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Homework Help: How pH would affect the dative bonding

  1. Apr 14, 2008 #1
    1. The problem statement, all variables and given/known data

    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.

    2. Relevant equations



    3. The attempt at a solution

    I really have no idea how pH would affect the dative bonding.
     
  2. jcsd
  3. Apr 14, 2008 #2
    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.
     
  4. Apr 14, 2008 #3

    chemisttree

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    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?
     
  5. Apr 14, 2008 #4
    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?
     
  6. Apr 14, 2008 #5

    chemisttree

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    Yep.
     
  7. Apr 14, 2008 #6
    nice, thnks!!!
     
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