NAVI D-25 Instruction Manual page 192

D-52/53/54/55
7.2 mV (oxidation-reduction potential [ORP]) measurement
For example, for a solution in which trivalent iron ions
coexist with bivalent iron ions, equations ① and ②
would be as follows.
3+ - 2+
⇔ Fe
Fe +e
RT aFe
E = E 0 - - - - ln - - - - - - - - - - - - - -
–
F
aFe
When only one type of equilibrium state ① exists in the
solution, the ORP of the solution can only be determined
by equation ② . What is important here is that ORP is
determined by the ratio of activity between the oxidant
(Fe 3+ ) and the reductant (Fe
2+ 3+
aFe /aFe ). In actuality, however, many kinds of states
of equilibrium exist simultaneously between various
kinds of ions, in most solutions. This means that under
actual conditions, ORP cannot be expressed using the
simple equation shown above and that the physical and
chemical significance with respect to the solution is not
very clear.
In this respect, the value of ORP must be understood to
be only one indicator of the property of a solution. The
measurement of ORP is widely used, however, as an
important index
(potentiometric titration) and in the disposal and
treatment of solutions.
Recently, various claims have appeared regarding this
matter. For example, that a high degree of ORP is
effective in sterilization, or that drinking water that has a
low ORP reduces the chance of illness by reacting with
the activated oxygen in the cells of the body. ORP is
used as an index for alkaline drinking water.
•••••• ① '
2 +
•••••• ② '
3 +
2+ )) (using the equation
in the analysis
7 Reference
of solutions
181