Determination Of NO_x in Automobile Exhaust Using UV-VIS Spectroscopy

1. Preparation of Nitrite (NO₂^-) Stock Solution

1. Weigh out 1.500 g NaNO₂ and add to a 1-L volumetric flask.
2. Dilute to the mark using nanopure water. (Check the distilled water from the tap – it may contain enough nitrite to interfere with the measurements.) This produces a 1,000 µg NO₂^-/mL stock solution.
3. To make a 5.0 µg NO₂^-/mL solution, take 1 mL of the 1,000 µg NO_2-/mL solution and dilute to 200 mL in a volumetric flask.

2. Preparation of NO_x Indicator Solution

1. Weigh out 5.0 g of anhydrous sulfanilic acid and add to a 1-L volumetric flask.
2. Add 500 mL of nanopure water.
3. Add 140 mL of glacial acetic acid.
4. Using a stir bar, stir the solution until the sulfanilic acid dissolves. This takes approximately 30 min.
5. Weigh out 0.020 g of N-(1-naphthyl)-ethylenediamine dihydrochloride and add to the volumetric flask.
6. Dilute to the mark using nanopure water.
7. Transfer to a dark bottle (to prevent photodecomposition) and stopper tightly (to prevent reaction with air).

3. Preparation of Calibration Standards

1. Put 1.0 mL of the 5.0 µg NO₂^-/mL solution in a 25 mL volumetric flask and dilute with the NO_x indicator solution to the mark. This makes a 0.2 µg NO₂^-/mL standard solution.
2. Put 2.0 mL of the 5.0 µg NO₂^-/mL solution in a 25 mL volumetric flask and dilute with the NO_x indicator solution to the mark. This makes a 0.4 µg NO₂^-/mL standard solution.
3. Put 3.0 mL of the 5.0 µg NO₂^-/mL solution in a 25 mL volumetric flask and dilute with the NO_x indicator solution to the mark. This makes a 0.6 µg NO₂^-/mL standard solution.
4. Put 4.0 mL of the 5.0 µg NO₂^-/mL solution in a 25 mL volumetric flask and dilute with the NO_x indicator solution to the mark. This makes a 0.8 µg NO₂^-/mL standard solution.
5. Put 5.0 mL of the 5.0 µg NO₂^-/mL solution in a 25 mL volumetric flask and dilute with the NO_x indicator solution to the mark. This makes a 1.0 µg NO₂^-/mL standard solution.

4. Creation of the Standard Curve

1. Using a UV-VIS spectrophotometer, set the instrument to read Absorbance.
2. Set the wavelength to 550 nm on the spectrophotometer.
3. Using the NO_x indicator solution, zero the spectrophotometer.
4. Measure the absorbance of the 5 standard solutions. Record values on the data table (Table 1).

5. Automobile Exhaust Sample Measurement

1. Start the diesel-powered automobile. 
2. Using a 60 mL gas-tight syringe, insert it a few inches into the tailpipe. Avoid burns and don’t breathe in fumes. Draw in and expel the exhaust twice to condition the syringe.
3. Draw 25 mL of the NO_x indicator solution into the syringe. Expel any air from the syringe without spilling the indicator solution.
4. Draw 35 mL of exhaust into the syringe, pulling the plunger to the 60 mL mark.
5. Cap the syringe. Shake the solution in the syringe for 2 min. Cover the syringe with aluminum foil.
6. Measure the air temperature at the tailpipe when collecting the samples.
7. Repeat steps 5.1 – 5.6 using a gasoline-powered automobile. These steps can be repeated as many times as desired, using various models of automobiles.
8. Repeat steps 5.1 – 5.6 after the automobiles have been running at least 10 min.
9. Wait 45 min to allow the color to develop, before measuring the absorbance of the solution.
10. After the 45 min are up, expel the gas from the syringe, put the solution into a cuvette, and measure the absorbance using the spectrophotometer set at 550 nm. Record values on the data table (Table 1).

Sample	Absorbance
0.2 µg NO2-/mL standard
0.4 µg NO2-/mL standard
0.6 µg NO2-/mL standard
0.8 µg NO2-/mL standard
1.0 µg NO2-/mL standard
Diesel Car Exhaust (upon startup)
Diesel Car Exhaust (after running 10 min)
Gasoline Car Exhaust (upon startup)
Gasoline Car Exhaust (after running 10 min)

Table 1. Blank data table to record values of absorption.