[phinie_b]

Hi!
I'm trying to determine the concentration of a ~30% Ferric Nitrate solution. The solution also contains about 2.5% Sulphuric acid. The method used is as per ACS (9the edition and newer) titrating with EDTA, using Variamine blue b indicator.
The problem is I don't see an end point. I get a gradual change from red to dark orange to dark yellow, but in 4 tries I was unable to spot the moment the orange became dark yellow.
Perhaps it's me that's failing to spot the end point, or could it be the sulphuric acid is interfering? It occurs to me that the formation of ferric sulphate may be the cause.
Any suggestions? Have any of you done this before? Is the endpoint normally this hard to spot?
We are going to get a standard ferric nitrate solution to compare results, but in the meantime I would appreciate any help/suggestions/advice!
Thanks!

[Borek]

Generally speaking EDTA works better in high pH, so I would try to neutralize the solution first (well, don't get pH too high to not precipitate the ferric hydroxide). Doesn't have to help, but definitely worth trying.

[phinie_b]

Generally speaking EDTA works better in high pH, so I would try to neutralize the solution first (well, don't get pH too high to not precipitate the ferric hydroxide). Doesn't have to help, but definitely worth trying.

Thanks! Maybe the pH is a factor.... its weird though, the ACS manual says to do the titration at pH 2.5

[Borek]

Thanks! Maybe the pH is a factor.... its weird though, the ACS manual says to do the titration at pH 2.5

pH of 2.5% H₂SO₄ is well below 1.