The Visible Spectra of Soda Pops

Purpose: The purpose of this experiment is twofold: the first part is designed to determine the origin of the color of a soda pop. In the second part you will determine by how much a sample of grape soda has been diluted. Be specific, which soda are you measuring?

The spectrum of a soda pop is obtained by measuring the absorbance of a sample of the pop at different wavelengths using a spectrophotometer. This spectrum can then be related to the color of the pop. Next a calibration curve is prepared by measuring the absorbance of different dilutions of grape soda at a single wavelength. The absorbance of the unknown solution can then be measured at the same wavelength and compared to the calibration curve to determine its concentration.

Sample Preparation: Pour about 15-20 ml of pop into a beaker and stir to remove the carbonation. Dilute it to 50% by pouring 10 ml into a graduated cylinder and add an equal volume of water. Stir. (Any waste in this experiment can be poured down the drain).

Part I. Measuring the Spectrum:

  1. Be sure the spectrophotometer is turned on.
  2. Set the wavelength knob to 600 nm.
  3. Using the zero adjust knob on the left side, set the needle to read 0% transmittance (%T) on the top of the meter. [Nothing should be in the sample compartment].
  4. Fill one cuvette with distilled water and insert it in the sample compartment with the line facing the front. Close the top.
  5. Use the 100% adjust knob on the right hand side to set the needle to 100%T with the water-containing cuvette in the holder. Remove the cuvette and set it aside without emptying it.
  6. Fill the other cuvette with your soda solution.
  7. Picture of the Spectronic 20 (Click) --- Diagram of the Spectronic 20 Light Path (Click)

  8. Insert it in the instrument and close the cover. Read the absorbance from the bottom scale on the meter. Record, in the Results section of your notebook, the wavelength and absorbance readings. Be sure to indicate which sample you are using.
  9. Remove the cuvette, close the top and change the wavelength to a setting which is 20 nm lower.

  10. Reset the 0%T if it has changed (empty sample compartment).
  11. Insert the cuvette of distilled water and reset the 100%T.
  12. Replace the water cuvette with your sample-containing cuvette and read the absorbance again recording your results.
  13. Repeat steps 8 through 11 until you reach 360 nm.

Part II. Quantitative Analysis of the Grape Soda:

  1. Set the spectrophometer wavelength to 500 nm.
  2. Set the zero and 100%T as above.
  3. Measure the absorbance of each of the four standard solutions of grape soda provided. Record the absorbance and the solution concentration (in percent) in your notebook.
  4. Measure and record the absorbance of your unknown grape soda solution.

Part II.

Plot the visible spectrum of the soda on graph paper. Your graph should fill the entire page and use accurate markings. All graphs should contain a title and appropriate labels on the axes. An example is shown below. Include the diagram indicating the wavelength regions and the corresponding colors of visible light. Use this diagram and plot to explain the origin of the color of the soda in your conclusion.

Next make a plot of the absorbance of the different standard grape pop solutions versus their concentrations, where four open circles indicate the measurements of the four known concentrations. One single straight line is drawn that comes the closest to all these circles. See below for an example. Note the equally spaced divisions, the straight line through the points and the title on the top. Now you can use this calibration plot to determine the concentration of your unknown grape pop solution. Find the concentration of your unknown by determining where its absorbance crosses the calibration line on your graph.

Note in the example, the student created a fifth point--the one with the square--for the unknown which read 0.31. Following the line straight down from this point, the concentration for the unknown is determined (for the example, this was 25%). Report your value in your conclusions--along with your unknown number.

This activity has been copied, with permission, from the California State University at Stanislaus server to ours, to allow faster access from our Web site. We encourage you to explore the original site

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