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On-line determination of cyanide in the presence of sulfide by flow injection with pervaporation
Analytica Chimica Acta 390 (1999) 133±139
On-line determination of cyanide in the presence of sul®de by ¯ow injection with pervaporation Hermin Sulistyartia, Terence J. Cardwella, M.D. Luque de Castrob, Spas D. Kolev1,a,* a
Department of Chemistry, Centre for Scienti®c Instrumentation, La Trobe University, Bundoora, Vic. 3083, Australia b Department of Analytical Chemistry, Faculty of Sciences, University of Cordoba, E-14004 Cordoba, Spain Received 8 September 1998; received in revised form 28 January 1999; accepted 30 January 1999
Abstract A pervaporation-¯ow injection (PFI) method is described for the analysis of cyanide in the presence of sul®de. The interfering sul®de ion in the injected sample is precipitated on-line using an acidi®ed lead nitrate reagent solution before the donor stream enters the pervaporation cell. Using amperometric detection at a silver electrode set at ÿ50 mV (vs Ag/AgCl), linear calibration was obtained in the range 0.02±100.0 mg lÿ1 with a detection limit of 1.0 mg lÿ1. Sample throughput was of the order of 12±15 hÿ1. When the method was applied to the analysis of synthetic samples, there was no signi®cant interference from sul®de at concentrations up to 50 mg lÿ1. Thiocyanate did not interfere at levels up to 1000 mg lÿ1. When applied to industrial samples containing sul®de and thiocyanate ions where the cyanide ions are predominantly complexed with various metal ions the PFI method was found to give results close to those obtained by standard distillation methods for weak acid dissociable (WAD) cyanide. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Flow injection; Cyanide determination; Sul®de intereference; Distillation
1. Introduction Cyanides are extremely hazardous and their monitoring at very low levels is of great importance. They are produced in nature by plants containing cyanogenic glucoside [1]. However, most of all, cyanides are generated and released into the environment by industries involved in ore leaching (especially, gold or silver), electroplating, blast furnacing, petroleum re®ning, or ®ber synthesis [2,3]. *Corresponding author. Tel.: +61-3-9479-3747; fax: +61-39479-1399; e-mail: [email protected] 1 On leave from the Faculty of Chemistry, University of Sofia, 1 James Bourchier Ave, BG 1126 Sofia, Bulgaria.
Amperometric detection has been successfully implemented in ¯ow systems for the determination of cyanides using the following working electrodes: 1. glassy carbon with a linearity range from 0.26 to 26 mg lÿ1 [4]; 2. gold with a linearity range up to 100 mg lÿ1 [5]; 3. silver with a linearity range over six orders of magnitude from 0.5 mg lÿ1 to 1 g lÿ1 [6]. The silver electrode is preferred due to its wide linear working range, higher reproducibility, better longterm stability and low cost. Amperometric detection with a silver electrode is based on the measurement of the current produced
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during the anodic polarization of the electrode in the presence of cyanide (Eq. (1)), where the current is directly proportional to the cyanide concentration. Ag 2CNÿ $ Ag CN2 ÿ eÿ :
(1)
It is well established that most of the ¯ow injection methods for cyanide analysis suffer from the interference of sul®de, which is commonly removed by offline addition of lead salts [7] prior to injection. Some procedures developed for on-line removal of the sul®de interference use oxidising agents in the donor stream of the ¯ow injection membrane separation device [8,9]. One such gas diffusion ¯ow injection procedure was proposed by Frenzel [8] using a mixture of permanganate and dichromate solutions. When implemented by us, this procedure resulted in blockage of the membrane pores by manganese dioxide [9]. A procedure for preventing this blockage was reported and an alternative method for eliminating the sul®de interference using cerium(IV) sulfate was proposed [9]. The use of oxidizing agents in the donor stream can result in the positive errors when the cyanide samples contain thiocyanate ions because these ions generate cyanide ions when oxidized. Nobrega and Capelato [10] proposed an on-line procedure for the elimination of sul®de and thiocyanate interferences in the cyanide samples using dichromate oxidizing solution at pH4.7. High results for cyanide were obtained when sul®de or sul®de and thiocyanate concentrations were higher than 50 or 20 mg lÿ1, respectively. On the basis of the discussion above, it can be concluded that a gas diffusion ¯ow injection approach is not suitable for cyanide measurements when the sample contains both sul®de and thiocyanate. A recently introduced on-line separation technique termed pervaporation [11] seems to offer possibilities to implement an on-line ¯ow injection measurement of cyanide in the presence of sul®de and thiocyanate. Pervaporation, which can be de®ned as a combination of continuous evaporation and gas diffusion through a gas-permeable membrane in a single step, has become popular as a separation technique in analytical laboratories [11,12]. This technique is based on converting and/or evaporating the analyte in the donor stream to a molecular gas, which then diffuses across the air gap between the membrane and the liquid level in the donor chamber, and then through the membrane into the acceptor stream where detec-
