Non-dispersive infrared gas analysis, gas chromatography, volumetric titration, etc. are mainly used to measure carbon dioxide in the air.
E.1 Non-dispersive infrared gas analysis
E.1.1 Related standards and basis This method is mainly based on GB/T18204.24 "Measurement of carbon dioxide in air in public places".
E.1.2 Principles Carbon dioxide has a selective absorption of infrared light. Within a certain range, the absorption value is linearly related to the concentration of carbon dioxide. Determine the concentration of carbon dioxide in the sample based on the absorption value.
E.1.3 Measurement range
0~0.5 %; 0~1.5 % two gears. The lowest detected concentration is 0.01%.
E.1.4 Reagents and materials
E.1.4.1 discoloration of silica gel: drying at 120 ° C 2h;
E.1.4.2 anhydrous calcium chloride: analytically pure;
E.1.4.3 high purity nitrogen: purity 99.99%;
E.1.4.4 Caustic soda asbestos: analytically pure;
E.1.4.5 plastic foil composite film airbag 0.5L or 1.0L;
E.1.4.6 Carbon dioxide standard gas (0.5%): Stored in aluminum alloy cylinders.
E.1.5 Instruments and Equipment Carbon dioxide non-disperse infrared gas analyzer.
The main performance indicators of the instrument are as follows:
Measurement range: 0~0.5 %; 0~1.5 % two steps;
Reproducibility: ≤±1% full scale;
Zero drift: ≤ ± 3% full scale / 4h;
Span drift: ≤ ± 3% full scale / 4h;
Additional temperature error: ≤±2% full scale/10°C (at 10°C~80°C);
Carbon monoxide interference: 1000mL/m3 CO ≤ ± 2% full scale;
Additional error when the supply voltage changes: 220V±10% ≤±2% full scale;
Start time: 30min;
Response time: The pointer indicates 90% of full scale time <15s.
E.1.6 Sample plastic aluminum foil composite film airbags are used to extract air from the site for 3 to 4 times, with 0.5L or 1.0L air intake, sealed air inlets, and returned to the laboratory for analysis. It is also possible to bring the instrument to the site for intermittent injection or to continuously measure the concentration of carbon dioxide in the air.
E.1.7 Analysis Procedure
E.1.7.1 Instrument startup and calibration
E.1.7.1.1 Start-up and zero calibration: After the instrument is powered on, it is stable for 30min to 1h. After the high-purity nitrogen or air passes through the drying tube and the caustic soda asbestos filter tube, zero calibration is performed.
E.1.7.1.2 End point calibration: Use a CO2 standard gas (such as 0.50%) to connect to the inlet of the instrument for end-point calibration.
E.1.7.1.3 Zero and end point calibrations are repeated 2 or 3 times to keep the instrument in normal operation.
E.1.7.2 Sample determination The plastic aluminum foil composite film air bag containing the air sample is connected to the air inlet of the instrument equipped with discolored silica gel or anhydrous calcium chloride, and the sample is automatically drawn into the air chamber. In, and shows the concentration of carbon dioxide (%).
If the instrument is brought to the site, it can be measured intermittently. It can monitor the concentration of carbon dioxide in the air for a long time.
E.1.8 Calculation of results The concentration of carbon dioxide in the sample can be read directly from the gas analyzer.
E.1.9 Precision and Accuracy
E.1.9.1 The reproducibility is less than 2%, and the hourly drift is less than 6%.
E.1.9.2 Accuracy depends on the standard gas uncertainty (less than 2%) and instrument stability error (less than 6%).
E.1.10 Interferences and exclusions of non-test components in the indoor air, such as methane, carbon monoxide, and water vapor, affect the measurement results. The infrared filter has a wavelength of 4.26 μm, and carbon dioxide strongly absorbs this wavelength; carbon monoxide and methane and other gases do not absorb it. Therefore, the interference of carbon monoxide and methane is negligible; however, water vapor interferes with the determination of carbon dioxide, which can reduce the reflectivity of the air chamber, and thus reduce the sensitivity of the instrument and affect the accuracy of the measurement result. Therefore, the air sample must be dried. After that, enter the instrument again.
E.2 Gas chromatography
E.2.1 Relevant standards and basis This method is mainly based on GB/T18204.24 "Measurement of carbon dioxide in air in public places".
E.2.2 Principle After CO2 is completely separated from the other air components in the column, it enters the working arm of the thermal conductivity detector so that the change in the arm's resistance value is not equal to the change in the reference arm's resistance value. The Wheatstone bridge is lost. Balanced signal output. In the linear range, the signal size is proportional to the concentration of carbon dioxide entering the detector. Thus qualitative and quantitative determinations.
