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MODEL 381pH/ORP SECTION 3.0
MAINTENANCE
3.3 MODEL 381
3.3.1 Platinum Electrode Check. The platinum elec-
trode may be checked as follows: There are two types
of standard solutions which may be used to check the
ORP electrode/transmitter system.
Type 1: One type of commonly used ORP standard
solution is the saturated quinhydrone solution. A com-
monly used ORP standard solution is a saturated quin-
hydrone solution. This can be made by simply adding
a few quinhydrone crystals to either a 4 pH or a 7 pH
buffer. Quinhydrone is only sightly soluble so only a
few crystals will be required. The solution will have a
yellow color. The resulting potentials should be within
±20 millivolts of the value shown in Table 3-1. The ORP
value of saturated quinhydrone solution is not stable
for long periods of time and therefore new solutions
should be made each time.
CAUTION
The solution used during the following check is
an acid and should be handled with care.
Follow the directions of the acid manufacturer.
Wear the proper protective equipment. If con-
tact with skin of clothing is made, immediately
rinse with plenty of clean water.
Type 2: A second ORP standard solution is the Ferric-
Ferrous Ammonium Sulfate Solution (PN R508-16OZ),
and it can be ordered as a spare part; otherwise, it can
be prepared from the following recipe: Dissolve 39.2
grams of reagent grade ferrous ammonium sulfate,
Fe(NH
4
)
2
(SO
4
)
2
• 6H
2
O and 48.2 grams of reagent
grade ferric ammonium sulfate, FeNH
4
(SO
4
)
2
•
12H
2
O, in approximately 700 milliliters of water (dis-
tilled 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 solu-
tion volume up to 1000 ml. This standard ORP solution,
although not as simple to prepare as the quinhydrone
recipe, is much more stable, and will maintain its milli-
volt value for approximately one year when stored in
glass containers. This solution (ferric/ferrous ammoni-
um sulfate) will produce a nominal ORP of 476 +20 mV
at 25°C when used with a saturated KCl/AgCl refer-
ence electrode and platinum measuring electrode.
Some tolerance in mV values is to be expected due to
the rather large liquid reference junction potentials
which can arise when measuring this strongly acidic
and concentrated solution. However, if the measuring
electrodes are kept clean and in good operating condi-
tion, consistently repeatable calibrations can be carried
out using this standard solution.
3.3.2 Cleaning Platinum Electrode. The electrode is
never exposed to these undesirable compounds. In the
event poisoning is suspected, the electrode can be
restored to normal operation by simply cleaning the plat-
inum electrode with baking soda. Polish it by rubbing it
with a damp paper towel and baking soda until a bright,
shiny appearance is attained.
TABLE 3-1. ORP of Saturated Quinhydrone Solutions
pH 4 pH 7
Temperature °C 20 25 30 20 25 30
Millivolt Potential 268 264 260 94 87 80