tion takes place. The constant air gap in the donor chamber prevents the direct contact between the sample and the membrane. In this way deterioration in the properties of the membrane is avoided (e.g. clogging by particles or macromolecules) and complex (e.g. biological ¯uids, fermentation and other industrial process liquids), corrosive or solid samples can be analyzed [12,13]. The application of auxiliary energies such as microwaves and ultrasound to the donor chamber extensively shortens the pretreatment step of samples containing solid particles and therefore, enables the automation of analysis [11]. One drawback of pervaporation, compared to gas diffusion, is the slower mass transfer process, resulting in higher detection limits. This drawback can be compensated to a considerable extent by increasing the donor stream temperature, using membranes of high permeability, or applying a stopped ¯ow mode [14,15]. Pervaporation has been successfully coupled to ¯owing systems and implemented to a wide range of analytes, such as ¯uoride in solid and liquid matrices (waste water and plant material) [14,16], sul®de in bleaching liquors [17], oxalate in urine [18], mercury in sludges [19], acetaldehyde in food [20], and trimethylamine in ®sh [21]. Cyanide is a suitable analyte for pervaporation because it can be easily converted into volatile hydrogen cyanide. This paper describes the application of pervaporation in a ¯ow injection system for cyanide determination in the presence of both sul®de and thiocyanate. The existence of the air gap of the pervaporation unit, which avoids the contact between the donor stream and the membrane, was expected to enable the utilization of lead salts in the donor stream for on-line elimination of sul®de interference by precipitation. 2. Experimental 2.1. Solution preparation Stock solutions of cyanide, sul®de, thiocyanate, sulfate, oxalate, iodide, and ¯uoride were prepared by dissolving the appropriate amount of KCN (AR, Ajax), Na2S9H2O (AR, Fison), KSCN (AR, Ajax), Na2SO4 anhydrous (AR, M&B), K2C2O4H2O (LR, M&B), KI (AR, BDH), and KF (AR, Ajax) in nano-
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135
Fig. 1. Flow injection manifold. For samples containing sulfide and cyanide, the reagent stream (dashed line) is used.
pure deionised water (Barnstead, 17.0 M cm). The standard cyanide and sul®de solutions were adjusted to pH12 with dilute sodium hydroxide solution. The solution used for the donor stream contained HNO3 and Pb(NO3)2 in the ranges from 0.01 to 0.2 M and from 0 to 0.2 M, respectively. Sodium hydroxide (AR, M&B) solution (0.1 M) was used as the acceptor stream. It was degassed with nitrogen for 15 min prior to use.
The liquid level in the donor chamber of the pervaporation cell, which affected considerably the performance of the system, was in¯uenced mainly by the siphoning effect, the ¯uid pressure in the upper chamber and the difference between the inlet and outlet ¯ow rates. An important requirement for obtaining reproducible measurements was to maintain a constant liquid level in the donor chamber throughout the
2.2. Flow injection system The PFI system constructed for this work is shown in Fig. 1. This system can be used for the analysis of samples containing cyanide in the presence of sul®de by introducing a solution of Pb(NO3)2 through the channel labeled ``reagent stream'' in Fig. 1. Two peristaltic pumps (Gilson Minipuls-2 and Minipuls-3), a type 50 injection valve (Rheodyne), and PTFE tubing (0.8 mm i.d.) were used to construct the FI-manifold. A home-made perspex pervaporation unit (Fig. 2), equipped with a 0.5 (Alltech), 2.0 or 5.0 (Gelman Sciences) mm pore PTFE ®lter membrane (47 mm diameter) and a PTFE membrane support, was used as the separation unit. The temperature of the donor stream was controlled by thermostating the supply reservoir of the donor stream and immersing the donor chamber of the pervaporation module in the water bath of the thermoregulator (RATEK Instruments, Australia). The current produced was continuously monitored at ÿ50 or 0 mV using a ¯ow-through amperometric cell (Metrohm 656 Electrochemical Detector) consisting of a silver working electrode, an Ag/AgCl reference electrode and a platinum auxiliary electrode. Potentials were applied using a potentiostat (Metrohm 641 VA) and the detector output was recorded on a strip chart recorder (BAS RYT).
Fig. 2. Pervaporation cell. Hexagonal donor chamber depth 0.5 cm, length 1.5 cm, width 1.0 cm. Hexagonal acceptor chamber depth 0.1 cm, other dimensions as for donor chamber.
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experiments. This can be achieved only by maintaining identical volumetric ¯ow rates in the inlet and outlet of the donor chamber. This condition was ful®lled successfully by aspirating the outlet of the donor chamber through a separate pump (Fig. 1, pump 2). 2.3. Determination of total, weak acid dissociable (WAD) and free cyanide in industrial samples Total and WAD cyanide were determined using microdistillation as described previously [9] while free cyanide was measured by a cyanide selective electrode (model 9406, Orion) coupled to an ion analyzer (EA 940, Orion). 3. Results and discussion 3.1. Optimization of the amperometric detector Hydrodynamic voltammograms of cyanide solutions were obtained using the silver working electrode in the potential range ÿ300 to 300 mV (vs Ag/ AgCl). It was found that the anodic current increased with increasing potential until a plateau was observed in the region ÿ100 to 0 mV. At potentials more positive than 0 mV, the current increased again reaching a maximum at 200 mV. At potentials more positive than 200 mV, the current started to decrease accompanied by the formation of a white precipitate which was thought to be AgCN [6,22,23]. In subsequent experiments, measurements were performed at ÿ50 or 0 mV since small ¯uctuations around these potentials did not affect the current signi®cantly. These operating potentials are similar to those previously reported [22,23].
