Anal. Chem. 1996,67,318-323
Electrochemical Detection of Trace Hydrogen Sulfide in Gaseous Samples by Porous Silver Electrodes Supported on IonlExchange Membranes (Solid Polymer Electrolytes) Gilbert0 Schiavon* and Gianni Zotti C.N.I?.-I.P.E.L.P., corso Stati Uniti 4, 35020 Padova, Italy
Rosanna Toniolo and Gino Bontempelli* Department of Chemical Sciences and Technology, University of Udine, via Cotonificio 108, 33100 Udine, Italy
A highly sensitive and fast-responding electroanalytical sensor for the determination of hydrogen sulfide in gaseous atmospheres is described which eliminates oxygen interferences. It consists of a porous silver working electrode (facing the sample) supported on one face of an ion-exchange membrane, which serves as a solid polymer electrolyte. The other side of the membrane faces an internal electrolyte solution containing the counter and reference electrodes. The performance of this sensor has been tested for the electroanalysisof H2S by amperometric monitoring, cathodic stripping measurements, and flow injection analysis. In all cases, the device displays a high current sensitivity and a low background noise, so that quite low detection limits (45ppb v/v, 0.07ppb v/v, and 3.7 x mol in amperometric, cathodic stripping, and flow injection measurements, respectively) are estimated for a signal-to-noiseratio of 3. The responses are found to be characterized by both a good reproducibility and a linear dependence on the concentration of H2S over fairly wide ranges, as well as by a short response time (ca. 0.5 s to attain a 95% response). This fast response time arises from the lack of a gas-permeable membrane and direct gas contact to the triple interphase among the gaseous an-, the porous working electrode, and the solid polymer electrolyte. The absence of any effect due to the most important potential interfering species and the possibility of adopting such a device for the direct detection of H2S in ambient air and for industrial hygiene measurements are discussed. Increasing concern over atmospheric environmental problems suggests the design and development of new analytical techniques in order to introduce alternatives to the presently employed methods. In particular, since the analysis of hydrogen sulfide in the atmosphere and in technological gases is especially important, the possibility of performing its determination by techniques qualified to provide in situ and instantaneous measurements appears to be particularly advisable. The analysis of such a reducing species, which is air-unstable but survives for rather long times owing to its slow reaction with oxygen, requires the use of analytical techniques characterized by quite low detection limits 318 Analytical Chemisfry, Vol. 67,No. 2,January 75,7995
below the small threshold levels allowed by law in many countries. Thus, for instance, both the American Conference of Governmental Industrial Hygienists and the Italian government have set in the neighborhood of industrial plants a time-weighted average W A ) of 10 ppm (15 pg L-l) and a short-term exposure limit (mL)of 15 ppm (23 pg L-9, while in ambient air a TWA of 0.03 ppm (0.04 pg L-I) and a STEL of 0.07 ppm (0.10 pg L-I) have been set by the Italian government. In experimental practice, these low detection limits are frequently attained by exploiting a preliminary adsorption or absorption accumulation step. But it is just the presence of this preconcentration step that makes almost all these approaches scarcely suitable for continuous monitoring procedures. On the contrary, continuous monitoring of H2S is possible by electroanalytical methods, thanks to their peculiar sensitivity, which allows very low detection limits to be attained without the need for preconcentration. Unfortunately, electroanalytical procedures cannot be directly applied to air samples in which no supporting electrolyte is present and none may be added. To overcome this obstacle, metalized membrane electrodes have been developed which consist of porous Teflon membranes with one side facing the sample, while the other surface is covered by a thin metal film acting as the working electrode.1,2This film is frequently prepared by evaporation of a metal resinate solution and contacts an internal electrolyte solution in which the auxiliary and reference electrodes are dipped. Thus, the use of this type of electroanalytical sensor metalized with Pt,Au, or Ag has been suggested for monitoring hydrogen sulfide in gaseous media by direct amperometric,3 ~oltammetric,~ or cathodic stripping5 measurements, as well as for detecting such a polluting species by gas chr0matographic6,~or flow analysis. In spite of the fairly good results obtained with these sensors, their performance is conditioned by diffusion of the analyte through the (1) Cao, 2.; Buttner, W. J.; Stetter, J. R Electroanalysis 1992,4, 253-266. (2) Chang, S. C.; Stetter, J. R; Cha, S. C. Tulunta 1993,40,461-477. (3) Sedlak, J. M.; Blurton, K. F. Talunta 1976,23,445-448. (4) Bergman, I. J. Electroanal. Chem. 1983,157,59-73. (5) Opekar, F.; Bruckenstein, S. Anal. Chem. 1984,56, 1206-1209. (6) Stetter, J. R; Sedlak, J. M.; Blurton, K F. /. Chromatogr. Sci. 1977,15, 125-128. (7) Blurton, K F.; Stetter, J. R 1.Chromatogr. 1978,155,35-45. (8) Nygaard, D. D.Anal. Chim.Acta 1981,127,257-261. (9) Opekar, F.; Bruckenstein, S. Anal. Chim.Acta 1985,169,407-412. (10) Langmaier, J.; Opekar, F.; Pacakova. V. Talanfa 1987,34, 453-459.
