[thetada]

Hello all,

I am writing an analytical chemistry question for my A-level chemistry students. I would like them to suggest why EDTA forms more stable complexes with Ca2+ ions than does Patton-Reeder's indicator. I've found it extremely difficult to find any information about how PR forms complexes, although I did find that it can form 5 or 6 membered chelation rings. EDTA has no aromatic rings and the only double bonds are those in the carboxylic acid groups so I assume it's more flexible and therefore, able to get the coordination sites closer to the metal ion than PR, which has four aromatic rings, joined in the middle by an azo group. Since all the coordination sites on PR are effectively on one side of the compound, I'm wondering if it's a simple case of steric hindrance, ie that PR can not arrange itself around the Ca2+ ion so that the average bond length is as short as those exhibited in the EDTA complex. What does anyone else think?

I'd be really grateful for any responses.