Possible causes for flawed results of an EDTA titration for the Ca content of milk

Earlier today I was trying to find the calcium content of different milks as a part of a school assignment. I used [this setup](http://www.outreach.canterbury.ac.nz/chemistry/documents/calcium.pdf) 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.