0003-2700/95/0367-0318$9.00/0 0 1995 American Chemical Society
membrane, whose porosity thus affects markedly their sensitivity and response time. Moreover, an alkaline internal electrolyte is adopted in the majority of them,45>*-l0so that its renewing after any measurement is required to avoid the progressive trapping of increasing amounts of sul6de ions, which leads to significant positive errors. More recently, promising results in the monitoring of electre active analytes present in gaseous media or in supportingelectrolyte-free solvents have been gained by a quite different approach based on the use of moist ionexchange membranes as solid polymer electrolytes (SPES).11-20These sensors are usually prepared by coating the side of an ionexchange membrane facing the analyte sample with a porous conductive film (working electrode), while the other side contacts an electrolyte solution containing the counter and reference electrodes. In these devices, any membrane permeation step is avoided since the membrane separating the sample from the internal electrolyte does not act as a filter for gaseous analytes but serves to ensure the transfer of charged species from the working to the counter electrode, thus playing the role usually played by supporting electrolytes. In this paper we propose the use of a suitable modification of this type of sensor for the detection of hydrogen sulfide at trace levels in gaseous media. Its suitability for both continuous amperometric monitoring and for cathodic stripping analysis has been tested, together with its ability to act as a detector in flow injection analysis. EXPERIMENTAL SECTION
Chemicals and Instrumentation. All the chemicals used were of reagent grade quality, and they were employed without further purification. Gaseous hydrogen sulfide was obtained by injecting known volumes of a stock sodium sulfide solution into 1 M HClOd solution. The stock aqueous solution of Na2S was standardized by titration with hexacyanoferrate(III) according to the literature.21 In all cases, water purified with a Milli-Q system Millipore (“Milli-Q water”) was used as the solvent. Unless otherwise stated, fully deoxygenated nitrogen was used as the carrier gas. Voltammetric and amperometric tests were performed by an EG&G PARC model 273 potentiostat driven by EG&G PARC model 270 software installed on an IBM System/2 computer. Electrode and Apparatus. The ion-exchange material used as the SPE was a porous Nafion 417 cationic perfluorinated membrane, 0.425 mm thick, reinforced with Teflon. It was cleaned by boiling in concentrated nitric acid for 1h and then in Milli-Q (11) Kaaret, T. W.; Evans, D.H. Anal. Chem. 1988,60, 657-662. (12) De Wulf, D.W.; Bard, A J. J. Electrochem. SOC.1988, 135, 1977-1985. (13) Schiavon, G.; Zotti, G.; Bontempelli, G. Anal. Chim. Acta 1989,221, 2741. (14) Harth, R;Mor, U.; Ozer, D.; Bettelheim, A J. Electrochem. SOC.1989,136, 3863-3867. (15) Schiavon, G.; Zotti, G.;Bontempelli, G.; Farnia, G.;Sandona, G. Anal. Chem. 1990, 62, 293-298. (16) Schiavon, G.; Zotti, G.; Toniolo, R;Bontempelli, G. Electroanalysis 1991, 3, 527-534. (17) Weisshaar, D. E.; Lamp, B.; Merrick, P.; Iichty, S. Anal. Chem. 1991, 63, 2383-2386. (18) Schiavon, G.;Zotti, G.;Toniolo, R; Bontempelli, G.Analyst 1991,116,797801. (19) Xing, X. K; Liu, C. C. Electroanalysis 1991,3, 111-117. (20) h u b , L.; Opekar, F.;Pacakova, V.; Stulik, K Electroanalysis 1992,4,447451. (21) Charlot, G . Les Methodes de la Chimie Analitique, Analyse Quantitative Minerale; Masson: Paris, 1966; p 918.
