pH glossary

85 technical terms of pH measurement, each explained in one or two sentences - for quick reference and further reading.

Look it up instead of searching: every term is explained in a self-contained way. Where a matching topic page exists, a link leads straight to it.

A

Acid: Substance that releases hydrogen ions in water and lowers the pH below 7. Examples: citric acid, hydrochloric acid, carbonic acid. Learn more →
Acid error: Measurement deviation in extremely acidic solutions (below about pH 1), where the glass membrane shows values that are too high. In practice much less often relevant than the alkaline error. Learn more →
Activity: The effective concentration of ions in a solution. Strictly speaking, pH electrodes measure the activity of hydrogen ions, not their pure concentration - in dilute solutions the difference is negligible.
Adjustment: Strictly speaking, setting the meter to the electrode based on the calibration values. In everyday use, the whole process is usually simply called “calibration”. Learn more →
Alkaline error (sodium error): Measurement error at very high pH values (from about pH 12): sodium ions fake a pH that is too low to the glass membrane. Special glasses reduce the effect. Learn more →
Antimony electrode: pH electrode made of metallic antimony instead of glass. Less accurate than glass electrodes, but insensitive to hydrofluoric acid - its classic field of use.
Asymmetry potential: Small residual voltage a glass membrane delivers even when the same solution is present inside and outside. It arises from manufacturing differences between the membrane sides and is compensated during calibration.

B

Base (alkali): Substance that binds hydrogen ions in water and raises the pH above 7. Examples: caustic soda, ammonia, soap solution. Learn more →
BNC connector: The de-facto standard connector for pH electrodes on aquarium controllers, pool equipment and many hand-held meters. BNC electrodes are interchangeable across brands. Learn more →
Buffer capacity: The ability of a water or solution to absorb additions of acid or base without changing pH strongly. Water with low buffer capacity is prone to pH jumps.
Buffer solution: Solution with a precisely known, stable pH (commonly pH 4, 7 and 10) for calibrating pH electrodes. Buffers keep their pH constant even with small contaminations. Learn more →

C

Calibration: Adjustment of the electrode against buffer solutions of known pH so the meter can correct the electrode's individual deviations. Without regular calibration, every electrode eventually measures wrongly. Learn more →
Calomel electrode: Historic mercury-based reference system, now largely replaced by silver/silver chloride. Old calomel electrodes belong in hazardous-waste collection because of the mercury.
Carbonate hardness (KH): The hydrogen carbonate content of water, which stabilises the pH as a buffer. In aquaristics, KH, CO₂ content and pH are directly linked. Learn more →
Ceramic junction: The standard junction design: a porous ceramic pin in the shaft. Low electrolyte outflow, but sensitive to clogging by proteins or suspended solids. Learn more →
Cleaning solution (pepsin-HCl, thiourea): Special solutions for chemically cleaning pH electrodes: pepsin-HCl dissolves protein deposits, thiourea removes sulphide deposits on the junction. Learn more →
Combination electrode: Design that combines measuring and reference electrode concentrically in one shaft. It is the standard today - when people say “pH electrode”, they almost always mean a combination electrode. Learn more →
Conductivity: Measure of the ion content of a water. Very low-ion water (ultrapure, reverse-osmosis water) conducts poorly and makes pH measurement unstable - special electrodes help here. Learn more →
CO₂ control via pH: Control principle in planted aquariums: a pH controller measures continuously and steers the CO₂ supply via a solenoid valve, since dissolved CO₂ lowers the pH. Learn more →

D

Digital sensor: pH electrode with built-in electronics that digitises the signal directly in the head and stores its own calibration data. It can be calibrated in the lab and simply plugged in at the measuring point. Learn more →
DIN connector: Multi-pin connector type on some laboratory and branded devices, sometimes with integrated temperature-sensor contacts. Check the device connector carefully before buying an electrode. Learn more →
Double-junction system: Reference system with two separate electrolyte chambers and junctions. It protects the lead-off system from sample components - for example in silver-, sulphide- or protein-containing media. Learn more →
Drift: Slow, steady wandering of the reading although the sample does not change. Common causes: dirty junction, ageing electrode or temperature fluctuations. Learn more →

F

Fibre junction: Junction made of a fibre bundle, often found in inexpensive electrodes. Simple and functional, but more sensitive to clogging than sleeve or open-hole junctions. Learn more →
Flat membrane: Membrane shape for measurements on surfaces such as skin, paper or agar plates, where nothing can be immersed. Learn more →
Flow-through fitting: Fitting that routes a defined partial stream of the line past the electrode membrane - for stable measuring conditions, for example in pool measuring cells and drinking-water systems. Learn more →