Fig. 3. Variation of detector current with sample volume. Conditions: FI manifold, Fig. 1; donor and acceptor stream flow rates, 1.250.2 ml minÿ1; detector potential, 0 mV; membrane, 0.5 mm pore.
The in¯uence of the sample volume in the range 100±1000 ml is shown in Fig. 3. It can be seen that this in¯uence decreases with increasing sample volume and vanishes for sample volumes greater than 600 ml. A sample volume of 300 ml was selected as most appropriate. The effect of variation of the donor and the acceptor stream ¯ow rates when both ¯ow rates are maintained at identical settings in the range from 0.65 to 3.35 ml minÿ1 is shown in Fig. 4. As expected, the best sensitivities are achieved at lower ¯ow rates. When one of the streams was held constant at 0.65 ml minÿ1 and the other was varied in the range from 0.65 to 3.35 ml minÿ1, the performance of the system could not be improved using different ¯ow
3.2. Influence of some important physical parameters The following major design and operation parameters were studied in order to improve the sensitivity and sample throughput of the experimental pervaporation FI system: the injection volume, the ¯ow rates of the donor and the acceptor streams, the coil length, the membrane porosity and the temperature of the donor stream.
Fig. 4. Variation of detector current with stream flow rate. Conditions: sample volume, 300 ml; other conditions, as in Fig. 3.
H. Sulistyarti et al. / Analytica Chimica Acta 390 (1999) 133±139
rates in the two channels. Taking into account that the sampling frequency decreases on lowering the ¯ow rate, it was considered that a ¯ow rate of 1.25 ml minÿ1 in both channels ensured both acceptable sensitivity and sample throughput. As expected, it was found that the sensitivity and the sampling rate decreased as the length of the reaction coil increased. Coil lengths up to 75 cm did not affect the performance of the system signi®cantly because the overall sample dispersion was dominated by dispersion in the donor chamber under these conditions. A 50 cm coil length was selected as optimal since the reproducibility of the measurements deteriorated at shorter lengths. PTFE membranes were reported to provide greater stability to NaOH solutions as temperature increased compared to cellulose acetate and PVDF membranes [14]. In this work, three PTFE membranes with pore sizes of 0.5, 2.0 and 5.0 mm and thicknesses of 0.16, 0.18 and 0.20 mm, respectively, were compared with respect to permeability to hydrogen cyanide. The results revealed that the membrane with 2.0 mm pore size and 0.18 mm thickness gave the highest permeability (Fig. 5). The 5.0 mm pore size did not give the best permeability as expected, since the larger pore size of the membrane only allows better diffusion of larger molecular gases, but not of the small molecular gases such as HCN. In this case, other considerations such as the thickness and the pore density of the
Fig. 5. Effect of membrane pore size on detector output. Conditions: donor and acceptor stream flow rates, 1.250.2 ml minÿ1; detector potential, ÿ50 mV; other conditions, as in Fig. 4.
137
membrane should be taken into account. The thickness is inversely proportional to the permeability while the pore density is proportional to it. It is expected that a larger pore size will require lower pore density to ensure acceptable mechanical stability of the membrane. This means that the overall area of the pores through which gas diffusion takes place could even decrease with increasing pore size and hence give lower permeability. An increase in the temperature increases both the rate of the volatile compound formation and its diffusion coef®cient in the air gap and within the membrane. The mass transfer through the membrane is further enhanced by an increase in the vapor pressure in the donor chamber. However, elevated temperatures reduce the membrane life [14]. In the present studies, when the donor stream reservoir and the pervaporator donor chamber were thermostated, the sensitivity improved by about 100% compared to the results at room temperature (208C) and the detector signal showed insigni®cant increase above 408C. However, when only the donor stream reservoir was thermostated, the temperature effect was negligible showing that thermostating of the donor chamber was necessary. However, the reproducibility of the measurements was poor at temperatures above 208C and for convenience, room temperature was used in remaining experiments. 3.3. Influence of the chemical parameters A number of different supporting electrolytes such as potassium nitrate, sulfate, carbonate and phosphate in combination with sodium hydroxide have been investigated for the amperometric detection of cyanide [6]. Sodium hydroxide (0.1 M) was found to provide satisfactory current±voltage characteristics [6] and therefore was used in this work for the acceptor stream without the addition of other electrolytes. The concentration of nitric acid in the donor stream required for converting cyanide ion into HCN was varied in the range from 0.01 to 0.40 M. Under the above experimental conditions, the detector current increased up to an acid concentration of about 0.1 M before leveling out at higher concentrations. 0.2 M nitric acid was selected as appropriate for the full protonation of all CNÿ ions in the samples and standards.
