I proceeded according to this paper: https://pubs.acs.org/doi/pdf/10.1021/ja01684a028
and synthesised 3-nitrophthalic acid. I recrystallised the reaction mixture from water two times. The first recrystallization batch had melting points of 186-187°C open tube, while the second one was 207-208°C open tube, much closer to the theoretical 210°C. That would mean I got rid of the unwanted isomer, 4-nitrophthalic acid, which melts we much lower temperatures (160°C). So far it all looks idyllic, but I'm concerned with a couple things: the recrystallised solid doesn't really form yellow solutions, more like a murky greenish/greyish. It also forms crystals very similar to those of unnitrated phthalic acid (my crystals photo 1, crystals of phthalic acid photo 2). I'd like to find some way to check for the presence of nitro group in my product, preferably chemical, as I don't have the access to things like NMR.
Thanks for all suggestions!
edit: forgot to add that the melting point for phthalic acid is around 207C, dangerously similar to my sample, hence the concern.