Monitoring and controlling pH reside at the heart of many scientific applications and requires precise testing. Because pH can control the behavior of chemicals, the availability of nutrients, biological functions, and microbial activity, this measurement is one of the most commonly used across multiple industries, fields of study, and scientific laboratories.

General pH Equipment

Ensuring reliable data from your measurements is paramount to the success of your projects, regardless of industry or purpose. This means purchasing and maintaining the best equipment available. Common pH measurement equipment includes pH meters or testers, electrodes, and storage and cleaning solutions.

Typically, meter and tester maintenance and calibration can be done as needed, depending on the frequency with which the unit is used. Each manufacturer creates and publishes its operation manuals for the equipment it produces and sells. User manuals will contain information on cleaning and maintenance for these meters, whether benchtop or handheld.

pH Electrodes

pH electrodes require a little more maintenance than a testing unit. A wide range of different types of electrodes can be used with meters, depending on the nature and application of the pH test being conducted. The electrode type being used will dictate the maintenance required for its proper function. The following information provides good, general guidelines for the care and maintenance of testing and reference pH electrodes.

Like a car, quality pH electrodes require routine care such as flushing, refilling, and topping off necessary fluids. A pH electrode should never be stored dry; the pH-sensing bulb and reference junction must not dry out. Therefore, when not in use, it is recommended for the sensing bulb and reference junction to be immersed in a solution.

In keeping pH electrodes long-term (> 1 week), the pH electrode should be placed into a storage bottle with a protective cap and an appropriate storage solution.

If it is refillable, a pH electrode should be topped off with an appropriate fill solution and closed at the point of fill. If you have carefully taken these steps, your pH electrode should be primed to for the preparation steps outlined below as if it were a new electrode. If your pH electrode has not been stored properly, there is a chance the pH glass bulb has dried out and may require reconditioning steps to bring back the functionality.

Electrode Preparation

1. Unscrew the cap on the storage bottle and remove the electrode from the storage bottle. Save the bottle for storage after preparation if the electrode will not be used immediately.

2. Rinse the electrode with distilled water to remove any salt precipitate.

3. If the electrode is refillable, open the fill hole, drain the chamber, and refill with the appropriate fill solution.

4. The electrode should be ready for calibration; typically, you should calibrate with fresh pH 4.01 and pH 7.00 buffers traceable to NIST standards.

5. For calibration, the ideal slope from calibration is 92–102%, and an ideal offset (E0) from calibration is +/- 59 mV. Note: offset is the mV value reading of pH 7 buffer.

6. If calibration results are outside these ranges, follow the manufacturer’s electrode maintenance steps to help clean and recondition your electrode. If recalibration is not successful, try using fresh unopened pH buffers. If using fresh buffers does not result in a successful calibration, please consider replacing your pH electrode.

7. If the electrode will not be used immediately, soak it in pH electrode storage solutions.

Electrode Cleaning Tips

Soak electrodes in a solvent that will remove deposits. Thermo Scientific™ Orion™ 900020 pH Electrode Cleaning Solutions are designed to simplify pH electrode maintenance. When purchasing a cleaning kit, bottles of ready-to-use cleaning solutions are provided along with a small beaker to hold the solution and a plastic pipette for removing any internal electrode-filling solutions prior to cleaning.

Kit contains the following cleaning solutions:

• 0.1 M HCl for General Cleaning
• 1% Pepsin in HCl for Proteins
• Bleach for Disinfecting
• Detergent for Grease and Oil

Role of Solution Quality in pH Measurement

Using high-quality solutions to calibrate and maintain your sensors is the best way to ensure your data is accurate and reproducible. Avoid using low-quality, homemade, or expired buffers. Low-quality cleaning and maintenance products can lead to measurement errors, which require lengthy troubleshooting if identified. Unfortunately, these errors may sometimes go unnoticed without proper care and attention.

Recommended Shelf Life for Buffers and Solutions

TLD pH buffers and storage solutions are NIST traceable. All buffers and solutions have a two-year shelf life, and because they have a carbonate base, should remain stable and functional within that time frame regardless of whether they have been opened. Never pour used pH buffers, electrode-filling, or storage solutions back into the bottle.