Flow Injection Electrochemical Hydride Generation of Hydrogen Selenide on Lead Cathode: Critical Study of the Influence of Experimental Parameters

ANALYTICAL SCIENCES MARCH 2003, VOL. 19 367 2003 © The Japan Society for Analytical Chemistry

Eduardo BOLEA, Francisco LABORDA,† and Juan R. CASTILLO

Analytical Spectroscopy and Sensors Group, Department of Analytical Chemistry, University of Zaragoza, Zaragoza, Spain

A flow injection system for the determination of selenium by electrochemical hydride generation and quartz tube atomic absorption spectrometry is described. The generator consists of an electrolytic flow-through cell with a concentric arrangement and a packed cathode made of particulated lead. The influences of sample flow rate, carrier gas flow rate and electrolysis current on the hydrogen selenide generation have been critically studied. Both sample flow rate and electrolysis current play important roles in the efficiency of the hydride generation process. A characteristic mass of 2.4 ng and a concentration detection limit of 17 µg lÐ1 were obtained for a sample volume of 420 µl.

(Received January 22, 2002; Accepted September 25, 2002)

The electrochemical generation of different hydrides on lead Introduction cathodes, the material that offers the highest hydrogen overpotential among those commonly used, has been studied by The formation of volatile covalent hydrides of a number of several authors.3Ð5,7Ð11 Sand3 and Thompson4,5 obtained elements (As, Bi, Ge, Sn, Sb, Se, Te and Pb) by reaction with generation efficiencies of 100%, whereas efficiencies of 86,11 sodium tetrahydroborate in acidic media has been widely used 928 and 60%11 were obtained for arsine, stibine and hydrogen for sample introduction in atomic spectrometry. This technique selenide generation, respectively, by Ding and Sturgeon. has proved to be extremely useful when low detection limits Platinum has been one of the metals most used as cathode have to be achieved.1 However, it is known that this technique material for hydride generation.2Ð8,11Ð19 However, there are some has some disadvantages. The reagent is expensive and may be a discrepancies in the literature about the performance of this source of contamination, which may worsen detection limits. In material. Arsine generation efficiencies of 1003 and 7%11 have addition, its solutions are unstable and have to be prepared been reported by different authors and hydrogen selenide has daily. been successfully generated from a platinum cathode by some The electrochemical generation of hydrides has become an authors18,19 but not by others.11 alternative to the chemical process, where the tetrahydroborate Carbon has also been used as cathode in several forms is exchanged by the electrons coming from the cathode of an (vitreous carbon,6 reticulated vitreous carbon,20 graphite,21 electrolytic cell. Since 1861, when Bloxan2 described the pyrolitic graphite8,11 and fibrious carbon15,22). The efficiencies determination of arsenic and antimony based on the electrolytic obtained depended on the type of carbon, and ranged from generation of their hydrides, several authors have developed 93%20 for a reticulated vitreous carbon to 40%11 for pyrolytic different electrochemical systems, and different cathode graphite on arsine generation. materials have been employed. Sand and Hackford3 in 1904 In addition to the material selected as cathode, one must take were the first researchers who studied the effect of different into account that the method of generation can also influence cathode materials (Pt, Cu, Fe, Pb, Zn and Hg) on arsine the efficiency of the process. Papers published before 1992 generation. They suggested that the influence of the cathode used electrolytic cells working in batch mode, whereas later material could be due to what they called “supertension” of the works use a flow-through cell working in continuous or flow material (the hydrogen overpotential) and its catalytic effect. In injection modes. In batch systems, generation efficiency is not 1909, Thompson4,5 studied the generation of arsine from limited by kinetics factors if reaction times are long enough. In cathodes of Pb, Zn Cd, Sn, Ag, graphite, Fe, Pt, Al, Au, Co, Ni, contrast, the efficiency in flow-through arrangements depends and Pd. He concluded that the overpotential did not appear to on hydrodynamic parameters of the system (flow rate, electrode be the controlling factor of the generation efficiency. More surface and mass transfer).23 A flow-through cell with a tubular recently, several authors have confirmed experimentally the arrangement and a packed cathode of reticulated vitreous carbon direct relation between hydrogen overpotential of the cathode were recently designed by the authors to achieve a high material and the analytical sensitivity6,7 or the generation generation efficiency in a short period of time.20 This efficiency.8 arrangement ensured a high mass transfer rate of the analyte to the cathode surface and it produced a small dispersion when it † To whom correspondence should be addressed. was used in a flow injection system. 368 ANALYTICAL SCIENCES MARCH 2003, VOL. 19

