Tools & Techniques Analytical science, Imaging

Chemical Reaction Monitoring with a Spectrometer

Reaction Monitoring with the QE Pro Spectrometer: Chemical Stoplight Reaction

The chemical stoplight reaction is a reversible oxidation reduction reaction featuring the redox indicator dye indigo carmine. During the reaction, the solution changes from green to red to yellow as the indicator dye is oxidized and then reduced when oxygen levels decrease. Vigorous mixing to reintroduce oxygen restarts the reaction. While the color change is the star of many classroom demonstrations and online videos, the change in absorbance during the reaction can be used to characterize the kinetics of the reaction. In this application note, we describe the use of the QE Pro spectrometer for measuring the change in absorbance during the chemical stoplight reaction. Enhanced spectrometer features including onboard buffering for data integrity are described.


The study of chemical kinetics provides important information on the rate and mechanism of the chemical reactions that occur all around us -- from inside the cells of the human body to the ozone layer in the atmosphere. Characterizing the impact of parameters such as reactant concentration, temperature, pH and the presence of a catalyst are vital to optimizing reaction conditions and understanding the mechanism of the reaction. In an industrial or process setting, detailed knowledge of the chemical kinetics for a reaction enables the use of the optimum conditions and reactant concentrations to maximize product yield while minimizing reactant waste. In the human body, chemical kinetics measurements are made to characterize the impact of enzyme catalysts on metabolism and to understand the factors critical to the accurate dosing and release of a medication.

In this application note, the QE Pro is used to collect the absorbance data needed to characterize the chemical kinetics for a reversible oxidation reduction reaction featuring indigo carmine indicator dye. The redox (reduction oxidation) indicator dye indigo carmine exists in oxidized, reduced and intermediate forms depending on its environment. Each form has a slightly different chemical structure resulting in the absorption of different wavelengths of light. When indigo carmine is mixed with a reducing agent (dextrose) in a basic solution (NaOH), it acts an indicator of the state of the redox process. When the solution is first mixed by shaking the solution to introduce oxygen, the indicator is in its green oxidized form (most exposed to oxygen in the air). The reaction mixture color changes to red and then to yellow as the indigo carmine goes from an oxidized to a reduced state.

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