Introduction to Spec 20

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Introduction to Spec 20
2
Educational Objectives

• Understand the operation of a spectrophotometer
• Understand and apply the mathematical
  relationship between transmittance and
  absorbance
• Operate the Spec 20 instrument available in the
  UD general chemistry instructional laboratory

3
Experimental Objectives

• Identify the characteristic wavelengths of
  different colors of visible light
• Investigate the variation in the response of the
  Spec 20 at different wavelengths
• Prepare a spectrum of a copper complex and
  identify the wavelength of maximum absorbance of
  the complex
• Prepare a calibration curve relating copper
  complex concentration and absorbance.

4
Basic Components of a Spetrophotometer

• Intensity of light differs for different
  wavelengths
• Sensitivity of detector differs for different
  wavelengths
• User must identify 100 transmittance

5
Intensity

• Io intensity of light from source
• I intensity of light reaching detector

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Transmittance and Transmittance

• Transmittance, T
• fraction of light from source reaching detector

• Percent Transmittance, T percent of light from
  source reaching detector

Io intensity from source I intensity
reaching detector
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Basic Components of a Spetrophotometer

• Intensity of light differs for different
  wavelengths
• Sensitivity of detector differs for different
  wavelengths
• User must identify 100 transmittance for each
  wavelength

8
Absorbance, A

• Measure of the quantity of light absorbed by the
  sample
• Related to transmittance
• A -log(T)
• A 2 log(T)

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Spectronic 20 Do not move the instruments

• 1. On/Off switch
• 2. LCD display
• 3. Sample compartment door
• 4. Keyboard
• 5. Optional built-in printer
• 6. Lamp compartment door

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Spectronic 20 Keypad Layout

• 1. LCD display
• 2. Escape, Back up or Clear
• 3. Enter, Accept or Continue
• 4. MENU DO NOT CHANGE.
• 5. Wavelength controls - increase or decrease
• 6. BLANK 100T or 0 Abc
• 7. MODE A/T/C - switches between absorbance,
  transmittance and concentration modes.

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The Visible Spectrum

• Observe the color of various wavelengths in the
  visible spectrum
• Dim lights in the lab
• Adjust location of cuvet to see the color at a
  wavelength greater than 380 nm
• Reset wavelength to 380 nm once cuvet is adjusted

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Response investigation

• Compare response at range of wavelengths
• Instrument most responsive at 580 nm
• Do not blank between wavelength changes in order
  to compare to 580 nm
• Cuvet must be free of scratches, fingerprints,
  and air bubbles

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Spectrum of copper complex

• Blanking required with each wavelength change due
  to response variation
• Reference Blank
• Cuvet filled with solution containing all
  substance EXCEPT the species of interest, in this
  case WATER
• Cuvet free of scratches, fingerprints, and air
  bubbles.
• Plot spectrum
• Identify the wavelength of maximum absorbance of
  the copper complex from plot of correct data

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Copper Complex Calibration Curve

• Use wavelength of maximum absorbance, lmax, from
  part c
• Blank instrument once at lmax
• Determine T and Abs at lmax for all copper
  standards provided
• Construct calibration curve
• Complete Title
• Appropriate axis scales and labels

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