Filter
Top Products
125
MODEL 3081 pH/ORP SECTION 14.0
ORP MEASUREMENTS
The ORP of the iron (II) - iron (III) standard when meas-
ured with a platinum electrode against a saturated sil-
ver-silver chloride reference is 476 ± 20 mV at 25°C.
The range of values is caused primarily by the high and
variable liquid junction potential generated in solutions
containing high acid concentrations.
Quinhydrone - hydroquinone ORP standards are also
used. They are prepared by dissolving excess quinhy-
drone in either pH 4.00 or pH 6.86 buffer. The ORP of
the standards at a platinum electrode against a silver -
silver chloride reference has been measured at 20°C,
25°C, and 30°C.
Temperature ORP in ORP in
pH 4.00 buffer pH 6.86 buffer
20°C 268 mV 92 mV
25°C 263 mV 86 mV
30°C 258 mV 79 mV
There are two disadvantages to using quinhydrone
standards. First, the shelf life is only about eight hours,
so fresh standard must be prepared daily. Second,
hydroquinone is highly toxic, so preparing, handling,
and disposing of the standards requires care.
Unlike pH calibrations, which are generally done using
two calibration buffers, ORP calibrations are almost
always single point calibrations.