KINETICS OF CRYSTAL VIOLET AND HYDROXIDE

Introduction

Crystal violet is a common, beautiful purple dye. In strongly basic solutions, the bright color of the dye slowly fades and the solution becomes colorless. The kinetics of this "fading" reaction can be analyzed by measuring the color intensity or "absorbance" of the solution versus time to determine the rate law.

Background Information

Crystal violet is used to dye paper and is a component of blue and black inks for inkjet printers and ballpoint pens. It is also added to consumer products like fertilizers, detergents, and leather to turn these items a blue or purple color. Crystal violet is also used as a biological stain. The process of Gram staining to classify bacteria exposes bacteria to crystal violet. Depending the type of bacteria, the stain containing crystal violet will cause the bacteria to turn one color or another. Crystal violet can be used is DNA gel electrophoresis and fingerprinting. Crystal violet stain has some medicinal uses, as it has antibacterial and anti-fungal properties.

Chemistry of Crystal Violet

Crystal violet belongs to a class of intensely colored organic compounds called triphenylmethane dyes. The structure and color of crystal violet depend on pH, making it a valuable acid-base indicator as well as an excellent dye. Crystal violet absorbs electromagnetic radiation (light) in the ultraviolet and visible light range with an optimal absorbance around 590nm.

Looking at the chemical structure (below) we can see what parts of the molecule are involved with absorbing light and reflecting the wavelength that we see. The auxochrome group (labeled in green) consists of the central carbon connecting the three aromatic carbon rings (labeled in gold). This central carbon atom can be ionized which helps this molecule dissolve in water. 

The chromophore groups are responsible for reflecting light and give the molecule its characteristic color. The chromophore groups contain carbon rings that have alternating single & double bonds in these Lewis Dot Structures. Modifying groups (labeled in purple) are adjacent to the chromophores and in this case are responsible for the purple color we see. In other similar molecules, if some the CH3 ("methyl") groups were removed, the color we would observe would change.

The major structural form of crystal violet is the monovalent (+1 charge) cation abbreviated, CV+, which is shown in Figure 1a. CV+ is the predominant form of crystal violet in the solid state and in aqueous solution across a broad range of pH values from pH 1 to 13. The positive charge shown on the central carbon atom in figure 1a is delocalized via resonance to the three nitrogen atoms. Figure 1b, 1c, and 1d shows the three resonance structures with the positive charge on a nitrogen atom. 

Looking at the Lewis structures one can see that there are multiple carbon rings that have alternating single and double bonds. These are called aromatic rings. These aromatic rings the p-orbitals of all carbon atoms bond together to form a large delocalized orbital. In this delocalized orbital, electrons from all 6 carbons can move freely around the orbital. This delocalized orbital makes an aromatic ring extremely stable. 

Delocalized of the charge across the system of double bonds in the benzene rings stabilizes the carbocation (see Figure 1a) and is responsible for the vibrant purple color of the dye. Additional information on other dyes and their chemical structures can be found at the Stainsfile website.

In strongly basic solutions, the purple CV+ cation slowly combines with hydroxide ions to form a neutral product, CVOH, which is colorless (see Figure 2). The rate of this reaction (Equation 1) is slower than typical acid-base proton transfer reactions and depends and depends on the initial concentration of both crystal violet and hydroxide ions.

Equation 1                                         CV+   +  OH-  → CVOH

                                                         purple                 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

Equation 2                                         Rate = k [CV+]n [OH-]m

The exponents n and m are defined as the order of reaction for each reactant and k is the rate constant for the reaction at a particular temperature.  The values of the exponents n and m must be determined by experiment.  If the reaction is carried out where we used so much extra NaOH Equation 2 will reduce to the form

Equation 3                                        Rate =  k' [CV+]n

Equation 4                   where               k' = k [OH-]m

Using an much more of one reactant in a reaction is called "swamping" because the system is "swamped" with excess OH- ions. The system has so much extra OH- ions that the concentration of OH- ion essentially doesn't change.  The amount of CV+ ions does significantly drop as the reaction proceeds. So the constant k' is a new "psuedo" rate constant created to incorporate both the "true" rate constant k and the [OH-]m term (since that is essentially constant also).  Equation 3 is called the psuedo-rate law because it is really a simplification of the actual rate law  (Equation 2).

Again, this psuedo-rate law is only valid when the concentration of OH- ions is much greater than concentration of CV+ ions.   

Recall that 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.

Equation 5                                     A =  ε b c 

where A is absorbance, ε is the molar absorptivity coefficient, b is the path length in cm, corresponding to the distance light travels through the solution, and c is the concentration of the solution.  Beer's law provides the basis of using spectroscopy in quantitative analysis.  Using this relationship, concentration and absorbance may be calculated if one variable is known while keeping ε and b constant.  This relationship is also 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.