Horace Mann School

Chemistry
Delanty/Feigin/Harwzinskie
Hatch/Pearlmutter
2005-06

Properties of Light Laboratory

INTRODUCTION

When light strikes an object, it may be transmitted (pass through unaltered), absorbed, or reflected. Some objects, like windowpanes, transmit almost all of the light that strikes them. Most objects, however, absorb some wavelengths of light and reflect others. The colors of light seen are those that are reflected. Those colors of light not seen are absorbed by the object.

We can observe and measure absorbed and reflected light in the laboratory. Colored aqueous solutions reflect the color seen and absorb the other colors of light. For example, a solution that appears blue is reflecting blue light and absorbing other colors of light. You will use a spectrophotometer to measure how much light different solutions absorb. The spectrophotometer aims a ray of light of a specific wavelength (i.e. color) at a vial of liquid. The light that strikes the solution is absorbed, transmitted, or reflected.

We will be measuring the absorbances using a colorimeter, a simple spectrophotometer that is interfaced with a computer. While spectrophotometers allow you to use a large range of light wavelengths (thus the whole range of colors in the visible spectrum, for example), our colorimeters have just four colors at four specific wavelengths:

Color / Wavelength (nm)
violet / 430
blue / 470
yellow / 565
Red / 635

You will measure how much blue and red light that solutions of several colors absorb. In addition, you will be conducting an experiment to test the relationship between a solution’s absorbance and its concentration (how much of a substance is dissolved in the mixture, measured in grams dissolved/mL of water). Dilute solutions absorb light proportionally to the concentration of the substance dissolved (the solute) in solution. The principle is known as Beer’s Law. The mathematical equation for Beer’s Law is given below:

Absorbance = (constant)(concentration)

The “constant” represents the slope of the line that is generated by graphing the absorbances of several solutions against their concentrations. Once the graph is generated, any solution that has an absorbance in the range of the graph has an associated concentration. You will generate a graph of absorbance versus concentration for a set of cobalt(II) nitrate solutions of known concentration. You will then measure the absorbance of two cobalt(II) nitrate solutions of unknown concentration, and, based on their absorbances and the Beer's Law plot, find the concentrations of these solutions.

PRELAB

1.  What happens to light that is “absorbed”? Where does it go?

2.  A red light is shone on a blue ball in a completely dark room. Would you be able to see the ball? Explain your reasoning.

3.  Read the lab procedure carefully. Set up a data table for Part I of the experiment.

PROCEDURE

PART I – Relationship between the color of a solution and its absorbance of different colors.

Setting up the colorimeter

1.  Open your computer and turn it on. Click on the LoggerPro icon in the dock (if prompted, choose LabProUSB and click OK). Click the Folder icon on the menu bar, find the folder “Chemistry with Computers”, then open “11 Beer’s Law.cmbl”.

2.  Prepare the cuvette (a small square plastic cylinder) by filling it 3/4 full with distilled water using your squirt bottle. Place a mark with a marking pen on the top of one of the clear sides of the cuvette (if there is no mark present already). To get the best results, make sure to do the following:

> Keep the outside of the cuvette clean and dry with a KimWipe paper towel.

> Handle the cuvette only at the top..

> Make sure there are no air bubbles in the cuvettr.

> Always position the cuvette in the colorimeter with the marked side facing the white hash mark inside the colorimeter.

3.  Place the filled cuvette in the colorimeter and close the cover. Turn the colorimeter wavelength switch to 470 nm (blue) with the arrow buttons on the colorimeter. Press “CAL” and wait for the warning light to stop flashing. The absorbance should read “0.000” or nearly 0 on the computer screen.

Measuring how much light solutions absorb.

4.  Rinse the cuvette with one of the four colored solutions. Fill the cuvette with the same solution and place it in the colorimeter. Record its absorbance in your notebook along with the color of the solution. Withdraw the sample from the cuvette and place in your waste beaker Repeat the above with each of the other solutions.

5.  Recalibrate the colorimeter to measure the solutions’ absorbance of light at 635 nm (red light). Repeat step 4 with the four solutions.

PART II – Relationship between absorbance and a solution’s concentration.

  1. You will now measure the absorbance of the red solution, Co(NO3)2, at various concentrations. Based on the data you collected in Part I, choose the wavelength that would best be absorbed by the red solution and recalibrate your colorimeter according to steps 2-3. Click on the value “0.50” on the x-axis of the graph until a box appears around it, type in the value “20” and press “Return”

7.  Click the red arrow button in the upper right corner to begin the experiment. Using a small Beral pipette, rinse the cuvette once with sample 1. Dispose of the rinse in your waste beaker. Fill the cuvette 3/4 full with the 5 g/L Co(NO3)2 solution. Wipe the cuvette clean and dry, and place it in the colorimeter in the correct orientation.

8.  Once the absorbance has stabilized, click on the “Keep” button in the upper right corner. A box will appear, into which you can enter the concentration of the sample. Ignore the units of concentration given by the computer. The units we are using are g/L.

9.  Withdraw the sample from the cuvette and place in your waste beaker.

10.  Repeat the above steps for samples with concentrations of 10, 15, and 20 g/L. You need not rinse the cuvette with water between samples. Simply add a portion of that sample. Click on "Stop" to end the experiment and to begin analyzing the data.

11.  You should now have a collection of data points in your Graph Window. In the tool bar, click on the icon with a capital A. This autoscales your graph, spreading the points evenly over the whole graph. In the menu bar, click on "Analyze",then “Linear Fit". A best-fit line will appear with an equation in slope-intercept form. Under the File menu, choose “Print Graph” (DO NOT click on the print icon in the tool bar). Print one copy for each member.

Measuring the absorbances of Co(NO3)2 solutions of unknown concentration.

12.  Place a sample of one of the unknowns into the cuvette and measure its absorbance. Your colorimeter should still be measuring absorbances even though you have stopped the original experiment. Record the absorbances of each of your unknowns in your notebook.

13.  Using the line equation generated on your graph, plug the absorbances you measured into the equation to calculate the concentrations of each of your solutions. Record these concentrations in your notebook.

14.  Dispose of your liquid waste as directed by your teacher. Return your cuvette, clean and dry, to the colorimeter. Clean up.

POST-LAB

  1. Analyze your results from Part 1. Can you detect any patterns about the absorbance of light by the solutions?
  1. Predict which solution(s) would absorb yellow light best based on your results in Part I. Explain your reasoning.
  1. Based on your results, predict the concentration of a solution whose absorbance is 0.198.
  1. What are the maximum and minimum concentrations measurable by your experiment? How can the range of measurable concentrations be increased?

5.  Discuss any sources of error in this experiment.