Table 1 Operating conditions

Lamp current/mA 16 Wavelength/nm 196.0 Slit width/nm 2.0 Nitrogen flow rate/l minÐ1 0.58 Catholyte 0.5 M HCl Anolyte 0.5 M HCl 20% hidroxylamine hydrochloride Sample flow rate/ml minÐ1 5.5 Electrolysis current/A 1.0

dissolving SeO2 (Merck, Elmsford, NY, USA) in ultrapure water. A standard solution (1000 mg lÐ1) of Cu(II) was prepared Fig. 1 Diagram of the electrochemical hydride generation system by dissolving high purity metal (Probus, Barcelona, Spain) in 10 coupled to the flame-heated quartz tube AAS. (1) Particulated lead mL of concentrated HNO and diluting with ultrapure water. cathode, (2) porous ceramic tube, (3) platinum wire anode, (4) 3 graphite electric contact. Working standard solutions were prepared by further dilution in 0.5 mol lÐ1 HCl. Electrolytes. A solution of HCl (Merck) 0.5 mol lÐ1 was used as catholyte and a 20% (w/v) hydroxylamine hydrochloride The aim of this work is to evaluate the use of lead as cathode (Sigma, St. Louis, MO, USA) solution in 0.5 mol lÐ1 HCl as material with the electrolytic hydride generator described anolyte. Hydroxylamine hydrochloride was added to the above,20 and to understand the effect of the experimental anolyte in order to avoid oxidation of chloride to chlorine. parameters on the performance of the electrochemical hydride Reagents for chemical hydride generation. The reductant was a generation. A flow injection system for hydride sample 0.5% sodium tetrahydroborate (Sigma) solution and the carrier introduction in quartz tube atomic absorption spectrometry has was 0.5 mol lÐ1 HCl. also been developed and optimized for the determination of selenium. Procedures

Experimental General procedure for the hydride generation Electrochemical hydride generation was accomplished in flow Instrumentation injection mode. The catholyte and anolyte were continuously A Perkin Elmer (Norwalk, CT, USA) Model 2380 atomic pumped through the cathodic and anodic compartment, absorption spectrometer, equipped with a flame heated quartz respectively, and a voltage was applied to deliver a constant tube atomizer (168 mm length, 12 mm i.d.) was used for atomic current. Injection of a pre-filled 420 µl sample loop into the absorption measurements. Selenium absorption was measured catholyte stream was done manually. The catholyte and at 196.0 nm. The spectral bandwidth was set at 2.0 nm and the reaction products were delivered to the gas-liquid separator hollow cathode lamp current at 16 mA. from which the gases were directed to the atomizer by a nitrogen stream. Operating conditions are summarized in Table Hydride generation system 1. The electrochemical hydride generation system has been For chemical hydride generation, sodium tetrahydroborate described in an early publication;20 a schematic diagram is (1.5 ml minÐ1) and HCl (4.0 ml minÐ1) were continuously shown in Fig. 1. The generator consisted of a tubular pumped through a T-piece. The electrolytic hydride generator electrolytic flow-through cell, where the cathode and anode was exchanged by a reaction coil (length 3.5 cm) and this spaces show a concentric configuration. The generator was stream was conducted to the gas-liquid separator as described connected to a dc power supply (Promax FAC-662B), operated above. in constant current mode. Particulated lead (1 Ð 2 mm2) was used as cathode. Because of the reliable behavior shown by the Procedure for the measurement of the cathode potential cell during all the experiments, no additional pretreatment was In order to measure the real potential applied to the cathode, necessary in order to prepare the surface of the material. we placed an Ag/AgCl reference microelectrode before the inlet The generation system also included a peristaltic pump to the cathodic space by a specially arranged T-piece. Under (Gilson Minipuls III, Villers le Bel, France), a six-ways the standard operating conditions, the potential between the injection valve (Omnifit, Cambridge, UK) and a gas-liquid reference electrode and the cathode was measured with a separator with forced outlet described previously.24 Nitrogen voltameter (Fluke, WA, USA). (99.998%) was used as the carrier gas, being introduced in the gas-liquid separator and controlled by a flow meter, PTFE Procedure for the measurement of the hydride generation tubing (i.d. 0.8 mm) was used for all connections. efficiency The efficiencies measured are referred to the fraction of Reagents and standard solutions analyte not volatilized and thus remaining in the solution during All standard stock solutions were prepared from analytical hydride generation. A solution containing 1 mg lÐ1 of analyte grade reagents. Ultrapure water was obtained from a Milli-Q was continuously pumped through the cathode. The generator system (Millipore-Waters, Milford, MA, USA). was operated under the selected experimental conditions and the A standard solution (1000 mg lÐ1) of Se(IV) was prepared by solution that drained in the gas-liquid separator was collected. ANALYTICAL SCIENCES MARCH 2003, VOL. 19 369

