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# 03Post - OBJECTIVES What is the quantitative basis for the...

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9/17/2011 1 Spectrophotometry & Beer’s Law of F o o d D y e s 1 Last Update: 9/17/2011 4:23 PM How is the absorption/transmission of light measured? How is the intensity of color related to the concentration of colored solutions? What is the quantitative basis for the color of substances? OBJECTIVES What considerations go into graphing experimental data? Questions 2 Concepts, Techniques Concepts: Absorbance Absorption Absorptivity Analytical Wavelength Beer’s Law Blank Concentration Electromagnetic Spectrum Incident Intensity Spectroscopy Spectrum Transmitted Transmittance Wavelength Purpose 3 Techniques: Visible Spectroscopy Quantitative dilution Graphing Plotting /Curve fitting Apparatus: Recording Spectrophotometer Spectronic 20 Pipet/Syringe Cuvette NO ADMITTANCE TO THE LAB WITHOUT SAFETY GOGGLES Gorilla 4 For a simple, transparent, C O L O R E D material, e.g. Can plot a graph showing the percent of light of each discrete C O L O R (wavelength, λ ) transmitted by the solution Transmitted light VIOLET CLEAR SOLUTION Incident white light I t ( λ ) I i ( λ ) I t ( λ ) 100 X I i ( λ ) %T( λ ) = R O Y G B V R OYG B V Percent Transmittance 5 BACKGROUND 0 10 20 30 40 50 60 70 80 90 100 Violet Red Green Yellow Blue UV IR Wavelength (nm) 6 % Transmittance of White Light by a Violet Solution

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9/17/2011 2 0 10 20 30 40 50 60 70 80 90 100 WAVELENGTH (nm) Violet Red Blue Green % Absorbed Yellow % Absorbed = 100% - % Transmitted 7 % Absorbed of White Light by a Violet Solution VISIBLE AND ULTRAVIOLET: Electronic structure of molecules INFRARED REGION: Vibrations of molecules What determines INTENSITY of absorption? EFFICIENCY of INTERACTION between Molecules & Electromagnetic Radiation λ = 100 nm – 800 nm 800 nm – 100 m 8 What determines the WAVELENGTHS of light that substances absorb? A SPECTROPHOTOMETER measures the RELATIVE absorption of light of DIFFERENT WAVELENGTHS by a substance DETECTOR to measure intensity of light transmitted through sample, and ELECTRONICS to translate detector output into numbers SOURCE of light, MONOCHROMATOR –an element to select particular wavelengths SAMPLE & CONTAINER 9 Which DEVICE measures this phenomenon?
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03Post - OBJECTIVES What is the quantitative basis for the...

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