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3.4. Detection limit, sampling rate and linear detection range Under the selected optimal conditions outlined above (i.e., donor and acceptor ¯ow rates of 1.25 ml minÿ1, sample volume of 300 ml, 2.0 mm pore PTFE membrane, 0.2 M nitric acid in the donor stream and 0.1 M NaOH in the acceptor stream), the sampling rate was measured to be 12±15 samples per hour. The calibration curves were linear for concentrations of CNÿ in the sample in the range from 0.02 to 100.0 mg lÿ1 (R20.9992), with a detection limit of 1.0 mg lÿ1 [24]. Poor reproducibility was observed when the samples contained cyanide more concentrated than 100 mg lÿ1 as a result of the precipitation of AgCN in the detector ¯ow cell. 3.5. Interference studies Since the samples are normally preserved in alkaline solution, the donor stream should be acidic to avoid precipitation of lead hydroxide. On the other hand, the acidity should not be too high, otherwise the solubility of PbS increases and S2ÿ ions from the sample will not be fully precipitated. The theoretical in¯uence of pH and Pb(NO3)2 concentration on the equilibrium, total sul®de concentration calculated using the equations for dissociation of H2S and PbS is shown in Fig. 6. It can be seen that for pH values less than 1.5 and Pb(NO3)2 concentrations in the range from 0.1 to 0.4 M, the equilibrium sul®de in the
Table 1 Cyanide recovery in the presence of sulfide using a donor stream containing 0.1 M Pb2 and 0.01 M HNO3 (pH2.0) (FI configuration as in Fig. 1; other conditions, as in Fig. 5) [CNÿ] (mg lÿ1) 1 0.1 0.1 0.05 0.05 a
[S2ÿ] (mg lÿ1)
Cyanide recoverya (%)
50 50 5 5 1
100.61.5 102.12.1 99.61.8 100.31.7 100.71.2
The results are the average of three determinations.
sample plug is negligible. To achieve the optimum condition for sul®de elimination, the concentrations of HNO3 and Pb(NO3)2 were varied in the ranges 0.01± 0.2 M (pH 2.0±0.7) and 0.05±0.20 M, respectively. A series of synthetic samples containing both cyanide and sul®de were investigated. Excellent recoveries of cyanide in the whole series of samples were obtained when the donor stream contained 0.1 M Pb(NO3)2 and 0.01 M HNO3 (Table 1). Sul®de concentrations greater than 100 mg lÿ1 resulted in peak tailing and poor reproducibility after several injections because of a build-up of un¯ushed PbS precipitate in the donor chamber. Thiocyanate did not interfere with the cyanide determinations at concentrations up to 1000 mg lÿ1. The interference of other anions, which form precipitates with lead, such as oxalate, sulfate, ¯uoride, and iodide were also investigated. It was found that all these anions did not affect the measurements in concentrations up to 500 mg lÿ1. 3.6. Cyanide analysis in industrial samples
Fig. 6. Theoretical equilibrium sulfide concentrations at different pH values and Pb2 concentrations.
The total, WAD and free cyanide were determined in three real samples obtained from the gold recovery industry. The samples gave positive reaction to both sul®de and thiocyanate. Total and WAD cyanide were determined by microdistillation [9] while free cyanide was measured potentiometrically (i.e., using the cyanide selective electrode). The cyanide content in all three samples were also determined by the PFI method without pretreatment of the samples prior to injection. The analysis data presented in Table 2 show that the proposed PFI method gives results close to those obtained for WAD cyanide.
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Table 2 Analysis of total, WAD and free cyanide in industrial samplesa Method ÿ1
Total cyanide [9] (mg l ) WAD cyanide [9] (mg lÿ1) Free cyanide (mg lÿ1) PFI method (mg lÿ1) a
Sample 1
Sample 2
Sample 3
45.111.49 29.240.24 9.470.16 30.620.83
179.505.51 115.001.82 57.302.02 115.650.38
86.852.02 44.370.31 24.850.34 44.470.96
The results are the average of three determinations.
4. Conclusions The PFI system outlined in this paper allows sensitive and selective determination of cyanide, which can be easily automated. An on-line method for reducing the sul®de interference was successfully developed. When industrial samples are analyzed directly by the PFI method, the results obtained are close to those given by a standard distillation method for WAD cyanide determination. On the basis of the results obtained, it is concluded that the PFI system mentioned above can be used successfully for on-line cyanide monitoring of industrial waste waters and especially for those from the mining industry. Acknowledgements We are grateful to AusAID for a scholarship for Hermin Sulistyarti and to the Australian Research Council for ®nancial support. References [1] [2] [3] [4] [5]
J.H. Bradbury, Chem. Aust. 64(11) (1997) 30. A. Nagy, G. Nagy, Anal. Chim. Acta 283 (1993) 795. N.K. Kutseva, A.N. Kashin, Ind. Lab. 61 (1995) 575. A.G. Fogg, R.M. Alonso, Analyst 112 (1987) 1071. H.D. Suschke, H. Kaden, U. Unseleit, Fresenius' J. Anal. Chem. 349 (1994) 597.
