Crystal Violet Bleaching

In this experiment, you will observe the reaction between crystal violet and sodium hydroxide. One objective is to study the relationship between concentration of crystal violet and the time elapsed during the reaction. The equation for the reaction is shown here:

A simplified (and less intimidating!) version of the equation is:

The rate law for this reaction is in the form: rate = k[CV⁺]^m[OH^-]ⁿ, where k is the rate constant for the reaction, m is the order with respect to crystal violet (CV⁺), and n is the order with respect to the hydroxide ion. Since the hydroxide ion concentration is more than 5000 times as large as the concentration of crystal violet, [OH^-] will not change appreciably during this experiment. Thus, you will find the order with respect to crystal violet (m), but not the order with respect to hydroxide (n).

As the reaction proceeds, a violet-colored reactant will be slowly changing to a colorless product. Using the green (565 nm) light source of a colorimeter, you will monitor the absorbance of the crystal violet solution with time. We will assume that absorbance is proportional to the concentration of crystal violet (Beer's law). Absorbance will be used in place of concentration in plotting the following three graphs:

• Absorbance vs. time: A linear plot indicates a zero order reaction (k = -slope).
• ln Absorbance vs. time: A linear plot indicates a first order reaction (k = -slope).
• 1/Absorbance vs. time: A linear plot indicates a second order reaction (k = slope).

Once the order with respect to crystal violet has been determined, you will also be finding the rate constant, k, and the half-life for this reaction.

MATERIALS

CBL System
0.10 M NaOH
TI-8X Graphing Calculator
2.0 X 10^-5 M crystal violet
Vernier Colorimeter
distilled water
Vernier adapter cable
stirring rod
TI-Graph Link
two 10-mL graduated cylinders
100-mL beaker
one plastic cuvette

PROCEDURE

1. Obtain and wear goggles.

2. Use a 10-mL graduated cylinder to obtain 10.0 mL of 0.10 M NaOH solution. Use another 10-mL graduated cylinder to obtain 10.0 mL of 2.0 X 10^-5 M crystal violet solution.

3. Plug the colorimeter into the adapter cable in Channel 1 of the CBL System. Connect the CBL System to the TI-8X calculator with the link cable using the port on the bottom edge of each unit. Firmly press in the cable ends.

4. Turn on the CBL unit and the TI-8X calculator. Press PRGM and select CHEMBIO. Press ENTER, then press ENTER again to go to the CHEM MAIN MENU.

5. Set up the calculator and CBL for the colorimeter.

• Select SET UP PROBES from the CHEM MAIN MENU.
• Enter "1" as the number of probes.
• Select COLORIMETER from the SELECT PROBE menu.
• Enter "1" as the channel number.
• PERFORM a NEW CALIBRATION.

6. You are now ready to calibrate the colorimeter. First prepare a blank by filling a cuvette 3/4 full with distilled water. To calibrate the cuvette at 0% and 100% transmittance:

• 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 the voltage reading displayed on the CBL screen stabilizes, press TRIGGER on the CBL and enter "0" (the "reference") in the TI-8X calculator.
• Turn the wavelength knob of the colorimeter to the Green LED position (565 nm). In this position, the colorimeter is calibrated to show 100% of the green light being transmitted through the blank cuvette. When the displayed voltage reading stabilizes, press TRIGGER and enter "100" in the TI-8X calculator. Leave the wavelength knob of the colorimeter set to the Green LED position for the remainder of the experiment.
• Press ENTER to return to the CHEM MAIN MENU.

7. Set up the calculator and CBL for data collection.

• Select 2:COLLECT DATA from the CHEM MAIN MENU.
• Select 2:TIME GRAPH from the DATA COLLECTION menu.
• Enter "2" as the time between samples, in seconds.
• Enter "90" as the number of samples (the CBL will collect data for a total of 3 minutes).

8. You are now ready to begin monitoring data. To initiate the reaction, simultaneously pour the 10-mL portions of crystal violet and sodium hydroxide into a 100-mL beaker and stir the reaction mixture with a stirring rod. Empty the water from the cuvette. Rinse the cuvette with ~1-mL of the reaction mixture and then fill it 3/4 full. Place the cuvette in the cuvette slot of the colorimeter and close the lid. Monitor the percent transmittance reading on the CBL for about 10 seconds (the percent transmittance reading should be gradually increasing). Then press ENTER to begin collecting data. During the 3-minute data collection, observe the solution in the beaker as it continues to react. When data collection stops after 3 minutes ("DONE" appears on the CBL screen), press ENTER to display a graph of absorbance vs. time. Discard the contents of the beaker and cuvette as directed by your teacher.

9. Analyze the data graphically to decide if the reaction is zero, first, or second order with respect to crystal violet.

• Zero Order: If the current graph of absorbance vs. time is linear, the reaction is zero order.
• First Order: To see if the reaction is first order, it is necessary to plot a graph of the natural logarithm of absorbance vs. time. First select QUIT from the CHEM MAIN MENU to leave the CHEM program Then take the natural logarithm (ln) of the absorbance values and store them in another list. Plot a graph of ln absorbance vs. time. If this plot is linear, the reaction is first order.
• Second Order: To see if the reaction is second order, plot a graph of the reciprocal of absorbance vs. time. Take the reciprocal (x^-1) of the absorbance values and store them in another list. Plot a graph of reciprocal of absorbance vs. time. If this plot is linear, the reaction is second order.

10. Calculate the linear regression equation for the linear plot in Step 9.

• Press STAT, RIGHT ARROW to display the CALC menu. Select LinReg(ax+b).
• To identify the lists that correspond to the two variables, press [2nd],[L1], [,],[2nd],[Ln], ENTER where Ln is the list (L 2, L 3, or L4) that gave a linear plot in Step 9. The statistics are displayed for the equation in the form:

  y = ax + b

  Record the value of the slope, a, to use in the calculation of the rate constant, k, in Processing the Data.

11. To display a best-fit regression line on the linear plot:

• Press [2nd][STAT PLOT], then select 1:Plot1. Set Xlist to L1 and the mark type to +. Then set Ylist to L 2, L 3, or L 4 (use the one that gave a linear plot in Step 9).
• Press Y=. Press CLEAR to clear the Y1= equation, then press VARS. Select 5:Statistics, then press RIGHT ARROW, RIGHT ARROW to display the EQ menu. Select 7:RegEQ to copy the linear regression equation to Y1=.
• Press ZOOM, then select ZoomStat.

PROCESSING THE DATA

1. Was the reaction zero, first, or second order, with respect to the concentration of crystal violet? Explain.

2. Calculate the rate constant, k, using the slope of the linear regression line for your linear curve (k = -slope for zero and first order and k = slope for second order). Be sure to include correct units for the rate constant. Note: This constant is sometimes referred to as the pseudo rate constant, because it does not take into account the effect of the other reactant, OH^-.

3. Write the correct rate law expression for the reaction, in terms of crystal violet (omit OH^-).

4. Using the printed data table, estimate the half-life of the reaction; select two points, one with an absorbance value that is about half of the other absorbance value. The time it takes the absorbance (or concentration) to be halved is known the half-life for the reaction. (As an alternative, you may choose to calculate the half-life from the rate constant, k, using the appropriate concentration-time formula.)