ORP Calibration 101

[scottyreef]

well over the years I have calibrated my orp probe dozens of times but when i was talking to some of my reef friends latly they told me they never have cal'd there probe. so I've tracked down a great plce to buy all the necessary items to get it done. the main items which a solution of 7pH and 4pH and the really hard item to find is quinhydrone which makes the ORP reaction

 

http://en.wikipedia.org/wiki/Quinhydrone_electrode

 

https://www.pulseinstruments.net/index.asp?...amp;ProdID=1692

 

the whole set up cost about $45 but it does about 30 calibration or each batch of high and low calibration standard

 

How to calibrate your probe ( general for stand alone ORP controllers and probes)

 

Instructions:

1. The buffer / Quinhydrone mixtures should be freshly made each time the ORP electrodes are

calibrated. Do not store the mixtures or use after 2 hours as their values change with time.

 

2. Fill a beaker with de-ionized or distilled water to use for rinsing the electrode.

 

3. Fill a second beaker with pH 7 buffer to approximately ½ oz mark and label the beaker pH 7.

 

4. Add to this buffer enough Quinhydrone that stays on about ¼ inch long (6mm) of the wood

applicator.

 

5. Use the wood applicator to stir the Quinhydrone into the buffer.

 

6. A small amount of Quinhydrone MUST remain un-dissolved; if all the Quinhydrone dissolves

add a small amount and stir. Repeat as necessary until a small amount of Quinhydrone

remains un-dissolved.

 

7. Fill a third beaker with pH 4 buffer to approximately ½ oz mark and label the beaker pH 4.

 

8. Add to this buffer enough Quinhydrone that stays on about ¼ inch long (6mm) on the wood

applicator.

 

9. Use the wood applicator to stir the Quinhydrone into the buffer.

 

10. A small amount of Quinhydrone MUST remain un-dissolved; if all the Quinhydrone dissolves

add a small amount and stir. Repeat as necessary until a small amount of Quinhydrone

remains un-dissolved.

 

11. Rinse the ORP electrode and pat it dry with a soft tissue.

 

12. Put the electrode in the pH 7 mixture and stir the electrode gently.

 

13. Allow the reading to stabilize, this typically takes 30 to 60 seconds, and note the reading.

 

14. The reading should be within about ±15mV from the following values:

 

Table 1: Readings In pH 7 Buffer / Quinhydrone Mixture

 

Temperature: 20°C(68°F) 25°C(77°F) 30°C(86°F)

Reading: +96mV +90mV +83mV

 

 

 

15. Rinse the ORP electrode and pat it dry with a soft tissue.

 

16. Put the electrode in the pH 4 mixture and stir the electrode gently.

 

17. Allow the reading to stabilize, this typically takes 30 to 60 seconds, and note the reading.

 

18. Add 170mV and 185mV to the reading from step 13. Write down the calculated values.

 

FOR EXAMPLE:

If the reading from step 13 is 90mV, then the calculated values should be

260mV = 90mV + 170mV and

275mV = 90mV+185mV.

 

19. The reading should be between the two calculated values from step 18.

 

20. With time and/or use, the value in the pH 7 mixture may change, however the reading from the

pH 4 mixture must be within the two calculated values (step 18). If the reading is within the

range, it means that the electrode has good span and should be able to be calibrated along

with the meter to reflect the proper ORP potential.

 

21. If the reading (step 17) is less than the calculated value, pH 7 mixture value + 170mV (260mV),

the electrode may be coated. Remove the coating using the following suggestions. If the

reading is still less than the calculated value after cleaning, replace the electrode.

Wipe the surface clean with a soft cloth or tissue.

Soak the electrode in a chemical known to dissolve the suspected coating material.

Very gently polish the surface with a paper towel.

After cleaning, let the electrode soak in one of the calibrating solutions for about five

minutes.

 

22. If the reading (step 17) is more than the calculated value, pH 7 mixture value + 185mV

(275mV), replace the electrode.

 

23. Once the ORP electrode is verified and is in good working order put the electrode into the pH 7

mixture.

 

24. Follow the ORP meter calibration procedure to calibrate the ORP value to one of the values in

Table 1.

 

Next calibration for a Aquacontroller

 

ORP Calibration

It is not necessary to calibrate the ORP probe of the AquaController III. It has been properly calibrated at the

factory to maintain accurate ORP readings for the lifetime of the controller. However, it is possible to calibrate

the probe if so desired. Quinhydrone, pH 4.00 and pH 7.00 calibration solutions are required for the calibration.

 

The following procedure should be used to calibrate the ORP:

 

1. Create a saturated solution of Quinhydrone and pH 7.00 calibration solution.

 

2. Select Setup:ORP Setup:ORP Calibrate from the AquaController III’s menus.

 

3. Place the ORP probe into the Quin-7.00 solution. Wait for the numbers on the bottom of the

LCD screen to stop changing. It does not matter what value is displayed only that it is not

changing. When the display stops changing press the select button.

 

4. Create a saturated solution of Quinhydrone and pH 4.00 calibration solution.

 

5. Place the ORP probe into the Quin-4.00 solution. Wait for the numbers on the bottom of the

LCD screen to stop changing. It does not matter what value is displayed only that it is not

changing. When the display stops changing press the select button.

 

6. The ORP probe is now calibrated.