[6] B. Pihlar, L. Kosta, B. Hitrovski, Talanta 26 (1979) 805. [7] Standard Methods for the Examination of Water and Waste Water, 17th ed., American Public Health Association (APHA), Washington, DC, 1992. [8] W. Frenzel, Fresenius' J. Anal. Chem. 336 (1990) 21. [9] H. Sulistyarti, T.J. Cardwell, S.D. Kolev, Anal. Chim. Acta 357 (1997) 103. [10] J.A. Nobrega, M.D. Capelato, J. Braz. Chem. Soc. 5 (1994) 91. [11] M.D. Luque de Castro, M.P. da Silva, Trends Anal. Chem. 16 (1997) 16. [12] Z. Zhi, A. Rios, M. Valcarcel, Crit. Rev. Anal. Chem. 26 (1996) 239. [13] M.D. Luque de Castro, I. Papaefstathiou, Trends Anal. Chem. 17 (1998) 41. [14] I. Papaefstathiou, M.T. Tena, M.D. Luque de Castro, Anal. Chim. Acta 308 (1995) 246. [15] I.L. Mattos, M.D. Luque de Castro, Anal. Chim. Acta 298 (1994) 159. [16] I. Papaefstathiou, M.D. Luque de Castro, Anal. Chem. 67 (1995) 3916. [17] I. Papaefstathiou, M.D. Luque de Castro, M. Vakarcel, Fresenius' J. Anal. Chem. 354 (1996) 442. [18] P. Canizares, M.D. Luque de Castro, Fresenius' J. Anal. Chem. 357 (1997) 777. [19] D.W. Bryce, A. Izquierdo, M.D. Luque de Castro, Anal. Chim. Acta 324 (1996) 69. [20] I. Papaefstathiou, U. Bilitewski, M.D. Luque de Castro, Fresenius' J. Anal. Chem. 357 (1997) 1168. [21] J.A. Garcia-Garrido, M.D. Luque de Castro, Analyst 122 (1997) 663. [22] U. Backofen, F.M. Matsyik, G. Werner, Fresenius' J. Anal. Chem. 356 (1996) 271. [23] R.E. Meyer, P.M. Lantz, Electronal. Chem. 61 (1975) 155. [24] A.I. Vogel (Ed.), Vogel's Quantitative Chemical Analysis, 5th ed., Longman, New York, 1989, p. 804.
On-line determination of cyanide in the presence of sul®de by ¯ow injection with pervaporation Hermin Sulistyartia, Terence J. Cardwella, M.D. Luque de Castrob, Spas D. Kolev1,a,* a
Department of Chemistry, Centre for Scienti®c Instrumentation, La Trobe University, Bundoora, Vic. 3083, Australia b Department of Analytical Chemistry, Faculty of Sciences, University of Cordoba, E-14004 Cordoba, Spain Received 8 September 1998; received in revised form 28 January 1999; accepted 30 January 1999
Abstract A pervaporation-¯ow injection (PFI) method is described for the analysis of cyanide in the presence of sul®de. The interfering sul®de ion in the injected sample is precipitated on-line using an acidi®ed lead nitrate reagent solution before the donor stream enters the pervaporation cell. Using amperometric detection at a silver electrode set at ÿ50 mV (vs Ag/AgCl), linear calibration was obtained in the range 0.02±100.0 mg lÿ1 with a detection limit of 1.0 mg lÿ1. Sample throughput was of the order of 12±15 hÿ1. When the method was applied to the analysis of synthetic samples, there was no signi®cant interference from sul®de at concentrations up to 50 mg lÿ1. Thiocyanate did not interfere at levels up to 1000 mg lÿ1. When applied to industrial samples containing sul®de and thiocyanate ions where the cyanide ions are predominantly complexed with various metal ions the PFI method was found to give results close to those obtained by standard distillation methods for weak acid dissociable (WAD) cyanide. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Flow injection; Cyanide determination; Sul®de intereference; Distillation
1. Introduction Cyanides are extremely hazardous and their monitoring at very low levels is of great importance. They are produced in nature by plants containing cyanogenic glucoside [1]. However, most of all, cyanides are generated and released into the environment by industries involved in ore leaching (especially, gold or silver), electroplating, blast furnacing, petroleum re®ning, or ®ber synthesis [2,3]. *Corresponding author. Tel.: +61-3-9479-3747; fax: +61-39479-1399; e-mail: [email protected] 1 On leave from the Faculty of Chemistry, University of Sofia, 1 James Bourchier Ave, BG 1126 Sofia, Bulgaria.