E.2.3 Measurement range When the sample volume is 3 mL, the concentration range is 0.02%~0.6%, and the minimum detection concentration is 0.014%.
E.2.4 Reagents
E.2.4.1 Carbon dioxide standard gas: Concentration 1% (aluminum alloy cylinders) with nitrogen as background gas;
E.2.4.2 Macroporous polymer: GDX-102, 60-80 mesh for chromatographic stationary phase;
E.2.4.3 Pure nitrogen: Purity 99.99%.
E.2.5 Instruments and Equipment
E.2.5.1 Gas chromatograph: Gas chromatograph equipped with a thermal conductivity detector;
E.2.5.2 syringe: 2mL, 5mL, 10mL, 20mL, 50mL, 100mL volume error <± 1%;
E.2.5.3 plastic aluminum foil composite film sampling bag: volume 400~600 mL;
E.2.5.4 Column: GDX-102 high-molecular porous polymer is filled in a stainless steel tube with a length of 3m and an inner diameter of 4mm, and glass wool is filled at both ends of the column tube. Prior to use, the newly installed column should be aged for 12 hours at a column temperature of 180°C and a nitrogen atmosphere of 70 mL/min until the baseline is stable.
E.2.6 Sampling Rubber twin ball will be used to drive the air in the air into the plastic foil-film composite airbag, allowing it to fill up and release it. This is repeated four times. After the last full time, seal the injection port and write a label indicating the sampling location and time.
E.2.7 Analysis Procedure
E.2.7.1 Chromatographic conditions Since chromatographic conditions often vary depending on the experimental conditions, the optimal chromatographic conditions for the analysis of carbon dioxide should be established based on the type and performance of the gas chromatograph used.
E.2.7.2 Draw the standard curve and determine the calibration factor Under the same conditions as for sample analysis, draw a standard curve or determine the calibration factor.
E.2.7.2.1 Prepare standard gas in five 100-mL syringes and inject 2%, 4mL, 8mL, 16mL, and 32mL 1% carbon dioxide standard gases, and dilute to 100mL with pure nitrogen to obtain concentrations of 0.02% and 0.04%. 0.08%, 0.16%, 0.32% of gas. Another pure nitrogen is used as zero concentration gas.
E.2.7.2.2 Draw the standard curve The standard gas for each concentration is passed through the six-way sampling valve of the chromatograph and the sample is taken in 3mL to obtain the chromatographic peaks and retention time of each concentration. Three times for each concentration, the average of peak heights (peak areas) was measured. A standard curve was plotted against the average peak height (peak area) of the carbon dioxide concentration (%), and the slope of the regression line was calculated. The reciprocal Bg of the slope was used as a calculation factor for the determination of the sample.
E.2.7.2.3 Determination of correction factors The correction factor is determined using a single point correction method. Take a standard gas that is close to the concentration of carbon dioxide in the sample air. Measure the average peak height (peak area) and retention time of the chromatogram peaks by operating in Section E.2.7.2.2. The correction factor is calculated using the following equation.
In the formula:
f - correction factor;
C0 - standard gas concentration, %;
H0 - average peak height (peak area).
E.2.7.3 Sample analysis Enter the sample gas through the chromatograph six-port sampling valve and proceed to section 3.2.7.2.2 to determine the peak height (peak area) of carbon dioxide. Each sample was analyzed three times and the peak height (peak area) averaged. And record the temperature and atmospheric pressure during the analysis. High concentration samples were diluted with pure nitrogen to less than 0.3% and analyzed again.
E.2.8 Result Calculation
E.2.8.1 Quantify the standard curve using the standard curve method, or calculate the concentration using the following formula.
c=h×Bg
In the formula:
c - carbon dioxide concentration in the sample air, %;
h - the average of the sample peak height (peak area);
Bg - Calculated factor derived from E.2.7.2.2.
E.2.8.2 Calculate the concentration using the correction factor
c=h×f
In the formula:
c - carbon dioxide concentration in the sample air, %;
h - the average of the sample peak height (peak area);
f - Correction factor obtained from E.2.7.2.3.
E.2.9 Precision and Accuracy
E.2.9.1 When the reproducible carbon dioxide concentration is 0.1% to 0.2%, the coefficient of variation for repeated measurements is 5% to 3%.
E.2.9.2 Recovery Carbon dioxide concentration is between 0.02% and 0.4%. The recovery rate is between 90% and 105%. The average recovery rate is 99%.
E.2.10 Disturbance and Elimination Due to the use of gas chromatography separation technology, air, methane, ammonia, water, and carbon dioxide do not interfere with the assay.
A.9 Capacity titration
E.3.1 Relevant standards and basis This method is mainly based on GB/T18204.24 "Measurement of carbon dioxide in air in public places".
