Question

The initial reduction of resazurin dye to resorufin in milk takes place at an oxidation-reduction potential of

• A. +0.3 V to +0.4 V
• B. +0.2 V to +0.05 V
• C. +0.04 V to +0.01 V
• D. -0.01 V to -0.06 V

Hint:

Resazurin Reduction Redox Potentials:
Blue Resazurin $\rightarrow$ Pink Resorufin = $\mathbf{+0.20\text{ V to } +0.05\text{ V}}$ (Irreversible).
Pink Resorufin $\rightarrow$ Colorless = $+0.04\text{ V to } -0.06\text{ V}$ (Reversible).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Electrochemical oxidation-reduction potential ($E_h$) kinetics of resazurin dye reduction: the first irreversible reduction from blue resazurin to pink resorufin occurs as $E_h$ drops from $+0.20 ext{ V}$ to $+0.05 ext{ V}$, followed by reversible reduction to colorless dihydroresorufin below $0.00 ext{ V}$.
Key Formula or Approach:
\[ \underbrace{\text{Resazurin (Blue)} \xrightarrow[E_h = +0.20\text{V to } +0.05\text{V}]{\text{Irreversible Reduction}} \mathbf{Resorufin \text{ (Pink)}}}_{\text{Step 1}} \xrightarrow[E_h < 0.00\text{V}]{\text{Reversible}} \text{Dihydroresorufin (Colorless)} \]

Step 2: Detailed Explanation:
In the electrochemistry and kinetics of the Resazurin Reduction Test in milk:
- Fresh aerated raw milk has a high positive oxidation-reduction potential ($E_h = +0.25\text{ to } +0.35\text{ V}$).
- As actively metabolizing bacteria consume dissolved oxygen and donate metabolic electrons, the redox potential steadily falls:
1. First Phase (Irreversible Reduction) (B): Blue Resazurin is irreversibly reduced to Pink Resorufin as the system potential drops through the range of +0.20 V to +0.05 V (+0.2 V to +0.05 V).
2. Second Phase (Reversible Reduction): Pink Resorufin is reversibly reduced to completely Colorless Dihydroresorufin (Hydroresorufin) when potential falls further between $+0.04\text{ V to -0.06\text{ V}$}.

Step 3: Final Answer:
Therefore, initial reduction takes place at +0.2 V to +0.05 V, corresponding to option (B).