The rate of transfer of hydrogen cyanide from the liquid to the gaseous phase increases with increasing concentration of sulfuric acid up to about 8 M (Fig. 2) but then decreases, probably because of the increased viscosity and foaming of the solution. A 6 M H2S04 sulfuric acid solution seems to be optimal. It has been recommended that hydrogen cyanide should be distilled from more complicated samples by purging with nitrogen through the acidified sample at an elevated temperature [3, lo]. Heating is simply done in the pneumatoamperometric determination of cyanide by injecting 1 ml of sample solution into 1 ml of 12 M sulfuric acid in the reaction vessel; the heat of dilution of sulfuric acid is sufficient. Under these conditions, a linear dependence of i, on the amount of cyanide was found in the studied range, 13260 ng, and the linear regression equation was i, = (20.87 k 0.86) nA ng-’ + (6.61 f. 61.86) nA. The sensitivity, expressed in terms of the slope of the dependence of current vs. amount of cyanide, is in this case about 2.3 times
410 higher than when a small volume of sample solution is injected into 6.0 M acid. A disadvantage of this method is the necessity of replacing the solution in the reaction vessel for each sample. Estimation of the electrolysis efficiency. About 10% of the electroactive material in the carrier gas is consumed at the electrode [ 71. The electrolysis efficiency for hydrogen cyanide can be found from the equation [ 91 E= i,R T/nFvgKCHcN, i
(1)
where vg is the carrier gas flow rate, K is the partition coefficient (K = pHCN/CHCN, where pnCN is the HCN partial pressure in the carrier gas) and c HCN. i is the initial HCN concentration in the solution. The other symbols have their usual significance. The value of K can be found from the equation describing the dependence of the electrode response on time [9] : it = iP exp(-v,Kt/VRT), where V is the volume of the solution. The K value was found from the slope of the log it vs. t dependence. From the intercept for t = 0, the i, value, unaffected by the time constant for the instrument, was then found. The K and i, values obtained in this way were used for the calculation of E. From measurements made at various initial concentrations, CncN, i, the value, K = 1.53 X lo4 Pa mol-’ was obtained; for the experimental conditions used in the determination of cyanide (v~ = 9.4 ml s-‘, V = 2 ml), the value of e was 0.14. The electrolysis efficiency was also calculated from the concentration dependence of the charge consumed during the electrolysis for various amounts of cyanide. This dependence was a straight line with a slope of 317.5 C g-‘. For 100% current efficiency, the reaction, Au + 2CN- = Au(CN);, requires 1856 C g-‘. Thus the slope found corresponds to the value E = 0.17. This value agrees satisfactorily with the value obtained from the equations based on the theory of the pneumatoamperometric method. Determination of sulfide. The i, dependence on the amount of sulfide was linear in the studied range, 8.7-1044 ng. The regression equation for this dependence was i, = (130 f 2.3) nA ng-’ + (1.6 + 45.10) ng. For 10 replicate experiments, the relative standard deviation was 1.81% for 21.2 ng of sulfide. The detection limit defined as twice the peak-to-peak background noise was ca. 0.7 ng of sulfide in 2 ml of solution. Simultaneous determination of cyanide and sulfide. By including a silicagel column between the reaction vessel and the AuPME, cyanide and sulfide can be determined in the presence of each other. The difference between the solubilities of hydrogen cyanide and sulfide in water and their partition on the silica-gel column are both utilized. After the column has been placed in the flow line, the carrier gas must be passed through the reaction vessel containing the acid and through the column for a few minutes to equilibrate the silica gel with water vapor. Linear dependencies of i, on the amounts of sulfide and cyanide in the solution were obtained for Sz- to CN- concentration ratios of 0.02-5. The sensitivity of the determination of cyanide was the same as for the measure-
411
Fig. 3. The AuPME response in the simultaneous determination of S’- and CN-: (a) 195 ng CN- + 17 ng S2-, S2-/CN- = 8.7 x lo-’ ; (b) 86.7 ng CN- + 435 ng S2-, W/CN- = 5 (for the given current scale, the experimental ip = 38.7 PA for H,S is off-scale).
ment technique without the column. The sensitivity of the determination of sulfide decreased by about 30% because of the zone broadening in the column. Thus the calibration line for sulfide must be prepared from measurements with the column. Alternatively, the sensitivity in the absence of the column and in the presence of the column can be evaluated at some sulfide level and the resulting factor used with the calibration obtained in the absence of the column. Examples of the AuPME response at two different ratios of S2-/CN- are given in Fig. 3. Up to a ratio of S2-/CN- of ca. 0.5, the two peaks are completely resolved (Fig. 3a). At higher ratios, the broadening of the sulfide peak is more perceptible, but the height of the cyanide peak corresponds to the calibration curve,provided that i, is measured as indicated in Fig. 3(b). This separation is, because of the higher sensitivity of the determination of sulfide, more suitable for samples with low S2-/CN- ratios. Interferences. The pneumatoamperometric method described above efficiently separates these anions from interferences and is equivalent to the separation by distillation of sulfide and cyanide from the sample [3, lo]. Among the common anions that form volatile and oxidizable substances in acidic media, only sulfite and nitrite interfere under the above experimental conditions. The interference from nitrite ions can be suppressed by sulfamic acid [2]. This work was supported by the Air Force Office of Scientific Research under Grant No. 84-0004.
