- 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
Determinação de NOx no escapamento do automóvel usando espectroscopia UV-VIS
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Overview
Fonte: Laboratórios de Margaret Workman e Kimberly Frye – Universidade Depaul
Na troposfera, o ozônio é naturalmente formado quando a luz solar divide o dióxido de nitrogênio (NO2):
NO2 + luz solar → NO + O
O + O2 → O3
O ozônio (O3) pode reagir com óxido nítrico (NÃO) para formar dióxido de nitrogênio (NO2) e oxigênio:
NÃO + O3 → NO2 + O2
Isso não resulta em nenhum ganho líquido de ozônio (O3). No entanto, com a produção antropogênica de precursores formadores de ozônio (NO, NO2e compostos orgânicos voláteis) através da combustão de combustíveis fósseis, foram encontrados níveis elevados de ozônio na troposfera. O escapamento de veículos automotores é uma fonte significativa desses precursores formadores de ozônio: NO, NO2e compostos orgânicos voláteis (VOCs). Por exemplo, as fontes móveis compõem quase 60% das emissões NO + NO2.
Nas altas temperaturas encontradas na câmara de combustão de um carro, nitrogênio e oxigênio do ar reagem para formar óxido nítrico (NO) e dióxido de nitrogênio (Nº2):
N2(g) + O2(g)→ 2 NO(g)
2 NO(g) + O2(g)→ 2 NO2(g)
O óxido nítrico (NO) emitido no escapamento do carro é gradualmente oxidado ao dióxido de nitrogênio (NO2) no ar ambiente. Esta mistura de NO e NO2 é frequentemente referida como NOx. Quando o NOx reage com compostos orgânicos voláteis na atmosfera na presença da luz solar, formas de ozônio troposférico, como visto nesta reação química simplificada:
NOx + VOCs + luz solar → O3 + outros produtos
Esta mistura nociva de poluição do ar, que pode incluir aldeídos, nitratos de peroxilato, ozônio, VOCs e NOx,é chamada de poluição fotoquímica. O ozônio é o maior componente da poluição fotoquímica. Essa poluição é encontrada em todas as cidades modernas, mas é encontrada especialmente em cidades com climas ensolarados, quentes, secos e um grande número de veículos motorizados. A cor amarelo-marrom da poluição no ar deve-se, em parte, ao dióxido de nitrogênio presente, uma vez que este gás absorve luz visível perto de 400 nm(Figura 1).
A exposição de curto prazo no2 (30 min a 1 dia) leva a efeitos respiratórios adversos em pessoas saudáveis e aumento dos sintomas respiratórios em pessoas com asma. NOx reage com amônia e outros compostos para formar partículas. Essas pequenas partículas podem penetrar nos pulmões e causar problemas respiratórios, incluindo enfisema e bronquite. Indivíduos que passam muito tempo na estrada ou que vivem perto de uma estrada experimentam uma exposição consideravelmente maior ao NO2.
Devido ao impacto que tem na saúde humana e no meio ambiente, a Agência de Proteção Ambiental dos EUA (EPA) classificou o NO2 como um poluente de critérios e estabeleceu o padrão primário em 100 ppb (98º percentil de concentrações máximas diárias de 1h, médias ao longo de 3 anos) e 53 ppb (média anual). Considerando que os veículos on-road são responsáveis por aproximadamente 1/3 das emissões NOX nos EUA, as emissões de automóveis são, portanto, reguladas através da Lei do Ar Limpo. Atualmente, os padrões de emissão de nível 2 estabelecem que os fabricantes devem ter emissões médias de NOx de frota de no máximo 0,07 g/milha.
Uma maneira de os fabricantes atenderem a este padrão é usando conversores catalíticos em seus carros. Este dispositivo é colocado entre o motor e o escapamento. O fluxo de escape passa pelo conversor catalítico e é exposto a um catalisador. Um catalisador de redução de platina e ródio é usado para reduzir a concentração NOx no escapamento. Quando uma molécula NO ou NO2 no escapamento entra em contato com o catalisador, o átomo de nitrogênio é retirado da molécula e mantido pelo catalisador. O oxigênio é liberado e forma O2. O átomo de nitrogênio no catalisador se liga com outro átomo de nitrogênio mantido no catalisador para formar n2.
Os conversores catalíticos reduziram consideravelmente as emissões de NOx do escapamento do carro – até 80% de redução, quando executados corretamente. No entanto, eles só funcionam quando atingiram uma temperatura bastante alta. Portanto, ao fazer uma partida fria de um carro, o conversor catalítico está removendo praticamente nenhum NOx. Não é até que o conversor catalítico atinja temperaturas mais altas que ele efetivamente remove o NOx do fluxo de escape. Os conversores catalíticos não funcionam em carros de passeio a diesel devido às condições magras em que operam. Além disso, o enxofre no diesel também desativa o catalisador. O NOx em motores diesel são reduzidos principalmente através da válvula de recirculação de gases de escape (EGR), que esfria a temperatura dos gases de combustão. Como resultado, os carros a diesel geralmente emitem mais NOx do que carros a gasolina.
Figura 1. Coloração característica para poluição na Califórnia no banco de nuvens bege atrás da Ponte Golden Gate. A coloração marrom é devido ao NOx na poluição fotoquímica.
Principles
Procedure
Results
Table 2 provides an example of proper results. Using the absorbance measurements of the standard solutions, a plot of Absorbance vs. Concentration of NO2– can 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:
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:
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,
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. 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.
Thanks for watching!