I added equal amount of malic or maleic or citric acid to the calcium sulphate precipitation solution to investigate their performance as an inhibitor. Citric acid acts better than malic acid because citric acid can release 3 protons so can make better complexes with calcium ions than malic acid which only releases 2 protons. But I don't know why maleic acid inhibits precipitation better than citric acid even maleic acid can release just 2 protons. Is there anything special about maleic acid from its structural point of view?
I am not sure if I am right. My sugestion is that higher inhibiting efficacy of maleic acid /as compared to citric acid/ might dwell in simplerand symetric molecule of maleic acid, in its cis-form of existence, and automaticaly in the presence of double bond between carbons linked with carboxyl groups. Cis-form of maleic acid molecule, is more convenient for complexes formation from sterical reasons, and does not represents any hindrance for reactant {Ca 2 cation }. The posibility of malein acid orientation towards cation is better than in the case of more robust molecule of citric acid conaining 3 carboxylic and one -OH groups. Localized double bond of maleic acid might under special conditions also serve like nucleophil, especially in case of its dissociated molecules.
I am not sure if I am right. My sugestion is that higher inhibiting efficacy of maleic acid /as compared to citric acid/ might dwell in simplerand symetric molecule of maleic acid, in its cis-form of existence, and automaticaly in the presence of double bond between carbons linked with carboxyl groups. Cis-form of maleic acid molecule, is more convenient for complexes formation from sterical reasons, and does not represents any hindrance for reactant {Ca 2 cation }. The posibility of malein acid orientation towards cation is better than in the case of more robust molecule of citric acid conaining 3 carboxylic and one -OH groups. Localized double bond of maleic acid might under special conditions also serve like nucleophil, especially in case of its dissociated molecules.
It is all about degrees of freedom as Ratislav has commented about. The maleic acid has a double bond that essentially locks the two carboxylate groups into the proper orientation the chelate the Calcium. The citrate has more carboxylate groups, but they can rotate around a single bond. So the maleic acid is a far better chelator than the citrate. If it chelates the calcium, it reduces the amount of free calcium so it is less likely to precipitate.
The above two answers are right from my point of view. More or less they are speaking the same language. It is all about the position of the active sites in space.
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