pH is a reference to how “acidic” or “alkaline” an aqueous (or water-soluble) solution is. SLP Sørensen is credited with creating the first pH scale back in 1909 as a way to denote this difference. This eventually evolved into the 0-14 pH scale scientists use today.
Liquids with more hydrogen (H+) ions compared to hydroxyl (OH-) ions have lower pH’s (pH < 7) and are more acidic, while substances with less H+ compared to OH- ions have higher pH’s (pH >7) and are more basic. Meanwhile, pure water contains equal H+ and OH- ions and is thus considered neutral with a pH equal to 7.
Obtaining sensitive pH measurements is integral to several biology, chemistry, medical, agricultural, and food science labs, amongst many others. For example, food scientists commonly add citric acid and lemon juice when canning their products to keep pH below 4.6.This acidic pH is crucial to preventing Clostridium botulinum spores’ growth, which release deadly neurotoxins that can cause botulism. But how can food scientists be sure their products are safe to consume? First, they’ll need to choose the right type of electronic pH meter!
Colorimetric vs Electronic pH Meters
There are two major measuring methods: colorimetric tests and electrochemical measurements.
Colorimetric tests involve adding dyes to the solution of interest, where the dyes change the color of the solution based on its pH. Examples of these dyes include phenolphthalein (measures pH from 8.2 to 10.0), bromothymol blue (measures from 6.0 to 7.6), and litmus (measures pH from 4.5 to 8.3).
Paper tests can also be used similarly, where one end of the paper, such as litmus paper, is dipped in the solution. The color of the paper then changes according to the pH of the solution.
Though often cheaper, these options tend to be limited in the pH range they can measure, where many options do not measure the full range in pH values. Furthermore, colorimetric measurements cannot detect specific pH values when used qualitatively. It is superior for most labs to have a digital pH meter for laboratory uses.
How Do Electronic pH Meters Work?
Electronic pH meters consist of two parts, including a probe and visual display. The probe contains two electrodes: a sensing electrode and a reference electrode. Commonly surrounding the electrodes is a thin glass membrane, which is further covered by a gel layer. When the electrodes are placed in a solution, H+ ions in the solution displace metal ions on the sensing electrode, thus generating an electrochemical flow. Using the Nernst equation, the pH meter’s software calculates this flow into a voltage converted into a corresponding pH value and compared to the reference electrode. The exact pH is then outputted on the visual display of the electronic pH meter.
Electronic pH meters are sensitive instruments capable of highly accurate measurements and therefore require proper care and cleaning to maximize lifespan. This includes cleaning the probes suitably after use, calibrating often, and storing the electrodes appropriately.
Types and Features of Electronic pH Meters
When deciding on the best laboratory pH meters, the user must consider the following aspects:
- Accuracy: Some labs can get away with pH measurements where accuracy is +/- 0.10, such as in scholastic learning environments, while others demand accuracy within +/- 0.001 pH, such as in pharmaceutical labs.
- Electrode type: Common electrodes use silver/silver chloride systems, but these systems can react with some substances, such as heavy metals and organic compounds. Mercury/calomel systems can then be used, but this system is more hazardous and requires additional care. Furthermore, some labs, such as food-grade labs, must avoid electrodes with glass membranes.
- Calibration: Some pH meters are specific to certain applications and may only calibrate and measure accurately within specific pH ranges. For example, plant scientists may only need pH meters ranging from 4 to 7, which most crops grow in.
- Memory: Depending on the size, software, and connections to the pH meter, memory can range from a few seconds to long-term storage.
- Special features: Some pH meters target specific features, where some labs require pH meters that measure across various temperatures, pH meters for viscous liquids, or maybe a specific pH meter used in microbiology labs is needed.
There are three major types of electronic pH meters: pen, handheld, and benchtop. Pen pH meters are the smallest type of electronic pH meter and are easily portable. However, these pH meters range in accuracy levels. Furthermore, replacing electrodes if they become old or damaged is not always possible..
Handheld pH meters tend to be more accurate compared to pen pH meters. However, they are not the most accurate type of electronic pH meter, and are better suited for field experiments than stable laboratory environments.
Benchtop pH meters are the most accurate type of electronic pH meter, offering electrode types that can fit the laboratories need, easy calibration settings, the most memory, and even additional features, such as temperature adjustments and viscosity compensation, depending on the supplier and model.
Manual vs Automated Electronic pH Meters
Like most things, benchtop lab pH meters can vary in their design and usefulness. Amongst the more advanced pH meters, automated pH meters tend to outperform manual pH meters across several characteristics and features. Automated pH meters reduce the risk of human error and offer more consistency between measurements. Being hands-off can be easier to manage while allowing the user more time to tend to more demanding tasks. Furthermore, automated electronic pH meters outperform human technicians in speed and sample processing within a given time for high throughput labs.
Hudson Robotics has been the leading supplier of automated products for over 38 years. To learn more on how your lab can benefit from automated electronic pH meters, speak to a representative today!