Biology – Respiration | e-Consult

Redox indicators are substances that change colour depending on the redox potential of the solution. They exist in two forms: a coloured form and a colourless form. The colour change occurs as a result of the equilibrium between these two forms, which is influenced by the redox potential of the solution.

DCPIP is a redox indicator that is blue in its reduced form and develops a pink/purple colour in its oxidized form. It's commonly used to detect the presence of reducing agents. The reaction involves the reduction of DCPIP to its radical anion, which then dimerizes to form the coloured product. The colour change is due to the loss of electrons (reduction) by the analyte, which causes the indicator to change its electronic structure and absorb light differently.

Methylene blue is another redox indicator, typically blue in its oxidized form and purple in its reduced form. It's often used to indicate the presence of oxidizing agents. The reduction of methylene blue involves the gain of electrons, leading to a change in its electronic configuration and a shift in its absorption spectrum, resulting in the colour change. The colour change is directly linked to the change in oxidation state of the analyte – reduction leads to a decrease in oxidation state, and oxidation leads to an increase.

In monitoring the rate of a redox reaction, the indicator is added to the reaction mixture. As the reaction proceeds, the concentration of the redox species changes, causing the indicator to change colour. The rate of colour change is directly proportional to the rate of the redox reaction. A faster rate of reaction will lead to a quicker colour change.