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JoVE Science Education Environmental Science
Determination Of NOx in Automobile Exhaust Using UV-VIS Spectroscopy
  • 00:00Overview
  • 01:08Principles of NOx Quantification in Automobile Exhausts
  • 02:41Preparation of Nitrite Stock Solution and Indicator Solutions
  • 04:10Preparation of Calibration Standards and Creation of a Standard Curve
  • 05:23Automobile Exhaust Sample Measurement
  • 07:38Applications
  • 09:32Summary

没有x在汽车尾气利用紫外-可见光谱法测定

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Overview

资料来源: 玛格丽特工人和金伯利弗莱-Depaul 大学实验室

在对流层臭氧是自然形成的时候阳光分裂二氧化氮 (2):

没有2 + 阳光 → 无 + O

O + O2 → O3

臭氧 (O3) 可以去与一氧化氮 (NO),形成二氧化氮 (2) 与氧气发生反应:

不 + O3 → 没有2 + O2

这会导致臭氧 (O3) 没有净增益。然而,随着汇的人为生产的臭氧形成前体 (没有、 没有2和挥发性有机化合物) 通过化石燃料的燃烧,对流层中的臭氧水平升高被发现。汽车废气是这些臭氧形成前体的一个重要来源:2和挥发性有机化合物 (VOCs) 不,不。例如,移动源组成几乎没有 + 没有2排放量的 60%。

在发现一辆汽车燃烧室内的温度高,氮和空气中的氧气发生反应,形成一氧化氮 (NO) 和二氧化氮 (2):

N2 (g) + O2 (g)→ 2 没有(g)

2 无(g) + O2 (g)→ 2 没有2 (g)

在汽车尾气排放一氧化氮 (NO) 逐渐氧化为二氧化氮 (2) 环境空气中。这种混合物的无2经常被称为没有x。当与挥发性有机化合物在阳光照射下大气中反应没有x时,对流层臭氧形式,如在此看到简化化学反应:

没有x + VOCs + 阳光 → O3 + 其他产品

这个有毒的混合物的空气污染,其中可以包括醛类、 过氧化乙酰硝酸盐、 臭氧、 挥发性有机化合物,没有x,称为光化学烟雾。臭氧是光化学烟雾的最大组成部分。这烟雾发现在所有的现代城市,但它发现特别是在阳光明媚、 温暖、 干燥的气候和大量的机动车辆的城市。黄棕色的颜色在空气中的烟雾是到期部分到目前,二氧化氮由于这种气体吸收可见光近 400 毫微米 (图 1)。

短期无2暴露 (30 分钟到 1 天) 会导致呼吸造成的不良影响,在健康人中,哮喘患者增加呼吸道症状。没有x与氨和其他化合物对窗体粒子反应。这些小颗粒可以穿透肺部,并导致呼吸系统问题,包括肺气肿和支气管炎。个人谁花了大量时间在路上或谁住在附近的巷道经验相当高暴露于没有2

由于它对人类健康和环境的影响,美国环境保护署 (EPA) 没有2作为一种标准的污染物进行了分类和已设置的基本标准在 100 ppb (1 h 每天的最大浓度,平均 3 年以上的第 98 个百分点值) 和 53 ppb (年平均)。考虑道路上车辆占大约 1/3 的没有x排放量在美国,汽车的排放量因此规管通过清洁空气法案 》。美国环境保护署成立汽车制造商必须遵循当生产汽车的废气排放标准。目前,层 2 排放标准设置,制造商必须有车队平均没有x的排放量不超过 0.07 g/英里。

单程制造商符合这个标准是在他们的汽车上使用催化转换器。本装置被放发动机和排气管之间。排气流通过催化转换器和暴露的催化剂。铂和铑的还原催化剂用于减少尾气中的 NOx排放浓度。当废气中的没有或没有2分子接触催化剂时,氮原子是抓走分子和催化剂作用下上举行。氧气释放,并形成 O2。在催化剂上的氮原子与催化剂对形式 N2日举行的另一个氮原子结合。

催化转换器有大幅减少的排放量没有x从汽车尾气 — — 最多减少 80%,正确执行时。然而,他们只工作时他们已经达到了一个相当高的温度。因此,当做汽车冷启动,催化转化器去除几乎没有没有x。它不是直到催化转化器达到更高的温度它从排气流有效去除无x 。催化柴油客车由于精益的条件下工作不工作。此外,柴油中的硫含量也将停用该催化剂。柴油发动机中的 NOx主要通过排气气再循环 (EGR) 阀,冷却燃烧气体的温度降低。其结果是,柴油车一般发出更多没有x比汽油车。

Figure 1
图 1。为在加利福尼亚的米色云烟雾特征着色银行在金门大桥后面。棕色着色是由于没有x在光化学烟雾中。

Principles

Procedure

1.亚硝酸盐 (没有2-) 股票溶液的制备 权衡了 1.500 g 纳米2并添加到 1-L 容量瓶。 稀释到使用 nanopure 水的标记。(检查蒸馏水从水龙头 — — 它可能包含足够的亚硝酸盐,干扰测量。)这将产生 1,000 µ g2-的 / 股票无解。 若要使 5.0 µ g2-的 / 无解,取 1 毫升的 1,000 微克没有2-的 / 解决方案和稀释至 200 毫升容量?…