G

Gel electrode: pH electrode whose reference electrolyte is bound as a gel and permanently sealed. Maintenance-free and position-independent - the first choice for aquariums, pools and continuous use. Learn more →
Glass membrane: Wafer-thin membrane of special glass at the tip of the pH electrode, usually shaped as a small bulb. The pH-dependent voltage evaluated by the meter arises at its surface. Learn more →
GLP (Good Laboratory Practice): Quality framework for laboratories that prescribes, among other things, documented calibrations, electrode history and acceptance criteria for slope and zero point. Learn more →
Ground-glass (sleeve) junction: Junction made of a ground glass ring with a large transition area. It can be cleaned mechanically by sliding and is therefore suited to fouling, protein- or particle-rich media. Learn more →

H

High impedance: Property of the electrode signal: the glass membrane has a very high internal resistance (megaohm range). This is why pH measurements need special inputs and suffer from moisture or poor connections.
Hydrated layer: Nanometre-thin, water-containing layer on the glass membrane into which hydrogen ions embed themselves. It makes the measurement possible in the first place - which is why the membrane must never dry out or be rubbed mechanically. Learn more →
Hydrofluoric acid (HF): Acid that chemically attacks glass and permanently destroys any glass membrane. In HF-containing media, antimony or ISFET electrodes are used. Learn more →
Hydrogen ions (H⁺): Positively charged hydrogen particles in a solution. Their concentration determines the pH value: the more H⁺ ions, the more acidic the solution. Learn more →

I

Immersion fitting: Holder that hangs the electrode from above into basins or vessels and protects it mechanically - standard in treatment plants and open basins. Learn more →
Inner buffer: Buffer solution with a known, constant pH sealed inside the glass electrode. It forms the reference side of the glass membrane. Learn more →
Ionic strength: Measure of the total concentration of all ions in a solution. It influences the activity of the hydrogen ions and thus slightly the measured pH value.
IP rating: Marking of how well a connector head or meter is protected against dust and water (e.g. IP65, IP67). Relevant for outdoor use, pool equipment and humid environments. Learn more →
ISFET: Ion-sensitive field-effect transistor: semiconductor-based pH sensor without a glass membrane. Unbreakable, dry-storable and very fast - ideal for food and rough environments. Learn more →

J

Junction (diaphragm): Porous transition point in the electrode shaft that electrically connects the reference system to the sample. A clogged junction is the most common cause of sluggish or drifting readings. Learn more →

L

Lifespan: The service life of a pH electrode: one to three years with good care. Electrodes also age chemically when unused; heat, strong alkalis and proteins shorten the lifespan considerably. Learn more →
Liquid junction potential: Small interfering voltage at the junction that arises when ions migrate at different speeds between electrolyte and sample. KCl as electrolyte keeps it low because both ions are similarly mobile.
Liquid-electrolyte electrode: Refillable pH electrode with liquid reference electrolyte. The continuous electrolyte outflow keeps the junction clear and delivers faster, more precise measurements - the laboratory standard. Learn more →
Logarithmic scale: The pH scale is logarithmic: one unit of difference means ten times the hydrogen ion concentration. pH 5 is therefore ten times more acidic than pH 6. Learn more →

M

Measuring electrode: The half-cell of a pH measuring chain that delivers the pH-dependent signal - in classic electrodes, the glass electrode with its membrane. Learn more →
Membrane glass: The special glass of the pH membrane, whose composition is matched to the application: standard glass, low-alkaline-error glass for high pH values, or temperature-resistant variants for hot media. Learn more →
Micro electrode: Particularly slim pH electrode for tiny sample volumes from a few hundred microlitres or narrow vessels such as NMR tubes. Learn more →
Multi-point calibration: Calibration with three or more buffer solutions for measurements across a wide pH range, common in laboratories and analytics. Learn more →
mV mode: Display mode of the pH meter showing the raw electrode voltage in millivolts. Useful for diagnosis: in pH 7 buffer, a healthy electrode should deliver close to 0 mV. Learn more →

