[Seascratch]

So I'm measuring ORP of a brine sample while killing the strong oxidizers (HOCl) with NaHSO3. I noticed that in the literature and the experiment that until I COMPLETELY remove the HOCl, the ORP stays very high (~1100mv). I was just wondering why the ORP stays so high until total removal instead of it coming gradually down as concentration of HOCl goes down.

I've done titration to measure HOCl concentration while killing it confirm that it is indeed going down.

Thank you!

[Borek]

Do you know Nernst equation? Potential doesn't change linearly with the concentration.

[Seascratch]

Yup, E=E^0-  59.16/n log (〖[X]〗^x 〖〖[Y]〗^y [Z]〗^z…)/(〖[A]〗^a 〖〗^b 〖[C]〗^c…) at room temp given aA+bB+cC+⋯+n e^-=xX+yY+zZ+⋯

I was thinking about this and basically it looks like the ORP shouldn't change as long as the equilibrium constant (which is (〖[X]〗^x 〖〖[Y]〗^y [Z]〗^z…)/(〖[A]〗^a 〖〗^b 〖[C]〗^c…) ) doesn't change. So since equilibrium constant is a function of temperature, gibbs free energy change and pressure, changing the concentration of one of the species wouldn't change anything since it'll just adjust itself to equilibrium. But I guess the huge ORP shift is when the system gets to the point where the system can't regenerate the strong oxidizer you've been removing.

Does that sound about right?

[Borek]

What you observe is not different from titration curves. Oxidizer is not regenerated, it is only removed. As the potential is a function of the ratio of the oxidized and reduced forms (raised to a necessary powers) it behaves in a specific way - it changes relatively slowly far from the equivalence, but very fast near the equivalence point. It is just a property of this system.