Remember that the standard potential E° is calculated with the assumption that there is 1 M of the reactant and product. Because there is more reactant than product, there is higher reaction potential than the standard E°, indicating the reaction is more favorable in these conditions.
What is ORP?
In terms of water quality, ORP is a measure of all reaction potentials present in the sample solution. Therefore, ORP is considered non-specific because it does not measure any unique species, unlike pH that specifically measures hydrogen ion concentration.
ORP is expressed in volts or millivolts and typically ranges from -1000 to +1000 mV. A positive voltage indicates that the solution is more likely to oxidize, or take electrons, from a species (compound, atom, or ion) with lower oxidative potential. For example, a +1000 mV solution can oxidize a +800 mV solution and a +800 mV solution can oxidize a +100 mV solution. The opposite is true for a negative voltage.