Amperometric detection has been successfully implemented in ¯ow systems for the determination of cyanides using the following working electrodes: 1. glassy carbon with a linearity range from 0.26 to 26 mg lÿ1 [4]; 2. gold with a linearity range up to 100 mg lÿ1 [5]; 3. silver with a linearity range over six orders of magnitude from 0.5 mg lÿ1 to 1 g lÿ1 [6]. The silver electrode is preferred due to its wide linear working range, higher reproducibility, better longterm stability and low cost. Amperometric detection with a silver electrode is based on the measurement of the current produced
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during the anodic polarization of the electrode in the presence of cyanide (Eq. (1)), where the current is directly proportional to the cyanide concentration. Ag 2CNÿ $ Ag CN2 ÿ eÿ :
(1)
It is well established that most of the ¯ow injection methods for cyanide analysis suffer from the interference of sul®de, which is commonly removed by offline addition of lead salts [7] prior to injection. Some procedures developed for on-line removal of the sul®de interference use oxidising agents in the donor stream of the ¯ow injection membrane separation device [8,9]. One such gas diffusion ¯ow injection procedure was proposed by Frenzel [8] using a mixture of permanganate and dichromate solutions. When implemented by us, this procedure resulted in blockage of the membrane pores by manganese dioxide [9]. A procedure for preventing this blockage was reported and an alternative method for eliminating the sul®de interference using cerium(IV) sulfate was proposed [9]. The use of oxidizing agents in the donor stream can result in the positive errors when the cyanide samples contain thiocyanate ions because these ions generate cyanide ions when oxidized. Nobrega and Capelato [10] proposed an on-line procedure for the elimination of sul®de and thiocyanate interferences in the cyanide samples using dichromate oxidizing solution at pH4.7. High results for cyanide were obtained when sul®de or sul®de and thiocyanate concentrations were higher than 50 or 20 mg lÿ1, respectively. On the basis of the discussion above, it can be concluded that a gas diffusion ¯ow injection approach is not suitable for cyanide measurements when the sample contains both sul®de and thiocyanate. A recently introduced on-line separation technique termed pervaporation [11] seems to offer possibilities to implement an on-line ¯ow injection measurement of cyanide in the presence of sul®de and thiocyanate. Pervaporation, which can be de®ned as a combination of continuous evaporation and gas diffusion through a gas-permeable membrane in a single step, has become popular as a separation technique in analytical laboratories [11,12]. This technique is based on converting and/or evaporating the analyte in the donor stream to a molecular gas, which then diffuses across the air gap between the membrane and the liquid level in the donor chamber, and then through the membrane into the acceptor stream where detec-
tion takes place. The constant air gap in the donor chamber prevents the direct contact between the sample and the membrane. In this way deterioration in the properties of the membrane is avoided (e.g. clogging by particles or macromolecules) and complex (e.g. biological ¯uids, fermentation and other industrial process liquids), corrosive or solid samples can be analyzed [12,13]. The application of auxiliary energies such as microwaves and ultrasound to the donor chamber extensively shortens the pretreatment step of samples containing solid particles and therefore, enables the automation of analysis [11]. One drawback of pervaporation, compared to gas diffusion, is the slower mass transfer process, resulting in higher detection limits. This drawback can be compensated to a considerable extent by increasing the donor stream temperature, using membranes of high permeability, or applying a stopped ¯ow mode [14,15]. Pervaporation has been successfully coupled to ¯owing systems and implemented to a wide range of analytes, such as ¯uoride in solid and liquid matrices (waste water and plant material) [14,16], sul®de in bleaching liquors [17], oxalate in urine [18], mercury in sludges [19], acetaldehyde in food [20], and trimethylamine in ®sh [21]. Cyanide is a suitable analyte for pervaporation because it can be easily converted into volatile hydrogen cyanide. This paper describes the application of pervaporation in a ¯ow injection system for cyanide determination in the presence of both sul®de and thiocyanate. The existence of the air gap of the pervaporation unit, which avoids the contact between the donor stream and the membrane, was expected to enable the utilization of lead salts in the donor stream for on-line elimination of sul®de interference by precipitation. 2. Experimental 2.1. Solution preparation Stock solutions of cyanide, sul®de, thiocyanate, sulfate, oxalate, iodide, and ¯uoride were prepared by dissolving the appropriate amount of KCN (AR, Ajax), Na2S9H2O (AR, Fison), KSCN (AR, Ajax), Na2SO4 anhydrous (AR, M&B), K2C2O4H2O (LR, M&B), KI (AR, BDH), and KF (AR, Ajax) in nano-
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Fig. 1. Flow injection manifold. For samples containing sulfide and cyanide, the reagent stream (dashed line) is used.
pure deionised water (Barnstead, 17.0 M cm). The standard cyanide and sul®de solutions were adjusted to pH12 with dilute sodium hydroxide solution. The solution used for the donor stream contained HNO3 and Pb(NO3)2 in the ranges from 0.01 to 0.2 M and from 0 to 0.2 M, respectively. Sodium hydroxide (AR, M&B) solution (0.1 M) was used as the acceptor stream. It was degassed with nitrogen for 15 min prior to use.
The liquid level in the donor chamber of the pervaporation cell, which affected considerably the performance of the system, was in¯uenced mainly by the siphoning effect, the ¯uid pressure in the upper chamber and the difference between the inlet and outlet ¯ow rates. An important requirement for obtaining reproducible measurements was to maintain a constant liquid level in the donor chamber throughout the
2.2. Flow injection system The PFI system constructed for this work is shown in Fig. 1. This system can be used for the analysis of samples containing cyanide in the presence of sul®de by introducing a solution of Pb(NO3)2 through the channel labeled ``reagent stream'' in Fig. 1. Two peristaltic pumps (Gilson Minipuls-2 and Minipuls-3), a type 50 injection valve (Rheodyne), and PTFE tubing (0.8 mm i.d.) were used to construct the FI-manifold. A home-made perspex pervaporation unit (Fig. 2), equipped with a 0.5 (Alltech), 2.0 or 5.0 (Gelman Sciences) mm pore PTFE ®lter membrane (47 mm diameter) and a PTFE membrane support, was used as the separation unit. The temperature of the donor stream was controlled by thermostating the supply reservoir of the donor stream and immersing the donor chamber of the pervaporation module in the water bath of the thermoregulator (RATEK Instruments, Australia). The current produced was continuously monitored at ÿ50 or 0 mV using a ¯ow-through amperometric cell (Metrohm 656 Electrochemical Detector) consisting of a silver working electrode, an Ag/AgCl reference electrode and a platinum auxiliary electrode. Potentials were applied using a potentiostat (Metrohm 641 VA) and the detector output was recorded on a strip chart recorder (BAS RYT).