C
R
wl lw a
b C
d
/
\J H,S/
e
Figure 1. Schematic view of the Ag-covered Nafion electrode. (a) internal compartment filled with aqueous 0.01 M HCIO4 0.99 M NaC104 and equipped with (C) a Pt counter electrode and (R) an aqueous HglHg2S04 reference electrode. (b) Nafion ion-exchange membrane. (c) Porous Ag coating. (d) Ag-ring collector. (e) Gaseous sample.
+
water for 1h. The membrane was cut into discs 1cm in diameter which were equilibrated in aqueous 1 M NaC104 for 3 h. One face of these discs was then covered by a porous silver film by metal vapor deposition with a vacuum coating unit Edwards model E 306 A, which was used to produce 0.1 pm thick silver deposits at a deposition rate of 0.5 nm s-l. SPE electrodes were assembled by clamping one of the metalcoated Nafion discs, with the porous silver layer directed downward, at the bottom of a Pyrex cylinder, the end of which was threaded for connection to a drilled Teflon holder sealing the assembly by means of an elastic O-ring resistant to acids and bases. Such an assembly allowed ca. 0.4 cm2of silver film to be exposed to external media. As shown schematically in Figure 1, the uncoated side of the membrane was put in contact with an aqueous electrolyte solution (0.01 M HC104 0.99 M NaC104) contained in an internal compartment equipped with a platinum counter electrode and an aqueous mercury/mercury sulfate reference electrode. The constructive details of this type of electroanalytical sensor have been reported p r e v i ~ u s l y . l ~ , ~ ~ , ~ ~ , ~ ~ As illustrated in Figure 1,when the silver electrode is held at an appropriate potential, its oxidation according to reaction 1 becomes possible just as the H2S molecules from the working sample reach the Ag/Nafion interphase. Hydrogen ions released
+
by this anodic reaction are trapped in the membrane, coupled with an ionic migration through the membrane to maintain electroneutrality. Concurrently,hydrogen ions in the internal electrolyte are reduced at the counter electrode, thus restoring its ionic content. Hydrogen produced in this cathodic counter-reactionwas periodically removed by purging with nitrogen. The electroanalytical sensors were used as gas-tight stoppers for the glass flow cell (V m 20 mL) in which all voltammetric, continuous-monitoring, and flow injection experiments were Analytical Chemistry, Vol. 67, No. 2,January 15, 1995
319
I
PUMP
I
1
I
I
l
l
,,' -1.0
pJ&)!:&\
' -0.5
E/V
I
/
I
, I
I
II
/ a
/ b
'
1-05
I
Figure 2. Scheme of the flow apparatus employed.
I
conducted. A schematic view of this cell is shown in Figure 2, which also reports a scheme of the closed-loop flow system adopted (V x 1.25 L). This system, which was assembled by connecting all devices with short Teflon tubing, was preliminarily fed with oxygen-free nitrogen at 1 atm. The sample (NaB solution) was introduced into the generation vessel by injection through the septum covering the input opening using a microsyringe. The H2S produced was hence fed to the flow cell by a nitrogen stream whose flow rate was kept constant, unless stated otherwise, at 50 mL min-' by a peristaltic pump. A flowmeter was inserted in the stream to monitor this flow rate, and, when desired, two threeway valves allowed the system to be purged with nitrogen. Flow injection experiments were conducted in the same flow apparatus,where the generation vessel was shut out by connecting directly the cell output to the sample buffer. In these measure ments, the injection device employed was a rubber septum covering an input opening, placed directly in 'the cell input, through which known volumes of gaseous samples were introduced by a gas-tight microsyringe. In these tests, as well as in continuous monitoring experiments, the SPE was employed as an amperometric detector working at a controlled potential. All the tests were conducted at room temperature, and all potentials are quoted versus an aqueous Hg/HgZSO1, saturated Na2S04 reference electrode.