Results

Table 2 provides an example of proper results. Using the absorbance measurements of the standard solutions, a plot of Absorbance vs. Concentration of NO2can be made (Figure 4). Then, the best fit line of the data can be determined. Using the best-fit line of the standard curve, the concentration of NO2 in each unknown solution (µg/mL) can be calculated. This value can be converted to the concentration of NO2 in the exhaust gaseous sample using the following equation:

Equation 1

Based on the balanced equation of NO2 in H2O seen previously, a 2 mol NO2/1 mol NO2 ratio is expected. In empirical experiments, it has been found to be nearer a 1.39:1 ratio. The volume of solution used was 25 mL. The volume of the gas sample was 35 mL.

The concentration of NO2 calculated actually represents all of the NOX in the exhaust sample (Table 3). The equation for conversion between ppmV and µg/L depends on the temperature and pressure at which the samples were collected. The conversion equation is:

Equation 2

Where R = universal gas constant = 0.08206 atm·L/mol·K, P = atmospheric pressure in atm, T = temperature in K, and MW = molecular weight of NOx (as NO2) = 46.01 g/mol. Therefore,

Equation 3

It’s important to input T in K and P in atm.

Sample Absorbance
0.2 µg NO2/mL standard 0.22
0.4 µg NO2/mL standard 0.43
0.6 µg NO2/mL standard 0.60
0.8 µg NO2/mL standard 0.79
1.0 µg NO2/mL standard 1.05
Diesel Car Exhaust (upon startup) 1.03
Diesel Car Exhaust (after running 10 min) 1.03
Gasoline Car Exhaust (upon startup) 0.10
Gasoline Car Exhaust (after running 10 min) 0.04

Table 2. Data table with representative results of absorption.

Figure 4
Figure 4. A standard curve plot of Absorbance vs. Concentration of NO2.

Vehicle NOx Concentration (ppm)
Diesel Car (upon startup) 500
Diesel Car (after running 10 minutes) 500
Gasoline Car (upon startup) 48
Gasoline Car (after running 10 minutes) 21

Table 3. NOx concentration (ppm) per vehicle.

Applications and Summary

The measurement of nitrite using the modified Saltzman reaction is very common and useful in many different fields. As described, the method can be used to measure NOx concentrations in air samples – car exhaust, laboratory rooms, air quality of cities, etc. In addition, this method can be used to monitor NOx in cigarette smoke. The procedure would be very similar to this experiment, except instead of drawing car exhaust into the syringe, cigarette smoke would be drawn in. There is often a higher concentration of NOx in cigarette smoke than coming out of the tailpipe of automobiles, which tends to be surprising to many. Typical values for NOx in cigarette smoke range from 500-800 ppm.

This method can also be used to test the levels of nitrate produced in the presence of nitrification bacteria. Nitrification bacteria are found in soil and water and play an important role in the nitrogen cycle – oxidizing ammonia to nitrite and then nitrate. The nitrate in the sample is first converted to nitrite by the enzyme nitrate reductase. Then the nitrite is measured using the modified Saltzman reaction. Lastly, this method can be used to determine the concentration of nitrates and nitrites in food products. Nitrites and nitrates are added to food mainly to preserve meats and meat products. A typical value for nitrite in cured meats is approximately 125 µg/mL.

Transcript

A mixture of nitric oxide and nitrogen dioxide is generally referred to as NOx. As a by-product found in automobile exhaust, NOx can be harmful to the environment, forming damaging tropospheric ozone.

At high temperatures in the combustion chamber of an engine, nitrogen and oxygen from the air can react to form nitric oxide and nitrogen dioxide. In the presence of sunlight, NOx reacts with volatile organic compounds in the atmosphere to form ozone and other products. Tropospheric ozone is a health risk, potentially causing lung and eye irritation amongst other complaints, and it is a major component of photochemical smog.

This video will illustrate the principles behind NOx and tropospheric ozone production, how to fabricate indicator solutions, and how to measure and quantify NOx production from automobile exhausts.

On-road automobiles account for approximately one-third of NOx emissions in the US, and emissions are strictly regulated through the Clean Air Act. Catalytic converters, located between a car’s engine and tailpipe, can reduce NOx concentration in the exhaust significantly, but these require high temperatures to function, so will only reduce NOx after an automobile has been running long enough to warm the converter.

Because of this difference in the ability of catalytic converters to remove NOx at different temperatures, NOx emissions are typically read upon vehicle start up, and after running for 10 min. This gives a quantification of the NOx emission produced by the automobile, and also an indication of the ability of the catalytic converter to remove the NOx.

When NOx is added to a solution containing sulfanilic acid and naphthyl-ethylenediamine, the resultant reaction forms a pink colored azo dye molecule. The intensity of this pink is directly proportional to the concentration of NOx in the solution, and can be measured using a UV-VIS spectrophotometer to give a quantification of the amount of NOx when plotted against standards in a calibration curve.

