4.3: Procedures

Experimental Procedures

There are four parts to this experiment.  First you will use the technique of dilutions to make a total of 5 solutions (stock plus 4 diluted solutions).  Then you will take a spectrum of your stock and choose a wavelength for your Beer's Law plot.  You will then record the absorbance of all 5 solutions and make a Beer's law plot that has 6 data points (when you calibrated the instrument you set A=0 for the pure solvent, the point 0,0 is a value on your graph).

Part I: Dilutions

1. Obtain stock solution of CV⁺ \(2.0x10^{-5}\)M  (Note that this is half the concentration we will use in Part IV)
2. Using a burette transfer 5 mL of \(2.0x10^{-5}\)M CV⁺ to a 25mL volumetric flask
3. Fill the flask a bit over half way to mark with deionized water and swirl to mix all of the solution. Continue filling to mark, using an eye dropper to add the last few drops.  Be sure all the solute is fully mixed. Pour solution into a clean 50mL beaker.
4. Fill cuvette 3/4ths full with solution and label the cap with number 1
5. Repeat steps 2-4 for volumes of 10mL, 15mL, and 20mL using a new cuvette and labeling each cuvette 2,3,4
6. Fill a cuvette with stock \(2.0x10^{-5}\)M CV⁺ and label cap 5

Part II: Spectrum

1. Calibrate the spectrometer 
   • Warm spectrometer for 5 minutes
   • Fill cuvette 3/4ths full of solvent (water), cap and label the cap 0 (zero)
   • Place in cuvette in the cavity so the light path goes through the clear side
   • From Experiment menu choose Spectrophotometer/calibrate
   • Follow the instructions until the calibration is OK.
   • Keep this "blank" solution in the cuvette until the experiment is over, as you may need to recalibrate the spectrometer
2. Generate a spectrum
   • After calibrating spectrometer place stock solution (cuvette #5) into cuvette cavity
   • Click <Collect> and once the spectrum is displayed click <Stop> 
   • Choose a wavelength for Beer's Law plot where A=1 for the stock solution, write this down in your data sheet
   • Up on the top menu select File>Save. Enter name and save
   • To Export: Plug in flash drive, File>Export. Enter name and save. Make sure to share file with all group members

Part III: Beer's Law plot

1. Calibrate the spectrometer if needed (you can read the absorbance of the blank (solvent), if it is zero at the wavelength you are measuring you do not need to recalibrate.
2. Place each cuvette into the spectrometer and read the absorbance at the chosen wavelength (where A=1 for stock).
   • Record values in data sheet
   • You should have 6 values (5 for each of the solutions, and the blank, which should read 0)

Part IV: Transient Kinetics

1. Calibrate the spectrometer if needed
2. From the meter tab click on "Mode:" and select "Time Based"
3. Set Interval to 5 seconds and Duration to 800s
4. Use a graduated cylinder to measure 5mL of \(4.0x10^{-5}\)M CV⁺ (Double check label! this conc is different from Beer's Law plot)
5. Use another graduated cylinder to measure 5mL of 0.100 M NaOH(aq)
6. Make sure Lab Quest is ready to take measurements.
7. Pour both solutions into a clean beaker and swirl to mix. Fill cuvette 3/4ths full with solution and place in spectrometer and press play. (You have about 15s from mixing)
8. If the initial absorbance is below 0.9 reset and try again.
9. When the absorbance is below 0.05 you can stop the labquest, save the data, then export the data to a flashdrive