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Journal of Automatic Chemistry, Vol. 20, No. (January-February 1998) pp. 17-21
Flow injection analysis of nitrogen dioxideusing a galvanic detector
Shan-jun Liu,*, Han-xi Shen,* Jian-xing Feng?and M. TubinoDepartment of Chemistry, " Department of Environmental Sciences, NankaiUniversity, Tianjin, 300071, PR China; Instituto de Quimica, Universidade
Estadual de Campinas, Caixa Postal 6154, CEP 13083-970, Campinas, SP,Brazil
A flow injection configuration (FIA) based on a galvanic detectorfor the determination of nitrogen dioxide is described. The gaseoussample is directly injected into a gaseous carrier. The sample istransported toward the detector. The steady state measurements arenot required to obtain the reproducible peak signals. The featuresof FIA are compared with that of continuous flow monitoringapplication. Theflow injection system is simple, rapid and capableof detecting NO2 in the range of 1-500ppm (v/v). Themeasuring range and sensitivity of the galvanic detector in FIAdepend on the sample volume. A relative standard deviation is2.4% (n 10)for 200ppm (v/v) of nitrogen dioxide. Thesampling frequency is about 24 h-1
Flow injection analysis (FIA) is based on the injection ofa liquid sample into a moving unsegmented continuouscarrier stream of a suitable liquid. The injected sampleforms a zone, which is transported toward a detector.The hallmark of FIA is that dispersion is reproducibleand controllable. Now FIA has evolved into a generaltechnique for solution handling and data gathering,applicable to many areas of chemical research andtechnology . FIA was mainly used in the analysis ofliquid samples. The analysis of gaseous samples is at-tended by special difficulties . Absorption by suitablesolutions is usually used to collect and/or preconcentratethe analyte prior to the measurement. Only a few flowinjection methods have been reported for the determina-tion of gaseous samples. These methods can be assembledinto these groups: (1) the gaseous sample is directlyinserted into a flow injection system and reacts at agas/liquid interface [2-5], or at a gas/solid interface ;(2) a dual phase gas diffusion/permeation techniquewhich involved a liquid donor and gaseous acceptor[7-10] or the reverse [11-15] is employed by means ofa separated membrane; and (3) a gas sensor is directlyused as the detector in FIA [16, 17].
Nitrogen dioxide is a major air pollutant that plays adominant role in acid deposition chemistry, as well as inthe production of ozone and hydroxyl radicals . Anumber of analytical methods has been developed suit-able for emission control and the determination of lowlevels occurring in the atmosphere. The widely adopted
Address for correspondence.This paper was presented at the Seventh International Conference onFlow Analysis, Piracicaba, Brazil, 25-28 August 1997.
instrumental chemical method for gaseous pollutants iscoulometric analysis . This involves measurement ofthe electrical current produced when strongly oxidizingor reducing pollutant gases react with potassium iodideor bromide solution in an electrochemical cell [20-22].The gaseous sample is drawn through the detector con-taining a platinum cathode and a carbon anode with agalvanic potential difference between them. The currentin the anode-cathode circuit is proportional to theamount of NO2 entering the cell [21-24]. This type ofgalvanic detector was designed for continuous monitor-ing of nitrogen dioxide for a steady state measurement.The present paper describes a study on the features of thegalvanic detector of nitrogen dioxide in flow injectionanalysis. The results of FIA are compared with the usualcontinuous flow monitoring. While the method of sampleintroduction changes from continuous flow monitoring(steady state) to FIA, the reproducible peak signals areobtained. The FIA method is simple, rapid and extend-ing the measuring range of the galvanic detector. Theeffect of FIA variables, i.e. flow rates of sample andcarrier, sample volume, delay time and calibrationcurve, is examined.
A certified gas of nitrogen dioxide was obtained from theNational Research Centre for Certified Materials(NRCCM, Beijing, China), and the certified value was0.103% NO/N, mol/mol (No. 406850). The mixturesof nitrogen dioxide with air were also prepared in a 10-litre glass bottle connected to an air-pump and samplingcoil of the injected valve in the sample enclosure, andwere calibrated by means of the certified nitrogendioxide. Air filtered by the scrubbers of silica gel andactivated charcoal was used as the gaseous carrier. CO(500ppm in N) was obtained from a cylinder(NRCCM). The gases SO, H2S and CO were pro-duced by chemical reactions of their respective salts witha solution of sulphuric acid, and were separately pre-pared in the 10-1itre glass bottle containing air.