Fig. 2. Pervaporation cell. Hexagonal donor chamber depth 0.5 cm, length 1.5 cm, width 1.0 cm. Hexagonal acceptor chamber depth 0.1 cm, other dimensions as for donor chamber.
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experiments. This can be achieved only by maintaining identical volumetric ¯ow rates in the inlet and outlet of the donor chamber. This condition was ful®lled successfully by aspirating the outlet of the donor chamber through a separate pump (Fig. 1, pump 2). 2.3. Determination of total, weak acid dissociable (WAD) and free cyanide in industrial samples Total and WAD cyanide were determined using microdistillation as described previously [9] while free cyanide was measured by a cyanide selective electrode (model 9406, Orion) coupled to an ion analyzer (EA 940, Orion). 3. Results and discussion 3.1. Optimization of the amperometric detector Hydrodynamic voltammograms of cyanide solutions were obtained using the silver working electrode in the potential range ÿ300 to 300 mV (vs Ag/ AgCl). It was found that the anodic current increased with increasing potential until a plateau was observed in the region ÿ100 to 0 mV. At potentials more positive than 0 mV, the current increased again reaching a maximum at 200 mV. At potentials more positive than 200 mV, the current started to decrease accompanied by the formation of a white precipitate which was thought to be AgCN [6,22,23]. In subsequent experiments, measurements were performed at ÿ50 or 0 mV since small ¯uctuations around these potentials did not affect the current signi®cantly. These operating potentials are similar to those previously reported [22,23].
Fig. 3. Variation of detector current with sample volume. Conditions: FI manifold, Fig. 1; donor and acceptor stream flow rates, 1.250.2 ml minÿ1; detector potential, 0 mV; membrane, 0.5 mm pore.
The in¯uence of the sample volume in the range 100±1000 ml is shown in Fig. 3. It can be seen that this in¯uence decreases with increasing sample volume and vanishes for sample volumes greater than 600 ml. A sample volume of 300 ml was selected as most appropriate. The effect of variation of the donor and the acceptor stream ¯ow rates when both ¯ow rates are maintained at identical settings in the range from 0.65 to 3.35 ml minÿ1 is shown in Fig. 4. As expected, the best sensitivities are achieved at lower ¯ow rates. When one of the streams was held constant at 0.65 ml minÿ1 and the other was varied in the range from 0.65 to 3.35 ml minÿ1, the performance of the system could not be improved using different ¯ow
3.2. Influence of some important physical parameters The following major design and operation parameters were studied in order to improve the sensitivity and sample throughput of the experimental pervaporation FI system: the injection volume, the ¯ow rates of the donor and the acceptor streams, the coil length, the membrane porosity and the temperature of the donor stream.
Fig. 4. Variation of detector current with stream flow rate. Conditions: sample volume, 300 ml; other conditions, as in Fig. 3.
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rates in the two channels. Taking into account that the sampling frequency decreases on lowering the ¯ow rate, it was considered that a ¯ow rate of 1.25 ml minÿ1 in both channels ensured both acceptable sensitivity and sample throughput. As expected, it was found that the sensitivity and the sampling rate decreased as the length of the reaction coil increased. Coil lengths up to 75 cm did not affect the performance of the system signi®cantly because the overall sample dispersion was dominated by dispersion in the donor chamber under these conditions. A 50 cm coil length was selected as optimal since the reproducibility of the measurements deteriorated at shorter lengths. PTFE membranes were reported to provide greater stability to NaOH solutions as temperature increased compared to cellulose acetate and PVDF membranes [14]. In this work, three PTFE membranes with pore sizes of 0.5, 2.0 and 5.0 mm and thicknesses of 0.16, 0.18 and 0.20 mm, respectively, were compared with respect to permeability to hydrogen cyanide. The results revealed that the membrane with 2.0 mm pore size and 0.18 mm thickness gave the highest permeability (Fig. 5). The 5.0 mm pore size did not give the best permeability as expected, since the larger pore size of the membrane only allows better diffusion of larger molecular gases, but not of the small molecular gases such as HCN. In this case, other considerations such as the thickness and the pore density of the
Fig. 5. Effect of membrane pore size on detector output. Conditions: donor and acceptor stream flow rates, 1.250.2 ml minÿ1; detector potential, ÿ50 mV; other conditions, as in Fig. 4.
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membrane should be taken into account. The thickness is inversely proportional to the permeability while the pore density is proportional to it. It is expected that a larger pore size will require lower pore density to ensure acceptable mechanical stability of the membrane. This means that the overall area of the pores through which gas diffusion takes place could even decrease with increasing pore size and hence give lower permeability. An increase in the temperature increases both the rate of the volatile compound formation and its diffusion coef®cient in the air gap and within the membrane. The mass transfer through the membrane is further enhanced by an increase in the vapor pressure in the donor chamber. However, elevated temperatures reduce the membrane life [14]. In the present studies, when the donor stream reservoir and the pervaporator donor chamber were thermostated, the sensitivity improved by about 100% compared to the results at room temperature (208C) and the detector signal showed insigni®cant increase above 408C. However, when only the donor stream reservoir was thermostated, the temperature effect was negligible showing that thermostating of the donor chamber was necessary. However, the reproducibility of the measurements was poor at temperatures above 208C and for convenience, room temperature was used in remaining experiments. 3.3. Influence of the chemical parameters A number of different supporting electrolytes such as potassium nitrate, sulfate, carbonate and phosphate in combination with sodium hydroxide have been investigated for the amperometric detection of cyanide [6]. Sodium hydroxide (0.1 M) was found to provide satisfactory current±voltage characteristics [6] and therefore was used in this work for the acceptor stream without the addition of other electrolytes. The concentration of nitric acid in the donor stream required for converting cyanide ion into HCN was varied in the range from 0.01 to 0.40 M. Under the above experimental conditions, the detector current increased up to an acid concentration of about 0.1 M before leveling out at higher concentrations. 0.2 M nitric acid was selected as appropriate for the full protonation of all CNÿ ions in the samples and standards.
