Chlorophyll is a substance in plants that is a key part of the complex process that allows green plants to convert sunlight, carbon dioxide, and water into food. It appears green because it absorbs other colors of light allowing greenish-yellow light to pass through. This unabsorbed light, also called transmitted light, is the light that our eye detects. Strawberry Kool-Aid® appears red because it absorbs the rest of the visible spectrum much more strongly (see Figure 3): (Figure 3)Visible spectra of strawberry Kool-Aid® (at 2 different concentrations above) show that the high-wavelength (600 – 700 nm (red light, see spectrum in Figure 1)) ) end of the spectrum is least absorbed, so the transmitted light is predominantly red. Light with wavelengths corresponding to blue (475 nm), through green (510 nm) towards yellow (570 nm) are most strongly absorbed. Each solution has ≥1 wavelength (l) at which it absorbs the most light -- its lambdamax(lmax). At lmaxthe instrument is most sensitive to differences in concentration, since the solute absorbs the most light at this wavelength; smallchanges in the amount of solute will yield largechanges in the absorbance. For copper sulfate solution, lmaxis 740 nm – see why it could be blue? In designing an experiment you want to use the spectrophotometer in a way that maximizes its sensitivity. The Vernier colorimeter offers four choices of wavelength: 430, 470, 565, and 635 nm. Choosing a wavelength beyond 600 nm in the case above would lead to zero absorbance in the concentration range of interest – a poor choice. Of the remaining choices which appears to have the most absorbance? Three other Kool-Aid flavors besides strawberry will be used in the experiment. Their visible spectra will be posted in lab. You will analyze two flavors, so be sure to consult the appropriate spectrum to determine the optimum choice of wavelength for the colorimeter. BEER’S LAWCONCENTRATION (c)EFFECTSIf a substance absorbs light, the more substance present, the more light is absorbed. If you’re accustomed to one teabag steeped for 3 minutes in a cup, tea prepared for the same length of time using an equivalent teabag, but in a 6-cup teapot full of water will taste weaker and look paler. The concentration of the dissolved solute is less. We will use this direct relationship between absorbance and concentration of dissolved solute tol help us determine the concentration of samples of unknown concentration. 0.00.51.01.52.02.5400425450475500525550575600625650675700725Absorbance (unitless)Wavelength (nm)Strawberry Kool-Aid® Visible Spectrum (as taken with a Vernier spectrophotometer)Abs (1 pkg/L)Abs (dil 1:4)
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- Spring '10