Fig. 3 Effect of the sample flow rate on the relative standard Fig. 2 Effect of the sample flow rate on the analytical signal of deviation of the analytical signals of Se(IV). ( ) Peak area, ( ) peak Se(IV). ( ) Peak area, ( ) peak height. 200 mg lÐ1 Se. height. 200 mg lÐ1 Se.

The analyte concentration of the collected solution was determined by electrochemical hydride generation and related to its initial concentration.

Sample preparation In order to determine the concentration of Se(IV) in a vitaminic complex containing selenium as sodium selenite, we placed the content of a capsule of the sample in a beaker with 50 ml of ultrapure water and allowed it to stand for 8 h with magnetic stirring. Then the supernatant was filtered and the solution was transferred into a 100 ml calibrated flask and made up to volume with 0.5 mol lÐ1 HCl. Hydrogen selenide is only generated chemically and Fig. 4 Effect of the carrier gas flow rate on the analytical signal of electrochemically from Se(IV) and not from Se(VI). Then, a Se(IV). ( ) Peak area, ( ) peak height. 200 mg lÐ1 Se. pre-reduction step would be necessary for samples containing Se(VI) to determine the total selenium content.

concentration, which is related to the generation efficiency and Results and Discussion the dispersion of the analyte in the flow stream. The decrease of generation efficiency at high flow rates is compensated by Sample flow rate the smaller dilution of the analyte and a plateau was observed For all experiments, equal flows of anolyte and catholyte were for flow rates higher than around 5 ml minÐ1. used. The effect of the flow on both peak area and peak height In spite of the increase of generation efficiency, very poorly was evaluated and it is shown in Fig. 2. It can be noted that the defined peaks were obtained when working at low flow rates. peak area decreases, whereas the peak height increases slightly Relative standard deviations at different flow rates are with the electrolyte flow rate in the range studied. This summarized in Fig. 3, showing the highest values at low flow behavior was in agreement with other works both in flow rates. Finally, a flow rate of 5.5 mL minÐ1 was chosen as a injection6 and in continuous mode.16,20 compromise among efficiency, dilution and relative standard Peak area signals are proportional to the mass of analyte deviation both for peak area and height. detected and they are directly related to the hydride generation efficiency. Several authors have studied the effect of flow rate Carrier gas flow rate 11,19,20 on generation efficiency, and a decrease of efficiency with Figure 4 shows the effect of N2 flow rate on analytical signals. increasing flow rate is always observed. In order to explain this No significant differences were found between the behavior of decrease of the efficiency, some approximations should be the height and area of the peaks, both decreased when the done. If a flow-through porous electrode is considered, the carrier flow rate was increased. This effect can be explained by efficiency (R) of the reaction carried out at the electrode can be the dilution of the hydride into the flow stream. Then, the expressed, under limiting-current conditions, as:25 mininum flow rate of 0.58 l minÐ1 was selected. The generation process produces, in addition to the hydride, a  m sAL  large amount of hydrogen, which is directly related to the R = 1 Ð exp Ð ——o —— (1)  Q  electrolysis current circulating in the generator. The effect of the carrier gas composition on the selenium signal was where mo is the mass transfer coefficient, s the specific area of investigated by connecting an external hydrogen flow to the the electrode, A and L the area and length of the electrode and Q gas-liquid separator. At the optimum carrier gas flow rate, the the flow rate, respectively. Then any parameter which external addition of a 10% of hydrogen produced 60% of signal contributes to decrease the time of contact between the analyte supression. The percentage of hydrogen electrochemically and the electrode surface (increase of sample flow rate, decrease generated in the carrier gas ranged from 1.2% at 1 A to 2.8% at of the specific area of the electrode and/or decrease of its 2.5 A. When the hydride was generated chemically, under the volume) will decrease the efficiency of generation. conditions described in the experimental section, 3.3% of Peak height signals are proportional to the analyte hydrogen gas from the tetrahyborate decomposition reaction 370 ANALYTICAL SCIENCES MARCH 2003, VOL. 19

