In back titration, which statement best describes how the metal concentration is determined?

In back titration, which statement best describes how the metal concentration is determined?

• A. By measuring pH changes throughout the titration
• B. By comparing the volume of EDTA added before and after the endpoint
• C. By calibrating against a colorimetric endpoint
• D. By determining the metal concentration by comparing total EDTA to EDTA in excess

Answer: (D)

Back titration determines metal concentration by using more EDTA than needed to bind the metal and then measuring how much EDTA remains unreacted. The metal in the sample is present in a 1:1 relationship with EDTA, so the amount of EDTA that actually reacted with metal equals the amount of metal. In practice, you add EDTA in excess to the sample, allow all metal ions to form metal-EDTA complexes, and then titrate the leftover free EDTA with a standard solution (often a metal ion that binds EDTA). The endpoint tells you how much EDTA is left unreacted; subtracting that from the total EDTA added gives the moles of EDTA that bound metal, which, by the 1:1 stoichiometry, equals the moles of metal in the sample. Divide by the sample volume to obtain the metal concentration. This approach contrasts with methods that rely on pH changes or a colorimetric endpoint for the EDTA amount itself, and it aligns with the description of determining metal concentration by comparing total EDTA to EDTA in excess.

Back titration determines metal concentration by using more EDTA than needed to bind the metal and then measuring how much EDTA remains unreacted. The metal in the sample is present in a 1:1 relationship with EDTA, so the amount of EDTA that actually reacted with metal equals the amount of metal.

In practice, you add EDTA in excess to the sample, allow all metal ions to form metal-EDTA complexes, and then titrate the leftover free EDTA with a standard solution (often a metal ion that binds EDTA). The endpoint tells you how much EDTA is left unreacted; subtracting that from the total EDTA added gives the moles of EDTA that bound metal, which, by the 1:1 stoichiometry, equals the moles of metal in the sample. Divide by the sample volume to obtain the metal concentration.

This approach contrasts with methods that rely on pH changes or a colorimetric endpoint for the EDTA amount itself, and it aligns with the description of determining metal concentration by comparing total EDTA to EDTA in excess.