PROCEDURES

You will be working in teams of two as assigned by your instructor. Each student is required to submit a complete lab report, including an original graph (no photocopies).

1. Obtain and wear goggles.

2. Obtain about 30 mL of 0.400 M NiSO₄ stock solution in a 100-mL beaker; cover with a watch glass to prevent evaporation. Add about 30 mL of distilled water to another 100-mL beaker.

3. Label four clean, dry, 50 mL beakers 1-4 (the fifth solution is the beaker of 0.400 M NiSO₄). Pipette 2, 4, 6, and 8 mL of 0.400 M NiSO₄ solution into Beakers 1-4, respectively. With a second pipette, deliver 8, 6, 4, and 2 mL of distilled water into Beakers 1-4, respectively.

NOTE: read all volumes to hundredths of an mL. Thoroughly mix each solution with a stirring rod. Clean and dry the stirring rod between stirrings. Keep the remaining 0.400 M NiSO₄ in the 100-mL beaker to use in the fifth trial. Approximate volumes and concentrations for the trials are summarized below. Actual values of volume should be recorded on the data entry sheet, Page 12-9; these will then be used to calculate the actual concentrations needed for Page 12-11.

Trial Number	0.400 M NiSO4 (mL)	H2O (mL)	Concentration (M)
1	2	8	0.0800
2	4	6	0.160
3	6	4	0.240
4	8	2	0.320
5	~10	0	0.400

CALIBRATION OF THE COLORIMETER

Choose "CALIBRATE" from the "EXPERIMENT" menu and then click "PERFORM NOW." 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. When “INPUT 1” in the column labeled “READING 1” stabilizes, click the "KEEP" button.

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. When “INPUT 1” in the column labeled “READING 2” stabilizes, click the "KEEP" button. Click on the "OK" button. Leave the colorimeter set to the Red LED position for the remainder of your measurements.

PREPARATION OF CALIBRATION CURVE FROM STANDARD SOLUTIONS

6. You are now ready to collect absorbance data for the five standard solutions. Carefully pour each of your 4 diluted solutions, plus the standard solution, into 5 separate clean, dry cuvettes. Wipe the optical surfaces clean with a Kimwipe. Click “COLLECT” from the “EXPERIMENT” menu. Place the first cuvette in the colorimeter. After closing the lid, wait for the absorbance displayed in the “live-display” window to stabilize. Click on "KEEP" (just above the live display of absorbance). The box will clear and you should enter “0.0800” (or the concentration of your particular solution) in the box. Then, press the enter key on the computer keyboard. The data pair you just collected should now be plotted on the graph and entered in the data table.

7. Repeat this process for each of your remaining solutions. If you need to repeat readings, you should click on “STOP” and then “Delete Run _ latest” under the “DATA” menu. This will allow you to retain your calibration, which you will lose if you choose to quit or close the program.

8. When you have finished reading and entering all five of your solutions, click on "STOP" under the “EXPERIMENT” menu.

9. Copy the absorbance and concentration data pairs from the Data Table on the screen, into the Data and Calculations Table of your report.

10. Examine the graph of absorbance versus concentration presented on your computer screen. Does it represent a direct relationship between absorbance and concentration? That is, does increased concentration correspond to higher absorbance? Is the relationship linear, as predicted by Beer’s law? Test the linearity of your data by selecting “LINEAR FIT” from the “ANALYZE” menu.

DETERMINATION OF UNKNOWN

11. Keep your experimental data, including the line from the curve fit, displayed on your computer screen. Obtain an unknown from your instructor. The unknown will be NiSO₄, just like the solutions you have been using, except that its concentration is unknown. Record the number of the unknown in the Data and Calculations Table. Fill a clean, dry cuvette about 3/4 full with unknown. Wipe the outside of the cuvette, place it into the colorimeter, and close the lid. The live display will give you the absorbance of the unknown. When the displayed absorbance value stabilizes, record its value in Trial 6 of the Data and Calculations Table.

12. Click “INTERPOLATE” from the “ANALYZE” menu. A vertical cursor will appear on the screen. As you move the cursor back and forth across the screen with your mouse, you will see a small “zero” moving up and down your graph. Look in the information box connected to your graph. You will see changing values. Adjust the position of the cursor until you see the value of the y-axis equal to the value of the absorbance of your unknown. The corresponding value of the x-axis is the concentration of your unknown. Write this value into the appropriate space in your Data and Calculation Table. Leave the “INTERPOLATE” cursor on the screen so it is a part of the graph you print.

13. Select “PRINT” from the “FILE” menu. Enter your name and your partner’s name. You may also enter comments. You need to print two copies of the graph, one for each partner. Several parts of your graph will need to be labeled: the calibration line prepared with known concentrations; the interpolation line; and the value of the unknown’s concentration.