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Reference Manual
00809-0100-3389
4.3
ORP
4.3.1
Check platinum electrode
Check the platinum electrode 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 (PN R508-16OZ). Refer to
WARNING
The solution used during the following check is an acid.
Handle the solution with care.
Follow the manufacturer's directions.
Wear the proper protective equipment.
If contact with skin or clothing is made, immediately rinse with plenty of clean water.
Type 2: A second ORP standard solution can be prepared from the following recipe:
Procedure
1. Dissolve 39.2 grams of reagent grade ferrous ammonium sulfate, Fe(NH
2. Slowly and carefully add 56.2 ml of concentrated sulfuric acid.
3. 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) produces a
nominal ORO of 476 +20 mV at 77 °F (25 ºC) 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 that can arise
when measuring this strongly acidic and concentrated solution. However, if measuring
electrodes are kept clean and in good operating condition, consistently repeatable
calibrations can be carried out using this standard solution.
4.3.2
Cleaning platinum electrode
To restore the electrode to normal operation, clean the platinum electrode with baking
soda. Polish it by rubbing it with a damp paper towel and baking soda until it appears
bright and shiny.
Reference Manual
6H
O and 48.2 grams of reagent grade ferric ammonium sulfate, FeNH
2
H
O in approximately 700 ml of water.
2
Distilled water is preferred, but tap water is acceptable.
™
Rosemount
389/389VP ORP
Maintenance
February 2019
calibration.
)
(SO
)
●
4
2
4
2
(SO
)
● 12
4
4
2
33
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