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Q: pH&ORP Cleaning Techniques
A: pH & ORP Cleaning Techniques

Cleaning and care of pH/ORP electrodes has been written about for decades. There are some points that need to be reviewed.

After use even in benign samples, the pH electrode performance may deteriorate, become slow and impede performance. At the outset of placing a new pH electrode into service, the user should make note of the response time, Eo and slope of the new pH electrode. These values should be used as benchmarks to determine the condition of your pH electrode.

After some period of use,the pH electrode may show signs of slow response, difficulty in calibrating, inaccurate readings or differences in Eo and slope from new performance. When your pH or ORP electrode gets slow, fouled and coated with contaminants, there are several things that you can do to restore performance.

The reason for this is that the sensing glass membrane becomes coated. These coatings range from organics such as oil, grease, proteins to metal ions or other contaminants.Removing them usually requires soaking or washing in a solution which will remove the coating; similar to cleaning your eyeglasses.
Metal ions, some proteins and organics can be removed by soaking the electrode in 0.01
N. HCl for 10-20 minutes. Follow this by soaking in pH electrode storage solution for 1-2 hours. Rinse the electrode in DI water, as usual, and recalibrate.

Next, the reference may become contaminated with various foulants. A single junction style pH electrode may become fouled with silver sulfide. This contamination is difficult to remove. The HCl treatment may help, but often, the best practice is to replace the electrode. If samples contain sulfides or similar silver-complexing agents, a double junction style electrode is recommended.

Proteins can be removed with HCl containing pepsin. Pepsin digests the protein and restores the glass surface. Soak the pepsin cleaned pH electrode in pH electrode storage solution and recalibrate.

Finally, grease and oils can be removed
from the measuring elements by washing with acetone or methanol and then warm tap water and dishwashing liquid. Use care if cleaning mechanically as the pH or ORP glass is fragile. Electrodes washed in this manner also need to be conditioned by storing in pH/ORP electrode storage solution and recalibrated.

When not in use, pH electrodes should be stored in pH electrode storage solution.
This insures that the electrode glass remains hydrated, ready to measure accurately and quickly. The storage solution also helps keep the reference junction fully charged with KCl. Storing pH electrodes in distilled or deionized water is not recommended.

As a last resort, you can etch your pH glass with a 0.01M HF solution. It is suggested that you only allow the glass to be in contact with HF for 30 seconds or less. Once you have dipped your electrode in HF, clean it immediately with deionized water and place it in a heated (40 ˚C) KCl saturated pH 4 buffer and allow the electrode to remain in the solution over night as the solution cools (once you place the electrode in the solution, remove it from the heat). Once you have completed this last step, recalibrate your electrode. It should be noted that if you are attempting this extreme measure, it is very possible that your electrode is at the end of its service life and should be replaced.


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