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Another comment about electrode cleaning. You may see procedures that use contact-lens enzymatic cleaning fluid, or ones that use various concentrations of sodium hydroxide.
These do have a purpose in a laboratory; if you're using a pH electrode in a bioreactor, for example, then the junction gets clogged with protein and lipids over time. The contact-lens enzymatic cleaners are generally pepsin or trypsin in an acidic solution, and these enzymes break down protein. Sodium hydroxide will turn oily lipids/fats into water-soluble compounds, and will also break down proteins to some extent.
The reason that I didn't mention these two is that in a reef the typical material clogging the junction in a pH electrode is calcium carbonate, either abiotically precipitated or associated with small critters like micro tube worms. Dilute HCl will dissolve this away.
It's also possible that the electrode junction has become clogged with fats/lipids/proteins, so you can try these other solutions. However, and in my experience, I rarely find that a 30 minute soak in 0.1N HCl doesn't do the job. So I'd recommend trying enzymatic solutions and/or sodium hydroxide as a 2nd attempt. The enzyme solution can't damage the electrode, but I simply find that it's ineffective for pH probes used in a reef. Soaking a probe in sodium hydroxide can work, but be aware that sodium hydroxide will actually dissolve crush-glass junctions, so you want to be cautious - use the specified concentration of sodium hydroxide in the procedure, and only soak the pH probe for the time specified.
These do have a purpose in a laboratory; if you're using a pH electrode in a bioreactor, for example, then the junction gets clogged with protein and lipids over time. The contact-lens enzymatic cleaners are generally pepsin or trypsin in an acidic solution, and these enzymes break down protein. Sodium hydroxide will turn oily lipids/fats into water-soluble compounds, and will also break down proteins to some extent.
The reason that I didn't mention these two is that in a reef the typical material clogging the junction in a pH electrode is calcium carbonate, either abiotically precipitated or associated with small critters like micro tube worms. Dilute HCl will dissolve this away.
It's also possible that the electrode junction has become clogged with fats/lipids/proteins, so you can try these other solutions. However, and in my experience, I rarely find that a 30 minute soak in 0.1N HCl doesn't do the job. So I'd recommend trying enzymatic solutions and/or sodium hydroxide as a 2nd attempt. The enzyme solution can't damage the electrode, but I simply find that it's ineffective for pH probes used in a reef. Soaking a probe in sodium hydroxide can work, but be aware that sodium hydroxide will actually dissolve crush-glass junctions, so you want to be cautious - use the specified concentration of sodium hydroxide in the procedure, and only soak the pH probe for the time specified.