pH is one of the most frequently measured properties of water. It governs how chemicals behave, whether aquatic life thrives and how quickly pipes corrode. This guide explains what pH is, how glass electrodes measure it, why they drift and need regular calibration, and how solid-state, calibration-free sensors differ.
What is pH?
pH is a measure of how acidic or alkaline a water-based solution is. Strictly, it describes the activity of hydrogen ions (H⁺) in the solution, expressed as a negative base-10 logarithm: pH = −log₁₀ a(H⁺). In plain terms, the more hydrogen ions there are, the lower the pH.
The scale is usually drawn from 0 to 14. Pure water at 25 °C sits at pH 7, which is neutral. Values below 7 are acidic and values above 7 are alkaline (or basic). Most natural fresh waters fall somewhere between about pH 6 and 8.5, while seawater is mildly alkaline, typically around pH 8.
Why the logarithmic scale matters
Because the scale is logarithmic, each whole pH unit represents a tenfold change in hydrogen ion activity. Water at pH 6 is ten times more acidic than water at pH 7, and a hundred times more acidic than water at pH 8. Even a 0.1 pH shift is a change of roughly 26% in hydrogen ion activity, so small, sustained changes matter.
Temperature and pH scales
pH depends on temperature, so a meaningful pH reading always comes with a temperature reading. There is also more than one pH scale in use. Most conventional electrodes, and ANB sensors, report on the NBS scale, whereas oceanographers often use the Total scale, so comparing readings across scales needs a conversion. Our FAQ covers this in more detail.
Why pH matters in water
Oceans and coastal waters
The ocean absorbs a large share of human carbon dioxide emissions. Dissolved CO₂ forms carbonic acid, which lowers pH and reduces the carbonate available to shell-building organisms such as oysters, mussels and corals. This is ocean acidification: average surface ocean pH has fallen by about 0.1 units since the industrial revolution, an increase in acidity of roughly 30%. Tracking it needs long, unattended deployments where returning to recalibrate a sensor is expensive or impossible.
Rivers, lakes and estuaries
In fresh and transitional waters, pH responds to geology, rainfall, photosynthesis and pollution. Agricultural runoff, wastewater discharges, misconnected drains and acid mine drainage can all push pH out of its normal range. Continuous monitoring catches short-lived events that spot samples miss.
Aquaculture
In recirculating aquaculture systems (RAS), fish respiration adds CO₂ and biofilter nitrification consumes alkalinity, and both push pH down. pH also controls how much ammonia is present in its more toxic, un-ionised form. Keeping pH stable supports fish welfare, feed efficiency and healthy biofilters.
Water treatment and industry
In drinking water treatment, pH affects coagulation, the effectiveness of chlorine disinfection and corrosion control in distribution networks. In industry and mining it is central to chemical dosing, discharge compliance and protecting equipment from corrosion and scaling. See how pH monitoring is used across these sectors on our applications page.
How is pH measured?
There are three broad families of method:
- Indicator dyes and test strips change colour with pH. Quick and cheap, but approximate and unsuited to continuous monitoring.
- Spectrophotometric methods measure the colour of an indicator dye precisely with an optical instrument. Very precise and used for reference-grade seawater work, but they need reagents and more complex equipment.
- Potentiometric (electrochemical) sensors measure a voltage that changes with pH. Almost all pH meters and online sensors work this way, and for around a century the dominant design has been the glass electrode.
How a glass pH electrode works
A glass pH electrode is really two half-cells in one body. The measuring half is a thin bulb of special pH-sensitive glass filled with a buffer solution. When the bulb is immersed, a hydrated gel layer on the glass surface exchanges hydrogen ions with the sample, generating a potential that depends on the difference in pH across the membrane.
The second half is a reference electrode, typically silver/silver chloride in a concentrated potassium chloride electrolyte, which is meant to provide a fixed, stable potential. It connects to the sample through a porous liquid junction that lets a tiny amount of electrolyte seep out.
The meter measures the voltage between the two: ideally about 59 mV per pH unit at 25 °C (the Nernstian slope).
The limits of glass electrodes
Glass electrodes work well in the laboratory. In the field and in process plants, their weaknesses become costly.
Fragility and storage
The sensing bulb is thin glass that can crack from impact, pressure, freezing or debris. The glass must also be kept hydrated: an electrode that dries out loses performance and needs reconditioning or replacement.
