Determination method of carbon dioxide in the air

Determination methods of carbon dioxide in indoor air

the determination methods of carbon dioxide in air mainly include non dispersive infrared gas analysis, gas chromatography, volumetric titration, etc

e.1 non dispersive infrared gas analysis method

e.1.1 relevant standards and basis

this method is mainly based on gb/t18204.24 determination of carbon dioxide in the air of public places

e.1.2 principle

carbon dioxide has selective absorption of infrared ray. Within a certain range, the absorption value is linear with the concentration of carbon dioxide. Determine the concentration of carbon dioxide in the sample according to the absorption value

e.1.3 measurement range

0~0.5%; 0~1.5% two gears. The lowest detectable concentration is 0.01%

e.1.4 reagents and materials

e.1.4.1 color changing silica gel: dry at 120 ℃ for 2h

e.1.4.2 anhydrous calcium chloride: analytically pure

e.1.4.3 high purity nitrogen: 99.99% purity

e.1.4.4 caustic soda asbestos: analytically pure

e.1.4.5 plastic aluminum foil composite film gas bag 0.5L or 1.0L

e.1.4.6 carbon dioxide standard gas (0.5%): stored in aluminum alloy steel cylinders

e.1.5 instruments and equipment

carbon dioxide non dispersive infrared gas analyzer

the main performance indicators of the instrument are as follows:

measurement range: 0~0.5%; 0~1.5% two gears

reproducibility: ≤± 1% full scale

zero drift: ≤± 3% full scale/4h

span drift: ≤± 3% full scale/4h

additional temperature error: ≤± 2% full scale/10 ℃ (at 10 ℃ ~80 ℃)

carbon monoxide interference: 1000ml/m3 Co ≤± 2% full scale

additional error when power supply voltage changes: 220V ± 10% ≤± 2% full scale

start time: 30min

response time: the time when the pointer indicates to 90% of the full scale is less than 15s

e.1.6 sampling

use plastic aluminum foil composite film gas bag, draw on-site air for washing for 3~4 times, extract 0.5L or 1.0L gas, seal the air inlet, and bring it back to the laboratory for analysis. The instrument can also be brought to the site for intermittent sampling or continuous determination of carbon dioxide concentration in the air

e.1.7 analysis steps

e.1.7.1 startup and calibration of the instrument

e.1.7.1.1 startup and zero calibration: after the instrument is powered on, stabilize for 30min~1h, and conduct zero calibration after passing high-purity nitrogen or air through the drying tube and Caustic asbestos filter tube

e.1.7.1.2 end point calibration: connect carbon dioxide standard gas (such as 0.50%) to the sample inlet of the instrument to calibrate the end point scale

e.1.7.1.3 repeat the zero and end point calibration for 2~3 times to keep the instrument in normal working state

e.1.7.2 sample determination

connect the plastic aluminum foil composite film air bag containing the air sample to the filter containing color changing silica gel or anhydrous calcium chloride and the air inlet of the instrument. The sample is automatically pumped into the air chamber and displays the concentration of carbon dioxide (%)

if the instrument is brought to the site, it can be measured by intermittent sample injection through the mutual cooperation between the base, reducer, electromechanical, pulley, eccentric, sensor and control cabinet. It can also monitor the concentration of carbon dioxide in the air for a long time

e.1.8 results calculate the concentration of carbon dioxide in the sample, which can be directly read out 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 the accuracy depends on the uncertainty of the standard gas (less than 2%) and the stability error of the instrument (less than 6%)

e.1.10 interference and elimination

non measured components in indoor air, such as methane, carbon monoxide, water vapor, etc., affect the determination results. The wavelength of infrared filter is 4.26 μ m. Carbon dioxide has strong absorption of this wavelength; Gases such as carbon monoxide and methane are not absorbed. Therefore, the interference of carbon monoxide and methane can be ignored; However, water vapor interferes with the determination of carbon dioxide, which can reduce the reflectivity of the gas chamber, thus reducing the sensitivity of the instrument and affecting the accuracy of the determination results. Therefore, the air sample must be dried before entering the instrument

e.2 gas chromatography

e.2.1 relevant standards and basis

this method is mainly based on gb/t18204.24 determination of carbon dioxide in the air of public places

e.2.2 principle

after carbon dioxide is completely separated from other components of the air in the chromatographic column, it enters the working arm of the thermal conductivity detector, so that the change of the resistance value of the arm is not equal to the change of the resistance value of the reference arm, and the signal output generated by the loss of balance of the Wheatstone bridge. The signal size is proportional to the concentration of carbon dioxide entering the detector within the range of polarity. So as to carry out qualitative and quantitative determination

e.2.3 determination range

when 3ml of sample is injected, the determination concentration range is 0.02%~0.6%, and the minimum detection concentration is 0.014%

e.2.4 reagent

e.2.4.1 carbon dioxide standard gas: concentration 1% (aluminum alloy steel cylinder), with nitrogen as the background gas

e.2.4.2 high molecular porous polymer: GDX-102 lays the classical strength theoretical foundation of fatigue failure, 60~80 mesh, as 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 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 chromatographic column: stainless steel tube with length of 3M and inner diameter of 4mm is filled with GDX-102 high molecular porous polymer, and both ends of the column tube are filled with glass wool. Before using the newly installed chromatographic column, it should be aged for 12h at the column temperature of 180 ℃ and 70ml/min of nitrogen until the baseline is stable