E.3.2 Principle Use excess cesium hydroxide solution to react with carbon dioxide in the air to produce cesium carbonate precipitate. The remaining cesium hydroxide after sample is titrated with standard oxalic acid solution until the phenolphthalein reagent red just fades. The concentration of carbon dioxide in the air can be measured by dividing the volumetric titration result by the volume of air sample collected.
E.3.3 Measurement range When the sampling volume is 5L, the measurable concentration range is 0.001% to 0.5%; the minimum detection concentration is 0.001%.
E.3.4 Reagents and materials
E.3.4.1 Absorbent
E.3.4.1.1 Dilute absorbent (for sampling when the air carbon dioxide concentration is lower than 0.15%): Weigh out 1.4 g of barium hydroxide [Ba(OH) 2?8H2O] and 0.08 g of barium chloride (BaCl2?2H2O). In 800 mL of water, 3 mL of n-butanol was added, shaken, and diluted with water to 1000 mL.
E.3.4.1.2 Concentrated absorption solution (used when the air carbon dioxide concentration is between 0.15% and 0.5%): Weigh 2.8g barium hydroxide [Ba(OH) 2?8H2O] and 0.16g barium chloride (BaCl2?2H2O) ) Dissolve in 800 mL of water, add 3 mL of n-butanol, shake well and dilute to 1000 mL with water.
The above two kinds of absorbing liquid should be prepared two days before sampling. The vials should be capped, sealed and kept away from the air. Prior to sampling, the stopper was connected to a sodium lime tube, and the absorbing solution was sucked into the absorbing tube with a siphon tube.
E.3.4.2 oxalic acid standard solution: Weigh 0.5637g oxalic acid (H2C2O4? 2H2O), dissolve and dilute to 1000mL with water, 1mL of this solution is equivalent to 0.1mL carbon dioxide in the standard condition (0°C, 101.325kPa).
E.3.4.3 Phenolphthalein indicator
E.3.4.4 n-butanol
E.3.4.5 Pure nitrogen (purity 99.99%) or air after removal of carbon dioxide via soda lime tubes.
E.3.5 Instruments and Equipment
E.3.5.1 Air sampler;
E.3.5.2 50mL porous glass plate absorption tube;
E.3.5.3 Acid burette: 50mL;
E.3.5.4 Iodine bottle: 125 mL.
E.3.6 Sampling Take an absorption tube (air that has been previously filled with nitrogen or charged with soda lime) and add 50 mL of cesium hydroxide absorption solution to the flow rate of 0.3 L/min for 5 to 10 min. Before and after the sampling, the absorption tube enters Both outlets are connected with latex hoses to prevent air from entering.
E.3.7 Analytical steps After sampling, the absorption tube is sent to the laboratory, the middle sand core tube is removed, and the stopper is allowed to stand for 3 hours to complete the precipitation of cesium carbonate. The supernatant is sucked into the iodine volume bottle (the iodine volume bottle should be filled with nitrogen in advance). Fill with soda-lime-treated air), add 2 drops of phenolphthalein indicator, set to a solution with oxalic acid standard drop from red to colorless, and record the volume of oxalic acid standard solution consumed. At the same time, 25 mL of unsampled cesium hydroxide absorption solution was taken as a blank titration, and the volume (mL) of consumed oxalic acid standard solution was recorded.
E.3.8 Result Calculation
E.3.8.1 Convert the sampling volume to the standard volume in accordance with 4.7.7.
E.3.8.2 The carbon dioxide concentration in the air is calculated as follows:
In the formula:
c - carbon dioxide concentration in the air, %;
a - sample titration using oxalic acid standard solution volume, mL;
b - blank titration using oxalic acid standard solution volume, mL;
V0 - Sampling volume in standard state, mL.
E.3.9 Sensitivity, Precision and Accuracy
E.3.9.1 Sensitivity The sample solution consumes 1 mL of standard oxalic acid solution, equivalent to 0.1 mL of carbon dioxide (standard condition 0°C, 101.325 kPa).
E.3.9.2 Precision and Accuracy The recovery rate of standard gas containing 0.04% to 0.27% of carbon dioxide is 97% to 98%, and the coefficient of variation for repeated measurements is 2% to 4%.
E.3.10 Interference and elimination of acid gases such as sulphur dioxide, nitrogen oxides and acetic acid in the air. Neutralization of the absorption solution of this method, but the general indoor air carbon dioxide concentration is above 500mg/m3, compared to the above-mentioned acid gases in the air. The concentration is much lower, even if the concentration of sulfur dioxide in the air exceeds 100 times the 0.15 mg/m3, and assuming that it is completely converted to sulfuric acid, the interference caused by this method is less than 5%.
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