Advanced Placement Chemistry
An Investigation of Crystal Violet Kinetics
Laboratory Exercise
Background
Crystal violet belongs to a class of intensely colored organic compounds called triphenylmethane dyes. The structure and
color of crystal violet is dependent on pH, making it a valuable acid−base indicator as well as an excellent dye.
CV+ (Figure 1a) is the predominant form of crystal violet in the solid state and in aqueous solutions ranging from pH 1-13.
The positive charge shown on the central carbon atom in Figure 1a is delocalized via resonance to the three nitrogen
atoms (Figure 1b). Delocalization of the charge across the system of double bonds is responsible for the vibrant purple
color of the dye.
+
+
In strongly basic solutions, the purple CV+ cation slowly combines with hydroxide ions to form a neutral product, CVOH,
which is colorless (Figure 2).
The rate of this reaction (Equation 1) is slower than typical acid–base proton transfer reactions and depends on the initial
concentration of both crystal violet and hydroxide ions.
CV+ + OH– → CVOH
Equation 1
Purple Colorless Colorless
Exactly how much the rate changes as the reactant concentration is varied depends on the rate law for the reaction. In the
case of the reaction of CV+ with OH– ion, the rate law has the general form:
rate = k [CV+]n[OH–]m
Equation 2
Many substances absorb or emit light at specific wavelengths of the electromagnetic spectrum. Absorbance for a specific
concentration of a solution with a fixed path length varies directly with the absorptivity coefficient of the solution. This
relationship is known as Beer’s law
A = abc
Equation 3
A - absorbance
a - molar absorptivity coefficient for a particular wavelength (sometimes called the extinction coefficient)
b - path length in cm (the distance light travels through the solution)
c - concentration of the solution.
Beer’s law provides the basis of using spectroscopy in quantitative analysis. Since a and b are mathematical constants, c
can be calculated if A is measured. This relationship is extremely valuable in kinetics experiments, making it possible to
follow the rate of disappearance of a colored substance by measuring its absorbance as a function of time.
1cm wide
cuvette
Internal Components of
a UV/Vis Spectrometer
Xenon
Lamp
Diffraction
Grating
Mirror
Detector
Figure 3.
There is a maximum absorbance value of
about 1.56 at 590 nm (Figure 4). This
wavelength would not be optimal because
the absorbance is too high and will yield
inaccurate results. Absorbance
measurements are most accurate and
sensitive in the range 0.2–1.0. Absorbance
values of 1.0 occur near both 540 nm and
610 nm. Either wavelength would be an
option for a Beer’s law plot because the
absorbance will be in an appropriate range
for both the stock solution and more dilute
solutions.
Figure 4.
Title: An Investigation of Crystal Violet Kinetics
Abstract: Student writes this after completing the rest of the laboratory report. Provide a brief report summary, not more
than one paragraph, containing less than 200 words. The abstract should address three points. The first point should
summarize the purpose and/or objective of the investigation. The second point summarizes the procedure. The third point
summarizes the results and conclusions.
Introduction: Crystal violet, C25N3H30Cl, is a common, purple dye. In strongly basic solutions, the bright color of the dye
slowly fades and the solution becomes colorless. The purpose of this investigation is to use spectroscopy and graphical
analysis to determine the rate law for the color-fading reaction of crystal violet with sodium hydroxide. The experiment
begins with constructing a calibration curve of absorbance versus concentration for crystal violet. A series of standard
solutions is prepared from a stock solution of crystal violet and the absorbance of each solution is measured at 540 nm. A
plot of absorbance versus time is used to determine the rate law and orders of reaction for the experiment.
Materials:
Crystal violet stock solution, 25 μM (2.5 × 10–5 M)
Sodium hydroxide solution, NaOH, 0.02 M
Water, distilled
6 Beakers, 50 mL
7 Cuvettes
Pipet pump
2 Pipets, 10 mL
UV/Vis Spectrometer
Stirring rod
Timers
Methods:
Constructing a Calibration Curve for Crystal Violet
In order to do a spectrophotometric analysis, the spectrometer must be calibrated to known concentrations of the material
being investigated. In order to do this, a “calibration curve” comparing absorbance to concentration is constructed. A
proper calibration curve should be a straight line with zero absorbance of light at zero concentration. If the line is not
straight, the instrument is not calibrated.
