Measuring pH in ferric oxalate solutions

I have a problem with measuring $\mathrm{pH}$ in ferric oxalate solutions ($\sim 5~\text{to}~\pu{10 g L-1}$ $\ce{Fe},$ $\pu{0.8 M}$ oxalic acid, $\pu{25 °C}$). While I am sure about the $\mathrm{pH}$-meter device and buffer solutions ($4$ and $7$) and also the good function of my electrode (Sentek) in other solutions, the $\mathrm{pH}$ value does not stabilize at all in such a way that it continuously decreases as far as the pH becomes negative and eventually, under-ranged.


Is there any interference or any requirements for $\mathrm{pH}$ measurement in oxalate solution? 


