Glass electrodes have been the standard way to measure pH for around a century, and they still do a good job on the laboratory bench. Out in the field, the picture changes: glass must be kept wet, its reference drifts, and it needs regular calibration with buffers. Solid-state, calibration-free sensors such as ANB’s AQ² take a different approach. They contain no glass, calibrate themselves in situ, and can be stored wet or dry and deployed wet or dry. This page compares the two side by side, sets out when each is the right choice, and summarises what our testing and field deployments show.
The short answer
- Choose a glass electrode for bench-top laboratory work and short-term measurements, where an operator can calibrate before each session and keep the electrode in solution between uses.
- Choose a solid-state, calibration-free sensor for long-term, unattended or hard-to-reach monitoring in rivers, estuaries, seawater, aquaculture, treatment works and subsea, where calibration visits are costly and drift between calibrations goes unnoticed.
- The biggest practical difference is storage and handling. A glass electrode must stay in solution at all times. An ANB sensor can sit dry on a shelf, travel dry and go straight into the water, with no loss of performance.
- Calibration-free is not maintenance-free. ANB sensors need a brief abrasion of the transducer surface when the sensor’s health number says so, instead of buffer calibration.
How glass and solid-state pH sensors work
Glass electrodes
A glass pH electrode develops a potential across a thin, hydrated layer of pH-sensitive glass in response to hydrogen ion activity. That potential is measured against a reference electrode, typically silver/silver chloride in a potassium chloride electrolyte, which connects to the sample through a porous liquid junction. The voltage follows the Nernst equation. The design is well established, but the glass must stay hydrated and the reference changes over time as electrolyte leaks out and the junction clogs or is contaminated. That change is the drift that regular calibration has to correct. Our guide to how a glass pH electrode works covers this in more detail.
ANB’s solid-state sensors
ANB’s patented solid-state sensors measure pH electrochemically too, but without glass. A pH-sensitive redox-active molecule on a working electrode shifts its peak potential as hydrogen ion concentration changes, and that shift gives the pH reading.
On its own, a redox-based sensor would still be exposed to reference drift. ANB’s design adds a second working electrode carrying a pH-independent redox molecule. It shares the same counter and reference electrodes, so its signal reflects only the reference potential. The sensor measures this reference tracker before each pH measurement cycle and compensates for any reference drift in real time. That is what allows ANB sensors to calibrate themselves in situ, with no buffers or operator intervention.
Glass vs solid-state pH sensor: side by side
| 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 in solution at all times | Stored wet or dry, deployed wet or dry |
| Out of the water | Degrades if it dries out | Immersion detection pauses measurement until the sensor is back in the water |
| Biofouling | Fouls glass and junction | Locally generated biocide inhibits biofilm |
| Routine maintenance | Cleaning, recalibration, electrode replacement | Brief surface abrasion when the health number indicates |
| Health feedback | No built-in indication of sensor health | Health number shows when abrasion or transducer replacement is needed |
| Fragility | Glass can crack from impact, pressure, freezing or debris | Mechanically robust; nothing fragile to break |
| Best for | Bench-top lab work and short-term measurements with regular calibration | Long-term, unattended, remote or intermittently submerged monitoring |
When a glass electrode is the right choice
Glass electrodes are a mature, widely understood technology, and for many jobs they are the sensible choice:
- Bench-top laboratory work, where an operator calibrates before each session and the electrode lives in its storage solution between measurements.
- Short-term and spot measurements, where the electrode is freshly calibrated and drift has no time to build up.
- Measurements outside pH 2 to 10. ANB’s AQ² measures pH from 2 to 10, so for samples outside that range a conventional electrode is the appropriate tool.
- Reference measurements. We use glass ourselves: our laboratory validation of AQ² benchmarks it against a daily calibrated, laboratory-grade glass pH electrode. For reference-grade seawater work, spectrophotometric methods remain the benchmark.
The limitations show up when a glass electrode has to work unattended for weeks or months. Between calibrations, drift, fouling and reference junction degradation can introduce unknown offsets, so the true accuracy of a glass electrode between calibration events is typically unknown.
When a solid-state, calibration-free sensor is the better choice
Long-term and unattended deployments
On moorings, buoys, rivers, boreholes and subsea infrastructure, every calibration means a site visit or vessel time, and any drift between visits ends up in the data. A calibration-free sensor removes the calibration visit and continuously tracks and compensates for reference drift in situ. AQ² is available in 5 m, 50 m, 300 m and 1,250 m depth ratings, has 8 GB of on-board storage and integrates with buoys, sondes, FerryBox systems and underwater vehicles.
Seawater, estuaries and changing salinity
AQ² measures pH in waters from 0.1 to 40 ppt salinity without recalibration, so the same sensor works in rivers, estuaries and the open sea. Low-salinity water is inherently harder for electrochemical sensors, because lower ionic strength increases solution resistance. In our laboratory testing, ANB sensors maintained stable and accurate pH measurements in both freshwater and seawater.
