Gravimetric preparation of NO 2 primary reference gas mixtures at the NMISA Tshepiso Mphamo 03 September 2012 NMISA 2012
Contents Introduction Experimental Verification Uncertainty CCQM- Pilot study results for NO 2 (2010-2011) Conclusions NMISA 2012
What is air pollutant? Introduction Any substance in the atmosphere that can cause harm to humans and the environment Primary pollutants and secondary pollutants The toxic pollutants are those primary chemical substances which are reactive with the other constituents of air, forming secondary pollutants which are also toxic and leading to adverse atmospheric conditions Results of air pollution Unpredictable weather patterns Photochemical smog Acid rain Health effects Difficulty in breathing Coughing Aggravation of existing respiratory and cardiac condition
Introduction (cont.) Formation of NO 2 2 NO + O 2 2 NO 2 Primary source Coal fired power stations
Introduction (cont.) Considerations before the preparation Choice of cylinder type (material) Required cylinder treatment Purity analysis of high purity parent gases Required amount of substance fraction Considerations during the preparation Adequate flushing Accurate weighing process Ensure excess addition of oxygen Considerations after preparation Storage conditions Stability period/shelf life
Experimental Standard procedure for the preparation of a gas mixture in a cylinder
Experimental (cont.) m i x i P cyl V cyl R T Z f M i x i m i P cyl V cyl M i R T Z f Intended mole fraction of component i to be added Mass(g) of component i to be prepared Final fill pressure(pa) of the mixture Volume(m 3 ) of the cylinder used Molar mass(g.mol -1 ) of the component to be prepared Gas constant Temperature in Kelvin Compressibility factor = 1
Experimental (cont.) The mass comparator balance Capacity: 10 kg Readability: 1 mg Accuracy: 2 parts in 10 7 Borda substitution weighing Figure 2: High Accuracy Mass Comparator Balance Mettler toledo
Experimental (cont.) The filling station Construction Flushing Target by mass Figure 3: Filling Station which consists of the turbo molecular vacuum pump, Rotary vane fore pump, Electro polished stainless steel tubing, Pressure indicator, Needle valves.
Verification Techniques that are used to verify the NO 2 Fourier transform infrared spectroscopy (FTIR) Chemiluminescence (CLD)
FTIR Verification 2801-2906cm -1 1540-1650cm -1 Figure 5: FTIR spectra of 10 ppm NO 2 in N 2
Verification (cont.) CLD is used to determine the concentration of NO and NO 2 Uses the chemiluminescence method of detection Figure 6: Basic NO-NO X Chemiluminescent Instrumentation.
Gravimetric uncertainty Fishbone Diagram Pressure Temperature Buoyancy Compressibility factor Mass in grams Atomic Weights (IUPAC) Gas Purity Balance resolution Mass piece contribution Linearity Repeatability Weighing Process
Gravimetric uncertainty (cont.) Parameter Best estimate Standard uncertainty Sensitivity coefficient Uncertainty contribution Degrees of freedom Sensitivity 0.99949025 0.00028289 0.04945833 0.00001399 1 Weighing difference 0.04945833 0.00302678 0.99949025 0.003025242 Mass pieces 143.00001000 0.00005545 0.99987100 0.00005545 infinity Air density 1.03199645 0.00018264-0.03575000-0.00000653 infinity Volume difference -0.01787500 0.00000845 1.03199645 0.00000872 infinity Density 8000.000000 0.00200000-0.00000231 0.00000000 infinity Mass 143.01254925 3.02580065
CCQM- K74 CLD results NO 2 10 µmol/mol
CCQM-P110 FTIR Pilot study results NO 2 10 µmol/mol
Conclusions NO 2 is monitored intensively throughout the world Contributes towards unfavourable weather patterns Contribute to many health effects Gravimetric preparation of NO 2 requires attention due to its reactive nature Traceable gas standards of NO 2 have been tested in a pilot study globally Stability of NO 2 standards is a concern due to their reactive nature
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