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Dying to Know
Purpose: Colored water will be analyzed with a spectrometer to create a calibration curve. The concentration of dye in an unknown solution will be determining using information from the calibration curve.

Time Required: 1 day

Group Size: Students will work in groups of 4

Materials Needed Per Group:
  • 100 mL graduated cylinder
  • 6 cups, large enough to hold 100 mL of liquid
  • 6 pipets
  • Blue food coloring
  • TI -83 calculator
  • Vernier colorimeter
  • 6 cuvettes
Prior Knowledge:
The student should be able to find the equation of a linear plot and be able to determine an x-value on the line when a y-value is known.

Light of a particular frequency (635 nm, which corresponds to red light) is passed through a solution that contains blue food coloring. The amount of light absorbed is directly proportional to the amount of dye in the solution.
  1. Obtain 5 cups and label them 0,1,2,3,4, which represents the number of drops of food coloring to be added. Each cup contains approximately 100 mL of water. Add no blue food coloring to the cup labeled #0. Add 1 drop of blue food coloring to cup #1. Continue this process until each cup has the necessary drops of blue food coloring. Each successive cup will have a deeper, more intense color.

  2. Each cup of solution contains a pipet. Use the pipet in each solution to transfer the solution to a cuvette. Position each cuvette in front of the cup to which it corresponds.

  3. Set up a Logger pro. Place the colorimeter probe into channel 1. The colorimeter will be calibrated at a wavelength of 635 nm. To do so, press the "or" button on the colorimeter to select the wavelength of 635 nm. You are going to begin by calibrating the probe using water from cup #0. Insert the cuvette filled with this solution into the colorimeter. Each time a cuvette is placed into the colorimeter, it must be positioned so the the CLEAR side of the cuvette is aligned with the white arrow. Close the lid of the colorimeter. Press the CAL button on the colorimeter, releasing the button when the red LED light begins to flash. When the LED stops flashing, the unit is ready to collect data.

  4. On the calculator screen’s main menu, press start.

    (The logger pro is, by default, set up to collect in this mode, known as “Events with Entry”.)

  5. Remove the cuvette of water from the colorimeter. Replace it with the cuvette containing solution #1. When the readout is stabilized, press enter. The absorbance is now recorded.

  6. Continue replacing cuvettes of solution, until the absorbance of each has been recorded. When you are finished, press STO.

  7. Go to the main menu and select analyze. Fit the curve to a linear regression. Record the equation of the line of best fit. Press enter to see this plot and equation, which is your calibration curve.

  8. Now, obtain an unknown from your instructor. You are going to determine the number of drops of blue food coloring per 100 mL of solution. Using a pipet, transfer some of the unknown to a clean, dry pipet. Put the cuvette into the colorimeter. Record the absorbance of the unknown.

λ at which data is being collected = ______________

   0 drops of dye in 100 mL H2O 
  1. Create a graph by hand with the number of drops of dye on the x-axis versus the absorbance on the y-axis. Find the line of best fit.

  2. Record the equation of the line of best fit.

  3. Deduce the # of drops of blue food coloring in the cup labeled “unknown” by:

    1. Using the equation for the line of best fit with the absorbency of the solution from the cup labeled “unknown”.

    2. By inputting the data of the drops of dye and the absorbency into L1 and L2 to acquire a scatter plot and line of best fit. Have the calculator paste the equation for the line of best fit into y1, type the value of the absorbency of the “unknown” into y2, and use the calculate menu on the calculator to determine the intersection point of these two equations

  4. Record the # of drops of food coloring that exists in the 100 mL of unknown.

E Saylor

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