Secondly, not only does the
enzyme reaction of sulfite oxidase with sulfite remove sulfite
from the solution but the product of the enzyme reaction,
hydrogen peroxide, also oxidizes sulfite. Therefore, unlike
the glucose biosensor, hydrogen peroxide does not reach the
detector electrode because it is reduced by sulfite which survived
the passage over the enzyme layer. This dual oxidation
of sulfite by both the enzyme and hydrogen peroxide produced
in the enzyme reaction leads to a significant difference
in currents between predictor and detector electrodes and thus
allows operation of the sensor (if electrochemically active
hydrogen peroxide simply replaced electrochemically active
sulfite, there would be only a small difference in currents).
An expression for the difference in currents can now be formulated.