24
MODEL 396P and 396PVP
SECTION 6.0
MAINTENANCE
6.3 MODEL 396P and 396PVP ORP
4.3.1 Platinum Electrode Check.
The platinum elec-
trode may be checked as follows: There are three
types of standard solutions which may be used to
check the ORP electrode/transmitter system.
Type 1:
A prepared ORP standard solution
(PN R508-160Z)
Type 2:
Another type of commonly used ORP stan-
dard solution is the saturated quinhydrone solution.
Refer to Section 5.2.
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 contact with skin of clothing
is made, immediately rinse with plenty of
clean water.
Type 3:
A third ORP standard solution can be pre-
pared from the following recipe: Dissolve 39.2 grams
of reagent grade ferrous ammonium sulfate, Fe(NH4)2
(SO4)2 • 6H2O and 48.2 grams of reagent grade fer-
ric ammonium sulfate, FeNH4(SO4)2 • 12H2O, 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 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 con-
dition, consistently repeatable calibrations can be car-
ried out using this standard solution.
6.3.2 Cleaning Platinum Electrode.
The electrode
can be restored to normal operation by simply cleaning
the platinum electrode with baking soda. Polish it by rub-
bing it with a damp paper towel and baking soda until a
bright, shiny appearance is attained.
