Welcome to the METTLER TOLEDOpH Electrode Troubleshooting Guide

The electrode is the key to successful analysis. Since the electrode is the only part of the analytical instrument that is in direct contact with the sample, its selection and maintenance has the strongest influence on precise and accurate measurements. However, a pH electrode that has been correctly selected and that has been working properly may nevertheless suddenly start performing badly. In this 'pH TroubleShooter' we will help you to identify possible reasons for this and suggest a number of procedures for restoring the electrode to its original performance.


Before beginning the detailed electrode diagnosis

Have you made sure that ...
...neither the instrument nor the cable are faulty, and the buffers are fresh ?
...the electrode is correctly filled with electrolyte, the connectors are clean and the tip of the electrode is free of air bubbles ?
...there are no visual signs of blockage of the reference junction ?

Step by step

1.Choose one ore more symptoms that describe your problem best.
2.Rate each symptom by moving the sliders.
3.One symptom may very often have various causes.These are listed according to their statistical probability.
4.Click on the according button to see possible restorative measures.


does not apply
possibly applies
weak symptoms
strong symptoms
reduced slope (80..90%)
very small slope (smaller than 80%)
slow or fluctuating reading
zero point shift

Possible diagnosis

Highly probable



IWith the meter set to read mV, dip the electrode in pH 7 buffer. The reading should be ±30mV with an Ag/AgCl reference. Next, read a pH 4 buffer. The reading should be greater than 150mV different from the 7 potential. If not, proceed as follows:
IIIf the system is not performing well, the next step is to find out whether the electrode cable and the instrument are working properly. This can be done by checking one element of the measuring chain at a time:
1pH meterTest the pH meter with the shorting clip (standard delivery) or the Test Plug Set (51302402). If the instrument does not perform as described in the operating instructions call METTLER TOLEDO Service.
2cableIf you are using an electrode with detachable cable, the best way to find out whether the cable is defective or not is to replace it by an identical one. If you do not have a spare cable, or if you are using a hard wired electrode watch whether there is a change in signal on the instrument while you bend the cable.
3buffersEnsure that you are using the correct buffers in the right sequence, and that they are fresh.

Before testing an electrode, examine it closely. Visual inspection can very often provide important clues about the cause of the problem. The cause might be something as simple as a dirty connector or an air bubble in the tip of the electrode.

1filling solutionEnsure that the electrolyte is above the internal elements. Empty, rinse and refill the electrode reference chamber, if the electrolyte looks contaminated. Ensure that you are using the correct electrolyte and that the electrolyte fill port is open.If you do not know the correct electrolyte of your electrode, you will find this information printed on the sensor shaft, in the operating instructions or on the Internet.
2dirty connectorInspect and clean all connectors, including the meter socket. If you are using an electrode with MultiPin™ or S7 connector make sure they are free from KCl crystals or other deposits. Dirty or corroded connectors lead to erranous readings.
3air bubblesIf you can see air bubbles inside the electrode, gently shake the electrode downward in the same manner as a clinical thermometer to remove them. Electrodes with gel electrolyte should be put upright in warm water to remove air bubbles.
blockage of junctionAre there signs of blockage or discoloration of the reference junction? To check whether the electrolyte flows through the junction, hang the electrode in the air for 30 minutes. If there is no KCl creeping out and crystallizing, proceed to 'how to clean the junction'. For hydration purposes, do not forget to put the electrode in storage solution after this test!

see how to clean the junction

see how to clean the junction

Type of contaminationColor of contaminationCleaning agentReaction timeRemarksApplies to:
Silver sulfideBlackThiourea solution, order no. 51340070 (250mL) or 51340082 (six-pack)5...60 minutesLeave until discoloration disappears.Ceramic junctions
ProteinsWhitish discoloration, hardly visiblePepsin-HCl solution, order no. 51340068 (250mL) or 51340069 (six-pack)> 1 hourCan also be used for internal cleaning.Ceramic junctions
ProteinsWhitish discoloration, hardly visibleNaOH 2%Approx. 20 minutesCeramic junctions
Lipophilic substancesEthanol or acetoneApprox. 30 minutesVery suitable for edible oils. Possibly with the help of a soft brush.All types of junction
Soaps and surfactantsHot water (80°C)Approx. 12 hoursRinse sensor well with hot water. Afterwards, immerse in hot water and leave to cool for approx. 12 hours. Only use tap water, not distilled or de-ionized water.Ceramic and open junctions
All possible types of contamination. First recommendation for removing unknown substances.Hot electrolyte(50-60°C)Hot electrolyte(50-60°C)Few minutesSoak the electrode in hot (50-60°C) reference electrolyte for a few minutes.Ceramic junctions
All possible types of contamination. Second recommendation for removing unknown substances.HCl 0.1 mol/LApprox. 12 hoursCan also be used for internal cleaning.All types of junction

see regenerating the membrane

1changes in the gel layerSlow response or a reduced calibration slope are a result of changes in the gel layer on the pH glass. This can very often be observed with older electrodes or electrodes that have been stored dry. Similar effects are noticeable when an electrode is used for non-aqueous applications because the gel layer is dehydrated. The pH-sensitive glass membrane is reactivated using an Ammonium Bifluoride Reactivation Solution, order no. 51350104 (25mL). Please note: Since this solution is extremely aggressive, make sure that you follow all the neccessary safety precautions, i.e. wear protective goggles, a laboratory coat and chemical-resistant gloves. Keep the volume of solution to a minimum by using a small acid-resistant vessel. Dip the tip of the electrode into the reactivation solution for 1 to 5 minutes. The maximum immersion point is level with the upper shoulder of the pH-sensitive glass. Never dip the sensor shaft into the solution because the acid will damage it. Then rinse the electrode thoroughly with water and condition it for approximately an hour in pH 7 buffer solution. Finally, place the electrode overnight in the reference electrolyte specific to that sensor.
2Calcium scaleSometimes a white Calcium scale is visible, this might affect the pH readings. It can easily be removed with acetic acid. The recommended treatment time is 30 minutes. For regeneration purposes, place the electrode overnight in the reference electrolyte specific to that sensor.
3irreparable damageDespite all corrective procedures mentioned, it is not always possible to repair a faulty electrode. If damage has occured to the wiring or casing, or if the electrode is too old, it is irreparable. In such cases the electrode must be replaced.