A porous gold membrane working electrode was evaluated for amperometric sensing of NO or NO 2 in air. The Au electrode was evaporated by E-beam onto a porous Teflon substrate and tested in two different geometries: a lab-built universal ͑U͒ and a housing ͑C͒ used in building sensors for commercial sale. The optimum bias for NO 2 reduction was determined to be Ϫ150 mV vs. Pt/air quasi-reference electrode ͑Pt/air-QRE͒ and was ϩ400 mV for NO oxidation. Sensor signals were independent of gas flow rate in both housings at flow rates over 30 cm 3 /min. NO sensitivity in the U and C housing was 180 and 43 nA/ppm, respectively, and the NO 2 sensitivity was Ϫ100 and Ϫ38 nA/ppm, respectively. The U sensor had a limit of detection ͑LOD͒ for NO 2 of 1.3 ppb ͑signal-to-noise ratio 3͒. The sensor housing design and operating conditions can be directly related to observed analytical performance characteristics.The air pollutant gases nitrogen oxide, nitrogen dioxide, nitrous acid, and peroxyacetyl nitrate constitute a nuisance to many people and a danger to those with respiratory conditions such as asthma and emphysema. Oxides of nitrogen are generated by internal combustion 1 and are therefore commonly encountered in urban atmospheres. Monitoring of these gases is important for the protection of human health. Moreover, because emissions are subject to government regulations, industry, especially the automotive industry, has an interest in monitoring for these chemicals.There are numerous methods for detecting NO/NO 2 gases including wet chemical, 2,3 electrochemical, 4-11 infrared ͑IR͒ and ultraviolet ͑UV͒ spectrometric, 12,13 chemiluminescent, 14,15 mass spectrometric ͑MS͒, 16 and gas chromatographic ͑GC͒ 17,18 techniques. Each method has advantages and limitations for particular applications. No method has sufficiently low power, high sensitivity, low cost, and still high performance to meet modern automotive requirements for monitoring in severe environments. Real-time electrochemical sensors, such as amperometric, 4-8 potentiometric, 9,10 and electric impedance based devices, 11 represent a convenient option for NOx measurement and can be incorporated into portable and process control instruments. The amperometric gas sensor ͑AGS͒ has attractive costs, selectivity, and low power. The AGS is limited by modest detection limits and slow response time for many applications. 7 Improvements in performance can be best achieved only by a thorough understanding of the chemical and physical processes that control the amperometric mechanism.Sedlak and Blurton 5 studied an AGS for NO and NO 2 and reported results in this journal. They used a composite Teflon-bonded gold powder working electrode and found the best potential for oxidation of NO at 1.5 V and reduction of NO 2 at 0.8 V ͑vs. a reversible hydrogen electrode, RHE͒. Electrode structure and the serpentine gas exposure geometry limited the sensors analytical performance to part-per-million level determinations. Here, we studied a gold membrane electrode and compared it to earli...
Cyclic voltammetry ͑CV͒ techniques were used to measure the concentration of nitric oxide ͑NO͒ and nitrogen dioxide (NO 2 ) in air using an amperometric gas sensor. A porous gold membrane working electrode in sulfuric acid electrolyte was used with a counter electrode of bare Pt wire and quasi-reference electrode of electrodeposited Pt on a Pt wire. The anodic peak current (i pa ) was proportional to the concentration of NO at various bias sweep rates. The limit of detection was about 4.8 ppm for NO in air or 31 ppm for NO 2 in air for the CV methods. A parameter related to the diffusion coefficient and membrane permeability can be determined from CV measurements and was at the high range, 2.4 ϫ 10 Ϫ4 cm 2 s Ϫ1 , of reported values of diffusion constants. Simple models of combined effects of gas and liquid mass-transport and electrokinetic current limiting mechanisms complicate explanation of observed currents with porous gas electrodes.Nitric oxide, NO, is a human hormone necessary for healthy metabolic processes at parts per billion levels, and NO inhalation is now used to treat some conditions, but at high levels it can damage the human respiratory system. 1 The U.S. Occupational Safety and Health Administration and Department of Labor recommended exposure limit is 25 ppm NO in air. Symptoms of excess NO exposure are eye, nose, and throat irritations, drowsiness, and unconsciousness. Chronic health effects are methemoglobinemia, central nervous system effects, and delayed lung damage. 2 NO is formed in processes of combustion and found in effluents from coal-fired power plants and automobiles. 3,4 NO 2 can also cause adverse health effects 5 and causes environmental impact when reacting with water vapor in the atmosphere to contribute to acid rain. Therefore, sensitive and selective detection methods for these gases are required.Detection methods from colorimetric to electrochemical have been reported for NO, and references to electroanalytical approaches 6-10 can be found, but it has been some time since the earliest amperometric gas sensors ͑AGS͒ for NO and NO 2 were reported 9 in this journal. This early commercial NO/NO 2 sensor had a large 25 cc electrolyte reservoir and a design adapted from the amperometric ethanol 11 and CO 12,13 sensors of the time that used a composite Pt/tetrafluoroethylene ͑TFE͒ porous fuel cell-type electrode. The early NO sensor replaced the working electrode ͑WE͒ of the CO sensor with a powdered Au/TFE composite electrode to avoid interference from CO. Selectivity was achieved by reducing NO 2 at a potential too cathodic for NO oxidation and too anodic for NO reduction. 6,9 Since then, this system for ambient NO/NO 2 detection has been studied and used in several different embodiments. Later an electrode of powdered Au on Nafion was built 14 and the performance of this system was characterized and compared 7 to a thin-film low surface area ͑LSA͒ Au electrode on Nafion, both at constant potential. It was found that the LSA electrode offered a faster response and parts per billion...
Electrochemical methods Electrochemical methods F 6000Amperometric Sensing of NO x with Cyclic Voltammetry. -A porous Au membrane working electrode in H 2 SO 4 is used with a counter electrode of bare Pt wire and a quasi-reference electrode of electrodeposited Pt on a Pt wire to measure the concentration of NO and NO2 in air by cyclic voltammetry techniques. The anodic peak current is proportional to the concentration of NO at various bias sweep rates. The limit of detection is about 4.8 ppm for NO in air and 31 ppm for NO 2 in air. -(ROH, S.-W.; STETTER, J. R.; J.
The lithium ion concentration dependant ionic conductivity and thermal properties of poly(ethylene glycol) methyl ether methacrylate (PEGMA)/acrylonitrile-based copolymer electrolytes with LiClO 4 have been studied by differential scanning calorimetry (DSC), linear sweep voltammetry (LSV) and AC complex impedance measurements. In systems with 11 wt% of acrylonitrile all liquid electrolytes were obtained regardless of lithium ion concentration. Complex impedance measurements with stainless steel electrodes give ambient ionic conductivities 8.1 × 10 −6~1 .4 × 10 −4 S cm −1. On the other hand, a hard and soft films at ambient temperature were obtained in copolymer electrolyte system consists of 15 wt% acrylonitrile with 6 : 1 and 3 : 1 of [EO] : [Li] ratio, respectively. DSC measurements indicate the crystalline melting temperature of poly(PEGMA) disappeared completely after addition of LiClO 4 in this system due to the complex formation between ethylene oxide (EO) unit and lithium salt. As a result, free standing film with room temperature ionic conductivity of 1.7 × 10 −4 S cm −1 and high electrochemical stability up to 5.5 V was obtained by controlling of acrylonitrile and lithium salt concentration.
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