[snoopmesh]

I've been researching this for weeks now but I still can't find the answer.
I'm going to do an experiment where I calculate the content of calcium in different samples of tofu. The tofu has been squeezed to its bare nutrients already, and while I want to do a simple titration with EDTA and the tofu solution, (EBT as the indicator), so I can easily calculate the calcium content based on the amount of EDTA used.
However, my teacher is telling me a more accurate way is to do a back-titration using MgCl. Does anybody know how this would work? Would I drown the tofu solution with MgCl, and then do the titration with EDTA? But then how would I calculate the calcium content?

[chenbeier]

No, it works in this way that you add a known value of EDTA to the tofu sample. The EDTA catches all the calcium. You use for titration MgCl₂ and you get a value of the remaining EDTA. The difference between the used original EDTA value and the amount by titration.will guide you to the calcium amount.
Let say you use 20 ml 0.1 M EDTA. The titration with Magnesium chloride 0,1 M will show 13 ml. It means 7 ml react with Calcium. With the factor of Ca you can find the mass.