pH probe maintenance is a labor-intensive process but can greatly prolong sensor life. Automated cleaning offers an effective alternative that reduces labor overhead and increases safety.
Proper maintenance makes a big impact on the lifespan of pH electrodes. Without regular care and upkeep, incompatible chemicals such as proteins, sulfites, and heavy metals can damage or poison the electrodes, cutting their useful life from years to months.
Whether stored in a cupboard or installed in a process, it’s easy to let maintenance slide until issues begin to surface — the pH sensor may be slow to respond, it may give erroneous or widely inconsistent readings, or calibration becomes difficult. By the time you get around to cleaning and maintenance, it may be too late and replacement is the only option.
In other words, a regular cleaning schedule can extend the life of electrodes. although the frequency of cleanings will depend on the chemical makeup of your process.
How to Clean pH Electrodes
There are some important rules to keep in mind when you clean pH electrodes. Glass electrodes are, needless to say, extremely sensitive and can be permanently damaged without careful handling. As long as you follow a few simple rules when cleaning, you should be able to get the most out of your electrodes.
1. Do not “wipe” or rub the electrode
The glass used in the pH sensitive bulb of electrodes is delicate and it can only take your fingers or a piece of cloth to mar it. Rubbing an electrode can also create an electrostatic charge which may make the signal unstable and pH readings inaccurate.
2. Swirl the electrode gently in the cleaning solution
An electrode only needs to be soaked — not pressure-washed — to chemically remove contaminants, but take care when swirling in a cleaning solution; knocking the fragile electrode into the sides of the container during cleaning can crack or break it.
3. Gently rinse with deionized or distilled water
Once cleaning is complete, lightly rinse the electrode with deionized or distilled water as leaving traces of the cleaning solution on the electrode can itself cause damage over time and eventually destroy the pH sensor. Only use deionized or distilled water as tap water can create an electrostatic charge.
4. Store in a storage solution
You should never let your pH electrodes dry out. Any reference cell diaphragm/interface must remain wetted and the quality of the hydrated layer on the glass bulb must be maintained. A poor hydrated layer will make calibrating an electrode difficult and cause readings to be inaccurate. To maintain the hydrated layer, store your electrodes in a storage solution (3 molar KCI) when not in use. DO NOT store your pH electrode in distilled or deionized water as this will cause ions to leach out of the glass membrane and reference electrolyte, causing a slow and sluggish pH response when the electrode is used.
5. When possible, use a specialized electrode
Traditional glass pH electrodes will inevitably become exhausted over time, even when cleaned and maintained. If your process contains proteins, heavy metals, or sulfites, consider using a specialized electrode to ensure accurate readings and to prevent damage to the electrode. A probe with a different diaphragm type might be better for your application. For really aggressive applications, an electrode with a non-porous reference junction might be the best choice.
Which Cleaning Solution to use?
Your particular process and the residues you are trying to remove will determine which type of cleaning solution to use. There are a wide range of pre-mixed cleaning solutions available or you can make your own. Some solutions are hazardous so make sure to follow all safety instructions and wear the appropriate protection equipment.
Immerse the electrode in a warm (~120°F) 4.0 molar KCI (potassium chloride) solution or warm 4.01 pH buffer solution for about one hour.
Immerse the electrode in 0.4M HCl (hydrochloric acid) for about 10 minutes.
Fats, Oils and Greases:
Carefully rinse the electrode in a 1% (0.5N) HCl solution. If the buildup is heavy, soak the electrode overnight in a warm detergent and water solution — standard dish soap or laundry detergent works well for this. Follow the overnight soak with a deionized or distilled water rinse, and soak again for 10 minutes in 1% HCl. As an alternative, Methanol or Ethanol can also be effective.
Proteins can get into and block the reference porous junction. Soak the pH electrode in a 1% pepsin and 0.1M HCl solution for one hour to dissolve the blockage and reopen the junction. Alternatively, use an enzymatic cleaning solution (such as contact lens cleaner) overnight.
Sulfides can react with the silver ions in your pH electrode’s electrolytes, creating a precipitate and blocking the reference junction. In all-glass electrodes, this is easy to diagnose, because the reference electrode electrolyte will become discolored. Immerse the electrode in 0.1M Thiourea/HCl solution for one hour or until the discoloration disappears.
Immerse in a weak acid or vinegar solution until the deposits are dissolved from the glass bulb.
Immerse in 0.1M NaOH until the deposits are dissolved from the glass bulb.
A solution with heavy metals can scratch the glass of the pH electrode and wear away the hydrated layer essential to electrode performance. To rectify the issue, you can try regenerating your probe using a solution of hydrofluoric acid. It eliminates the top layer of glass, exposing the glass beneath.
Note: Hydrofluoric acid is extremely dangerous and requires full adherence to all safety precautions (goggles, gloves, and long sleeves or lab coat). Keep in mind:
- Only use the minimum amount needed.
- Ensure that the container you’re using is resistant to hydrofluoric acid — glass and many other materials are not.
- When cleaning the electrode, only allow the glass bulb of the sensor, not the main shaft, to touch the liquid, because it will corrode whatever it touches.
- Only submerge the sensor bulb into the acid solution for no more than a minute as any longer could destroy the electrode entirely.
- Immediately rinse the electrode thoroughly with deionized or distilled water and leave it to soak in a pH 7 buffer for about an hour.
After cleaning with any solution, rinse thoroughly with deionized/distilled water and soak the electrode in a storage solution overnight. If the electrode is a refillable electrolyte type, you should drain and refill with a fresh reference solution. After an overnight soak, test and recalibrate the pH electrode. If the electrode response is slow or the electrode does not calibrate correctly, it might just be that the probe is at the end of its life and you may just need to replace it.
While electrodes used in laboratories don’t require quite as much work due to their portable nature and periodic use, pH electrodes in process applications demand more resources as they are used continuously and can be difficult to access.
Cleaning Sensors While in Process
For process installations, devices are available to withdraw probes from the process while the process line is live. Safety interlocks in these retractable housings properly contain the process while the probe is removed for cleaning and servicing. These devices are fitted with cleaning ports to allow process fluids to be rinsed off of electrodes before they are removed and handled.
To reduce the labor-intensive maintenance process of cleaning to prolong sensor life, automation offers an effective alternative. An automated retraction system with a cleaning controller will retract a pH (or other parameter) probe from a process and execute a pre-programmed cleaning cycle without the need for anyone to be present.
The maintenance requirement on such a system is just to ensure that a local supply of the required cleaning solution(s) remains stocked. Automated systems allow pH probes to be cleaned far more frequently than manual cleaning alone and this translates into much longer electrode life.
The Final Solution
pH electrodes are not made to last forever, nor should they be expected to. Even with regular cleaning and maintenance, they will need to be replaced after one to three years. But to get the most out of your pH sensors and to keep readings accurate, the importance of maintenance cannot be overlooked.
Frequent manual cleaning or the installation and utilization of an automated cleaning system will ensure you don’t cut the intended lifespan short.
If your electrodes don’t tend to last, investigate other electrode types as an alternative.
Maybe you are using a probe with a small, tight pore ceramic diaphragm when a probe with a larger surface area PTFE diaphragm would perform better. PTFE diaphragms are often better in “dirtier” applications. For aggressive, poisoning applications, electrodes with ion traps or those without porous reference junctions will improve longevity.(although they too will also need to be replaced periodically).
And, if your sensors are not performing as well as expected, check your cleaning method. One of the solutions outlined above for pH electrode cleaning may be all you need to boost their performance.
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