Lab_04_2011 - Spectrophotometry& Beer’s Law N N O O SO...

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Unformatted text preview: Spectrophotometry & Beer’s Law N N O O SO 3-- SO 3 H H A = ε b c Absorbance vs Wavelength 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 3 5 3 8 4 1 4 4 4 7 5 5 3 5 6 5 9 6 2 6 5 6 8 7 1 7 4 Wavelength (nm) Absorbance Absorbance vs Concentration Red Dye #2 at 470 nm 0.2 0.4 0.6 0.8 1 1.2 5 10 15 20 25 30 35 40 45 50 Concentration (mg/L) Absorbance of F o o d D y e s NO ADMITTANCE TO THE LAB WITHOUT SAFETY GOGGLES Gorilla 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? ? QUESTIONS ? What considerations go into the graphing of experimental data? Questions Concepts: Concentration Transmittance Absorbance Analytical Wavelength Beer’s Law Blank Electromagnetic Spectrum Absorptivity Techniques: Quantitative dilution Visible Spectroscopy Graphing Curve fitting Slopes Apparatus: Spectronic 20 Pipet Syringe Purpose Purpose 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 that is transmitted by the solution BACKGROUND Transmitted light PURPLE SOLUTION Incident white light I t I i I t 100 X I i %T = R O Y G B V R OYG B V Percent Transmittance Violet Red Or, instead, we can plot the percent of light absorbed Green Yellow Blue % Transmitted % Transmittance of White Light by a Violet Solution Violet Red Blue Green % Absorbed Yellow % Absorbed of White Light by a Violet Solution VISIBLE AND ULTRAVIOLET : Electronic structure of molecules INFRARED REGION: Vibrations of molecules What determines the WAVELENGTHS of light that substances absorb? What determines INTENSITY of absorption? EFFICIENCY of INTERACTION between MOLECULES and ELECTROMAGNETIC RADIATION 100 nm – 800 nm 800 nm – 100 m Which DEVICE measures this phenomenon? A SPECTROPHOTOMETER measures the RELATIVE ABSORPTION of light of DIFFERENT WAVELENGTHS by a substance SOURCE MONO- CHROMATOR DETECTOR SAMPLE 37.7 SLIT DETECTOR to measure intensity of light transmitted through the sample, and ELECTRONICS to translate detector output into numbers SOURCE of light, MONOCHROMATOR – an element to select particular wavelengths SAMPLE CONTAINER Detector output depends on intensity, wavelength & instrumental factors....
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This note was uploaded on 11/28/2011 for the course CHE 143 taught by Professor Raleigh during the Spring '09 term at SUNY Stony Brook.

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Lab_04_2011 - Spectrophotometry& Beer’s Law N N O O SO...

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