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Illustration of pH values and calipers for article on pH Meter calibration.
Published On: March 15th, 2022
pH is an integral aspect to many scientific disciplines, where most scientists will likely consider pH at some point in their research or industry. For instance, did you know corals cannot properly grow their calcium skeletons when pH is too low? Ocean acidification directly threatens coral species, affecting all life in coral reefs, even microbes! Marine biologists may want to culture corals in a lab to study the effects of ocean acidification.To do this, they need access to a sensitive pH meter to measure various pH conditions. To ensure accurate measurements, they also need to understand how to care for and use ph meter calibration solutions to calibrate the pH meter properly.
Read on to understand what these solutions are and how they work!

How Do pH Meters Work?

Before understanding how to use liquid pH meters and pH meter calibration solutions, you must understand how common electronic pH meters work. Digital pH meters for laboratory consist of two major parts- a probe and an electronic visual display that work together to measure H+ ion concentrations in a solution. The probe contains two electrodes, including a sensory electrode and a reference electrode. When the probe is placed in a solution, the H+ ions in the solution remove the metal ions from the sensory electrode, thereby generating an electro-chemical flow.

The pH meter measures this flow and converts it into a corresponding pH value using the Nernst equation (where E= Eo – (2.3RT/nF) x pH) by comparing the generated voltage with that from the reference electrode’s voltage. This pH value is then presented on the electronic display.Based on this explanation, measuring pH is a sensitive process. Given electrodes can lose H+ with time and usage, routine calibration using pH meter calibration solutions is thus needed to ensure the pH meter can adjust itself accordingly and continue producing accurate readings.

What are pH Meter Calibration Solutions?

pH meter calibration solutions are buffers with pre-determined pH values that are used to ensure field and lab pH meters are reporting pH accurately. These solutions range across pH values, with the most common values being a pH of 4.01 (an acidic buffer), pH 7 (neutral buffer), and 10.0 (a basic buffer).

The range needed to calibrate pH meters in your lab should be described in the meter’s user manual or found under the calibration setting on the pH meter’s visual display. For example, hydroponic growers may use pH meters that only measure pH values between 4 and 7, encompassing the optimal pH for most crops.

Most calibration solutions will be pre-purchased from a supplier. These solutions will come in a large bottle, but the pH probe should not be placed directly in this bottle to avoid contamination. Instead, a smaller aliquot should be placed in a glass beaker with enough buffer to submerge the probe’s tip completely. The buffers used should be properly discarded after use.

Why Do We Use pH Meter Calibration Solutions?

pH meters measure aqueous (or water-based) liquid solutions. Therefore, liquid calibration solutions are critical to ensure the pH meter is properly working. Calibration should be done often and routinely, where it is recommended to calibrate the pH meter before each use. By doing so, the user is confident that their pH measurements are accurate.

By setting the pH meter to calibration mode and using multiple calibration buffers, the pH meter’s software can determine if it is working properly depending on two factors: the offset (which is an indication of the electrode’s millivolts (mV)) and the slope (which is the change in millivolts per pH unit). In neutral solutions (pH = 7), the pH meter should theoretically measure 0 mV, while acidic solutions at pH 4 will measure +177.48 mV, and basic solutions at pH 10 will measure -177.48 mV. If true, then the slope percentage should be 100%. (Note: This is the theoretical scenario when actual values in the lab are unlikely to be perfect.)

If the readings are off, this could indicate a few different scenarios, including the following: the pH meter calibration solutions are expired and need to be replaced; the pH meter probe is dirty and needs to be cleaned, or the pH meter probe is damaged and needs to be replaced.

How To Use pH Meter Calibration Solutions in Your Lab?

It is best to use pH meter calibration solutions at room temperature (25°C) for the best accuracy, given that changing temperatures can result in different pH values for some solutions. Specific pH meters may require particular instructions, but the general procedure for using pH meter calibration solutions is as follows:

  • Set the visual display to calibration mode and obtain the first pH meter calibration solution listed on the display (typically pH 4)
  • Remove the pH probe from the storage buffer
  • Rinse the probe with de-ionized water and gently pat dry with a Kimwipe
  • Aliquot a small volume of the first calibration solution in a small beaker
  • Submerge the electrodes of the probe in the solution without touching the glass
  • Continue calibrating until the visual display asks for the next pH meter calibration solution (usually pH 7).
  • Repeat until the process ends
  • Save the calibration data so the pH meter will use it for future readings

pH applications vary across the sciences, however, ensuring your pH meter is working accurately for your research or industry is vital to your lab’s throughput.

If you have any questions or inquiries about pH meters or pH meter calibration solutions, do not hesitate to contact Hudson Robotics!