- 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
Détermination des NOx dans les gaz d'échappement des voitures à l'aide de la spectroscopie UV-VIS
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Overview
Source : Laboratoires de Margaret Workman et Kimberly Frye – Depaul University
Dans la troposphère, l’ozone est naturellement formé quand la lumière du soleil divise le dioxyde d’azote (NO2) :
PAS2 + lumière → NO + O
O + O2 → O3
L’ozone (O3) peut aller réagit avec l’oxyde nitrique (NO) pour former de l’oxygène et dioxyde d’azote (NO2) :
NE + O3 → aucune2 + O2
Cela se traduit par aucun gain net de l’ozone (O3). Cependant, avec la production anthropique de l’ozone, formant des précurseurs (NO, NO2et les composés organiques volatils) par le biais de la combustion de combustibles fossiles, ont été trouvés des niveaux élevés d’ozone dans la troposphère. Gaz d’échappement de véhicules automobiles est une source importante de ces ozone formant des précurseurs : NO, NO2et les composés organiques volatils (COV). Par exemple, les sources mobiles représentent près de 60 % des émissions de2 pas + pas.
Aux températures élevées dans la chambre de combustion d’une voiture, l’azote et l’oxygène de l’air réagissent pour former de l’oxyde nitrique (NO) et dioxyde d’azote (NO2) :
N2 g + O2 (g)→ 2 pas(g)
2 pas(g) + O2 (g)→ 2 aucun2 g
L’oxyde nitrique (NO) émis dans le gaz d’échappement de la voiture est peu à peu oxydé en dioxyde d’azote (NO2) dans l’air ambiant. Ce mélange de non et non2 est souvent dénommé NOx. Lorsque aucunx réagit avec les composés organiques volatils dans l’atmosphère en présence de lumière du soleil, des formes de l’ozone troposphérique, comme on le voit dans ce simplifié réaction chimique :
AUCUNx + COV + lumière → O3 autres produits
Ce mélange nocif de la pollution atmosphérique, qui peut inclure des aldéhydes, nitrate de peroxyacétyle, ozone, COV et de NOx, est appelé smog photochimique. L’ozone est la plus importante composante du smog photochimique. Ce smog se trouve dans toutes les villes modernes, mais on le trouve surtout dans les villes avec des climats secs, chauds et ensoleillés et un grand nombre de véhicules à moteur. La couleur jaune-brun de smog dans l’air est due en partie pour le dioxyde d’azote présent, étant donné que ce gaz absorbe la lumière visible près de 400 nm (Figure 1).
À court terme QU’AUCUNE exposition2 (30 min à 1 jour) ne conduit à des effets indésirables respiratoires chez les personnes saines et une augmentation des symptômes respiratoires chez les personnes souffrant d’asthme. AUCUNx réagit avec l’ammoniac et d’autres composés de particules de forme. Ces petites particules peuvent pénétrer dans les poumons et causer des problèmes respiratoires, y compris l’emphysème et la bronchite. Les personnes qui passent beaucoup de temps sur la route ou qui vivent près de la chaussée d’expérience exposition considérablement plus élevée à NO2.
En raison de son impact sur la santé humaine et l’environnement, la U.S. Environmental Protection Agency (EPA) a classé N°2 comme un polluant de critères et a fixé la norme primaire à 100 ppb (98e centile de concentrations maximales quotidiennes de 1 h, en moyenne sur 3 ans) et 53 ppb (moyenne annuelle). Considérant que les véhicules routiers représentent environ 1/3 d’aucune émissionx aux États-Unis, automobiles sont donc régis par la Loi sur l’assainissement de l’Air. L’EPA a établi des normes d’émission que les constructeurs automobiles doivent respecter lors de la production de voitures. Actuellement, les normes d’émissions Tier 2 valeur que les fabricants ne doivent avoir moyenne du parc aucune émissionx ne dépassant pas 0,07 g/mille.
Fabricants aller rencontrer ce standard est à l’aide de convertisseurs catalytiques sur leurs voitures. Ce dispositif est placé entre le moteur et le tuyau d’échappement. Le flux d’échappement passe par le convertisseur catalytique et est exposé à un rôle de catalyseur. Un catalyseur de réduction de la platine et le rhodium est utilisé pour réduire la concentration de NOx dans le gaz d’échappement. Quand une molécule2 aucun ou aucune dans le gaz d’échappement entre en contact avec le catalyseur, l’atome d’azote est attrapé au large de la molécule et s’agrippent à par le catalyseur. L’oxygène est libéré et formules O2. L’atome d’azote sur le catalyseur se lie avec un autre atome d’azote qui s’est tenu sur le catalyseur pour le formulaire N°2.
Convertisseurs catalytiques ont considérablement réduit les émissions de NOx de voiture d’échappement – jusqu’à à réduction de 80 %, lorsque vous effectuez correctement. Toutefois, ils ne fonctionnent que lorsqu’ils ont atteint une température assez élevée. Par conséquent, lorsque vous effectuez un démarrage à froid d’une voiture, le convertisseur catalytique est la suppression quasiment aucune NOx. Il n’est pas jusqu’à ce que les convertisseur catalytique atteint des températures plus élevées qu’il supprime efficacement le N°x du flux de gaz d’échappement. Convertisseurs catalytiques ne fonctionnent pas sur les voitures particulières diesel en raison des conditions maigres dans lesquelles elles opèrent. En outre, le soufre dans le carburant diesel désactive également le catalyseur. Le n °x dans les moteurs diesel sont réduites principalement par le biais de la soupape de recirculation des gaz d’échappement, qui refroidit la température des gaz de combustion. Ainsi, les voitures à moteur diesel émettent généralement plus aucunx que les voitures essence.
Figure 1. La coloration caractéristique de smog en Californie dans le nuage beige Banque derrière le Golden Gate Bridge. La coloration brune est due à la NOx dans le smog photochimique.
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!