I probably failed in my layman's chemistry terms to explain things.
So, quoting from Zoecklein et. al Wine Analysis and Production
, "the analysis for free and total SO2
is dependent on the redox reaction: H2
+ 2HI. Completion of the reaction is signaled by the presence of excess iodine in the titration flask. This excess iodine can complex with starch (blue-black endpoint) or be sensed with a platinum electrode" (Zoecklein p.495).
One can use either an analog or digital pH meter. The analog is more "user friendly", as you are only looking for the needle inflection (mV) rather than the constantly changing numbers of a digital display. When I do the titration via analog or digital meter, the endpoint is usually a surge in voltage around 100 mV. I've sent samples to an outside lab, so I know my results are correct. The Orion 301 analog pH meter that I have has a scale of -400 to 400 mV, so a change of ~100mV, moves the needle about .75".
According to the Nernst equation, shouldn't the endpoint be far greater than 100mV? I don't understand how to calculate the potential with the Nernst equation. Help appreciated. Thank you.