412 REFERENCES 1 2 3 4 5 6
P. Beran and S. Bruckenstein, Anal. Chem., 52 (1980) 1183. D. D. Nygaard, Anal. Chim. Acta, 127 (1981) 257. D. B. Easty, W. J. Blaedel and L. Anderson, Anal. Chem., 43 (1971) 509. D. R. Canterford, Anal. Chem., 47 (1975) 88. A. M. Bond, I. D. Heritage and G. G. Wallace, Anal. Chem., 54 (1982) 582. W. G. Sherwood, G. A. Martinchek, L. Gal-Or and S. Bruckenstein, First Semi-Annual Tech. Progress Rep., US Bureau of Mines, Grant No. 155 007, March 1, 1975. 7 P. R. Gifford and S. Bruckenstein, Anal. Chem., 52 (1980) 1024. 8 P. R. Gifford and S. Bruckenstein, Anal. Chem., 52 (1980) 1028. 9 P. Beran and S. Bruckenstein, Anal. Chem., 52 (1981) 2207. 10 B. Pihlar and L. Kosta, Anal. Chim. Acta, 114 (1980) 275.
in The Netherlands
Short Communication
PNEUMATOAMPEROMETRIC DETERMINATION SULFIDE AND THEIR MIXTURES
FRANTISEK
OPEKAR
J. Heyrovsky Institute Academy of Sciences, STANLEY
OF CYANIDE,
of Physical Chemistry and Electrochemistry, Jilska 16,110 00 Prague 1 (Czechoslovakia)
Czechoslovak
BRUCKENSTEIN*
Chemistry Department, State New York 14214 (U.S.A.)
University
of New York at Buffalo,
Buffalo,
(Received 22nd August 1984)
Summary. A rapid pneumatoamperometric method for quantifying cyanide in the presence or absence of sulfide is described. The gaseous mixture of hydrogen cyanide and hydrogen sulfide is separated by inserting a short chromatographic column packed with silica gel between the reaction vessel used to generate the volatile acids and the porous gold electrode that detects them. The detection limit is ca. 5 ng cyanide in 2 ml of solution regardless of sulfide content. The detection limit for sulfide is ca. 1.0 ng in 2 ml of solution when cyanide is present and ca. 0.7 ng in absence of cyanide. Both sulfite and nitrite interfere.
The determination of cyanide described by Beran and Bruckenstein [l] is based on quantitative conversion of cyanide to iodine monocyanide and the pneumatoamperometric measurement of the equivalent amount of iodine liberated in the decomposition of ICN by a strong acid. The determination is sensitive and has a detection limit of 1.25 ng CN- ml-‘. If such sensitivity is necessary, the fact that the procedure requires eleven steps and the time is tens of minutes is unimportant. When such sensitivity is not required, the simpler and shorter pneumatoamperometric method described below is preferable. Another advantage of this pneumatoamperometric method is that sulfide, the main interference in most determinations of cyanide, does not interfere and can in fact be determined simultaneously. A pneumatoamperometric determination of sulfide having a detection limit of 2.73 ng S*- ml-’ has been developed by Nygaard [ 21. The present communication describes two pneumatoamperometric determinations of cyanide at a gold porous-teflon membrane electrode (AuPME), one in the absence of sulfide and the other in its presence. In both determinations, the sample is injected into a reaction vessel containing acid. If sulfide is absent, the hydrogen cyanide liberated is led to the AuPME where an anodic current corresponding to the formation of the Au(CN); complex [3] is measured. Sulfide ions present in the sample produce hydrogen sulfide 0003-2670/85/$03.30
o 1985 Elsevier Science Publishers B.V.
