Titration Mg2+, Ca2+, and pH

name         Alex
status       student
grade        9-12
location     N/A

Question -   When testing for water hardness, we titrated EDTA
into a water sample to find the [Mg2+] and [Ca2+]. Why did the did
the solution have to be adjusted to a pH of 10 for the reaction to occur?
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Alex-
  A fine question.
    These chelating-complexing agents always have a 3-way decision to make:
    1) have a dangling negative charge (be ionized),
    2) bind with an H+ from the water-solution they are dissolved in,
    3) bind with a positive metal ion from the water-solution  they 
 are dissolved in.

If the water is very acidic that favors (2), to the exclusion of (1) 
of course,
but also to the exclusion of (3), binding to metal ions, which is 
what you wanted it for.
So the solution needs to be basic enough to go after the metal you 
intend to complex.

If the solution is too basic, the chelate ion (i.e. EDTA4-)
may go after other harder-to-bind metals you did not intend and 
would prefer to be ignored.
It also will be a fairly strong base in its own right.
Mg and Ca are about the hardest to bind, so your prescribed pH (10) 
is pretty strongly basic.
Na+, K+, and similar are too difficult; I don't think you can get 
them with EDTA at any pH.
Zn+2  is easier, and still easier are {Cu+2, Fe+2, Fe+3} I think.
You could probably titrate for dissolved iron in hard water, 
ignoring the Mg and Ca,
by setting pH nearer neutral, maybe pH=~8.

I would like to have heard: what did you use to detect the endpoint 
of your titration?
A pH transition, using a pH meter?
If you used a pH-sensitive dye,
I am sure that setting up the dye to properly change colors
had much to do with your pH 10, as well.
Each pH-sensitive dye has a particular pH range over which it changes color.
(Probably pH 9-10 in your case,  if you used a dye.)
Iron and copper ions have colors of their own
(at least in solutions much more concentrated than tap water),
and will probably change color or fade greatly when chelated,
and that might be used to detect the endpoint,
so with them the pH could be used solely to select the metal of interest.
(EDTA by itself in solution is colorless.)

Jim Swenson
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