analysis of a substance is selected by measuring the visible spectrum of the substance, corresponding to a plot of absorbance (A) versus wavelength (A, "lambda"). The Food and Drug Administration approves only seven unique dyes for use in foods, drugs and cosmetics. These seven FD&C dyes give rise to the entire palette of artificial food colors. The structure of FD&C Blue 1 is shown in Figure 3. Notice the extensive series of alternating single and double bonds (also called conjugated double bonds) in the center of the structure. This feature is characteristic of intensely colored organic dyes and pigments. A solution containing FD&C Blue 1 appears blue under normal white light - this is the color of light transmitted by the solution. The colors or wavelengths of light that are absorbed by this solution are complementary to the transmitted color. A color wheel (see Figure 4) provides a useful tool for identifying the colors or wavelengths of light absorbed by a substance. The blue solution absorbs yellow, orange and red light and we would expect the visible spectrum of FD&C Blue 1 to contain a peak in the 580-750 nm region. The optimum wavelength for spectrophotometric analysis of a dye solution generally determined from the wavelength of maximum absorbance (abbreviated λ max or "lambda max"). The value of lambda max for FD&C Blue 1 is 630 nm. The wavelength of light absorbed by a substance is dependent on its molecular or electronic structure. The intensity of light absorbed depends on the amount Adapted from Flinn Scientific November 14, 2014
Mrs. Nielsen AP Chemistry 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 0 ) 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. Absorbance values are most accurate within the range of 0.2-1. 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 ε ("epsilon") in the equation is a characteristic of a substance and is known as the molar absorptivity coefficient. Day 1: Introductory Activity – Constructing a Calibration Curve *Do NOT write these sections in your lab notebook Purpose: Adapted from Flinn Scientific November 14, 2014 A = ε b c Equation 3
Mrs. Nielsen AP Chemistry Students will work cooperatively to prepare a series of standard dilutions from an FD&C Blue 1 stock solution and measure the percent transmittance of each. The
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