Development of a coulometric method for the determination of gaseous sulfur compounds in urban atmospheres

Journal of the Brazilian Chemical Society

Silva, Magali¹  Dick, Luis F. P.¹  Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil¹ 

关键词: coulometric method;    tarnishing of Ag;    urban pollution;    SO2;    H2S;   

DOI  :  10.1590/S0103-50532000000200010

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Besides suspended particles, gaseous compounds, such as carbon monoxide (CO), carbon dioxide (CO2) and sulfur dioxide (SO2), are normally the main atmospheric pollutants in urban areas1, 2. Previous studies showed that in Porto Alegre- RS, approximatly 3.7 tons of SOx are daily emitted. Further 3.4 tons of SOx per day are produced in the industrial metropolitan area around the city3.The main SO2 source in urban areas is commonly accepted as resulting from the use of fossil fuels. In warm regions like Southern Brazil, the main source should be attributed to the transport activity and to the use of Diesel oil. Due to local variations in the traffic density and in the wind velocity in a city, a local monitoring of SO2 is more adequate to identify critical areas, which are harmful to the human health and are also very corrosive to metals, stones and concrete structures.The aim of the present study was to develop a method for the determination of atmospheric SO2 content. The method should be inexpensive and simple, making possible the local determination of the mean concentration in several points in an urban area. As showed below, the tarnishing rate of silver shows the desired characteristics for an indicator of the SO2 concentration.Pure silver, when exposed to H2S or SO2, forms a tarnishing layer consisting only of achantite (alpha Ag2S). This sulfide has an extremely low solubility product in water, Kps=1 10-50 mol L-1 3,4 and is very conductive and adherent to the silver surface, enabling its coulometric determination after the atmospheric exposure of silver3, 5. Moreover, due to the high mobility of Ag+ ions in the Ag2S salt, the tarnishing rate is time independent for tarnishing layers up to several microns thick, being directly proportional to the H2S and SO2 concentrations5. As an example, the tarnishing rate of Ag in 1ppm H2S, which is higher than the one in SO2, remains controlled by surface reaction for several days6, 7.The results presented here were measured during the southern hemisphere summer of 1999 in three urban points of Porto Alegre- RS, showing heavy traffic, where only SO2 is expected, and also in two sites near an oil refinery, with different distances from a H2S source. An increasing SO2/H2S concentration ratio is expected here for greater distances from the H2S source, resulting from the oxidation of H2S to SO2 during its transport from the source to the measuring site. A long-term study is now under way to determine the influence of traffic intensity and climate parameters in the correlation between the coulometric and pararosaniline methods.  ExperimentalFor the coulometric determination of tarnishing rates, coupons of pure silver (Sigma-Aldrich, 99.99% Ag) with geometric areas between 5 and 6 cm2 were cut and provided with a 0.5 mm diameter hole. After vacuum annealing (650 °C h-110-2 atm) the coupons were polished on both sides with emery paper up to 2400 mesh and degreased in ethanol. For the atmospheric exposure, the coupons were hanged by a PTFE thread in the vertical position at five "monitoring stations", where a good ventilation and rain protection were provided, and also other atmospheric data were being collected. Three monitoring stations were located in downtown sites, with dense traffic, Rodoviária, Borges and Azenha. Among these sites, Rodoviária, where the Central Bus Station is located, shows usually the highest SO2 emissions. The other two monitoring stations were located at 19 km far from downtown of Porto Alegre- RS, at the oil refinery "Refinaria Alberto Pasqualine-Petrobras", namely REFAP1 and REFAP2. The station REFAP1 lied 750 m far from an H2S-SO2-source, and REFAP2, respectively 1,500 m. At three of these stations, Rodoviária, REFAP1 and REFAP2, SO2 was also collected and the concentration determined by the pararosaniline method as described in the appropriate norm8. The exposures were proceeded from January to March 1999. The mean relative humidity and temperature during this period were 79.4% and 24.9 ºC respectively, as determined for Porto Alegre- RS by the local state climate station.After exposure for different times, the Ag coupons were rinsed with water and acetone and reduced potentiodynamically in a standard three electrodes cell. The electrolyte was a borate buffer of pH 10 (3.092 g L-1 H3BO3, 3.728 g L-1 KCl, 1.756 g L-1 NaOH), with addition of small quantities of Na2S and purged previously for 8 h with N2. Good results were obtained for sweep velocities in the negative direction of 0.1 mV s-1, starting from the Ag2S/Ag equilibrium potential. This low sweeping rate was necessary to avoid the superposition of the Ag2S reduction peak and the current related to the hydrogen evolution reaction. The equilibrium potential (Ag2S/Ag) was determined by a previous potentiodynamic sulfidation and reduction of a blank Ag probe in the same solution. The potentials were measured against an AgCl/Ag reference electrode in 3.5 mol L-1 KCl, but are referred in the text to the normal hydrogen electrode (NHE). After data acquisition by computer, the mass of Ag2S per area was calculated from the charge density under the observed cathodic peak, assuming Ag2S as the only tarnishing product and subtracting the charge related to the background current density.  Results and DiscussionThe voltammograms of the reduction of the exposed Ag coupons are shown in Figures from 1 to 5. A sharp reduction peak in the range between E(Ag2S/Ag) and –200 mV more negative than this, i.e (from –350 to 550 mV) is always observed. The peak current density grows with the exposure time, and it can be attributed undoubtedly to the reduction of the environmentally formed Ag2S. No reduction peaks could be observed for unexposed Ag samples. The background current density, measured on unexposed samples, or estimated on exposed samples as the current minimum between the Ag2S peak and the hydrogen evolution region, ranged between 0.2 and 1µA cm-2, indicating the reduction of residual O2 dissolved in the electrolyte. The coupons exposed at urban sites, i.e. Rodoviária, Borges and Azenha, (Figures 1-3, respectively) showed less pronounced reduction peaks, comparing to the ones exposed at the oil refinery sites, REFAP1 and REFAP2 (Figures 4 and 5). For exposures at the less aggressive sites Borges and Azenha, a clear identification of the reduction peaks, at a potencial sweep rate of 0.1 mV s-1 was only possible for exposure times of ca. 10 days or longer. For the more aggressive sites at the refinery, clear peaks were already identified after seven days of exposure.           Samples exposed for more then 20 days at the most aggressive sites showed a second reduction peak (Figure 1, 4, 5). Its current density, subtracting the background current, grows with time of exposure and with the aggressivity of the atmosphere, and its current was allways less than 3% of the main Ag2S peak. A second reduction peak was also observed by the reduction of Ag samples tarnished strongly in aqueous sulfide solutions of 0.01mol L-1 Na2S6,7. Only Ag2S could be detected in that

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