Just an update for all those who care: I prepared 5% Na2-EDTA solution and used 50% NaHSO4 stock solution to incrementally lower the pH. Starting at a pH of about 4.3 something started to precipitate at pH of about 3. The behavior was a bit odd, because every time I added NaHSO4 stock solution pH dropped to 2.6 and then went back up to about 3.3 or 3.4 within a few minutes. My interpretation is that pH rose back as EDTA free acid crystallized.

After a lot more NaHSO4 stock solution pH finally stayed stable below 2.5 and went down smoothly from then on until pH 0.5. Given the amount of NaHSO4 stock solution needed to reach pH 0.5 I ended the test run at this level. The EDTA did not settle but had to be filtered.

Subsequent washing with distilled water did not reduce the amount of compound in the filter. The pH at which the compound precipitated from solution would point at the EDTA2- ---> EDTA- transition. Another data point would be this book where Na-EDTA is shown to have pH 3.5 while EDTA free acid supposedly has pH 2.0, which would tell me that I precipitate EDTA free base when crossing the pH 3 line downwards. Both Na-EDTA and EDTA free acid are described as poorly soluble ...

I will try to find some way of determining whether I precipitated EDTA free acid or Na-EDTA or a mixture thereof with my method.