It's a safe bet that any chemical containing trivalent iron ("ferric") is non-stoichiometric. Even something as simple and common as rust varies wildly! Add oxalic acid to that, and you don't have to be a particularly bright scientist to guess that it's going to be diffcult to quantify "purity": The oxalic anion is a very strong chelating agent, and will cling to anything in just about any possible configuration.

Which got me to thinking: Oxalic acid is used as a rust remover in several different applications. Could ferri oxalate solution be made by dissolving rust (or a "purer" FeO(OH)*nH2O in oxalic acid? A small addition of dilute hydrogen peroxide to assore the oxidation state of the iron might be a good idea, but at least it would avoid a whole lot of tricky process steps and explosive chemicals?