408
which leads to an anodic current that interferes seriously. However, inserting a simple chromatographic column between the reaction vessel and the AuPME separates the hydrogen cyanide and sulfide so that both can be quantified from the i/t response curve of the electrode. Compared with other electrochemical methods for simultaneous determination of these ions (e.g., rapid d.c. polarography [4] or ion chromatography with electrochemical detection [5], this pneumatoamperometric procedure offers a lower detection limit. Experimental Reagents. All the solutions were prepared from analytical-grade chemicals and water obtained from a Milli-Q reagentgrade water system (Millipore). A 0.1 M stock solution of potassium cyanide in lo-’ M sodium hydroxide was standardized by titration with silver nitrate. A 0.1 M stock solution of sodium sulfide in lo-? M sodium hydroxide was standardized iodimetritally. Solutions with lower concentrations were obtained by dilution of the stock solutions with 10m2M sodium hydroxide. Electrode and apparatus. The preparation of the AuPME and the pneumatoamperometric apparatus have been described [6-81. The reaction vessel was filled with 2 ml of 6 M sulfuric acid and 10e2 M sodium hydroxide was used as the supporting electrolyte in the voltammetric vessel. The electrode response was measured at 0.1 V vs. Ag/AgCl electrode. Nitrogen carrier gas was used at a flow rate of 9.4 ml s-’ to purge the solution in the reaction vessel. To separate the analytes, a glass chromatographic column (4-mm i.d., llO-mm long) packed with silica gel (Baker A. R.; 40-140 mesh) was used without any pretreatment. The silica gel was held in the column by glass wool plugs. The column was connected between the reaction vessel and the electrode [8]. All measurements were made at laboratory temperature, 22 + 3°C. Procedure. A constant flow rate of nitrogen through the pneumatoamperometric apparatus was established. After the residual current became constant, 10 or 100 ~1 of a solution of potassium cyanide, sodium sulfide or their mixture was injected (microliter syringe) into the reaction vessel containing the acid through a rubber septum. The i/t response curve of the AuPME was recorded. Results and Discussion Determination of cyanide. The dependence of the peak current, i,, on the amount of cyanide was linear over the studied range, 13-1560 ng. The regression equation for this dependence was i, = (8.72 f 0.09) nA ng-’ + (0.5 & 18.9) nA. The relative standard deviation was 1.27% for 10 replicate samples containing 52 ng of cyanide. The detection limit, defined as twice the peakto-peak background noise, was ca. 5 ng of cyanide in 2 ml of solution (9.6 X lO* M). The time needed was ca. 150 s. Characteristic AuPME responses are shown in Fig. 1. The greater peak
I
IJJA
( 100 5
,
Fig. 1. The AuPME responses: (a) 450 ng CN-; (b) 45 ng W. Fig. 2. Dependence of i, on the concentration 260 ng CN-).
of sulfuric acid in the reaction vessel (for
width for cyanide is caused by the lower rate at which hydrogen cyanide is transferred from the solution in the reaction vessel into the carrier gas stream; hydrogen cyanide is much more soluble in water than hydrogen sulfide. The differing rates of transfer to the gaseous phase is also reflected in their respective i, values (see pneumatoamperometric theory [ 91). The rate of transfer of hydrogen cyanide from the liquid to the gaseous phase increases with increasing concentration of sulfuric acid up to about 8 M (Fig. 2) but then decreases, probably because of the increased viscosity and foaming of the solution. A 6 M H2S04 sulfuric acid solution seems to be optimal. It has been recommended that hydrogen cyanide should be distilled from more complicated samples by purging with nitrogen through the acidified sample at an elevated temperature [3, lo]. Heating is simply done in the pneumatoamperometric determination of cyanide by injecting 1 ml of sample solution into 1 ml of 12 M sulfuric acid in the reaction vessel; the heat of dilution of sulfuric acid is sufficient. Under these conditions, a linear dependence of i, on the amount of cyanide was found in the studied range, 13260 ng, and the linear regression equation was i, = (20.87 k 0.86) nA ng-’ + (6.61 f. 61.86) nA. The sensitivity, expressed in terms of the slope of the dependence of current vs. amount of cyanide, is in this case about 2.3 times
410 higher than when a small volume of sample solution is injected into 6.0 M acid. A disadvantage of this method is the necessity of replacing the solution in the reaction vessel for each sample. Estimation of the electrolysis efficiency. About 10% of the electroactive material in the carrier gas is consumed at the electrode [ 71. The electrolysis efficiency for hydrogen cyanide can be found from the equation [ 91 E= i,R T/nFvgKCHcN, i
(1)
