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Spectrophotometry

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

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Spectrophotometry

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  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. 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 0 10 25 50 75 100 2.0 1.0 0.6 0.3 0.1 0 Absorbance 1.0 0.5 0.0 e≈ 18,000 for our red dye Absorbance Slope = el [L/mol∙cm][cm] 0 0.00001 0.00002 0.00003 0.00004 0.00005 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: 0.0788g/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”

  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

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