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3.4. Detection limit, sampling rate and linear detection range Under the selected optimal conditions outlined above (i.e., donor and acceptor ¯ow rates of 1.25 ml minÿ1, sample volume of 300 ml, 2.0 mm pore PTFE membrane, 0.2 M nitric acid in the donor stream and 0.1 M NaOH in the acceptor stream), the sampling rate was measured to be 12±15 samples per hour. The calibration curves were linear for concentrations of CNÿ in the sample in the range from 0.02 to 100.0 mg lÿ1 (R20.9992), with a detection limit of 1.0 mg lÿ1 [24]. Poor reproducibility was observed when the samples contained cyanide more concentrated than 100 mg lÿ1 as a result of the precipitation of AgCN in the detector ¯ow cell. 3.5. Interference studies Since the samples are normally preserved in alkaline solution, the donor stream should be acidic to avoid precipitation of lead hydroxide. On the other hand, the acidity should not be too high, otherwise the solubility of PbS increases and S2ÿ ions from the sample will not be fully precipitated. The theoretical in¯uence of pH and Pb(NO3)2 concentration on the equilibrium, total sul®de concentration calculated using the equations for dissociation of H2S and PbS is shown in Fig. 6. It can be seen that for pH values less than 1.5 and Pb(NO3)2 concentrations in the range from 0.1 to 0.4 M, the equilibrium sul®de in the
Table 1 Cyanide recovery in the presence of sulfide using a donor stream containing 0.1 M Pb2 and 0.01 M HNO3 (pH2.0) (FI configuration as in Fig. 1; other conditions, as in Fig. 5) [CNÿ] (mg lÿ1) 1 0.1 0.1 0.05 0.05 a
[S2ÿ] (mg lÿ1)
Cyanide recoverya (%)
50 50 5 5 1
100.61.5 102.12.1 99.61.8 100.31.7 100.71.2
The results are the average of three determinations.
sample plug is negligible. To achieve the optimum condition for sul®de elimination, the concentrations of HNO3 and Pb(NO3)2 were varied in the ranges 0.01± 0.2 M (pH 2.0±0.7) and 0.05±0.20 M, respectively. A series of synthetic samples containing both cyanide and sul®de were investigated. Excellent recoveries of cyanide in the whole series of samples were obtained when the donor stream contained 0.1 M Pb(NO3)2 and 0.01 M HNO3 (Table 1). Sul®de concentrations greater than 100 mg lÿ1 resulted in peak tailing and poor reproducibility after several injections because of a build-up of un¯ushed PbS precipitate in the donor chamber. Thiocyanate did not interfere with the cyanide determinations at concentrations up to 1000 mg lÿ1. The interference of other anions, which form precipitates with lead, such as oxalate, sulfate, ¯uoride, and iodide were also investigated. It was found that all these anions did not affect the measurements in concentrations up to 500 mg lÿ1. 3.6. Cyanide analysis in industrial samples
Fig. 6. Theoretical equilibrium sulfide concentrations at different pH values and Pb2 concentrations.
The total, WAD and free cyanide were determined in three real samples obtained from the gold recovery industry. The samples gave positive reaction to both sul®de and thiocyanate. Total and WAD cyanide were determined by microdistillation [9] while free cyanide was measured potentiometrically (i.e., using the cyanide selective electrode). The cyanide content in all three samples were also determined by the PFI method without pretreatment of the samples prior to injection. The analysis data presented in Table 2 show that the proposed PFI method gives results close to those obtained for WAD cyanide.
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Table 2 Analysis of total, WAD and free cyanide in industrial samplesa Method ÿ1
Total cyanide [9] (mg l ) WAD cyanide [9] (mg lÿ1) Free cyanide (mg lÿ1) PFI method (mg lÿ1) a
Sample 1
Sample 2
Sample 3
45.111.49 29.240.24 9.470.16 30.620.83
179.505.51 115.001.82 57.302.02 115.650.38
86.852.02 44.370.31 24.850.34 44.470.96
The results are the average of three determinations.
4. Conclusions The PFI system outlined in this paper allows sensitive and selective determination of cyanide, which can be easily automated. An on-line method for reducing the sul®de interference was successfully developed. When industrial samples are analyzed directly by the PFI method, the results obtained are close to those given by a standard distillation method for WAD cyanide determination. On the basis of the results obtained, it is concluded that the PFI system mentioned above can be used successfully for on-line cyanide monitoring of industrial waste waters and especially for those from the mining industry. Acknowledgements We are grateful to AusAID for a scholarship for Hermin Sulistyarti and to the Australian Research Council for ®nancial support. References [1] [2] [3] [4] [5]
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