How to do complexometry titration of calcium oxalate?

My titration of calcium oxalate with NaEDTA doesn't work. I need to calculate the ion calcium within my sample. My essay is to test anticalculi activity.


Why do I say I have problem? Because the colour keeps turning back to its initial colour (pink) *more than twelve times*. It's never ending and the titration end point is so premature.


In my preparation, I added HCl 0.5 M to my calcium oxalate so it dissolves. Calcium oxalate has very low solubility.


* First, I used EBT indicator and ammonia buffer. When i add the ammonia the solution became foggy and there's a precipitation within the sample.
* So I used substitution titration with Mg-EDTA (EBT indicator) = Still happens
* Then change the indicator to hydroxynaftol blue and calcon = Still happens (I don't use buffer with this indicator, only NaOH, but when I add NaOH it's became foggy again)




Does it have to be complexometric?


You could acidify the sample and do a redox titration of the oxalate with permanganate or dichromate. And since the amount of oxalate is the same as calcium, bob's your uncle.


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