Food dyes in Sports Drinks Project (Report sheet)

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Pasadena City College *

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Chemistry

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Jun 13, 2024

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Introduction on Beer’s Law In this experiment, we will be determining the amount of red dye present in a drink by measuring the amount of light absorbed by the sports drink and relating it to a standard curve using a spectrophotometer. The amount of light absorbed (or absorbance) can be quantitatively related to the amount of material present using the Beer-Lambert’s Law (usually referred to as Beer’s Law). Beer’s Law relates the amount of light absorbed ( A ) of a chemical substance to its concentration ( C ): A = εl C In order to obtain the beer’s law equation for a chemical substance, a dilution set of known concentrations for that particular chemical substance is prepared and its absorbance data is measured. The resulting data is plotted on excel for analysis. Consider the following plot of absorbance versus concentration of FD&C Blue#1 dye. Figure 1. Beer’s Law Plot of FD&C Blue #1 On this plot, the Beer’s law equation is the linear trend line equation displayed above the curve in the form y = mx + b in which the dependent variable (or variable that is measured) “y” is A , the independent variable (or variable that is being changed or controlled) “x” is C , the slope m is εl , and the y-intercept is a value equal to or close to zero. This Beer’s Law equation represents the best fit line through the data points and can be used to solve for the unknown concentration ( C ) of a FD&C blue #1 dye solution as long as its absorbance A is measured. Notice that determining the concentration of the dye requires knowledge of the molar absorptivity ε , and the path length l . The path length l is the distance the light travels through the solution in centimeters. For this course, the path length is defined by thickness of the cuvette you used in lab, which is 1.00 cm. The molar absorptivity ε is unique for the substance and serves as a differentiating characteristic for the substance. The molar absorptivity is simply equal to the slope (typically in units of ଵ ெ∙௖௠ ) since the pathlength l is 1.00 cm.

Data sheet Wavelength at maximal absorbance (λ max ) for red#40 dye: ____________ Table 1. Absorbance of standard solutions and 10-fold diluted Gatorade sample Solutions Concentration of red#40 dye (M) Absorbance at λ max Standard solution 1 Standard solution 2 Standard solution 3 Standard solution 4 Standard solution 5 10-fold diluted Gatorade Unknown Post-lab questions Understanding relationship between λ max and color of dyes 1. What was the wavelength (in nm) at maximal absorbance (λ max ) that you recorded for red#40? _________ 2. Is the wavelength consistent with the red#40 dye being red? Explain. (Hint: think back to the color wheel) Estimating Red Dye Concentration in Gatorade by Visual comparison 3. Based on your visual comparison to the 5 standard solutions, which standard solution did the 10-fold diluted Gatorade sample most resemble? ________________________ 4. What was the concentration of that standard? ________________________ 5. This is the approximate concentration of the 10-fold diluted Gatorade sample. Since this was a 10-fold diluted sample, to determine the original concentration of the Gatorade, multiply the approximate concentration by 10 and write value below: ________________________

Quantitative comparison via Beer’s Law Plot 6. Similar to the Excel graphing assignment, produce a scatter plot of the absorbance versus concentration of the 5 standard solutions. This will serve as your standard plot (or reference curve) so that you can estimate the concentration of your Gatorade sample. Be sure to include the linear trendline equation and R 2 value. The x-axis and y-axis should be properly labeled with units. Adjust your x-axis and y-axis values to reflect proper significant figures from your data set. Attach your plot in the space below. 7. Write down your trendline equation here: ________________________________ This trendline equation (with the format of y = mx + b) is referred to as the Beer’s Law Equation. Refer to Introduction on this report sheet for more information. Recall that Beer’s law equation has the following format: A = ε l C Since you graphed Absorbance for your “y” and Concentration as your “x”, your trendline IS the Beer’s Law Equation for Red#40 and can be used to solve for the concentration of your Gatorade sample by simple substitution. 8. Calculate the concentration of the 10-fold diluted Gatorade using Beer’s Law Equation (with proper units) = ________________________. Show work here: 9. This is the approximate concentration of the 10-fold diluted Gatorade sample. Since this was a 10-fold diluted sample, to determine the original concentration of the Gatorade, multiply the approximate concentration by 10 and write value below: Concentration of original Gatorade sports drink (with proper units) : _____________________

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