Fig. 5 Effect of the electrolysis current on the analytical signal of Fig. 6 IntensityÐpotential curve at a lead cathode in 0.5 M HCl. Se(IV). ( ) Peak area, ( ) peak height. 200 mg lÐ1 Se.

generation conditions was low enough to produce all the would be present in the carrier. The decrease of the signals can reactions involved in the hydrogen selenide formation. As the be related to the atomization process in the quartz tube, as has applied cathodic potential was lower than that one been pointed out by Bax.26 This author concluded that the corresponding to the limiting current of the analyte reduction, overall reaction: the increment of the hydride generation efficiency cannot be due to an increment of the mass transfer rate, since the steady

SeH2 ←→ Se + H2 reaction (1) state is reached even from the lowest current. So any modification which affects this first step should be related to is shifted to the left by the hydrogen, and subsequently the hydrodynamic parameters, such as the flow rate or the specific efficiency of the atomic selenium formation is decreased. area of working electrode. With respect to the second step, Tomlinson28 proposed a Electrolysis current mechanism of stibine generation, which can be extended to As can be seen in Fig. 5, analytical signals increased with other hydrides. It consisted of (1) the deposition of the analyte increasing electrolysis current, in agreement with the in elemental form on the electrode surface and (2) the formation observations of other authors.8,11,20,22 This behavior reflects the of the hydride from the deposited analyte. This second process improvement on the hydride generation efficiency observed has been described by Salzberg29 for arsine generation from when the electrolysis current was increased. The efficiencies arsenic and arsenic-lead electrodes. He proposed that arsine obtained ranged from 52.2±0.5% at 1 A to 75.5±0.7% at 2.5 A. was formed by reduction of water molecules on arsenic atoms, In order to analyze the effect of the electrolysis current on the which became covered with chemisorbed hydrogen in the form generation efficiency, the process of hydride generation can be of monohydrogen and dihydrogen arsenic species. These schematically represented by three successive steps: (1) species would disproportionate, producing the arsine. The transport of the analyte to the cathode surface; (2) charge overall mechanism would be described for a trivalent analyte as transfer and/or catalytic reactions between the analyte, hydrogen follows: and the electrode; (3) transport of the reaction products from the electrode.23 So, a modification of the efficiency should depend A(III) → A(0) reaction (2) on one or more of these steps. The first step is limited by diffusion and convection processes, A(0) + H → AH reaction (3) responsible for transfering the analyte from the solution to the electrode surface. Under mass transfer limiting conditions, AH + H → AH2 reaction (4) where all the analyte reaching the electrode surface undergoes the electrochemical reaction, the reaction rate cannot be AH2 + AH → AH3 + A reaction (5) increased and the current intensity reaches a steady state although the applied potential was increased. Batanero27 has In this context, the rate of a reaction is directly proportional to described the reduction on a carbon electrode of Se(IV) to grey the number of reactant particles which possess a minimum Se(0) at Ð0.020 V (vs. SCE), followed by the reduction to activation energy, and the energy of the reactants (atomic hydrogen selenide at Ð0.45 V (vs. SCE). Hydrogen selenide hydrogen and mono and dihydrogen species) depends on the also reacts at this potential with selenite in order to produce red potential applied. Then, an increment on the electrolysis current selenium, a reaction which has been visually observed by the means an increment on the energy of the species involved on authors at around Ð0.500 V (vs. Ag/AgCl) on a lead cathode. the generation of the hydride, since the potential is greater, Figure 6 shows the variation of current intensity with respect to improving the rate of reaction and its efficiency. When the the potential applied to the cathode of the hydride generator system is operated under constant current mode, and reduction under standard conditions (flowing a solution of 0.5 M HCl). of hydronium ions or water to hydrogen is the main reaction, the The current intensity increased slowly with decreasing cathode potential applied to the cathode depends on the cathode material potential down to Ð2.3 V (vs. Ag/AgCl), where the intensity and its hydrogen overpotential. In this respect, the high showed a fast increase due to the bulk reduction of hydronium overpotential of lead would contribute to improve the ions or water to hydrogen. This voltammogram is not limited generation efficiency in comparison to other materials with by mass transfer because of the high concentration of lower overpotentials (platinum, carbon) operating at the same hydronium ions and water, which are reduced at those negative current, as has been pointed out by other authors.3,6,7 potentials due to the high overpotential of lead. Therefore, the Finally, the third step is related to the efficiency of the potential applied to the cathode of the generator under standard transport of the products from the electrode, and the release of ANALYTICAL SCIENCES MARCH 2003, VOL. 19 371

