The crystal violet (CV) reaction with NaOH is great for measuring kinetics. The reaction proceeds fast enough that we can do an experiment multiple times in one lab period, and yet slow enough to get multiple data points as the reaction progresses. In addition: CV is a bright purple color that converts into a clear, colorless product when reacted with hydroxide ions. Thus, the reaction can be monitored spectrophotometrically.
The concentrations used for this experiment are very dilute, thus it is very important to keep in mind the following:
- Do not let the CV come into contact with any rubber or plastic, as it will initiate decomposition.
- Rinse all vials and glassware with RO water thoroughly.
- Rinse any vials or beakers to be used to contain a standard with the prepared solution to rinse out any RO residue.
- Glassware that will be used to dilute with RO water do not need to be dried (so long as any liquid left in the containers after rinsing is just RO water).
- Be very careful not to cross-contaminate any samples and to be very careful with your pipetting!
Generating a Beer's Law Plot
Appendix III in the Lab Manual has a section on Beer's Law Analysis and on Analysis of MicroLab Data using Excel. Please read carefully through these pages before coming to lab.
Be very careful to pay attention to the usage of the words "Standard," "Stock," and "Solution" - these each have different meanings!
A series of dilutions will be made by pipetting from a stock solution into multiple volumetric flasks. Obtain stock CV solution from the front of the room in a clean, dry 125 mL Erlenmeyer flask (shown below with the pipet used to obtain the stock solution).
Dilutions of CV into 100 mL volumetric flasks along with the stock CV solution in the Erlenmeyer at top. You can avoid cleaning multiple volumetric flasks by simply using one volumetric flask and then just make your solutions in an increasing order of concentration. The dilute CV residue won't effect the concentrations of the more concentrated solutions and you can store the solutions in different labeled beakers.
The solutions are made by pipetting various amounts of the stock CV solution into the volumetric flasks before diluting to the mark with RO water. Be sure to use a transfer (plastic) pipet dropper to get just to the mark on the volumetric glassware. When pipetting, it is best to use a TD Graduated pipette. A selection of pipettes are shown below. Notice the difference between the TD and TC pipettes: the former has graduations past the "9" toward the tip, whereas the latter stops at the "10" with no markings along the tip. The TD pipette has been calibrated To Deliver the amount all the way to the tip. The numbers are typically in an order opposite what you would expect. To deliver 2 mL, you would fill to the 8 mL mark and dispense to the tip. The calibration takes into account the small amount of liquid left in the tip once it has emptied. Do not blow this out using a bulb.
Click to enlarge.
Once you have inverted the volumetric flask 3-4 times, rinse out a clean, dry beaker with a few mL of the prepared solution and discard the rinse. Then fill the beaker with the remaining prepared CV solution. You do not need to wash the volumetric flask between making various standard solutions, although a quick rinse with RO water is recommended. Make the standards in order from most dilute to most concentrated. This ensures that any residue left in the volumetric flask will not affect the stronger concentrations being made (the reverse is more likely to contaminate more dilute standards).
Prepare the vials to be used in the spectrophotometer (MicroLab) by rinsing them with RO water, then rinsing them with the solution to be contained 3-4 times. Finally, fill the vial with the standard solution.
Follow the instructions in the manual on how to generate a Beer's Law curve in MicroLab beginning with taking a blank reading of RO water (shown below).
Sample of a Beer's Law plot at 590 nm. You should select the highest absorbance wavelength for your Beer's Law plot.
Performing a Kinetics Experiment
The instructions for performing a MicroLab Kinetics Experiment are detailed in Appendix III. Begin by selecting the "Spectro-Kinetics" experiment from the initial MicroLab screen. There will be two experiments performed: one with 10 mL of CV, and one with 5 ML of CV. In both cases, the dilutions will be done by pipetting the CV from the stock solution. Be sure that you are monitoring the reaction at the same wavelength that you selected for your Beer's Law plot.