Emerson Rosemount 385+ -02, Reference Manual, Page: 24 / 42

3.3.2 Ferric-Ferrous Ammonium Sulfate Solution
Although this solution is not as easy to prepare as the quinhydrone solution in Section 3.3.1, it
offers a much more stable solution which will maintain its millivolt value for approximately one
year when stored in a glass container.
To prepare solution, dissolve 39.2 grams of reagent grade ferrous ammonium sulfate [Fe(SO4) •
(NH)2SO4 • 6H2O] and 48.2 grams of reagent grade ferric 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 1,000 milliliters. This solution (ferric-ferrous
ammonium sulfate) will produce a nominal ORP of 476 ±20 mV at 25°C. 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, consistent repeatable calibrations can
be achieved.
The solution used during the following check is an acid and should be handled with care. Follow the directions
of the acid manu facturer. Wear the proper equip ment. Do not let the solution come in contact with skin or
clothing. If contact with skin is made, immediately rinse with clean
water.
CAUTION
Most industrial applications have a number of ORP reactions occurring in sequence or simultaneously. There
can be several components that are oxidized or reduced by the reagents that are used. Theoretically, the ORP
potential is absolute because it is the result of the oxidation-reduction equilibrium. However, the actual meas-
ured potential is dependent on many factors, including the condition of the surface of the ORP platinum elec-
trode. Therefore, the sensor should be allowed 1-2 hours to become “conditioned” to the stream to be
measured when first setting up or after being cleaned.
NOTICE
Startup and Calibration Reference Manual
February 2019 00809-0100-3386
18 Startup and Calibration