A standard pH sensor placed in ultrapure water does not measure the pH of the water. It measures a drifting, unstable number that is influenced by the sensor’s own reference cell leaching ions into the sample, by CO₂ absorption at the liquid junction, and by the extremely low ionic strength of the water itself. The reading looks like a pH value — it appears on the transmitter display, it logs, it trends — but it does not correspond to the actual pH of the process. In ultrapure water applications, a standard pH sensor is not just inaccurate. It is measuring a different thing entirely.
What Makes Ultrapure Water Difficult for pH Measurement
pH measurement in a standard electrode relies on two things: the glass membrane, which generates a millivolt potential proportional to hydrogen ion activity, and the reference cell, which provides a stable, known potential against which the glass potential is measured. The difference between the two potentials is the pH signal. For this to work, the reference cell must maintain a stable junction potential where its electrolyte meets the sample.
In a normal process fluid — an acid, a buffer solution, a cooling tower blowdown — there are abundant ions in the sample. The reference electrolyte diffuses out through the junction at a controlled rate, and the junction potential stabilises quickly. In ultrapure water, there are almost no ions. The conductivity of pharmaceutical-grade ultrapure water is 0.055 µS/cm at 25°C (the theoretical minimum for pure water). The ionic strength is so low that even the minute leaching of potassium chloride from a standard reference junction dominates the ion composition of the sample near the junction. The reference contamination creates a localised pH zone at the junction that differs from the bulk water. The sensor is measuring its own contamination.
CO₂ makes this worse. Ultrapure water has no buffering capacity — it cannot resist pH shifts from dissolved gas. Atmospheric CO₂ dissolves into ultrapure water and forms carbonic acid, dropping pH toward 5–6 in a matter of minutes at an open surface. A flow-through cell reduces this exposure, but in a typical pH measurement installation with open access to the atmosphere, the CO₂ effect overwhelms any meaningful pH signal from the process.
How the LECOL B64 Hydrionis Solves This
The LECOL B64 uses a non-flow reference design, protected by US Patent 11860117 B2 (with equivalent patents in Taiwan M628015 and China ZL 202122720763.1). The reference cell in the B64 does not flow electrolyte into the sample through a porous junction. Instead, the reference maintains a stable, fixed concentration of KCl at the junction without continuous outward diffusion. This is the fundamental engineering departure from standard reference cell design: the reference is stable not because it replaces the electrolyte it loses, but because it is not losing it in the first place.
The practical result is that the B64 can operate stably in water at conductivities down to 0.069 µS/cm — approximately 25% above the theoretical minimum for pure water — without reference contamination affecting the sample. The sensor does not introduce ions into the measurement zone, which means the reading reflects the actual hydrogen ion activity of the process rather than the superimposed effect of reference electrolyte outflow.
The B64 is built with an ABS body and is rated to 80°C and 6 bar at 80°C. pH range is 0–10, which covers the full range encountered in ultrapure water systems. Process connection is 3/4″ or 1″ NPT. Temperature compensation uses a PT100 or PT1000 RTD. The glass option is standard hemi-glass for ultrapure water — not a low-ionic-strength glass, because the reference design handles the low-ionic-strength problem, not the glass membrane.
Where the B64 Is Used
Ultrapure water applications include pharmaceutical water-for-injection (WFI) systems, semiconductor fab rinse water, high-pressure boiler feedwater, and laboratory-grade reagent water preparation. In all of these, the downstream process or regulatory requirement demands a meaningful pH measurement, not a reading dominated by sensor artefacts.
In pharmaceutical WFI systems, pH measurement is a pharmacopoeial requirement — both USP and EP specify pH limits for WFI and purified water. A sensor that introduces ions into the sample and produces a drift-affected reading does not satisfy the intent of the requirement, even if it produces a number. The B64’s non-flow reference design is directly suited to this application: stable measurement without sample contamination.
In semiconductor wafer rinsing, the ionic content of the rinse water directly affects the number of metallic contaminants deposited on the wafer surface. A standard pH sensor leaching potassium and chloride into the rinse water is an ionic contamination source. This is not a theoretical concern in a 5-ppb ionic budget environment — it is a real process impact.
The Comparison Against a Standard Sensor
A standard pH sensor placed in ultrapure water typically shows unstable readings — drifting 0.5–1.5 pH units over hours, with sensitivity to flow rate, sample temperature, and proximity to the liquid junction. Re-calibration helps temporarily but does not address the fundamental instability, because the calibration buffer has sufficient ionic strength to stabilise the sensor, but the process water does not. The sensor that calibrates at pH 7.00 in a standard buffer is a different device in ultrapure water: its junction potential is different, its response time is different, and its reading has no reliable relationship to the process pH.
The B64 resolves this not by using different calibration buffers or a different glass membrane, but by removing the source of the instability — the flowing reference junction — and replacing it with a fixed-potential, non-contaminating reference design. The result is a sensor that behaves predictably in ultrapure water the same way a standard sensor behaves predictably in process water: stable, calibratable, and representative of the actual process pH.
The LECOL B64 Hydrionis is available through Autoflo Technology. Contact us at info@autoflotechnology.com for specifications and application advice.