Aquaculture
In hatcheries, RAS, flow-through tanks and sea cages, glass breakage is more than a replacement cost: it is a contamination risk in active culture tanks. ANB’s solid-state design removes the risk of breakage entirely. Routine upkeep is a simple 30-second abrasion that untrained site staff can carry out, compared with glass probe calibration that can take up to 30 minutes and needs a range of equipment and solutions. For farms, AQ² is packaged as AquaLink pH and AquaView pH.
Dry storage, intermittent immersion and event-driven sampling
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 be stored wet or dry and deployed wet or dry. In the field, built-in immersion detection suspends measurement whenever the sensor is out of the water; in continuous mode the sensor sleeps in a low-power state and checks again every five minutes, resuming automatically once it is immersed. 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.
Drinking water and industrial processes
In treatment works and process plants, Process pH combines AQ² with an integrated controller, transmitter and touchscreen display. It has no glass, needs no reagents or storage solution, and notifies operators when a surface refresh is due.
What the evidence shows
The results below come from ANB’s own laboratory testing, our published white paper and customer and field case studies. Where a study used an earlier ANB sensor model, we name it.
Laboratory validation against a calibrated glass electrode
We validated AQ², our second-generation calibration-free sensor, in controlled laboratory tests against a daily calibrated and verified laboratory-grade glass pH electrode, which has a stated accuracy of ±0.2 pH immediately after calibration. Through acid and base additions in 10 ppt water and in tap water, with several AQ² sensors running at once, spot measurements with the freshly calibrated glass probe agreed with the AQ² readings within ±0.2 pH across the tested range. Longer runs, in freshwater at hourly intervals for 4.5 weeks and in seawater with continuous monitoring for 6 weeks, showed stable and accurate measurements in both.
Calibration alone doesn’t make glass electrodes agree
In the same study, two commercial glass pH probes were calibrated with the same standard pH 4, 7 and 10 buffers and then placed in freshwater and seawater test tanks. Despite identical calibration, their readings differed by up to 0.24 pH in freshwater and 0.11 pH in seawater. Electrode ageing, reference junction effects, temperature mismatch, ionic strength differences between buffers and real samples, and uncertainty in the buffers themselves all contribute.
Four months in flow-through seawater aquaculture
ANB sensors logged pH every 15 minutes in flow-through seawater tanks stocked with halibut for four months. They gave readings consistent with the site manager’s expected values for local seawater, captured natural changes of around 0.2 pH, ran without manual cleaning or recalibration, and resumed monitoring automatically after power outages. Read the aquaculture white paper (PDF).
Side by side with a glass sensor in coastal water
Off Oban on the west coast of Scotland, an ANB calibration-free sensor was deployed at 2 m depth from a stationary research vessel alongside a glass-based oceanographic pH sensor. Over 26 hours of measurements every 15 minutes, the ANB sensor recorded the tidal rise and fall of pH between 7.7 and 7.9. The glass electrode probe performed poorly in the same conditions and produced scattered readings. Read the coastal monitoring case study (PDF).
Biofouling over six weeks in the Mediterranean
With Universitat Politècnica de Catalunya, two ANB sensors were deployed for six weeks at 20 m depth on the OBSEA coastal observatory, a biologically active site. One scanned every 15 minutes and the other was left dormant. At the end, the scanning sensor’s transducer showed no evidence of biofouling, while the dormant sensor was completely covered in biogrowth. The measurement itself produces a low concentration of hydrogen peroxide at the sensor surface, which inhibits biofilm and is quickly diluted and broken down. Read the ocean case study (PDF).
Remote and rugged deployments
In the Antarctic, an ANB AQ5 sensor was integrated into a FerryBox on a research vessel. Its calibration-free design removed the need for frequent maintenance and recalibration, and its ability to be stored wet or dry made it convenient on remote missions (FerryBox case study, PDF). Offshore, Stinger Technology AS fitted an ANB OC300 sensor to an ROV to detect cement returns during drilling. As they put it: “The robustness of the sensor is also a huge plus—it doesn’t require careful storage, which is a real bonus in the field” (Stinger case study, PDF).
What to plan for
Calibration-free does not mean maintenance-free. Over time the surface molecules become less responsive after many electrochemical scans, so the transducer needs a light abrasion. How often depends on measurement frequency rather than elapsed time, and the health number tells you when. The transducer is replaced in the field after approximately 300,000 measurements. Allow around 30 minutes after power-up for readings to stabilise; in continuous mode the first measurement takes 4 minutes while the sensor runs its reference checks, and readings then follow every 5 seconds.
Choosing between glass and solid-state
A few questions usually settle it:
- How long will the sensor be left on its own? Hours on a bench favour glass. Weeks or months in the field favour a calibration-free sensor.
- How easy is it to reach? If every calibration means a boat trip, a confined space or a site visit, drift and calibration cost dominate.
- Will it dry out? If the sensor will be stored dry, shipped, or sit in a tidal, splash or intermittently flowing location, glass is a poor fit.
- Does fouling or salinity change? Biologically active water and shifting salinity both favour ANB’s solid-state design.
- What range and accuracy do you need? AQ² covers pH 2 to 10 with an accuracy of ±0.2 pH and a resolution of 0.01 pH. Check this against your requirement.
New to the topic? Start with what pH is and how it is measured. You can also explore typical deployments on our applications page, or browse the sensor FAQ for operational details.