Residual method titration with disodium edetate VS is applicable to what metal ion:

• A. Aluminum
• B. Calcium
• C. Zinc
• D. Magnesium

Answer: (A)

In complexometric titration with EDTA, the metal ion amount is determined by how strongly it binds EDTA. The residual titration approach uses more EDTA than needed to bind all ions, then back-titrates to see how much EDTA was consumed by the target ion. Aluminum stands out here because it forms an exceptionally stable, 1:1 complex with EDTA and remains soluble under the conditions used for this method. That makes the amount of EDTA that has complexed with aluminum easy to quantify from the residual EDTA, giving a reliable measurement.

Calcium and magnesium tend to behave differently under these conditions, often involving hydrolysis to form insoluble hydroxides at the pH used, which complicates residual back-titration. Zinc can also complex with EDTA, but the residual method is less straightforward for zinc in typical samples, so aluminum is the best fit for this approach.