I am using EDTA to titrate Ca ions from a sample solution using Patton-Reeder indicator, aka calconcarboxylic acid. The original organic sample was ashed at 700°C for 5 hours. The ash was dissolved in 2M HCl, then made pH neutral by adding 2M NaOH, which made a floculent precipitate which I assumed to be MgOH, and subsequently filtered off. Titration with EDTA would reach the endpoint (change from pink to blue), but then revert to pink within 5 minutes. After having gone through that cycle several times, I realized that I needed to understand what was going on before I could rely on any result.
I assume something must be releasing Ca ions slowly, or is degrading the EDTA. Any idea what this could be? My standard test solutions (Ca from CaCO3 and Mg from MgO, same treatment) do not behave this way, so the problem in not with reagents or method.
I also have to wonder if the precipitate, which is soluble in acid but not at pH 7, is only MgOH, or may also be Ca and/or Mg pyrophosphate. Is there a preferred method of Ca determination, like gavimetric using oxalate? In this case, I don't know which precipitates first (at the lower pH), the oxalate or the pyrophosphate. Where might I look?