The samples of nitrogen dioxide were detected by a NOgalvanic detector (The Third Analytical InstrumentFactory of Tianjin, China) and a schematic diagram ofthe component is given in figure l(a). The principle ofthe detector was the coulometric internal electrolysis [21-24]. The detector utilized a cyclic oxidation-reductionprocess and contained a platinum gauze cathode andcarbon anode. The composition of the electrolyte con-sisted of 0.3mol/1 potassium iodide in a neutral
0142-0453/98 $12.00 (C) 1998 Taylor & Francis Ltd 7
Shan-jun Liu et al. Flow injection analysis of nitrogen dioxide using a galvanic detector
l.V. F D S VssAIR
Figure 1. (a) A side view of the galvanic detector of nitrogendioxide. sample inlet; 2 sample outlet; 3 activatedcharcoal anode; 4 gaseous bubble; 5 plexiglas receptacle;6 platinum gauze cathode; 7 neutral buffered electrolyte;8 small hole. (b) Scheme of system for the determination ofNO2 by flow injection. Flow direction is indicated by arrows.S] silica gel scrubber; $2 activated charcoal scrubber;I. V. six-port rotary injection valve; F1, F2 Jlow meter;D NO galvanic detector; V], V needle valve; P1, Pair pump and B 10-litre glass bottle.
(pH 7.0) phosphate buffer solution. The features of thedetector claimed by the manufacturer are: measuringrange of 0-4mg/m (about 2ppm); detection limit0.02 mg/m3; response time (90% of final value) less than4min; precision better than 4%. The analytical systemwas composed of two electromagnetic air-pumps and asix-port rotary valve of stainless steel (SP-2305 for gaschromatography, the Analytical Instrument Factory ofBeijing, China) supplied with 0.5, 1.0, 2.0 and 3.0mlsampling coilings used to introduce the gaseous samples.The carrier and sample streams were aspirated by thepumps. Flow rates of sample and carrier were indicatedby the two flowmeters (701 HB, range of 0.05-0.501/min,Beijing and LZB-3T, range of 0.1-1.01/min, Changzhou,China). The operational amplifier circuit was laboratory-made. The signal peak was displayed by a chart recorder(Dahua, Shanghai). All measurements were performed atroom temperature, about 15 C.
ManifoldThe scheme of the flow injection system is shown in figurel(b). The clean air obtained by flowing the air throughactivated charcoal ($1) and silica gel ($2) filters wascontinuously aspirated as carrier through the galvanicdetector (D). The gaseous mixture of nitrogen dioxidewas circulated within the sample enclosure and the loopof the injection valve was filled with the sample by meansof an air-pump (P2). The flow rates of carrier and samplewere adjusted by the adjusting valves (V1,V) to 0.25and 1.01/min, respectively. Various volume samples of0.5-3.0ml were injected into the air carrier. The peak
signals of the galvanic cell current were monitored as thepotential drop across a precision resistor.
Results and discussion
The galvanic detector of nitrogen dioxide was usuallyemployed in the continuous flow monitoring, and thesteady state signal was necessary. When the detector isused in a flow injection manifold, a typical peak graph ofFIA is observed . The printout of multiple injectionsof NO2 sample is reproducible for a given concentrationof nitrogen dioxide. The hallmark of a conventional FIAexperiment is that dispersion is reproducible and con-trollable. By observing the peak shapes and heights it wasclear that the sample zone reproducibly expanded ordiluted in the flowing stream. Since identical physicaland chemical conditions are consistently obtained inFIA, the steady state is not necessary. A six-port rotarysample valve and syringe were tested for the introductionof gaseous samples. For 200ppm (v/v) NO, the relativestandard deviations are 2.4% (n 10) for the quantita-tive sampling valve and 14.3% (n 8) for the syringe,respectively. The result shows that for the gaseousanalysis the method of sample introduction has greatlyinfluenced the reproducibility and that the six-portquantitative sample valve is better than the syringe.Similar results were also obtained for the determinationsof sulphur dioxide by means of a gas sensor in FIA .Therefore, the six-port rotary valve was used in all theexperiments.
Effect offlow ratesIn continuous monitoring analysis, the steady state isnecessary and the output current of the galvanic detectorfor NO is proportional to the nitrogen dioxide concen-tration in the following equation [22-24] (derived fromFaradays law):
IliA 0.0669 x f/(ml/min) x c/(ppm) (1)where I is the galvanic current in gA:f is the sample flowrate in ml/min at 20C and 1013mbar; and c is thenitrogen dioxide concentration in ppm (v/v). In a certainrange of flow rate, for a given concentration of nitrogendioxide the output current is dependent on flow rate andlinearly increases with flow rates .
When the detector is used in the flow injection mode, agraph of the signal as a function of flow rates, for sampleand car