1. Turn on the spectrophotometer and allow it to warm up for 15–20 minutes before use.
2. Prepare the series of standard solutions of the crystal violet stock solution.
0.00 µM (blank)
2.50 µM
5.00 µM
7.50 µM
10.0 µM
12.5 µM
Water (mL)
10
9.0
8.0
7.0
6.0
5.0
Stock Solution (mL)
0
1.0
2.0
3.0
4.0
5.0
3. Fill six cuvettes approximately ¾ full with each standard solution.
4. Measure and record the absorbance of the stock solution and each standard solution at 540 nm wavelength.
Select the following in order:








Select “Test”
Select “Standard Curve”
Enter Name (last name of a group member)
Set 540nm wavelength
Set number of samples at 5
Select “Run Standards”
Enter concentrations of five standard solutions
Select “Measure Standards”
4. Prepare a Beer’s law calibration curve of absorbance versus concentration for crystal violet. Record the absorbances
on this table:
Concentration (μM)
Absorbance at 540nm
0 (blank)
0
2.5
5.0
7.5
10.0
12.5
Insert graph of concentration versus absorbance here.
Part A. Rate of Reaction of Crystal Violet with Sodium Hydroxide
1. Install flashdrive in UV/Vis spectrometer
2. Press “Test” button.
3. Choose “kinetics.”
4. Set: wavelength = 540nm
time = 20s
total run time = 15:00 (do not change any other settings)
5. Prepare a “blank” of equal volumes of distilled or deionized water and 0.02 M NaOH and insert in carousel
6 Measure 2.0 mL of 25 μm crystal violet into a clean 50 mL beaker.
7 Measure 2.0 mL of 0.02 M sodium hydroxide and add to the 50 mL beaker with crystal violet.
8. Swish beaker and pour solution into a cuvette.
9. Place cuvette with solution into UV/Vis spectrometer within 40s of mixing, close lid, and press “Run Test” and then
“Measure Sample.”
10. At completion of test select in order: “tabular,” “edit data,” and “save data.” Enter a group member’s name when
prompted. Select “Enter.”
Part B. Order of Reaction with Respect to Sodium Hydroxide
1. Install flashdrive in UV/Vis spectrometer
2. Press “Test” button.
3. Choose “kinetics.”
4. Set: wavelength = 540nm
time = 20s
total run time = 15:00 (do not change any other settings)
5. Prepare a “blank” of equal volumes of distilled or deionized water and 0.02 M NaOH and insert in carousel (you can use
the same blank from Part A).
6. Measure 2.0 mL of 0.02 M sodium hydroxide and 2.0 mL of distilled water into a clean 50 mL beaker to create a 0.01 M
sodium hydroxide solution.
7 Measure 4.0 mL of 25 μm crystal violet into the 50 mL beaker.
8. Swish beaker and pour solution into a cuvette.
9. Place cuvette with solution into UV/Vis spectrometer within 40s of mixing, close lid, and press “Run Test” and then
“Measure Sample.”
10. At completion of test select in order: “tabular,” “edit data,” and “save data.” Enter a group member’s name when
prompted. Select “Enter.”
Safety Precautions:
Dilute sodium hydroxide solution is irritating to eyes and skin. Crystal violet is a strong dye and will stain clothes and skin.
Clean up all spills immediately. Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron.
Avoid contact of all chemicals with eyes and skin and wash hands thoroughly with soap and water before leaving the
laboratory.
Results:
Provide a data table Part A experimental results.
Provide graphs of 1/[CV+] and ln[CV+] for Part A
Provide a data table Part B experimental results.
Provide graphs of 1/[CV+] and ln[CV+] for Part B
Discussion:
Report the order of reaction with respect to CV+ and the order of reaction with respect to OH-.
Discuss how you could have reduced the error in the experiment or improved on the methods used.
Conclusions:
Discuss the importance of your findings. Do not simply repeat the abstract, results, or discussion sections. This is your
opportunity to show the significance of your findings in paragraph form. Include the following when appropriate:
Suggest applications or further research.