What is the optimum pH range for EDTA when used for oxidative enzyme stains?

• A. pH 2-3.
• B. pH 7-7.4.
• C. pH 9-10.
• D. pH 11-12.

Answer: (B)

The pH that optimizes EDTA’s role in oxidative enzyme staining is the neutral range because this balances effective metal chelation with preserving enzyme activity. EDTA works by binding divalent metal ions (like Ca2+ or Fe2+) that can interfere with the enzyme reaction or contribute to non-specific staining. For EDTA to chelate well, it needs to be in a deprotonated form, which occurs near neutral pH. At around pH 7–7.4, EDTA is sufficiently deprotonated to bind metals, while oxidative enzymes (such as peroxidases used in these stains) remain active and the substrate remains stable.

If the environment were too acidic, chelation would be less effective and enzyme activity could be diminished. If it were too alkaline, enzyme activity or substrate stability could be compromised and non-specific staining could increase. So, pH 7–7.4 provides the best compromise for optimal EDTA performance in oxidative enzyme staining.