Smoking in Lab: Some Chemistry Behind It

Nature’s kitchen for some really interesting chemicals

Before We Get Started

What is NOx?• NOx stands for the combination of NO and NO2.

• In smoke, NO is the original product of combustion of nitrogenous compounds.

• The NO is oxidized to NO2 within a few minutes in the presence of O2.

NO2

NO +(brown gas seen in smog)

[Shooter, 1993]

oxidants

Reaction Mechanism

NO2 + OH2 HNO2

CH3COOH

NH2

O2S

NH2

NH2

O2S

N

N+

sulfanilamide diazonium salt

NO2 -,

[Shooter, 1993]

CH3COOH

Reaction Mechanism

NH2

O2S

N

N+

+

NH3+

NH2+

Cl-

2

NH3+

NH2+

NH2

O2S

NN

Cl-

2

Azo Dye

diazonium salt

NEDA

[Shooter, 1993]

So what does this mean?• The amount of NO2 is related to the intensity of

the dye.

• More NO2 …

• …means more dye molecules…

• …means more intense color of solution.

• Now you can use a spectrophotometer to analyze the solution and see how color intensity relates to percentage transmittance or absorbance.

The Apparatus

¼ inch tubing Filter Disc

     

Cigarette 5/16 inch 60 ml Syringe

tubing • When assembling:

– Secure tubing to disk with copper wire– Note that cigarettes are fragile

Procedure Notes

• Allow cigarette to burn 10 seconds IN A HOOD

• When drawing up the smoke:– 35 mL in 2 seconds—THIS IS IMPORTANT!

• Draw up three 35 mL aliquots, in 1 minute intervals, to passivate the syringe

Procedure Notes• Then draw up 25 mL of indicator solution into

the syringe followed by an additional 35 mL smoke with the syringe held vertically (cigarette on top)

• Cap and shake VIGOROUSLY

• Allow the solution to develop 30 minutes– Shake occasionally

Procedure Notes• After 30 minutes expel the smoke and draw up

an additional 25 mL of indicator solution

• Measure the absorbance spectrum of the dye. Determine A at 550 nm.– Use absorbance mode, rather than %T– Ensure you have calibrated the SpectroVis– Use indicator solution as the blank

Notes on the Indicator• Take only the required amount

• RE-CAP THE BOTTLE!

• Store your sample of the solution in a Parafilm covered beaker outside the hood where your cigarettes are used.

Preparation of the Standards• Since the NOx is converted to nitrite during this

process, known concentrations of standard solutions can be made from potassium nitrite.

• These are used to create a calibration line

Beer’s Law

A=abc• A = Absorbance• a = absorptivity (units: often M-1cm-1)

– but can have different concentration units

• b = optical pathlength• c = concentration• A linear formula – a series of A’s at different c’s

should yield a straight line

Prepare Standard Solutions

• Stock Solution

• C1V1=C2V2

• Need a minimum of 3 standards but 4 or 5 are better

Beer’s Law Plot

• Acquire all of the A vs. c data

• Plot A vs. c

• Determine the best-fit line and the equation of the line

• From A of the solution from the cigarette, determine c of NOx using Beer’s Law Plot

Beer’s Law Plot: Example Data

Concentration

(M) A

1.2 x 10-5 0.292

9.0 x 10-6 0.194

6.0 x 10-6 0.144

3.0 x 10-6 0.086

0.00 0.000*

*Set when instrument calibrated

Beer’s Law Plot

A = 23.083 mM-1cm-1·l·c + 0.00476

Note the concentration scale

Analysis of Unknown Solution

Example: Unknown solution has a absorbance of 0.163 absorbance units. What is the dye concentration in the unknown?

Use the graph or equation of the line to get concentration:

Beer’s Law Plot

0.0069 mM

0.163

Beer’s Law Plot• Using the equation of the line (assuming the

same 1 cm cuvette):

c-1 -10.163 = 28.083 mM cm ×1cm× +0.00476

c-1 -1

0.163 -0.00476= = 0.0069 mM

28.083 mM cm ×1 cmc

-1 -1

0.163 -0.00476=

28.083 mM cm ×1 cm

For Today’s Lab• Standard solution preps are given

– You must calculate ACTUAL concentrations from your volume data

– Concentration will be in g/mL, not molarity

• Blank should be unexposed indicator solution

• Final value of NO2 should be reported in ppm

• You will need this website for post-lab question 1:http://www.arb.ca.gov/Aqd/almanac/almanac.htm