Flgure 3. Cyclic voltammogram recordedat a Ag-Nafion electrode, 0.99 M NaC104 as the internal with aqueous 0.01 M HC104 electrolyte, on a nitrogen stream containing M HzS (0.34 mg L-I, 220 ppm vh). Scan rate, 0.02 V s-l; flow rate, 50 mL min-'. Dashed lines are anodic and cathodic limits (a) in the absence of 02 and (b) in the presence of 02.Potentials are quoted versus a Hg/ HgZS04, saturated NazSO4 reference electrode.
RESULTS AND DISCUSSION
Anodic Behavior of Ag-SPEElectrodesin HzS-Containing Atmospheres. When the closed-loop flow cell (Figure 2) was fed with a nitrogen stream containing hydrogen sulfide, the typical steady-statecyclic voltammogram shown in Figure 3 was recorded at Ag-covered Naiion electrodes with 0.01 M HC104 0.99 M NaC104 as the internal electrolyte. A well-formed and quite reversible anodic-cathodic system was observed for the silver oxidation to silver sulflde and the associated reverse reduction, which was reproducible in the second and subsequent cycles. Such behavior appears to be in full agreement with that displayed by conventional silver electrodes in sulflde containing 0.2 M NaOH solutions,22once the potential shift (60 mV per pH unit) expected on passing from this basic medium to that used by us as the internal electrolyte (pH = 2) for wetting the rear side of the Nation membrane is taken into account. Figure 3 also shows the potential window available for the AgSPE in oxygen-free and oxygencontaining nitrogen. It is apparent
+
(22) Shimii, IC; Osteryoung, R A Anal. Chem. 1981,53,584-588. (23) Tierney, M. J.; Kim, H. L.Anal. Chem. 1993,65,3435-3440.
320 Analytical Chemistry, Vol. 67, No. 2, January 15, 1995
I
+
that the oxygen reduction interferes with the sulfidepromoted silver oxidation in the potential range from -0.75 to -0.5 V, which is hence impracticable for the sulfide monitoring of oxygencontaining atmospheres. This monitoring free from oxygen interferences is conversely possible in the rather wide potential region from -0.5 to -0.3 V. This useful region became narrower for pH values of the internal electrolyte lower than 2, while it did not increase appreciablyfor pH ranging from 2 to 7, owing to the scarce reversibility of the oxygen reduction, which did not shift cathodically as expected on the basis of the lower availability of protons. An abrupt increase of this useful potential range is attained only at basic pH values, which have been, however, intentionallyexcluded to avoid the possible trapping of H2S in the supporting electrolyte. These results led us to conclude that the most appropriate internal electrolyte for the Ag-SPE electrode was an aqueous solution at pH = 2 containing both 0.01 M perchloric acid and 0.99 M NaC104, this last added to ensure a high ionic conductivity, which also offered the advantage of a lower and more reproducible residual current in the potential region from -0.5 to -0.3 V, useful for HzS monitoring. Amperometric Monitoring. To test the performance of the silvercovered Na6on electrode as an amperometric sensor for hydrogen sulfide in gaseous atmospheres, the flow cell was fed with nitrogen streams containing known and increasing concentrations of H2S produced in the generation vessel. The H2S content was monitored by measuring the anodic current flowing when a potential of -0.4 V was applied to the working Ag film (see Figure 3). The results obtained are summarized in Figure 4,which shows a typical current-time response recorded during these measurements. Each addition of HzS causes a rapid rise in the current density,which attains a satisfactorily constant value in a fairly short time (about 100 s). This constant current density was found to be reproducible within f3% in replicate measurements. In connection with this precision, it must be remarked that the size of the flow apparatus (V x 1.25 L) was chosen so as to avoid appreciable depletion of the H2S content during amperometric measurements, especially at the lower concentrations
Table 1. Performance of the Ag-SPE Sensor
type of measurement
amperometric analysis
sensitivitp 39.5 A cm-2 M-l
cathodic stripping analysis
30 kA cm-2 M-'
flow injection analysis
240 kA cm-2 mol-'
detection limi@ 2
x
M
68 ng L-' 45 ppb v/v 3x M 0.1 ng L-' 0.07 ppb v/v 3.7 x 10-13 mol
a Current densities are referred to the geometric area (0.4 cmz). Estimated for a signal-tenoise ratio of 3.
