[kerryh]

Hi there,

I need to do a titration for the determination of calcium content in some water samples. I don't want the magnesium content so am I ok to leave out the magnesium chloride when preparing my 0.01M EDTA solution? Also I have found a couple of indicators which would be suitable (Calmagite or Calcon) but not sure which would be better.

Also, my samples may contain traces of iron and I know this can interfere with the result. I have found that you can add KCN to complex with the iron and hence stop it interfering but is there a safer or not so nasty chemical that will do the same job?

All suggestions much appreciated.  

[Borek]

I need to do a titration for the determination of calcium content in some water samples. I don't want the magnesium content so am I ok to leave out the magnesium chloride when preparing my 0.01M EDTA solution?

Do you mean you want to add MgCl₂ when preparing EDTA solution for titration?

[kerryh]

Yes. It says to add it but if I don't need the magesium concentration and my samples don't contain any then I don't need to add it. Is this right?

[Borek]

IMHO from the analytical point of view adding metal ions to the EDTA solution used for titration doesn't make sense. Reaction used in determination is just a complexation reaction, yet you want to add magnesium to titrant so that EDTA is already part of some other complex even before you start titration?

To mask Fe you may use trietanoloamine, much safer then KCN.

[kerryh]

I was wondering about the Magnesium Chloride because when calcium ions are titrated with EDTA a stable calcium complex is formed but with calcium alone (when using eriochrome black T indicator) no sharp end-point can be obtained. This was why the Mg is added. But if I use a different indicator there should be no need to add this then?

One other thing, Will a buffer solution pH 10 still be adequate when using calmagite indicator?

[kerryh]

Did you mean Triethanolamine?

[Borek]

Did you mean Triethanolamine?

Yeah. I have mixed Polish spelling (trójetanoloamina) with English.

As for titration: my textbooks doesn't mention Mg addition. They propose calcium titration against murexide.

I was a member of my faculty team for some competition. We did a determination of Mg and Ca in the same sample using EDTA (twenty years ago it was) and we tried different approaches - from what I remember we didn't came to any conclusions which combination of temperature/indicator gives best results. It was rather a matter of personal preference.

[ksr985]

i thought eriochrome black-T was a good indicator...ive used that for determining Calcium and magnesium ions present together in solution.

BUT, calcium titration is a special case.

Eriochrome black only forms a fairly weak calcium complex, and you often do
not get a good endpoint with calcium. One variant of the titration procedure is to use a back-titration. You add a measured excess of EDTA to the calcium-containing
solution, and then back-titrate by adding magnesium chloride solution until
you start to see the wine red colour from the magnesium complex. That means
that the measured amount of EDTA you added was used up in part by the
calcium ions originally present in the solution, and in part by the
measured amount of magnesium ions you added in the back titration.

i hope this explains it.

[kerryh]

Yip, that makes sense. cheers for your help.

[Garneck]

You can also mask Fe with fluoride ions.