How pH would affect the dative bonding

[Kushal]

Homework Statement

The concentration of Ca2+ in hard water can be determined by titration with a reagent X which forms a complex with Ca2+ giving a change in colour. Since the colour change is sensitive to pH, the titration has to be carried out in an alkaline buffer.

Th structure of X is similar to that of an amino acid. The way X acts as a ligand can be understood by considering how aminoethanoic acid forms co-ordinate bonds with a calcium ion.

Explain why the complex between aminoethanoic acid and aqueous calcium ions is more stable at pH 10 than at pH 4.

Homework Equations

The Attempt at a Solution

I really have no idea how pH would affect the dative bonding.

 

[LtStorm]

Well, as pH is based on how many hydrogen ions (and thus how few hydroxide ions) there are in the solution, I'd guess that hydroxide interferes in the bonding.

 

[chemisttree]

Remember that aminoethanoic acid is an amino acid. At pH 4 the amino functionality exists as an ammonium ion and the carboxylic acid exists largely as an acid (R-CO-OH).

Ligands are electron rich species and calcium in solution exists as a cation. Is an ammonium compound electron-rich? Is a carboxylic acid?

 

[Kushal]

bah no...

the ammonium compound is not electron rich because it has already accepted a proton... right?? (the nitrogen lone pair is used in dative bonding)

at pH 4 the dissociation of the organic acid is repressed. it cannot donate its proton and therefore is not electron rich.

but at pH 10, the carboxylic acid exists as COO-. and the lone pair on the nitrogen can coordinate with Ca2+.

is this correct?

 

[chemisttree]

Yep.

 

[Kushal]

nice, thnks!