Question 1

How does NOx form in automobile engines?

Accepted Answer

At high temperatures in the combustion chamber, nitrogen and oxygen from air react to form nitric oxide (NO) and nitrogen dioxide (NO2). Nitric oxide is gradually oxidized to nitrogen dioxide in ambient air. This mixture of NO and NO2 is referred to as NOx and accounts for approximately one-third of NOx emissions in the US.

Question 2

What is photochemical smog and how does NOx contribute to it?

Accepted Answer

Photochemical smog forms when NOx reacts with volatile organic compounds in the atmosphere in the presence of sunlight, producing ozone and other products. Tropospheric ozone is the largest component of this smog and poses health risks including lung and eye irritation. The yellow-brown color of smog is partly due to nitrogen dioxide, which absorbs visible light near 400 nanometers.

Question 3

Why are NOx emissions measured at vehicle startup and after 10 minutes of running?

Accepted Answer

Catalytic converters reduce NOx concentration significantly but only work at high temperatures. During cold start, the converter removes virtually no NOx. Measuring emissions at startup and after 10 minutes provides quantification of NOx production and indicates the catalytic converter's ability to remove NOx as it warms up.

Question 4

How does the colorimetric method detect and quantify NOx in exhaust samples?

Accepted Answer

When NOx is added to a solution containing sulfanilic acid and naphthyl-ethylenediamine, a pink colored azo dye molecule forms. The intensity of this pink color is directly proportional to NOx concentration. A UV-VIS spectrophotometer measures absorbance at 550 nanometers and compares it against a calibration curve to quantify NOx levels.

Question 5

What steps are involved in preparing the NOx indicator solution?

Accepted Answer

Weigh 5 grams of anhydrous sulfanilic acid and add to a 1-liter volumetric flask with 500 mL of nitrite-free water and 140 mL of glacial acetic acid. Swirl until dissolved, then add 20 mg of naphthyl-ethylenediamine. Fill to the 1-liter mark with nitrite-free water, transfer to a dark bottle to prevent photodecomposition, and label appropriately.

Question 6

How is the exhaust sample collected and prepared for NOx analysis?

Accepted Answer

Insert a 60 mL gas-tight syringe into the tailpipe and condition it by drawing in and expelling exhaust twice. Draw 25 mL of NOx indicator solution into the syringe, expel air, then draw 35 mL of exhaust to the 60 mL mark. Shake by hand for 2 minutes, cover with aluminum foil, and allow 45 minutes for color development before measuring absorbance.

Question 7

What other sources of NOx can be monitored using this colorimetric method?

Accepted Answer

Cigarette smoke contains 500-800 ppm NOx, significantly higher than automobile emissions. Nitrifying bacteria in soil oxidize ammonia to nitrite and nitrate, allowing soil NOx levels to be examined colorimetrically. Cured foods contain measurable nitrates and nitrites added as preservatives, and carbon and nitrogen analysis of environmental samples can quantify these compounds using modified colorimetric tests.

образец	Абсорбция
0,2 мкг NO₂^-/мл стандарт
0,4 мкг NO₂^-/мл стандарт
0,6 мкг NO₂^-/мл стандарт
0,8 мкг NO₂^-/мл стандарт
1,0 μг NO₂^-/мл стандарт
Выхлоп дизельного автомобиля (при запуске)
Выхлоп дизельного автомобиля (после 10 минут работы)
Выхлоп бензинового автомобиля (при запуске)
Бензиновый выхлоп автомобиля (после пробега 10 минут)

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