e.2.6 sampling

use a rubber duplex ball to inject the on-site air into the plastic aluminum foil composite film air bag to make it full and then release it. Repeat this for four times. After the last filling, seal the sample inlet and write a label indicating the sampling place and time

e.2.7 analysis steps

e.2.7.1 chromatographic analysis conditions

because chromatographic analysis conditions often differ due to different experimental conditions, the best chromatographic analysis conditions that can analyze carbon dioxide should be formulated according to the model and performance of the gas chromatograph used

e.2.7.2 draw the standard curve and determine the correction factor

under the same conditions when making sample analysis, draw the standard curve or determine the correction factor

e.2.7.2.1 prepare standard gas

inject 2ml, 4ml, 8ml, 16ml and 32ml of 1% carbon dioxide standard gas into five 100ml syringes respectively, and then dilute to 100ml with pure nitrogen to obtain gas with concentration of 0.02%, 0.04%, 0.08%, 0.16% and 0.32%. Take pure nitrogen as zero concentration gas

e.2.7.2.2 draw the standard curve

for each concentration of standard gas, measure 3ml of sample through the six way injection valve of the chromatograph respectively, and obtain the chromatographic peak and retention time of each concentration. Measure the average value of chromatographic peak height (peak area) three times for each concentration. Draw a standard curve for the average peak height (peak area) with the concentration (%) of carbon dioxide, and calculate the slope of the regression line. The reciprocal BG of the slope is used as the calculation factor for sample determination

e.2.7.2.3 determine the correction factor

calculate the correction factor by single point correction method. Take the standard gas that is close to the concentration of carbon dioxide in the sample air. According to e.2.7.2.2, measure the average peak height (peak area) and retention time of chromatographic peak. Use the following formula to calculate the correction factor

where:

f -- correction factor

c0 -- standard gas concentration,%

h0 - average peak height (peak area)

e.2.7.3 sample analysis

inject 3ml of sample air through the six-way sampling valve of the chromatograph, operate according to item e.2.7.2.2, qualitatively determine the retention time, and measure the peak height (peak area) of carbon dioxide. Each sample is analyzed three times to find the average value of peak height (peak area). And record the temperature and atmospheric pressure at the time of analysis. Dilute the high concentration sample with pure nitrogen to less than 0.3%, and then analyze

e.2.8 result calculation

e.2.8.1 use the standard curve method to check the standard curve quantification, or use the following formula to calculate the concentration

c=h × Achieve the completion and delivery of the plant and equipment installation within B years, G

where:

C - carbon dioxide concentration in the sample air,%

h - average value of peak height (peak area) of the sample

bg - Calculation factor obtained from e.2.7.2.2

e.2.8.2 calculate the concentration with the correction factor according to the following formula

c=h × F

where:

c - carbon dioxide concentration in the sample air,%

h - average value of peak height (peak area) of the sample

f -- correction factor obtained from e.2.7.2.3

e.2.9 precision and accuracy

e.2.9.1 reproducibility

when carbon dioxide concentration is 0.1% - 0.2%, the coefficient of variation of repeated determination is 5% - 3%

e.2.9.2 recovery rate

when the carbon dioxide concentration is 0.02%~0.4%, the recovery rate is 90%~105%, and the average recovery rate is 99%

e.2.10 interference and elimination

due to the use of gas chromatography separation technology, air, methane, ammonia, water and carbon dioxide do not interfere with the determination

a.9 volumetric titration

e.3.1 relevant standards and basis

this method is mainly based on gb/t18204.24 method for the determination of carbon dioxide in the air of public places

e.3.2 principle

use excess barium hydroxide solution to react with carbon dioxide in the air to generate barium carbonate precipitation, and titrate the remaining barium hydroxide after sampling with standard acetic acid solution until the phenolphthalein reagent just fades red. The concentration of carbon dioxide in the air can be measured by dividing the volumetric titration result by the volume of the collected air sample

e.3.3 determination range

when the sampling volume is 5L, the measurable concentration range is 0.001% - 0.5%; The lowest detectable concentration is 0.001%

e.3.4 reagents and materials

e.3.4.1 absorption solution

e.3.4.1.1 dilute absorption solution (for sampling when the concentration of carbon dioxide in the air is lower than 0.15%): weigh 1.4g barium hydroxide [ba (OH) 2 8H2O] and 0.08g barium chloride (BaCl2 2H2O) and dissolve them in 800ml water, add 3ml n-butanol, shake well, and dilute to 1000ml with water

e.3.4.1.2 concentrated absorption solution (used for sampling when the concentration of carbon dioxide in the air is 0.15% - 0.5%): weigh 2.8g barium hydroxide [ba (OH) 2 8h2o] and 0.16g barium chloride (BaCl2 2H2O) and dissolve them in 800ml water, add 3ml n-butanol, shake well, and dilute to 1000ml with water

the above two absorption solutions should be prepared two days before sampling, and the storage bottle should be covered and sealed to avoid contact with air. Before sampling, the stopper of the liquid storage bottle is connected with the sodium lime pipe, and the absorption liquid is sucked into the absorption pipe with a rainbow absorption pipe

e.3 speed correction method of tensile testing machine: 4.2 oxalic acid standard solution: weigh 0.5637g oxalic acid (H2C2O4 2H2O), dissolve with water and dilute to 1000ml. 1ml of this solution is equivalent to the standard condition (0 ℃, 1