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) solution.

For this project a Colorimeter was used. This device has a red LED light that passes through the solution and strikes a photocell. The higher the concentration, the more light. The lower the concentration, the less. 

Five nickel sulfate solutions of known concentrations were prepared. Each was placed in a small cuvette. The Colorimeter was hooked up to the computer and would show a graph of absorbance vs. concentration.

Procedure:

1. Obtain and wear goggles! CAUTION: Be careful not to ingest any NiSO2 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 Ni SO4 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 mi 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 an concentrations for the trials are summarized below:

Trial number	0.40 M NiSO4 (mL)	Distilled H20 (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. Open the file "Exp 11 Colorimeter" in the Experiment 11 folder of Chemistry with Computers. 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 o 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 uvette with its reference mark facing toward the white reference mark at the right of the cuvette slot on the Colorimeter.

6. Calibrate the Colorimeter.

a. Holding the cuvette by the upper edges, place it in the cuvette slot of the Colorimeter.

b. If your Colorimeter has an AUTO CAL button, set the wavelength on the Colorimeter to 635 nm (Red), press the AUTO CAL button, and proceed directly to Step 7. If your Colorimeter does not have an AUTO CAL button, continue with this step to calibrate your Colorimeter.

First Calibration Point

c. Choose Calibrate fromt he Experiment menu and then click "Perform No."

d. Turnt he wavelength knob on the Colorimeter to the "0% T" position.

e. Type "0" in the edit box.

f. When the displayed voltage reading for Input 1 stabilizes, click "Keep".

g. Turn the knob of the Colorimeter to the Red LED position (635 nm).

h. Type "100" in the edit box.

i. When the displayed voltage reading for Input 1 stabilizes, click "Keep", then click "Ok".

7. You are now ready to collect absorbance data for the five standard solutions. Click "Collect". 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 "Keep", type "0.80" in the edit box, and press the ENTER key. The data pair you just collected should now be plotted on the graph.

8. 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, pace it in the Colorimeter, and close the lid. When the absorbance value stabilizes, click "Keep", type "0.16" in the edit box, and press the ENTER key.

9. Repeat the Step 8 procedure to save and plot the absorbance and concentration values of the solution 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 "Stop".

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

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

12. 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. (The reading in the Meter window is live, so it is not necessary to click "Collect" to read the absorbance value.) When the displayed absorbance value stabilizes, record its value in Trial 6 of the Data and Calculations table.

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

PROCESSING THE DATA
1. 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 gloating box (x is concentration and y is absorbance). Move the cursor along the regression line until the absorbance (y) value is exactly the same as the absorbance value recorded in Step 12. 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

Trail	Concentration (mol/L)	Absorbance
1	0.08	0.186
2	0.16	0.255
3	0.24	0.51
4	0.32	0.595
5	0.40	0.726
6	Unknown Number 3	0.45

Concentration of unknown                                                            0.23 mol/L

Crystal Lab

Crystals!! 

For this experiment, the class did separate trials to see crystals form.

Step 1!
Find beaker, put in about 80 ml water.

Step 2!
Sprinkle some Aluminum Potassium Sulfate into the water and stir the mixture. Keep adding until Aluminum Potassium Sulfate does not dissolve into water any more.

Step 3!
Set onto hot plate (heater) and let heat until starting to bubble.

Step 4!
Remove with tongs and set to cool overnight. VERY HOT!

Step 5!
Next day! Collect a larger one of the crystals formed on the bottom of the glass and tied at the end of string. Tie to middle of a stick long enough to reach across the top of another small beaker.

Step 6!
Repeat steps 1, 2, and 3.
-As heating, add little Aluminum Potassium Sulfate, but take caution, as the mixture WILL bubble over-QUICKLY.

Step 7!
Once sick of watching heat and adding Aluminum Potassium Sulfate, remove beaker from hot plate and set to cool.

Step 8!
Once cooled, add stick with string to top of beaker and let tied crystal rest in mixture. Leave overnight!

Step 9!
Come back next day and repeat steps to change out water and repeat process.

Step 10!
Crystals should be starting to form! Be very proud!!
(ours didn't...we weren't...)