Reference junction drift
The weakest link is usually not the glass but the reference. Electrolyte steadily leaks out through the junction and is diluted; the junction can clog with sediment, biofilm or precipitates; and contaminants such as sulphides can poison the reference element. Each of these shifts the reference potential, so the reading slowly drifts away from the true pH even though nothing appears to be wrong. Low-conductivity fresh water and changing salinity add junction errors on top.
Drift is gradual and silent, which makes it particularly damaging to long-term datasets and dosing control.
pH sensor calibration: why glass electrodes need it
Calibration corrects for drift. The electrode is placed in buffer solutions of known pH (commonly pH 4, 7 and 10) and the meter's offset and slope are adjusted to match. A typical routine uses two or three buffers, with rinsing between each, temperature compensation and a record of the result.
Because drift never stops, calibration has to be repeated, often weekly and sometimes daily in demanding applications. Each calibration means a site visit or vessel time, fresh buffers, downtime and the risk of human error, and any drift between calibrations ends up in the data. Biofouling of the glass and junction makes things worse. Across a sensor network, the cost of calibration can quickly outweigh the purchase price.
Solid-state, calibration-free pH sensing
ANB Sensors was founded in Cambridge in 2015 to tackle these problems at their root. Rather than refining the glass electrode, ANB's patented solid-state technology replaces it.
- Stored wet or dry, deployed wet or dry. Glass electrodes must stay in solution at all times. Let one dry out and it degrades, so storage caps, wetting solutions and careful handling become part of every deployment. ANB sensors have no such limit. They can sit dry on a shelf, travel dry, and go straight into the water when needed, with no loss of performance. In the field, built-in immersion detection pauses measurements whenever the sensor is out of the water, and checks again every five minutes. That makes it well suited to tidal zones, splash zones, drainage outfalls and event-driven sampling, where conventional probes can't be left to dry out.
- No glass. The sensing element is a solid-state transducer, so there is nothing fragile to break.
- No reference drift to correct. ANB's technology is designed to eliminate the reference drift that forces glass electrodes into regular recalibration. The sensor runs internal reference checks and calibrates itself in situ, so no buffers or operator intervention are needed.
- Built-in biofouling resistance. A low concentration of biocide is produced locally at the transducer during measurement, inhibiting biofilm.
- Simple maintenance. The routine task is a brief abrasion of the transducer surface, prompted by the sensor's health number. The transducer can be replaced in the field.
- Comparable data. ANB sensors report pH on the NBS scale and are verified against standard buffer solutions and reference glass probes, so readings can be compared directly with conventional instruments.
The core sensor, AQ², measures pH from 2 to 10 with an accuracy of ±0.2 pH and a resolution of 0.01 pH, alongside temperature, in waters from 0.1 to 40 ppt salinity, so the same sensor works in rivers, estuaries and the open sea. Depth ratings run from 5 m to 1,250 m, and it integrates with buoys, sondes, FerryBox systems and underwater vehicles. The same sensor is at the heart of Process pH for drinking water and industrial process control, and of AquaLink pH and AquaView pH for aquaculture.
What “calibration-free” does and doesn’t mean
Calibration-free means you never calibrate the sensor with buffer solutions, because it handles calibration internally. It does not mean maintenance-free: plan for periodic surface refreshes, and allow around 30 minutes after power-up for readings to stabilise.
Glass electrode vs solid-state pH sensor
| Glass electrode | ANB solid-state sensor | |
|---|---|---|
| Sensing element | Thin pH-sensitive glass bulb | Solid-state transducer, no glass |
| Calibration | Regular manual calibration with buffers | Calibration-free; calibrates itself in situ |
| Reference drift | Gradual drift as junction and electrolyte change | Designed to eliminate reference drift |
| Storage | Must be kept wet | Wet or dry, no loss of performance |
| Biofouling | Fouls glass and junction | Locally generated biocide inhibits biofilm |
| Routine maintenance | Cleaning, recalibration, electrode replacement | Brief surface abrasion when the health number indicates |
Choosing the right approach
Glass electrodes remain a sensible choice for bench-top laboratory work, where an operator can calibrate before each session, and spectrophotometric methods remain the benchmark for reference-grade measurements. For long-term, unattended or hard-to-reach monitoring, such as moorings, rivers, RAS farms, treatment works and subsea infrastructure, the cost and risk of calibration and drift dominate. That is where a calibration-free, solid-state sensor earns its place.
Explore typical deployments on our applications page, or read our whitepapers on aquaculture, estuaries, drainage outfalls and vehicle deployment.