" I
0
10
I
I
20
30
t/min
Figure 4. Current-time profile recorded at a Ag-Nafion electrode, 0.99 M NaCIO4 as the internal with aqueous 0.01 M HClOl electrolyte, on nitrogen streams containing H2S in the following concentrations : (a) 6.5 x lo-* M (2.2pg L-I, 1.4ppm); (b) 1.3 x 1 O-' M (4.4pg L-I, 2.8ppm); (c) 2.6 x 1 O-' M (8.8pg L-', 5.6 ppm); M (19.6 (d) 3.9 x lo-' M (13.2pg L-I, 8.4 ppm); and (e) 5.8 x pg L i , 12.5ppm). Applied potential, -0.4 V; flow rate, 50 mL min-'.
+
(typically, 0.5%of the H2S content is consumed at any minute when the concentration is M, 3.4 pg L-l, 2.2 ppm v/v). At the same time, the working silver electrode was very large compared with the silver amount converted to AgzS during any measure ment, so that no si&cant change with time of the real electrode surface is expected, even at the higher HzS concentrations (2 x M, 0.68 mg L-l, 450 ppm v/v). To prevent any appreciable alteration of the electrode surface, it was periodically regenerated (usually every 10 measurements) by applying a potential of -0.75 V for 5 min to reduce the accumulated AgzS (see Figure 3), while the cell was purged with nitrogen. The steady-statecurrent density depended linearly on the HzS concentration over a fairly wide range which extends up to 2 x M (0.68 mg L-l, 450 ppm v/v). From its plot versus HzS concentration from 3 x to 2 x M, a sensitivity of 39.5 A cm-2 M-l (1.76 pA cm-2 ppm-l) was obtained, with a correlation coefficient of 0.998. As the residual current density at the working potential was about 0.7 pA cm-2, with a standard deviation of about 0.03 pA cm-2 (background noise), a detection limit of a p proximately 2 x M (68 ng L-l, 45 ppb v/v) could be evaluated for a signal-to-noise ratio of 3. These results are summarized in Table 1, where they are compared with those reported below and obtained by the Ag-SPE sensor in the other types of measurements. The long-term stability of these Agcovered Nafion electrodes in gaseous streams containing HzSappeared to be totally satisfactory in that no appreciable change in the current response was observed, even after 2 months of continuous use. Moreover, different Ag-Naiion electrodes led to very similar responses (&5%)when used on the same HzS samples, thus indicating that quite reproducible electrode surfaces are obtained by the construction procedure mentioned above. Note that no interference was observed at the sensing potential from oxygen intentionally added to the carrier gas, in full agreement with the expectations based on the voltammetric results reported above. Finally, it is worthwhile to remark that these measurements are characterized by a fast response time, which was investigated
by carrying out suitable experiments. A 95% response was estimated in about 0.5 s when the electrode was transferred rapidly from air to the cell shown in Figure 2 fed with a gas stream containing M (34 pg L-l, 22 ppm v/v) of HzS. This response time was practically unaffected when the electrode was transferred from air to gas streams with higher or lower contents. Such a short response time is probably the main advantage offered by these electrodes since it makes them suitable for flow injection analysis and all other applications requiring fast r e sponses, by contrast with conventional membrane electrodes. In SPEs any gas permeation step is avoided in that the gaseous analyte reaches the working electrode by transport through the air-filled pores of the porous metal substrate instead of by permeation through a gas-permeable membrane, and the anodic reaction begins as soon as the analyte reaches the three-phase site where the electrode, the polyelectrolyte, and the gas meet. Consequently, the relatively slow d ~ s i o of n the gaseous analyte across a polymeric membrane, which is the main obstacle to fastresponding electrochemical gas sensors, is thus removed. Also, the sensitivity of these SPEs is largely better than that characterizing conventional membrane electrodes, which is strongly affected by the fairly slow permeation of the gaseous analyte through the membrane. The lack of a gas-permeable membrane also characterizes an electrochemical gas sensor, employing likewise immobilized Naiion but with a rather different design, which has been proposed recently to achieve expressely fast response timesz3 However, the responses provided by such a sensor are largely dependent upon ambient humidity. Cathodic Stripping Measurements. To accede to the determination of H B concentration levels lower than those detectable by amperometric monitoring, the possibility of exploiting the cathodic strippingpeak associated with the electrochemical oxidation of silver in the presence of hydrogen sulfide (see Figure 3) has been explored. With this purpose, the flow cell shown in Figure 2 was fed with nitrogen streams containing known but very small concentrations of HzS produced in the generation vessel. This determination was performed first by holding the porous silver electrode at -0.4 V for 10 min and then by scanning the electrode potential in the negative direction at a linear scan rate of 20 mV s-l, so that the cathodic stripping peak of AgzS could be recorded. A preelectrolysis time of 10 min was adopted to reconcile the requirement for good sensititvity with that for a reasonably short analysis time. In fact, the dependence of the stripping peak current with the preelectrolysis time, whose typical trend is reported in Figure 5, shows a significant deviation from linearity Analytical Chemistry, Vol. 67, No. 2, January 15, 1995
321
10s
(e)
(b)
(C)
Figure 6. Flow injection peaks recorded at a Ag-Nafion electrode, 0.99 M NaC104 as the internal with aqueous 0.01 M HClO4 electrolyte, for increasingamounts of HzS injected: (a) 20 pL, (b) 50 pL, and (c) 100 pL of nitrogen containing M H2S (34 fig L-I, 22 ppm v h ) . Electrode potential, -0.4 V.
