max”). The value of lambda max for FD&C Blue 1 is 630 nm. The wavelength of light absorbed by a substance is characteristic of its molecular or electronic structure. The intensity of light absorbed depends on the amount of the substance in solution. Generally, the more concentrated the solution, the more intense the color will be, and the greater the intensity of light the solution absorbs. A digital spectrophotometer measures both the percent transmittance of light and the absorbance. When light is absorbed, the radiant power ( P ) of the light beam decreases. Transmittance ( T ) is the fraction of incident light ( P/P o ) that passes through the sample (see Figure 5). The relationships between transmittance and percent transmittance ( % T ) and between transmittance and absorbance ( A ) are given in Equations 1 and 2, respectively. % T = T × 100 = P/P o × 100% Equation 1 A = absorbance = –log T Equation 2 c P o P b Figure 5. The amount of light absorbed by a solution depends on its concentration ( c ) as well as the path length of the sample cell ( b ) through which the light must travel. See Equation 3, which is known as Beer’s law. The constant a in the equation is a characteris- tic of a substance and is known as the molar absorptivity coefficient. A = abc Equation 3 orange 600–640 nm blue 450–480 nm violet 400–450 nm green 480–560 nm red 640–700 nm yellow 560–600 nm Figure 4. N N + - O 3 S SO 3 - SO 3 - Figure 3. Na + Na + FD&C Blue 1
– 3 – IN7642 © 2015 Flinn Scientific, Inc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Analysis of Food Dyes in Beverages, Catalog No. AP7642, from Flinn Scientific, Inc. No part of this material may be reproduced or transmitted in any form or by any means, electronic or mechanical, including, but not limited to photocopy, recording, or any infor- mation storage and retrieval system, without permission in writing from Flinn Scientific, Inc. Experiment Overview The purpose of this advanced inquiry lab is to use spectroscopy and graphical analysis to determine the concentration of dye in a sports drink. The investigation begins with a an introductory activity for preparing a series of standard dilutions of an FD&C Blue 1 stock solution and measuring the percent transmittance of each. The results will be analyzed graphically to identify an optimum linear relationship among various functions ( T , % T , log T and A ) for a Beer’s law calibration curve. The procedure provides a model for guided-inquiry analysis of the concentration of food dye(s) in sports drinks and other consumer beverages. Additional dyes, FD&C Yellow 5 and FD&C Red 40, are also available for optional extension or cooperative class studies. Pre-Lab Questions The visible absorption spectrum for FD&C Blue 1 is shown in Figure 6. The estimated concentration of the dye was 7.0 μM (7.0 × 10 –6 M).
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