Determining the Concentration

of a Solution: Beer’s Law

The primary objective of this experiment is to determine the concentration of an unknown nickel (II) sulfate solution. You will be using the Colorimeter shown in Figure 1. In this device, red light from the LED light source will pass through the solution and strike a photocell. The NiSO4 solution used in this experiment has a deep green color. A higher concentration of the colored solution absorbs more light and transmits less light than a solution of lower concentration. The computer-interfaced colorimeter monitors the light received by the photocell as either an absorbance or a percent transmittance value.

Figure 1 Figure 2

You are to prepare five nickel sulfate solutions of known concentration (standard solutions). Each is transferred to a small, rectangular cuvette that is placed into the colorimeter. The amount of light that penetrates the solution and strikes the photocell is used to compute the absorbance of each solution. When a graph of absorbance vs. concentration is plotted for the standard solutions, a direct relationship should result, as shown in Figure 2. The direct relationship between absorbance and concentration for a solution is known as Beer’s law.

The concentration of an unknown NiSO4 solution is then determined by measuring its absorbance with the colorimeter. By locating the absorbance of the unknown on the vertical axis of the graph, the corresponding concentration can be found on the horizontal axis (follow the arrows in Figure 2). The concentration of the unknown can then be found using the slope of the Beer’s law curve.

MATERIALS

Macintosh or IBM-compatible computer

30 mL of 0.40 M NiSO4

Serial Box Interface or ULI

5 mL of NiSO4 unknown solution

Logger Pro

two 10-mL pipets (or graduated cylinders)

Vernier Colorimeter

pipet pump or pipet bulb

one cuvette

distilled water

five 20 X 150 mm test tubes

test tube rack

tissues (preferably lint-free)

two 100-mL beakers

stirring rod

 		 

 

PROCEDURE

1. Obtain and wear goggles! CAUTION: Be careful not to ingest any NiSO4 solution or spill any on your skin. Inform your teacher immediately in the event of an accident.

2. Add about 30 mL of 0.40 M NiSO4 stock solution to a 100-mL beaker. Add about 30 mL of distilled water to another 100-mL beaker.

3. Label four clean, dry, test tubes 1-4 (the fifth solution is the beaker of 0.40 M NiSO4). Pipet 2, 4, 6, and 8 mL of 0.40 M NiSO4 solution into Test Tubes 1-4, respectively. With a second pipet, deliver 8, 6, 4, and 2 mL of distilled water into Test Tubes 1-4, respectively. Thoroughly mix each solution with a stirring rod. Clean and dry the stirring rod between stirrings. Keep the remaining 0.40 M NiSO4 in the 100-mL beaker to use in the fifth trial. Volumes and concentrations for the trials are summarized below:

Trial

Number

0.40 M NiSO4

(mL)

H2O

(mL)

Concentration

(M)

1

2

8

0.08

2

4

6

0.16

3

6

4

0.24

4

8

2

0.32

5

~10

0

0.40

4. Prepare the computer for data collection by opening "Exp 11" from the Chemistry with Computers Logger experiment files of Logger Pro. The vertical axis has absorbance scaled from 0 to 0.6. The horizontal axis has concentration scaled from 0 to 0.5 mol/L.

5. You are now ready to calibrate the colorimeter. Prepare a blank by filling a cuvette 3/4 full with distilled water. To correctly use a colorimeter cuvette, remember:

• All cuvettes should be wiped clean and dry on the outside with a tissue.

• Handle cuvettes only by the top edge of the ribbed sides.

• All solutions should be free of bubbles.

• Always position the cuvette with its reference mark facing toward the white reference mark at the right of the cuvette slot on the colorimeter.

Choose Calibrate from the Experiment menu and then click . Place the blank cuvette in the cuvette slot of the colorimeter and close the lid. Turn the wavelength knob of the colorimeter to the 0%T position. In this position, the light source is turned off, so no light is received by the photocell. Type "0" in the % edit box. When the displayed voltage reading for Input 1 stabilizes, click .

For Reading 2, turn the wavelength knob of the colorimeter to the Red LED position (635 nm). In this position, the colorimeter is calibrated to show 100% of the red light being transmitted through the blank cuvette. Type "100" in the % edit box. When the displayed voltage reading for Input 1 stabilizes, click , then click .

6. You are now ready to collect absorbance data for the five standard solutions. Click . Empty the water from the cuvette. Using the solution in Test Tube 1, rinse the cuvette twice with ~1-mL amounts and then fill it 3/4 full. Wipe the outside with a tissue and place it in the colorimeter. After closing the lid, wait for the absorbance value displayed on the monitor to stabilize. Then click , type "0.080" in the edit box, and press the ENTER key. The data pair you just collected should now be plotted on the graph.

 

7. Discard the cuvette contents as directed by your teacher. Rinse the cuvette twice with the Test Tube 2 solution, 0.16 M NiSO4, and fill the cuvette 3/4 full. Wipe the outside, place it in the colorimeter, and close the lid. When the absorbance value stabilizes, click , type "0.16" in the edit box, and press the ENTER key.

8. Repeat the Step 7 procedure to save and plot the absorbance and concentration values of the solutions in Test Tube 3 (0.24 M) and Test Tube 4 (0.32 M), as well as the stock 0.40 M NiSO4. Wait until Step 12 to do the unknown. When you have finished with the 0.40 M NiSO4 solution, click .

9. In your Data and Calculations table, record the absorbance and concentration data pairs that are displayed in the Table window.

10. Examine the graph of absorbance versus concentration. To see if the curve represents a direct relationship between these two variables, click the Linear Regression button, . A best-fit linear regression line will be shown for your five data points. This line should pass near or through the data points and the origin of the graph. (Note: Another option is to choose Curve Fit from the Analyze menu, and then select Proportional. The Proportional fit (y = Ax) has a y-intercept value equal to 0; therefore, this regression line will always pass through the origin of the graph).

11. Obtain about 5 mL of the unknown NiSO4 in another clean, dry, test tube. Record the number of the unknown in the Data and Calculations table. Rinse the cuvette twice with the unknown solution and fill it about 3/4 full. Wipe the outside of the cuvette, place it into the colorimeter, and close the lid. Read the absorbance value displayed in the Meter window. (Important: The reading in Meter window is live, so it is not necessary to click to read the absorbance value.) When the displayed absorbance value stabilizes, record its value in Trial 6 of the Data and Calculations table.

 

12. Discard the solutions as directed by your teacher. Proceed directly to Steps 1 and 2 of Processing the Data.

PROCESSING THE DATA

1. Use the following method to determine the unknown concentration. With the linear regression curve still displayed on your graph, choose Interpolate from the Analyze menu. A vertical cursor now appears on the graph. The cursor’s x and y coordinates are displayed at the bottom of the floating box (x is concentration and y is absorbance). Move the cursor along the regression line until the absorbance (y) value is approximately the same as the absorbance value you recorded in Step 11. The corresponding x value is the concentration of the unknown solution, in mol/L.

2. Print a graph of absorbance vs. concentration, with a regression line and interpolated unknown concentration displayed. Print a copy of the Graph window. Enter your name(s) and the number of copies of the graph you want.

DATA AND CALCULATIONS

Trial

Concentration

(mol / L)

Absorbance

1

0.080

________ 

2

0.16

________ 

3

0.24

________ 

4

0.32

________ 

5

0.40

________ 

6

Unknown number

_____ 

________ 

Concentration of the unknown

________ mol/L