Beer's Law Lab

Part 1 - Preparing the solution

Part 2 - Dilutions

Part 3 - How the colorimeter works:

Read the following and then answer the questions below.

A colorimeter is designed to determine the concentration of a solution by analyzing its color intensity. It measures the amount of light that is transmitted or absorbed through a sample at a user selected wavelength. Light from a LED light source passes through a cuvette (the small container that holds your sample). Some of the incoming light is absorbed by the solution and the rest of the light passes through (this is also know as transmittance). As a result, light of a lower intensity strikes a photodiode detector on the other side of the cuvette. (See figure 1). When a cuvette is filled with pure water the colorimeter will show 100% transmittance and 0 absorbance.

1. What is a cuvette?

2. In your own words, in a sentence or two to sum up how a colorimeter works.

3. Look at your solutions of different concentrations that you prepared. What do you notice about the color of the solutions as they become more dilute?

4. Which of your solutions do you believe will absorb more light before the light reaches the detector?

5. Which of your solutions do you believe will transmit more light to the detector?

Part 4: Choosing the proper wavelength:

Read the following and then answer the questions as you go along.

On your colorimeter, you have various settings of different wavelengths (430 nm, 470 nm, 565 nm, and 635 nm). Each wavelength value represents a different color light. Using the arrows you can select a different wavelength. Hook up your colorimeter to a logger pro box and the computer. Open up logger pro.

1. Put the setting on 635 nm and look inside in the direction of the setting buttons. You should see a blinking LED light. What color is it? Change to each of the other wavelengths and write down the color associated with each wavelength in a chart.

Now you must choose the proper wavelength to use when examining your solution. READ this paragraph so you understand what you are about to do. DON'T start doing anything until you get to the bullets below. You never want to use the same color LED as the color of the solution. The best way to determine the proper wavelength to use is to place a sample of your solution that is closest to 1 M into the colorimeter and measure the absorbance at each wavelength. The highest absorbance that is recorded is the best wavelength to use. At this point you are going to determine the best wavelength to use. STOP - Call me over before proceeding. The steps below will help you to do this:

• • Go to logger pro.

  • Next, on the tool bar go into insert, meter, digital to turn on absorbance.

  • Fill a cuvette with water and wipe any fingerprints off using the Kim Wipes.

  • Place the sample of water into the colorimeter (flat side toward the light), shut the lid, and hit the CAL button (this calibrates the machine setting the absorbance at 0).

  • Be sure to wipe off any fingerprints each time you place a sample into the colorimeter.

  • Now place one of the concentration samples (ONLY THE ONE CLOSEST TO 1M) into the colorimeter, shut the lid, and record the absorbance at each wavelength.

1. Create a chart in your lab book for the absorbance at each wavelength for ONLY the solution with a concentration closest to 1M.

2. Which wavelength should we use to complete the rest of our experiment?

3. Why did we do this part of the lab?

Part 5: Creating a graph:

Now that you have picked the proper wavelength to conduct your experiment, measure the absorbance of each concentration solutions that you prepared. Create a data table for your results. Then create a graph. When you go to graph, be sure to unplug your colorimeter and do file new. Curve fit your graph and sketch it into your notes. Before continuing along, you can clean up all your beakers and cuvettes (except the water one) by dumping them down the sink. Call me over to see your graph.

Part 6: Determining unknown concentration:

See me for the unknown concentration of cobalt chloride hexahydrate. Determine the concentration of the sample.