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.
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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.
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Possible diagnosis
Highly probable
Probable
Possible
| I | With 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: |
| II | If 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: |
| 1 | pH meter | Test 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. |
| 2 | cable | If 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. |
| 3 | buffers | Ensure 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.
| 1 | filling solution | Ensure 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. |
| 2 | dirty connector | Inspect 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. |
| 3 | air bubbles | If 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 junction | Are 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 |
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see how to clean the junction
| Type of contamination | Color of contamination | Cleaning agent | Reaction time | Remarks | Applies to: |
|---|---|---|---|---|---|
| Silver sulfide | Black | Thiourea solution, order no. 51340070 (250mL) or 51340082 (six-pack) | 5...60 minutes | Leave until discoloration disappears. | Ceramic junctions |
| Proteins | Whitish discoloration, hardly visible | Pepsin-HCl solution, order no. 51340068 (250mL) or 51340069 (six-pack) | > 1 hour | Can also be used for internal cleaning. | Ceramic junctions |
| Proteins | Whitish discoloration, hardly visible | NaOH 2% | Approx. 20 minutes | Ceramic junctions | |
| Lipophilic substances | Ethanol or acetone | Approx. 30 minutes | Very suitable for edible oils. Possibly with the help of a soft brush. | All types of junction | |
| Soaps and surfactants | Hot water (80°C) | Approx. 12 hours | Rinse 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 minutes | Soak 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/L | Approx. 12 hours | Can also be used for internal cleaning. | All types of junction |