Everything you need to know about pH measurement — the pH scale, how glass electrodes work, temperature effects, calibration, common problems, and industrial applications.
The pH scale is logarithmic — each unit represents a tenfold change in hydrogen ion concentration. pH 7 is neutral (pure water at 25°C). Values below 7 are acidic and values above 7 are alkaline (basic). Because the scale is logarithmic, the difference between pH 5 and pH 6 is not just 1 unit — pH 5 has 10 times more hydrogen ions than pH 6, and 100 times more than pH 7.
| pH Value | Classification | Common Example |
|---|---|---|
| 0 to 2 | Strongly acidic | Hydrochloric acid, battery acid |
| 2 to 4 | Acidic | Vinegar (pH 3), lemon juice (pH 2) |
| 4 to 6 | Weakly acidic | Coffee (pH 5), rain water (pH 5.6) |
| 7 | Neutral | Pure water at 25°C |
| 8 to 9 | Weakly alkaline | Seawater (pH 8), baking soda (pH 9) |
| 10 to 12 | Alkaline | Milk of magnesia (pH 10), bleach (pH 12) |
| 13 to 14 | Strongly alkaline | Caustic soda (pH 14), drain cleaner |
The glass pH electrode is the sensing element of any pH measurement system. It consists of a thin glass membrane at its tip — specially formulated to be selectively permeable to hydrogen ions. When the electrode is immersed in a liquid, hydrogen ions from the solution exchange with ions in the glass membrane surface, creating an electrical potential difference across the glass.
This potential, typically ranging from about +414mV at pH 0 to −414mV at pH 14 at 25°C, changes by approximately 59.2mV for each pH unit change (at 25°C). This relationship is described by the Nernst equation. The pH meter reads this millivolt signal from the electrode and converts it to a pH reading using the electrode's calibration.
Modern pH electrodes are almost always combination electrodes — they incorporate both the glass measuring electrode and a reference electrode in a single housing. The reference electrode provides a stable reference potential against which the glass electrode potential is measured. The reference electrode is typically filled with potassium chloride (KCl) solution, which diffuses slowly into the process liquid through a junction. This junction must remain wet and free from contamination for accurate measurement.
Temperature affects pH measurement through two mechanisms. First, the Nernst response of the glass electrode is temperature-dependent — at 25°C the slope is 59.16mV/pH, but at 0°C it is 54.2mV/pH and at 50°C it is 64.1mV/pH. If the instrument assumes a constant slope of 59.16mV/pH when the actual temperature is different, the pH reading will be incorrect. Automatic temperature compensation (ATC) using a PT100 sensor corrects for this electrode response change.
The second effect is more subtle — the true pH of the solution itself changes with temperature. For example, pure water is pH 7.0 at 25°C but pH 6.99 at 40°C. ATC corrects for the electrode, but the solution's own temperature-dependent pH change must be considered for the most demanding accuracy requirements.
Every glass pH electrode ages over time — its slope (millivolts per pH unit) and zero point (the potential at pH 7) drift slowly. Calibration corrects the instrument's conversion formula to match the actual electrode characteristics. Without regular calibration, pH readings become increasingly inaccurate.
Standard pH calibration uses buffer solutions — liquids with a precisely known, stable pH. The most common calibration procedure is two-point calibration:
Countronics pH indicators and controllers support front-panel digital calibration for pH 7 and pH 4 — no tools or software required.
| Problem | Likely Cause | Solution |
|---|---|---|
| Slow electrode response | Contaminated or aged electrode | Clean electrode, replace if old |
| Cannot calibrate to buffer | Electrode damaged or depleted | Replace electrode |
| Reading drifts continuously | Reference junction blocked | Clean junction, refill KCl |
| Reading jumps erratically | Broken glass membrane or electrical interference | Check cable, replace electrode |
| Reading offset from correct value | Calibration drift or wrong buffer used | Recalibrate with fresh buffers |
A glass electrode generates a millivolt voltage proportional to hydrogen ion activity. The Nernst equation converts this voltage to pH. At 25°C, each pH unit change produces approximately 59.2mV change at the electrode. The pH controller reads this voltage and displays pH after calibration correction.
The electrode's millivolt-per-pH-unit response changes with temperature. ATC using a PT100 sensor automatically corrects the reading for this temperature effect, ensuring accurate pH measurement across varying process temperatures.
At minimum once per day for continuous online monitoring, or before each use for batch measurement. Use fresh pH 7 and pH 4 buffers. In dirty industrial processes, calibrate more frequently and inspect electrode condition regularly.
Membrane contamination by oils or chemicals, drying out of the reference junction, poisoning by sulphides or heavy metals, physical cracking, or KCl electrolyte depletion. Symptoms: slow response, inability to calibrate, or continuous drift.
CPCB standards require pH 6.5 to 8.5 for effluent discharged to inland surface water. The Countronics 112pH dual-limit controller automates neutralisation to maintain pH within this range continuously.
Countronics manufactures pH indicators, controllers, and combined pH+ORP systems for water treatment, ETP, STP, pharma, and food applications. Contact us for recommendations.