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I have a titration of Calcium Ions with EDTA. When I reach a volume of 9.6mL I the liquid turns slightly blue and after that the more I add the bluer it becomes. Now at what point is my titration finished? When I notice the first change or as soon as adding more EDTA doesnt change the color?

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  • $\begingroup$ Suggestion: Prepare a stem solution (i.e. of known $c(\ce{Ca^{2+}}$). Perform the titration several times. Once "as if you were unaware about $c(\ce{Ca^{2+}}$" till the point all Calcium is chelated (which may be, taking known concentration of your stem solution and its volume, calculated in advance). Keep a small amount of this solution, for example in a test tube. Repeat the titration, intentionally adding more EDTA solution, than necessary; equally keep some of this solution as visual reference in a test tube. Now perform a titration with your unknown, with the visual aides beside. $\endgroup$ – Buttonwood Apr 22 '17 at 12:23
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Assuming you've used Erichrome Black-T as an indicator, the second your whole solution changes from wine red to blue, as in the moment you realise there is a change in colour, however light it is, that's your end point and you should stop.

Make sure you use a white tile under your conical flask and remember to pause the titration and swirl the flask little bit every now and then as you near the end point. That should even out the colour and make the change noticeable before it's too late.

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  • $\begingroup$ I see, thanks. Could you also explain why this is so? As far as I know ErichromeBlack-T is pink as long as there are metall ions that it can bind to, so while there are pink and blue particles (first change) there whould still be some Calcium not bound to EDTA but to ErioT $\endgroup$ – newToEverything Apr 22 '17 at 16:34
  • $\begingroup$ I'm a bit unsure about what your question means but I'll try to answer.Erichrome Black-T is actually blue in colour in its protonated form but turns wine red on complexing. That is why it is imperative to swirl the the contents of the flask, to distribute the reagent evenly so that any colour change is uniform. It's only when the whole solution uniformly turns the lightest blue possible that you must stop. There should be no trace of pink of course, but just like it's unadvisable to have a deep phenolpthalein pink while titrating, it's the same here. Does this answer your question? $\endgroup$ – Reya Apr 22 '17 at 19:15

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