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Earlier today I was trying to find the calcium content of different milks as a part of a school assignment. I used this setup as the basis for my experiment:

I was using $\pu{0.1 M}$ EDTA solution as a titrant. The milk was diluted with destilled water and $\pu{8M}\ \ce{NaOH}$ solution to make sure magnesium won't interfere. The only diversion from the procedure above was that eriochrome black T was used as an indicator. After several attempts, starting with $\pu{10ml}$ and reducing the amount of milk with each trial, absurdly large quantities of EDTA were used without any change in color. Lastly, when a sample of $\pu{(0.1± 0.005) ml}$ milk was diluted in $\pu{0.4ml}$ water and $\pu{0.05ml}\ \ce{NaOH}$, the change in color occured after $\pu{68ml}$ of the titrant was added, which is equivalent to there being $\pu{240g}$ of calcium in the sample (fun fact: this would mean that the milk carton weighs 2.4 tons. I haven't weighed it, but I am pretty sure it does not).

Thus my question is: what could the possible mistakes that lead up to these terrible results be? Equipment was rinsed with distilled water before use, and everything else was done according to the procedure, except for the EDTA solution, which was made approximately an hour before the experiment (and it should be left alone for at least 4 hours after creation). Nevertheless, could this impact the experiment in such a severe way?

The teacher was uncertain of what caused it as well.

We chose our own topics, so no other students have done the same experiment.

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    $\begingroup$ Eriochrome Black T isn't a suitable indicator for the determination of calcium ions in 8M NaOH. Try with Murexide. $\endgroup$ – Marko Jan 18 '16 at 19:16
  • $\begingroup$ Thanks, I will try that! Could you, however, elaborate on why? It would really help me with my paper, as I need to describe my design in the paper. $\endgroup$ – Bruno KM Jan 18 '16 at 19:24
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    $\begingroup$ I don't have the time now to do the whole calculation, but I will just direct you towards the proof: compare the pCa at the equivalence point of the titration (approximately 6.3) with the pCa at which the color change of Erio Black T occurs (approximately 3.8). It is also possible that you don't see the change of color nicely as milk is colloidal. $\endgroup$ – Marko Jan 18 '16 at 20:14
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    $\begingroup$ Milk also has emulsified fat globules, so it is a two-phase system. For such a large over determination of calcium I'd wonder if Eriochrome Black T partitions between the two phases and preferentially prefers the fat globules. $\endgroup$ – MaxW Jan 19 '16 at 1:15
  • $\begingroup$ You could test the effect of milk by simply testing an aqueous solution of calcium. $\endgroup$ – MaxW Jan 19 '16 at 1:21

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