+
0
I
10
tlmin
Figure 5. Dependence on preelectrolysis time of the cathodic stripping peak current density recorded at a Ag-Nafion electrode, with aqueous 0.01 M HCIO4 0.99 M NaC104 as the internal electrolyte. Sample: M H2S (3.4 ng L-I, 2.2 ppb v/v) in nitrogen.
+
for times higher than about 10 min, thus pointing out that progressively lower increases of the preconcentrated sulfide are gained at longer times. Such a deviation is apparently due to the appreciable depletion of the HzS contained in the flow apparatus occurring at these quite low concentrations during the anodic accumulation, in agreement with the expectations based on the comparison of the charge involved in the cathodic peak (related approximately to the stripped amount of HzS) with the total amount of HzS present in the flow apparatus. These stripping measurements were well reproducible in that a relative standard deviation of 3.8%was obtained for seven replicate measurements with 4 x M (13.6 ng L-l, 9 ppb v/v) HzS samples. It was optimized by applying periodically at the working electrode a potential of -0.75 V for 5 min to reduce the accumulated AgzS, thus renewing the silver surface. Such a renewal, repeated usually after any five stripping measurements, was accompanied by the release of hydrogen sulfide, which was removed by a nitrogen stream passing through the acidic medium adopted as the internal electrolyte in the Ag-SPE. The dependence of the height of the cathodic stripping peak on the HzS concentration was linear from 5 x lo-” to 5 x M, with a slope (sensitivity) of 30 kA cm-2 M-l (1.34 mA cm-2 ppm-l). As the residual current density at the potential of the stripping peak was about 0.8 p A cm-2, with a standard deviation of about 0.03 p A cm-2 (background noise), a detection limit of 3 x M (ca. 0.1 ng L-l, 0.07 ppb v/v) was evaluated for a signalto-noise ratio of 3. Also, these findings are compared in Table 1 with those obtained by the Ag-SPE sensor in the other types of measurements performed in this work. It is worth noting that reliable results are obtained by these cathodic stripping measurements only when oxygen is absent during the analytical assay involved in the cathodic potential 322 Analytical Chemistry, Vol. 67, No. 2, January 15, 1995
sweep, to avoid the relevant interference (see Figure 3). This does not mean that 02 must be absent in the sample, because this interfering species may easily be removed with a nitrogen stream after the preelectrolysis step is completed, Le., when the risk of the simultaneous removal of the gaseous analyte is not run. Flow Injection Analysis. The favorable dynamic properties of the sensor described here makes it attractive for the sensitive detection of hydrogen sulfide in flow injection analysis, which is highly desirable for on-line and high-speed assays of environmental matrices. To examine this capability, the electrode was inserted in the flow cell shown in Figure 2, in combination with flow injection measurements as described in the Experimental Section. Based on voltammetric findings shown in Figure 3, an operating potential of -0.4 V was used in all these determinations. Figure 6 displays typical flow injection responses recorded for different amounts of gaseous samples injected. Well-defined and sharp peaks, with a rapid increase and decrease in the current, are observed. The peak base width of about 2 s reflects these rapid “wash-out”characteristics and permits very high injection rates (up to -600 sampledh) . The peak currents increase linearly with both the HzS concentration from to M (sample injected, 20 pL) and the injected volume from 2 to 200 pL (sample concentration M, 34 pgL-’, 22 ppm v/v). The resulting calibration plot exhibited a slope (sensitivity) of 240 kA cm-2 mol-’, while the residual current density at the working potential was about 0.7 p A cm-2, with a standard deviation of about 0.03 p A cm-2 (background noise). Based on a signal-to-noiseratio of 3, these Sndings lead to a detection limit of 3.7 x mol, which is compared in Table 1 with those of other types of measurements performed by the Ag-SPE sensor. The precision of these flow injection determinations was satisfactory in that a relative standard deviation of 3.0%was estimated for the peak height recorded for seven replicate measurements with 5 x M (17 pg L-l, 11 ppm v/v) H2S samples. Also in these measurements, the silver surface was
