UV-Vis Experiment 4: Beer-Lambert Law and the Crystal Violet-Sodium Hydroxide Reaction

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Background

Crystal Violet, CV, (structure below) is an intensely coloured dye. Reaction with NaOH results in a colourless product.

CV structure

CV structure

 

The absorbance of a reaction mixture containing CV and NaOH will be proportional to the concentration of unreacted dye still present in solution. The reaction can therefore be monitored and the kinetics studied by measuring the absorbance of the mixture as a function of time.

Kinetic Rate Law

The rate of the reaction between CV and NaOH can be written in a general form which is known as the rate law:

Rate = k [OH-] x [CV] y          Equation 1

where k is the rate constant (or rate coefficient) for the reaction and x is the order with respect to CV and y is the order with respect to OH-. In this experiment, the value of x and y will be determined experimentally. In order to do this the reaction (and the kinetics) must be simplified by using a vast excess of NaOH: it can therefore be assumed that the concentration of hydroxide ion does not change through the course of the reaction even though all of the CV may have been used up. Consequently, the [OH-] term in the general rate law is constant and can be grouped with k as in equation 2:

Rate = {k [OH-] x} [CV] y          Equation 2

where k’ = k [OH-] x (k’ is termed a pseudo rate constant or the observed rate constant.)
The rate equation can therefore be written:

Rate = k’ [CV] y          Equation 3

This approximation simplifies the reaction and allows k’ and y to be determined experimentally by carrying out two reactions: in the first kinetic run the concentration of NaOH is half that used in the second experiment. For both reactions the [OH-] is vastly in excess of that of CV.

Once k’ has been derived it is possible to determine k and x since the concentration of NaOH is known in both experiments.

Experiment 4, the  CV/NaOH reaction

The Beer-Lambert Law is used in the first part of the experiment which relates the absorbance to the concentration of the sample. Students will make up a series of solutions of CV in water using the virtual flask.

Sim4Web: The Virtual Flask

Sim4Web: The Virtual Flask

The absorbance spectra of the virtual solutions will be recorded using the spectrometer simulator:

UV-Vis Spectrometer Simulator

UV-Vis Spectrometer Simulator

 

A Beer-Lambert plot will be constructed in order to derive the molar extinction coefficient for CV. This information can then be used to determine the concentration of CV remaining in the reaction as a function of time in the second part of the experiment.

 

The CV/NaOH reaction will be performed in the virtual laboratory at two different NaOH concentrations in part two of the experiment.

 

Apparatus used in this experiment: Graduated flasks and various pipettes; timer; 1cm path length sample cell; pipette to release 9 cm3 of CV; pipette to release 1 cm3 of NaOH to start the reaction; absorption spectrometer simulator

 

The screen captures below show the experiment being performed:

Pipette CV into beaker

Pipette CV into beaker

 

 

A sample of the CV/NaOH reaction mixture is extracted from the reaction

 

Expt 4 Extract Sample

Extract a sample from the reaction mixture

 

and transferred to a cuvette.

Transfer to a cuvette

Transfer to a cuvette

 

The cuvette is then loaded into the spectrometer and absorbance readings taken as a function of time:

Start of the reaction

Start of the reaction

 

Reaction after 15 minutes

Reaction after 15 minutes

 

This allows the concentration of CV to be monitored as a function of time and for kinetic information about the reaction to be derived.