where vg is the carrier gas flow rate, K is the partition coefficient (K = pHCN/CHCN, where pnCN is the HCN partial pressure in the carrier gas) and c HCN. i is the initial HCN concentration in the solution. The other symbols have their usual significance. The value of K can be found from the equation describing the dependence of the electrode response on time [9] : it = iP exp(-v,Kt/VRT), where V is the volume of the solution. The K value was found from the slope of the log it vs. t dependence. From the intercept for t = 0, the i, value, unaffected by the time constant for the instrument, was then found. The K and i, values obtained in this way were used for the calculation of E. From measurements made at various initial concentrations, CncN, i, the value, K = 1.53 X lo4 Pa mol-’ was obtained; for the experimental conditions used in the determination of cyanide (v~ = 9.4 ml s-‘, V = 2 ml), the value of e was 0.14. The electrolysis efficiency was also calculated from the concentration dependence of the charge consumed during the electrolysis for various amounts of cyanide. This dependence was a straight line with a slope of 317.5 C g-‘. For 100% current efficiency, the reaction, Au + 2CN- = Au(CN);, requires 1856 C g-‘. Thus the slope found corresponds to the value E = 0.17. This value agrees satisfactorily with the value obtained from the equations based on the theory of the pneumatoamperometric method. Determination of sulfide. The i, dependence on the amount of sulfide was linear in the studied range, 8.7-1044 ng. The regression equation for this dependence was i, = (130 f 2.3) nA ng-’ + (1.6 + 45.10) ng. For 10 replicate experiments, the relative standard deviation was 1.81% for 21.2 ng of sulfide. The detection limit defined as twice the peak-to-peak background noise was ca. 0.7 ng of sulfide in 2 ml of solution. Simultaneous determination of cyanide and sulfide. By including a silicagel column between the reaction vessel and the AuPME, cyanide and sulfide can be determined in the presence of each other. The difference between the solubilities of hydrogen cyanide and sulfide in water and their partition on the silica-gel column are both utilized. After the column has been placed in the flow line, the carrier gas must be passed through the reaction vessel containing the acid and through the column for a few minutes to equilibrate the silica gel with water vapor. Linear dependencies of i, on the amounts of sulfide and cyanide in the solution were obtained for Sz- to CN- concentration ratios of 0.02-5. The sensitivity of the determination of cyanide was the same as for the measure-
411
Fig. 3. The AuPME response in the simultaneous determination of S’- and CN-: (a) 195 ng CN- + 17 ng S2-, S2-/CN- = 8.7 x lo-’ ; (b) 86.7 ng CN- + 435 ng S2-, W/CN- = 5 (for the given current scale, the experimental ip = 38.7 PA for H,S is off-scale).
ment technique without the column. The sensitivity of the determination of sulfide decreased by about 30% because of the zone broadening in the column. Thus the calibration line for sulfide must be prepared from measurements with the column. Alternatively, the sensitivity in the absence of the column and in the presence of the column can be evaluated at some sulfide level and the resulting factor used with the calibration obtained in the absence of the column. Examples of the AuPME response at two different ratios of S2-/CN- are given in Fig. 3. Up to a ratio of S2-/CN- of ca. 0.5, the two peaks are completely resolved (Fig. 3a). At higher ratios, the broadening of the sulfide peak is more perceptible, but the height of the cyanide peak corresponds to the calibration curve,provided that i, is measured as indicated in Fig. 3(b). This separation is, because of the higher sensitivity of the determination of sulfide, more suitable for samples with low S2-/CN- ratios. Interferences. The pneumatoamperometric method described above efficiently separates these anions from interferences and is equivalent to the separation by distillation of sulfide and cyanide from the sample [3, lo]. Among the common anions that form volatile and oxidizable substances in acidic media, only sulfite and nitrite interfere under the above experimental conditions. The interference from nitrite ions can be suppressed by sulfamic acid [2]. This work was supported by the Air Force Office of Scientific Research under Grant No. 84-0004.
412 REFERENCES 1 2 3 4 5 6
P. Beran and S. Bruckenstein, Anal. Chem., 52 (1980) 1183. D. D. Nygaard, Anal. Chim. Acta, 127 (1981) 257. D. B. Easty, W. J. Blaedel and L. Anderson, Anal. Chem., 43 (1971) 509. D. R. Canterford, Anal. Chem., 47 (1975) 88. A. M. Bond, I. D. Heritage and G. G. Wallace, Anal. Chem., 54 (1982) 582. W. G. Sherwood, G. A. Martinchek, L. Gal-Or and S. Bruckenstein, First Semi-Annual Tech. Progress Rep., US Bureau of Mines, Grant No. 155 007, March 1, 1975. 7 P. R. Gifford and S. Bruckenstein, Anal. Chem., 52 (1980) 1024. 8 P. R. Gifford and S. Bruckenstein, Anal. Chem., 52 (1980) 1028. 9 P. Beran and S. Bruckenstein, Anal. Chem., 52 (1981) 2207. 10 B. Pihlar and L. Kosta, Anal. Chim. Acta, 114 (1980) 275.