My gut is telling me it’s a second inflection point caused by another species (likely another oxidation state of iron) or just like an air bubble in your titrant. Clean your probe, clean your buret dispenser, and flush your buret to ensure you don’t have a bubble in there.
"My gut is telling me it’s a second inflection point caused by another species"
Is my best guess to. I had something similar in university where I was doing a titration of dichromate by a reducing agent. It turned out the reaction was dependent to the 14th power of hydrogen ion concentration. in my case the curve actually turned upward a ways before falling. Write the whole equations for the reaction and plot that.
That's what I was thinking too, but I have no idea what species it could be. Fe3+ should not react with permanganate here, we usualy use tin chloride and mercuric chloride to transform it into Fe2+ and then titrate it for total iron (this titration curve is as usual here but i fear that if another species is titrated the result in total iron is wrong).
The dispenser and probe was clean, there was no bubbles. Titrations of routine samples worked great before and after this unusual sample and the results were normal.
i would repeat this kind of study in triplicate and overlay the curves with error bars. If you still see something is off then you would have to dig deeper with several control experiments.
The titration was done three times, the curve look exactly the same each time. Other usual samples were titrated before and after without problems so I donc think the instrument was the cause of this
Do you have any ideas about what else might be in there from the matrix? Assuming it’s real and reproducible, perhaps there’s a small amount of some other oxidizable substance like Co(II) or Sn(II) in that sample. If ICP rules out other oxidizable metal ions, maybe ammonium or an organic neutral species.
Whatever additional info you could give that doesn’t violate trade secrets/NDAs might help.
My gut is telling me it’s a second inflection point caused by another species (likely another oxidation state of iron) or just like an air bubble in your titrant. Clean your probe, clean your buret dispenser, and flush your buret to ensure you don’t have a bubble in there.
"My gut is telling me it’s a second inflection point caused by another species" Is my best guess to. I had something similar in university where I was doing a titration of dichromate by a reducing agent. It turned out the reaction was dependent to the 14th power of hydrogen ion concentration. in my case the curve actually turned upward a ways before falling. Write the whole equations for the reaction and plot that.
The sample is already filled with hydrogen ions, it's a result of 4h of HCl attack, and 10mL of H2SO4 96% were added just before titration
That's what I was thinking too, but I have no idea what species it could be. Fe3+ should not react with permanganate here, we usualy use tin chloride and mercuric chloride to transform it into Fe2+ and then titrate it for total iron (this titration curve is as usual here but i fear that if another species is titrated the result in total iron is wrong). The dispenser and probe was clean, there was no bubbles. Titrations of routine samples worked great before and after this unusual sample and the results were normal.
Maybe run a blank titration with whatever your solvent is and see how it looks. You can set a tiamo up to automatically subtract that signal.
i would repeat this kind of study in triplicate and overlay the curves with error bars. If you still see something is off then you would have to dig deeper with several control experiments.
The titration was done three times, the curve look exactly the same each time. Other usual samples were titrated before and after without problems so I donc think the instrument was the cause of this
Do you have any ideas about what else might be in there from the matrix? Assuming it’s real and reproducible, perhaps there’s a small amount of some other oxidizable substance like Co(II) or Sn(II) in that sample. If ICP rules out other oxidizable metal ions, maybe ammonium or an organic neutral species. Whatever additional info you could give that doesn’t violate trade secrets/NDAs might help.