35
MODEL 1054B pH/ORP SECTION 5.0
START-UP AND CALIBRATION
TABLE 5-1
ORP of Saturated Quinhydrone Solution
(in Millivolts)
pH 4 pH 7
Temperature °C 20 25 30 20 25 30
Millivolt Potential 268 264 260 94 87 80
5.7. STANDARD ORP SOLUTION (1054B ORP only).
Procedures are given below for making (2) types of
standard ORP solutions with known oxidation reduc-
tion potentials. Either one will be used to calibrate the
ORP system following the steps at the end of the sec-
tion.
CAUTION
The following recipes contain chemical
components that require protective meas-
ures and precautions- Wear protective eye
gear and clothing while working with these
solutions.
A. QUINHYDRONE SOLUTION. Saturated quinhy-
drone is a commonly used ORP standard solution. It is
made by adding a few crystals of quinhydrone to
either pH 4 or pH 7 buffer. Quinhydrone is only slight-
ly soluble, so a few crystals are all that are needed.
The resulting solution will be yellow-colored. The milli-
volt potentials, measured with a clean platinum elec-
trode, should be within ± 20 millivolts of the values
shown. Solution temperature must be noted to assure
accurate interpretation of results. The ORP value of
saturated quinhydrone solution is not stable over long
periods of time; therefore, these solutions should be
made up fresh each time they are used.
B. FERRIC-FERROUS AMMONIUM SULFATE
SOLUTION. This standard ORP solution is not as easy
as the quinhydrone solution to prepare, but is much
more stable and will maintain its millivolt value for
approximately one year when stored in a glass con-
tainer.
To prepare the solution, dissolve 39.2 grams of
reagent grade ferrous ammonium sulfate
[Fe (NH
4
)
2
(SO
4
)
2
•6H
2
O]
and 48.2 grams or reagent ferrous ammonium sulfate
[FeNH
4
(SO
4
)
2
•12H
2
O]
in approximately 700 milliliters of water (distilled water
is preferred, but tap water is acceptable). Slowly and
carefully add 56.2 milliliters of concentrated sulfuric
acid. Add sufficient water to bring the total solution
volume up to 1000 milliliters.
This solution (ferric-ferrous ammonium sulfate) will
produce a nominal ORP of 476 ± 20 mV at 25°C when
used with a saturated KCl/AgCl reference electrode
and a platinum measuring electrode. Some variance
in mV values is to be expected due to the large liquid
reference junction potentials which can arise when
measuring this strongly acidic (and concentrated)
solution. However, if the measuring electrodes are
kept clean calibrations can be achieved.
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