Table 2 Analytical figures of merit for wet chemical and The attainable sensitivity, when analytes are introduced in electrochemical selenium hydride generation atomic spectrometry instruments as volatile hydrides generated Detection Characteristic Characteristic electrochemically, is directly related to the efficiency of the Hydride limita/ concentration/ mass/ RSD, % hydride generation process. When flow-through electrolytic generation µg lÐ1 µg lÐ1 ng generators are used, this efficiency depends on sample flow rate and electrolysis current, for a particular cathode material and Tetrahydroborate 2.3 3.5 1.5 3.2 specific cathodic area. Sample flow rate controls the residence Electrochemical 6.7 11.0 4.6 5.0 time and the mass transfer of the analyte along the generator, whereas electrolysis current controls the applied potential, the energy of the species involved in the generation of the hydride and the amount of the hydrogen formed. In its turn, hydrogen the hydride from the solution. An increment in the electrolysis contributes to the stripping of the gaseous hydride from the current implies an increment in the amount of hydrogen formed. solution, although its presence in the quartz tube reduces the As the gas is generated in situ, it can contribute to the release of atomization of the hydride. the hydride more efficiently than the carrier gas, which is externally added, as it has been suggested by other authors.30 Acknowledgements Figures of merit Analytical figures of merit for chemical and electrochemical This work has been sponsored by the DGICYT of the Spanish hydride generation are summarized in Table 2. Detection Ministry of Education, Culture and Sport (Project No. limits, defined as three times the standard deviation of the base BQU2000-0992). line divided by the peak height sensitivity, of 2.3 and 6.7 µg lÐ1 were obtained for chemical and electrochemical hydride generation, respectively. Similar differences were obtained for References the characteristic concentration and mass due to the attainable efficiency of the electrochemical generator, which was limited 1. J. Dedina and D. L. Tsalev, “Hydride Generation Atomic by the specific area of the packed cathode and the particle size Absorption Spectrometry”, 1995, John Wiley and Sons, of the lead used. Relative standard deviations better than 5% (n New York. = 10) were obtained by both generation modes. 2. C. L. Bloxam, Quart. J. Chem. Soc., 1861, 13, 12. 3. H. J. E. Sand and J. E. Hackford, J. Chem. Soc., 1904, 85, Determination of Se(IV) in a vitaminic complex 1018. A vitaminic complex containing selenium as sodium selenite 4. W. Thompson, Chem. News, 1909, 99, 157. was analyzed by the proposed method, which allows the 5. W. Thompson, Proc. R. Soc. [Edinburgh], 1909, 29, 84. determination of Se(IV). The sample could not be analyzed by 6. Y. Lin, X. Wang, D. Yuan, P. Yang, and B. Huang, J. 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Juláková, “Electrochemical 372 ANALYTICAL SCIENCES MARCH 2003, VOL. 19

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