Emerson Rosemount 385, Reference Manual, Page: 24 / 34

18 Maintenance
Maintenance Reference Manual
February 2019 00809-0100-3385
4.4 Rosemount 385 ORP
4.4.1 Platinum Electrode Check
The platinum electrode 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.
Refer to Section 3.3.
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 or reagent grade ferrous ammonium sulfate, Fe(NH4)2 (SO4)2 • 6H2O
and 48.2 grams of reagent grade ferric ammonium sulfate, FeNH4(SO4)2 • 12H2O, in approxi
mately 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 millivolt value for approximately
one year when stored in glass containers. 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 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 condition, consistently repeatable calibrations can be carried out using this standard
solution.
4.4.2 Cleaning Platinum Electrode
If the electrode is never exposed to undesirable compounds, the electrode can be restored to
normal operation by simply cleaning the platinum electrode with baking soda. Polish it by rubbing
it with a damp paper towel and baking soda until a bright, shiny appearance is attained.
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.
CAUTION
Figure 4-3: Male Connector Tightening Diagram