Analyzing Chemicals in Smoke: A Spectrophotometric Approach

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.

Reaction Mechanism

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 *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 0.163 0.0069 mM

Beer’s Law Plot • Using the equation of the line (assuming the same 1 cm cuvette):

For Today’s Lab • Standard solution preps are given • You must calculate ACTUAL concentrations from your volume data • Concentration will be in mg/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

Analyzing Chemicals in Smoke: A Spectrophotometric Approach