Now that we are familiar with the process of NOx formation, let’s look at how NOx production by automobiles can be quantified in an experimental setting.

To begin the experiment, detection solutions that will react with the NOx should be prepared. To prepare the nitrite stock solution, first weigh out 1.5 g of sodium nitrite and add it to a 1-L volumetric flask. Add nitrite-free water to the 1 L mark on the flask. This produces a stock solution of 1,000 μg nitrite per mL. Label this stock solution appropriately. To make a working solution of 5 μg nitrite per milliliter, take a fresh flask and add 1 mL of the stock solution. Dilute to 200 mL.

To prepare the NOx indicator solution, first weigh out 5 g of anhydrous sulfanilic acid, and add to a 1-L volumetric flask. To the same flask, add 500 mL of nitrite free water, then 140 mL of glacial acetic. Swirl the solution, until the sulfanilic acid dissolves.

Next, weigh out 20 mg of naphthyl-ethylenediamine and add it to the flask. Finally, fill the flask to the 1-L line with nitrite free water. Transfer the solution to a dark bottle to prevent photodecomposition, stopper tightly, and label appropriately.

To generate a standard curve, calibration standards need to be created. First, put 1 mL of the 5.0-μg nitrite stock solution into a 25-mL volumetric flask and dilute with the NOx indicator solution to the calibration mark. This makes a 0.2 μg NO2-/mL standard solution.

Next, prepare 0.4, 0.6, 0.8, and 1 μg NO2-/mL standard solutions by adding 2, 3, 4, and 5 mL nitrite solutions to separate 25-mL flasks, and fill each to the mark with NOx indicator solution.

Using a UV-VIS spectrophotometer, set the instrument to read absorbance. Next, set the wavelength to 550 nanometers. Add the NOx indicator solution to a clean spectrophotometer sample cell, and use this to zero the spectrophotometer. Finally, measure the absorbance of the five standard solutions, and record the values.

To begin the readings, start the diesel-powered automobile. Take a 60 mL gas-tight syringe and insert it a few inches into the tail pipe, taking care to avoid burns or inhaling fumes. Draw in and expel the exhaust twice to condition the syringe.

Next, draw 25 mL of the NOx indicator solution into the syringe. Expel any air from the syringe without spilling the indicator solution. Finally, draw 35 mL of exhaust into the syringe, pulling the plunger to the 60 mL mark, then withdraw and cap the syringe.

Shake the solution in the syringe by hand for 2 min. Cover the syringe with aluminum foil. Finally, measure the air temperature at the sample tail pipe. Repeat the sampling process with a gasoline powered automobile, and any other model or design of automobile desired.

Repeat the experiment after the vehicles have been running for at least 10 min. Once all the samples have been collected, wait 45 min to allow color to develop. Finally, expel the gas from the syringes, and place the sample indicator solutions into individual cuvettes. Measure the absorbance using the spectrophotometer set at 550 nm, and record the values.

Using the absorbance measurements of the standard solutions, make a plot of absorbance versus concentration of nitrite. Determine the best-fit line of the data. Using this best-fit line, calculate the concentration of nitrite in each test solution. This value can then be converted to nitrogen dioxide in the exhaust.

The concentration of nitrogen dioxide calculated actually represents all of the NOx in the exhaust sample. The ppmV, or parts per million by volume to μg/L conversion is dependent on the temperature and pressure at which the samples were collected.

Automobiles are not the only source of NOx. Monitoring its production is important in a wide range of fields.

Cigarette smoke often contains a higher concentration of NOx than emitted from automobile engines. Typical values for NOx in cigarette smoke range from 500-800 ppm, compared to 21-48 ppm for emissions from a gasoline car, or around 500 ppm for a diesel vehicle. This can result in a variety of personal health issues, including bronchitis, irritation of the nose and throat, respiratory infections, or blocking of oxygen transfer in the bloodstream. NOx levels in cigarette smoke can also be quantified using the methods shown in this video.

Nitrifying bacteria are found in soil and water, and play an important role in the nitrogen cycle, oxidizing ammonia to nitrite and then nitrate. As with exhaust fumes and cigarette smoke, the NOx levels in soil can also be examined and quantified colorimetrically.

Nitrates and nitrites can also be found in measureable amounts in food products. For cured foods, nitrates and nitrites may be added as a preservative, most commonly in meats and meat products. These have antimicrobial as well as color-fixing and preservation actions, and a significant indirect beneficial effect on flavor. However, too high of nitrite content can lead to medical complications including infant methemoglobinemia, or cause shortened shelf life of products due to effects like nitrite burn. Nitrite contents in cured foods therefore should be monitored closely, and this can be carried out using a modified version of the colorimetric test.

You’ve just watched JoVE’s introduction to the determination of NOx. You should now understand how NOx is formed in automobile engines, how to formulate NOx indicator solutions, and how to measure and quantify NOx from vehicle exhaust fumes.

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JoVE Science Education Database. JoVE Science Education. Determination Of NOx in Automobile Exhaust Using UV-VIS Spectroscopy. JoVE, Cambridge, MA, (2023).