Thanks for the replies.
Starting with phth's reply, by "organic extraction", do you mean something like adding ethanol and heating the solution at about 80'C? I've tried that, and the solution becomes clearer, with a dark green precipitate forming and sinking to the bottom of the tube. But on analysis of the remaining fluid, it seems like the calcium oxalate is also being removed from the solution.
Now Mitch. That doc you linked contained a substantial amount of information, but didn't really expand on any of it, and I'm having trouble researching the things it says.
"strong-acid solution (e.g hot 1 MOL HCl) leading to the dissolution of crystalline calcium oxalate" sounds promising, and it might be possible that if the crystals are better dissolved into the solution, the ethanol method might not remove it, but I'm not sure.
It also says "extraction of tissues with water", which sounds great but I can't find anything that says what this means.
Finally, it mentions the use of enzymology to determine concentration. Would this entail using oxalate oxidase enzymes as a catalyst to do the reaction:
oxalate + O2
And then record the volume of CO2 formed and then use that to determine concentration?
Just thought of another method; use the ethanol to produce the dark green solid (which I hope contains the calcium oxalate), remove the liquid and then heat up the remaining solid until it is red hot. This should burn away most impurities and turn the calcium oxalate into calcium oxide, which I can then use to determine concentration.
Thanks for all the help so far