pH Questions

Test Specifications for pH Electrodes


1. Millivolt Meter or pH/mV meter ("Test Meter")
2. Temperature bath at 30 C
3. Plastic beakers
4. pH Buffer 4.01
5. pH Buffer 7.00
6. Deionized water
7. Ohmmeter or DMM with 0.1 K-ohm resolution
8. Electrode leak rate guage (liquid electrolyte electrodes only)


a. 0 + 15 mV
b. 180 + 10 mV higher
c. 20 - 100 Megohms
d. 24.2 + 0.5  K-ohms   [temperature probes only]
e. 0.2 - 1.5 mL/day    [liquid electrolyte electrodes only]

PROCEDURE a  --  Isopotential point

1. Place pH 4.01 and 7.00 buffers in beakers and set in the  temperature bath.  Adjust bath to 30.0 C and verify during remainder of procedure that actual bath temperature is 30.0 + 0.5 C.
2. Allow buffers to come to temperature.
3. Connect the pH electrode to be tested to the Test Meter.
4. Place the electrode in the pH 7.00 buffer.  Swirl briefly.
5. When the potential stabilizes, record it.  Reading must be less than  + 15 mV.  This is the "isopotential point" check.

PROCEDURE b  --  Slope check

1. Rinse the electrode and place in the pH 4.01 buffer.  Swirl briefly.
2. When the potential stabilizes, record it.  The difference in potential between the two buffers must be 180 + 10 mV. 

PROCEDURE c  --  Glass membrane resistance

1. Measure the d.c. resistance of the glass in pH 4.01 buffer at 30.0 + 0.5 C, with the Electrode Test Stand.
2. Resistance must be between 20 and 100 Megohms.

PROCEDURE d  --  Temperature probe (thermistor) check

1. Place the temperature probe in buffer in the temperature bath at 30.0 + 0.2 C.  Verify the bath temperature is correct. Note: solution temperature is important in this test.
2. Connect an ohmmeter or DMM (resistance scale) to the 2 sections of the jack of the probe's temperature cable.
3. When the temperature and reading stabilize, read the thermistor resistance.
4. The resistance must be between 23.7 and 24.7 K-ohms.

PROCEDURE e  --  Electrolyte leak rate


1. Fill the electrode with calomel electrode filling solution. 
2. Suspend the electrode in a beaker of deionized water so that the lower electrode plug is level with the water.
3. After 24 hours use the graph paper leak rate gauge to determine how many mL of electrolyte have leaked out.
4. Leak rate must be between 0.2 and 1.5 mL/24 hours. (If a leak rate gauge is not available, an alternate method of measuring the flow rate is acceptable.  Use a fiber tip pen to mark the initial fill level, and the final level after 24 hrs.  Refill the electrode using a serological pipet, noting the volume of filling solution required.)

Technical information provided by Fox Scientific Company.

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Electrode Storage and Cleaning

Electrode Storage

Make sure to store the electrode in its Wetting Cap containing Electrode Fill Solution.  Electrode Fill Solution (3M KCl) can be purchased or made (dissolve 22.37 grams of potassium chloride into 100 mL of DI or distilled water).  Do NOT store the electrode in deionized water.


1. Clean the electrode.
2. Place the meter into absolute mV mode.
3. Place the electrode into a pH 7 buffer.  The mV reading should be 0 +/- 15 mV.
4. Rinse and place the electrode into a pH 4 buffer.  The mV reading should be 159 to 186 mV  more positive than mV  value in pH 7.
5. Optionally, place the clean electrode into a pH 10 buffer.  The mV reading should be -159 to -186 mV less than mV  value in pH 7.

Cleaning Procedure


1. Soak in 0.1M HCl for half an hour.
2. Drain and refill the reference solution.
3. Soak the electrode in filling solution for one hour.


1. Soak in 0.1M tetrasodium EDTA solution for 15 minutes
2. Drain and refill the reference solution.
3. Soak the electrode in filling solution for one hour.


1. Soak in 1% pepsin / 0.1M HCl for 15 minutes.
2. Drain and refill the reference solution.
3. Soak the electrode in filling solution for one hour.

Grease and Oil:

1. Rinse with detergent or ethanol solution.
2. Drain and refill the reference solution.

3. Soak the electrode in filling solution for one hour. (Electrode response may be enhanced by substituting a mixture of 1:1 pH 4 buffer and Filling Solution for the soaking solution.)

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Perfecting pH Measurements

  1. 1. Always use fresh buffers for calibration.  Choose buffers that are no more than 3 pH units apart and bracket the expected sample pH.
    2. Check electrode slope daily by performing a two-buffer calibration.  Slope should be 92-102%.
    3. Between measurements, rinse electrodes with distilled water.
    4. Stir all buffers and samples during measurement.
    5. Place a piece of insulating material (styrofoam, cardboard) between magnetic stirrer and beaker to prevent error from transfer of heat to sample. 
    6. Avoid rubbing or wiping electrode bulb, to reduce chance of error due to polarization.  Blotting is ideal.
    7. Store pH electrode in electrode storage solution when not in use.  Never store in deionized water.
    8. Always remove covering from electrode fill hole prior to measurement.
    9. Try to always soak glass electrodes in electrode storage solution overnight prior to first use.
    10.Be careful not to use incorrect fill solution for electrode or application.
    11. Do not allow salt crystals to form inside pH electrodes.
    12. Frequently inspect electrodes for damage and clogging of external junction.
    13. Use appropriate cleaning procedure if electrode becomes sluggish or drifts excessively.
    14. Technical information provided by Orion Research.
    15. Contact Orion Research Technical Support at 1-800-225-1480.

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Rejuvenating a "LOST" pH electrode

The following procedure should be attempted only as a last resort for a pH electrode that is otherwise unusable.  If this procedure does not result in a pH electrode that responds quickly and has a slope of 60mV +/- 5 mVper pH unit, the electrode is unrecoverable and should be thrown away.

Follow the procedures for cleaning given on the "Electrode storage and cleaning" section first.  If they fail:

1. Fill the electrode with filling solution.
2. Prepare a 10% nitric acid solution; approximately 50 mL in a 100 mL beaker.
3. Place the electrode in the solution on a hotplate.  Heat to boiling, and allow to stand for 10 minutes.
4. (THIS STEP SHOULD BE PERFORMED IN A FUME HOOD.)   Place 50 mL of filling solution in a second clean beaker.  Heat, although boiling is not necessary.
5. While the electrode is still hot, transfer it to the beaker of heated filling solution.   Set aside to cool.
6. When the electrode has cooled, test the electrode as described in the "Test Specifications for pH Electrodes" page.  This rejuvenating procedure is particularly effective with gel filled combination electrodes.  Do not be concerned if a small amount of the gel protrudes through the reference frit during the boiling in nitric acid step.  This is both acceptable and useful.

Technical information provided by Fox Scientific Company.

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