1. Spectrophotometry Electromagnetic Radiation = Light What is Light?
Visible light is a particular kind of electromagnetic radiation
X-rays, UV, Infrared, Microwaves, and Radio waves are all light forms
Light is a form of Energy
All light travels in waves at “the speed of light” = c = 3 x 108 m/s 2

2. Characteristics of Light
Wavelength = distance between two peaks in a wave
l (lambda) is the symbol
Meters = m is the unit
Frequency = number of complete waves passing a given point per second
n (nu) is the symbol
Hertz = Hz = s-1 is the unit
Amplitude = A = measure of the intensity of the wave, “brightness”
The speed of light is constant: c =  x 
l and n are inversely proportional
If one increases, the other decreases
Their product is always the speed of light = c =  x  = 3 x 108 m/s

3. II. Spectroscopy = using light to investigate a compound
How the experiment works
What color do we see?
a) What we see as the color of a compound is the complementary color to what the compound absorbs
b) Example: Absorbs red, we see green

4. Beer’s Law
Absorbance tells us how much light is absorbed by the molecule
B. A = elc (y = mx + b) describes the absorption of light in a solution
l = the length of the cell containing the solution, usual 1 cm
c = concentration in mol/L = M
e = Molar extinction coefficient = constant for a given molecule at a given wavelength of light = how well the molecule absorbs light
Percent T
Absorbance
1.0
0.5
0.0
e ≈ 18,000 for our red dye
Absorbance
Slope = el [L/mol∙cm][cm]
Concentration (mol/L)

5. Today’s Experiment
Make a Beer’s Law Calibration Curve using a Stock Solution
Calculating the concentration of your stock solution: (Ex: g/L)
Calculations for the calibration curve samples: M1V1 = M2V2
An Excel Template is available at faculty.swosu.edu/tim.hubin in the “Shared Folder” link called “Spectrophotometry Lab Excel sheet”
Sample # 1 2 3 4 5 6
mL Dye
Stock Soln
0.50
1.00
1.50
2.00
2.50
3.00
mL Water
9.50
9.00
8.50
8.00
7.50
7.00
Molarity

6. Find the concentration of an unknown sample of the red dye
We will give you an unknown amount of the stock solution
Dilute it to 25 ml in a volumetric flask
Calculate the absorbance (from %T) on the Spec. 20
Use your spreadsheet to calculate the concentration from the absorbance
Example Calculation: %T = 15% for your diluted unknown
Make 100ml of a solution of a known concentration (2.5 x 10-5 M)

7. Chromic Acid Splash in Eyes Frayed Electrical Cord Fire