NOTE: in most cases, calibration problems can be addressed by steps 1-6 below. If your electrode is slow or showing numbers way out of range, try the meter test FIRST (steps 13-16 below). The reconditioning steps (7 – 12) should not normally be needed on an electrode that is less than 6 months old.
1. Be sure your electrode has been stored at least 24 hours in a proper electrode storage solution (Vinmetrica’s product is 3M potassium chloride in 10 mM potassium hydrogen phthalate; other similar products may be used). The entire bottom 1 inch of the electrode needs to have been submerged for at least 24 hours. If this has NOT happened, wait until it has!
2. IMPORTANT! Always stir or gently agitate the solution when using the pH electrode; letting it sit static will cause drift and inaccurate readings!
3. When calibrating your pH electrode, remember that the displayed pH may not be correct until AFTER you press ENTER, and the “Good Cal” message finishes scrolling.
4. If the instrument signals stable pH (i.e., the Cal LED is flashing) but displays “Bad Cal” after pressing ENTER, try putting it flat on the table; when the next stable pH is signaled, press the ENTER button quickly without handling the instrument. Sometimes the instrument may pick up noise from its environment, particularly if you handle it at the last second, while it’s trying to achieve a stable reading.
5. If values appear to drift, leave the electrode in the pH 4.01 reference solution for 30 minutes.
6. If you intend to read pH values in samples that are at a different temperature than ambient, it’s best to have your reference solutions at that temperature also before calibrating.
7. You can restore the pH calibration and bias (DAC setting) values to factory defaults in Test Mode (see Appendix A, Section 13 in your manual). Contact tech support if you have questions beyond this.
Reconditioning and cleaning of pH electrodes
Note: before you try the procedures that follow in steps 8-12, try the simple step of soaking the electrode in your pH 4.01 reference solution for about 1 hour, then try to calibrate. Sometimes this is all that’s needed!
Even in normal use and storage, performance of pH electrodes may show deterioration over time, which typically shows up as noisy, erratic or sluggish electrode readings, and/or difficulty calibrating. Assuming the meter itself is working (see “Meter test” below), then there are two main causes for this:
A. Clogging of the reference junction (most likely).
B. Fouling of the glass membrane (happens occasionally, or after prolonged service).
The following procedures will often provide renewed stability and pH sensitivity. If the electrode cannot be restored by one of these methods, it needs to be replaced.
Unblocking the reference junction:
The reference electrode junction is usually the problem when the electrode can’t calibrate in its expected ranges. This junction is a fine-pored frit that allows electrical contact of a reference electrode with the solution being tested. It can become clogged over time.
8. Soak electrode in hot (NOT boiling!) water, about 60 °C, for 5 – 10 mins. Allow to cool to room temperature then place in pH 4 reference solution for 5 minutes. Try to recalibrate. If this does not work, try the next step.
9. Place electrode in electrode storage solution (from Vinmetrica, or 3M KCl with optionally added 0.01M potassium acid phthalate, KHP) at 60 °C and allow electrode and solution to cool to room temperature, then place in pH 4 reference solution for 5 minutes. Try to recalibrate. If this does not work, try the next step.
10. Soak in 0.1M HCl (note: this can be made by diluting 1 mL of the SO2 Acid Solution with 20 mL DI water) or HNO3 for 1 hour. Rinse with DI water, then place in pH 4 reference solution for 5 minutes. Try to recalibrate. If this does not work, try the next step.
11. Soak in 1:10 dilution of bleach in a 0.1 – 0.05 % solution of liquid detergent in hot water with vigorous stirring for 15 mins. Rinse with DI water, then place in pH 4 reference solution for 5 minutes. Try to recalibrate.
Cleaning the pH electrode’s glass membrane:
The glass bulb is a thin membrane of a special kind of glass that actually does the job of responding to the pH of the solution. It can sometimes become dirty and poorly responsive.
12. Immerse electrode tip in 0.1M HCl (see above for how to make) for about 15 secs., rinse with distilled water, then immerse in 0.1M NaOH (you can use a little of your TA Titrant for this) for another 15 sec. Cycle the electrode through these solutions several times (rinsing with DI water in between), then rinse and check for performance in pH buffer 4.00 and 7.00.
Meter test
You want to be sure that the instrument is responding correctly. A quick test is to simply short out the electrode connector:
13. Put the instrument in pH mode.
14. Remove the electrode to expose the BNC connector at the back of the instrument. Short out the terminals on the connector, using a paper clip or similar metal piece to touch the center pin of the connector to its outer metal sheath.
15. With the input shorted out, the reading should be pH 7.00 +/- 0.5. If out of this range, the meter is probably bad. Contact us.
16. Bear in mind that this test is not 100% fool-proof (the instrument might still have trouble reading pH values different from 7.00), but generally if this test passes, it is much more likely to be an electrode problem.
pH test with cream of tartar
A quick way to check your calibration and pH accuracy is to measure the pH of a saturated solution of cream of tartar which has a pH of 3.56 at 25 degrees celsius:
a. Get pure cream of tartar (grocery store stuff is fine, provided it’s pure), or even better is reagent grade potassium hydrogen tartrate, also known as potassium acid tartrate or potassium bitartrate. Call it KHT for short.
b. Place about 1/4 teaspoon of KHT in 20 mL of distilled water. Mix well for about 30 seconds. You want to be sure the solution is saturated, i.e., everything that can dissolve, has dissolved. There should be some undissolved solid left.
c. Decant or filter the solution off the solids.
d. This solution has a standard pH of 3.56 at 25 degrees C (78 degrees F). It should be within 0.02 pH of this value at temperatures from 20 to 30 degrees Celsius. Discard after 24 hours.