An important aspect to examine when caring for and maintaining a pH sensor are the maintenance and storage solutions. Let us review proper handling and evaluate the difference between storage, calibration and cleaning solutions for a pH sensor. Understanding the difference between each and knowing what solutions to use in certain circumstances will ensure you are receiving optimal performance and life out of your sensors!
Storage Solution
Never leave a glass pH electrode dry! Glass pH sensors come out of the box with a rubber boot around the bulb. This storage cap contains a solution that prevents the glass bulb of the sensor from drying out. The best solution to use when storing your pH sensor is 3 mol/L KCl (Potassium Chloride). The storage solution is similar to the internal electrolyte of the sensor and will guarantee the best hydration while your sensor sits on the shelf. If KCl is unavailable, you can store your pH sensor in a pH buffer 4.0 or 7.0.
Under no circumstances do you ever want to store a glass electrode in distilled or deionized water! Due to the minimal number of ions present in the water, storage in these solutions will cause the ions in the sensor’s electrolyte to “leach” out. This will impact accuracy and drastically shorten the life of the sensor.
Calibration Buffers
Accurate calibrations equal accurate measurements! Calibrate your pH sensors using pH buffers. A pH buffer solution removes hydrogen and hydroxide ions, thus ensuring a known stable value typically within ± 0.02 pH units. NIST (National Institute of Standards and Technology) and DIN (German Institute for Standardization) certify most pH buffers on the market and guarantee precise values.
Below are some helpful tips when working with pH buffers to achieve the best results:
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Always work with fresh unexpired buffers! pH buffers do have an expiration date. You should monitor them accordingly. When working with pH buffers you must prevent contamination and discard buffer solution after each use. Performing “offline” calibrations with single dose containers is highly recommended!
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Perform at least a 2-point calibration with pH buffers that bracket your measuring range (e.g. use pH buffers 7.0 and 4.0 when normal pH value of process is pH 5.0). Although not a must, we recommend using a pH buffer 7.0 during each calibration.
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pH buffers are also temperature dependent! Examine your paperwork or the label that comes with your pH buffer. Buffer values are commonly stated at a temperature of 20° or 25°C. As an example, a pH buffer of 4.00 at 20°C has a pH value of 4.04 at 5°! It is critical to use the temperature compensated value of the buffer when performing your calibration. We recommend using “automatic” calibration if available. This function takes care of the compensation for you.
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Cleaning Solutions
Contamination of the pH glass membrane and within the sensor’s junction can cause slow response time and inaccurate measured values. Cleaning your sensor chemically as opposed to using a brush or rag is recommended due to the potential for damage or static charge on the Hydrogen ion sensitive glass. Below are some best practice tips when chemically cleaning your pH sensor:
Contamination in Acidic Processes
Use a strong base solution such as 5% Sodium Hydroxide (NaOH) to clean coatings that occur in acidic processes. Soak the sensor for 5 to 10 minutes in the solution. Rinse the sensor with distilled or DI water then submerge in storage solution for at least another 10 minutes before using the sensor.
Contamination in Basic Processes
Clean the sensor with a strong acid such as 5% HCl or Muriatic Acid. Submerge both the pH glass membrane and junction in the solution for 5 to 10 minutes to dissolve or loosen any build-up. Rinse the sensor with distilled or DI water then submerge in storage solution for at least another 10 minutes before using the sensor.
Contamination due to Proteins
Proteins, although not visible, will coat the pH glass membrane and junction. Submerge the sensor in a weak HCl or Pepsin solution for several hours to remove the proteins. Rinse the sensor with distilled or DI water then submerge in storage solution for at least another 10 minutes before using the sensor.
Contamination from Fats and Oils
Remove fats and oils using isopropyl alcohol, detergent, or acetone. Submerge the sensor for 5 to 10 minutes in the cleaning solution. Clean off any residual solvent with distilled or DI water then submerge in storage solution for at least another 10 minutes before using the sensor.
By using these best practices when caring for your pH sensor you will ensure optimal performance of online measurements and long life from your sensors! The proper (fresh) solutions for storage, calibration, and cleaning are a must!
