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When Not to Use the pH Meter

by Roger Fleming

Overview

Don't use a pH meter when you are likely to break the electrode or when the electrode will yield an inaccurate result. Most pH meter inaccuracies are due to faulty electrodes or a mismatch of the electrode with the application.
Diagnosing Meter Problems

Meter failure is unusual. A pH simulator is an electronic device that plugs into the meter to simulate a signal from an electrode to diagnose meter malfunction. Most meter manuals for units using automatic temperature correction (ATC) contain instructions on using a volt meter to diagnose problems with that system.

Combination Electrodes

A pH electrode is really an electrode pair, consisting of a reference electrode and a pH-sensing electrode. The commonly used combination electrodes have the reference electrode inside the pH-sensing electrode. A ceramic frit typically provides the fluid junction between the reference electrode and the sample.
Conditions That Will Break Electrodes

Don't jam glass electrodes into thick substances such as butter or they will break. Do not immerse an electrode into samples containing large, hard chunks such as cement. Use electrodes with an epoxy body and barriers around the sensing bulb for materials that will flow (such as mayonnaise) or containing undissolved chunks. Alternatively, use a portable, hand-held pH meter (such as the Horiba B-213) for measuring these materials or materials that will not flow (including toothpaste). The electrode is very fragile on these units. So add the sample gently to the scoop shaped sample area, and clean in accordance with the manufacturer's instructions. Do not use pH electrodes with glass bulbs or solutions containing hydrofluoric acid (HF) or sodium fluoride (NaF) because they dissolve glass.
Inspect the Electrode

Do not use broken or cracked electrodes. Do not use a dry electrode without adequate prehydration. Store the electrodes in the solution recommended in the owner's manual. Verify the electrode has sufficient filling solution of the correct type for the electrode. Open the reference electrode filling port before use, and close it afterward.
Special Electrodes

Use special electrodes for accurate measurements of samples below pH 2 and above pH 12. Polymers and proteins often cause problems because they may precipitate in the frit.. A plugged frit causes pH readings to be unstable, drift and become inaccurate. Consider using flushable frit electrodes or double-junction sleeve electrodes for solutions that are known to clog frits. Solutions of very low ionic strength, such as pure water, may lead to unstable readings, requiring a special ionic-strength diluent to stabilize readings. A pH meter does not work well in organic solvents.
Calibration and Temperature Problems

Use a two-point calibration. If the calibration slope is grossly off (< 50 mV/decade or < 92 percent), change the electrode. Ensure that the temperature-corrected calibration value for the calibrators is entered into meters without ATC. Orion makes a special electrode for use with tris(hydroxymethyl)aminomethane (TRIS) because this commonly used biological buffer is very temperature-sensitive. A pH meter used for in-line process applications can experience grounding problems, which lead to unstable readings.
Pick an Electrode Suitable for Your Use

Use these troubleshooting rules as a means for selecting the correct type of electrode for your system. It is less expensive to have the right electrode to begin with than breaking the wrong one.