Which statement best describes Kf' in the EDTA system?

Which statement best describes Kf' in the EDTA system?

• A. Kf' is the rate constant for complex formation
• B. Kf' is the adjusted formation constant accounting for the deprotonation state of EDTA at working pH
• C. Kf' is the degradation constant for the ML complex
• D. Kf' is the partition coefficient of the complex

Answer: (B)

Kf' reflects the conditional or apparent formation constant for the metal-EDTA complex at a specific pH. EDTA can only bind a metal efficiently when it is in its fully deprotonated form, but at any given working pH only a fraction of EDTA exists in that binding form. Kf' incorporates that fraction, making the observed stability of the ML complex depend on the protonation state of EDTA. In other words, it’s the formation constant adjusted for how much EDTA is available to bind at the current pH. This is why the statement describing Kf' as the adjusted formation constant accounting for EDTA’s deprotonation state at working pH is the best choice. It’s not a rate constant, a degradation constant, or a partition coefficient.

Kf' reflects the conditional or apparent formation constant for the metal-EDTA complex at a specific pH. EDTA can only bind a metal efficiently when it is in its fully deprotonated form, but at any given working pH only a fraction of EDTA exists in that binding form. Kf' incorporates that fraction, making the observed stability of the ML complex depend on the protonation state of EDTA. In other words, it’s the formation constant adjusted for how much EDTA is available to bind at the current pH. This is why the statement describing Kf' as the adjusted formation constant accounting for EDTA’s deprotonation state at working pH is the best choice. It’s not a rate constant, a degradation constant, or a partition coefficient.