pH measurement in laboratory & pharma
Where the second decimal place counts: requirements, calibration practice and electrode choice for analytics and regulated environments.
Different league, different demands
In the laboratory, the pH value is rarely just a control figure - it is part of the result. Whether buffer for cell culture, release testing of an active ingredient, or titration: accuracies of ±0.01 to ±0.02 pH are required, reproducible and documented. No electrode achieves that incidentally; it is the interplay of the right measuring cell, clean calibration and disciplined handling.
The right laboratory electrode
- Design: A refillable liquid-electrolyte electrode - its continuously flowing electrolyte delivers the most stable reference potential and thus the most accurate readings.
- Junction by sample: Ceramic for clear solutions; a sleeve junction for protein-rich or fouling media (cell cultures, food analytics), because it can be cleaned.
- Special cases: Micro electrodes for sample volumes from a few hundred microlitres, special glasses for strongly alkaline media, temperature-resistant cells for hot samples.
- Temperature sensor: Integrated (ATC) saves the second probe and a classic source of error.
Calibrating at laboratory level
The standard is two-point calibration with buffers that bracket the measuring range (usually pH 7 and 4, alternatively 7 and 10). If you measure across a wide range, calibrate with three points. The quality of the buffers is decisive: traceable to standard reference materials, freshly opened, temperature-equilibrated - and never pour back into the bottle. The calibration data (zero point, slope, date) belong in a record; modern laboratory meters store them automatically.
GLP & co.: In regulated environments (GLP, GMP, ISO 17025), pH measurement is part of equipment qualification: defined calibration intervals, a documented electrode history, acceptance criteria for the slope (typically 95-105 % of the theoretical value). An electrode that fails the criterion is replaced - not talked into compliance.
Typical error sources in laboratory routine
The most common accuracy killers are banal: sample not stirred or stirred too hard (junction effects), buffer and sample at different temperatures, static charging on plastic vessels, ion-poor samples (ultrapure water needs special electrodes), and too little patience before the value settles. Master these five and you usually master the second decimal place as well.
Matching products: Laboratory electrodes, buffer solutions and analytics accessories are available in our shop aquaundco.com (German-language).