periodically renewed by applying a potential of -0.75 V for 5 min at the working electrode to reduce the accumulated AgzS, while the cell was purged with nitrogen to remove the hydrogen sulfide released in the cathodic reaction. As to the innuence on the response of the flow rate, a gradual M Hfi samples was increase (-20%) of the peak height for observed on decreasing the flow rate from 50 to 7.5 mL min-’. This trend can be accounted for by considering that a decrease of the flow rate also causes an increase of the yield of the AgzS deposition process with respect to the injected amount of H2S. In fact, the comparison of the charges involved in the oxidation process occurring at -0.4 V with the corresponding equivalents of hydrogen sulfide injected showed an increase from 76%to 94% for the efficiency of the anodic detection process on passing just from 50 to 7.5 mL min-’. This is the reason why a flow rate of 7.5 mL min-’ was adopted in all the flow injection measurements here reported. CONCLUSIONS
The performance of the Ag-SPEdescribed above makes this type of sensor particularly suitable for monitoring hydrogen sulfide in gaseous atmospheres, provided that both the working potential and the internal electrolyte are appropriately chosen. The amperometric approach cannot be adopted for direct measurements of HzS in ambient air, as a slightly lower limit of detection is required. However, it can be successfully used for the direct determination of HzS in industrial plants and for industrial hygiene measurements, without the need for preconcentration. Nevertheless, the detection of Hfi concentrationlevels as low as the threshold limit values for ambient air becomes accessible by employing cathodic stripping, which is also a procedure bound to be easily automatized. As to selectivity, this sensor offers the advantage of allowing the detection of Hfi by recording anodic currents at quite negative potential values, where the oxidation of many potential interfering gaseous species cannot occur. In fact, anodic currents at these potentials can be promoted practically by the sole hydrogen halides able to provide poorly soluble salts with Ag+, according to the generalized reaction 2.
Ag
+ HX
AgX + H+ + e-
--.
(2)
However, there is a large difference between the value of the solubility constant of AgzS and those for the silver salts formed with these potential interferents. Based on the corresponding pK, values, the difference in the oxidation potential of silver in the presence of identical concentrations of HzS and HI is almost 0.6 V, and an even greater Werence is expected when HI is replaced by HBr or HCl. Consequently, these gaseous species are not expected to interfere at the potentials at which silver is oxidized to form Ag2S. Such a statement does not apply to HCN, which is known to form a quite stable soluble complex with silver ions, according to reaction 3. Ag
+ 2HCN -Ag(CN),- + 2H++ e-
(3)
However, as verified by us, this possible interference may be easily recognized by combining suitably anodic amperometric monitoring with cathodic stripping measurements which do not provide, of course, cathodic strippingcurrents for the soluble silver complex formed in the anodic preelectrolysis, since it does not accumulate at the silver surface but diffuses into the internal electrolyte solution. ACKNOWLEDGMENT The authors thank S. Sitran of C.N.R-I.P.E.L.P. (Padova) and
A. Valentino of the University of Udine for skilful experimental assistance. Financial aid from the Italian National Research Council (C.N.R), from the Ministry of the University and of Scientific and Technological Research (M.U.RS.T.), and from Project “Sistema Lagunare Veneziano” is gratefully acknowledged. Received for review April 4, 1994. Accepted October 18, 1994.a AC940321 R m . Abstract published
in Advance ACS Abstracts, November 15, 1994.
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