N

Nernst equation: Physical equation describing the relationship between ion concentration and electrical voltage. It is the theoretical basis of pH measurement: about 59 mV per pH unit at 25 °C. Learn more →
Nernst factor: The theoretical voltage change per pH unit: 59.16 mV at 25 °C. It rises with temperature - at 60 °C it is already about 66 mV. Learn more →
Neutral point: pH 7 at 25 °C: the point at which an aqueous solution is neither acidic nor alkaline. Pure water sits here. Learn more →

O

Open-hole junction: Open bore as the transition between reference system and sample, typical of gel electrodes. Practically never clogs and makes the electrode particularly low-maintenance. Learn more →

P

PG13.5 thread: The widespread mechanical standard for installing process electrodes: PG13.5 screw-in thread with a 12 mm shaft and a typical length of 120 mm. Learn more →
pH measuring chain: The complete measuring arrangement of measuring electrode and reference electrode. In the combination electrode, both are united in one shaft. Learn more →
pH meter: The instrument that measures the electrode's high-impedance voltage and converts it into the pH value. The electrode only delivers the signal - display and calibration logic sit in the meter. Learn more →
pH value: A measure of how acidic or alkaline a liquid is, on a scale from 0 to 14. Values below 7 are acidic, 7 is neutral, and above 7 is alkaline. Learn more →
Polymer electrolyte: Solid plastic electrolyte in rugged process electrodes. Even less sensitive than gel to pressure and fouling, at the cost of a somewhat longer response time. Learn more →
Potassium chloride (KCl): The standard salt for reference electrolyte and storage solution, usually in 3-molar concentration. KCl conducts well and does not disturb the measurement because both ions migrate at similar speeds. Learn more →
Potentiometry: The measuring principle of the pH electrode: determining an ion concentration by measuring an electrical voltage between two electrodes without current flow. Learn more →
Protective cap (soaker cap): The cap over the glass membrane, filled with KCl storage solution. It protects the membrane mechanically and keeps the hydrated layer moist. Learn more →

R

Redox potential (ORP): Measure of the oxidising or disinfecting power of a water, measured in millivolts with a redox electrode. Not to be confused with pH: both electrodes look similar but measure different things. Learn more →
Reference electrode: The second half-cell of every pH measurement: it provides a constant comparison potential against which the voltage of the glass membrane is measured. Learn more →
Reference electrolyte: The conductive solution in the electrode's reference system, usually 3-molar potassium chloride solution. It establishes electrical contact with the sample via the junction. Learn more →
Reference system: The entirety of lead-off element (usually Ag/AgCl), reference electrolyte and junction that generates the constant comparison potential of a pH electrode. Learn more →
Refill opening: Side opening of refillable electrodes for topping up electrolyte. It stays open during measurement so electrolyte can flow out and no negative pressure builds up. Learn more →
Regeneration: Last rescue attempt for aged glass membranes: brief immersion in dilute hydrochloric acid followed by long soaking. Can reactivate the hydrated layer - without guarantee. Learn more →
Response time: The time until an electrode delivers a stable reading after immersion - under one minute for healthy electrodes. Considerably longer response times are a typical sign of ageing. Learn more →
Retractable assembly: Process fitting that can insert and retract the electrode during operation - for cleaning, calibration or replacement without a plant stop. Learn more →

S

S7 screw head: Widespread screw connection on process electrodes: the cable is screwed on separately and can be replaced independently of the electrode. Learn more →
Salt bridge: Conductive connection between reference system and sample via an intermediate electrolyte. It is used when the standard electrolyte would disturb the sample or vice versa.
Shaft: The body of the electrode, made of glass or plastic. The widespread standard diameter of 12 mm fits most measuring cells, fittings and holders. Learn more →
Silver/silver chloride system (Ag/AgCl): The common reference system in pH electrodes today: a silver chloride-coated silver wire in potassium chloride solution. Non-toxic, stable and reliable up to about 80 °C. Learn more →
Slope: The voltage change of a pH electrode per pH unit, theoretically 59.2 mV at 25 °C. It is the key health indicator: if the slope falls below about 90 % of the ideal value, the electrode is ageing noticeably. Learn more →
Soaking: Immersing an electrode in KCl solution for several hours so the hydrated layer builds up or regenerates - necessary before first use, after cleaning and for dried-on electrodes. Learn more →
Spear-tip electrode: Electrode with a conically tapering membrane for direct insertion into solid and semi-solid samples such as meat, cheese or dough. Indispensable in food production. Learn more →
Spherical membrane: The standard shape of the glass membrane: a small bulb with a large surface for stable readings in liquids. Learn more →
Storage solution: Potassium chloride solution (usually 3 mol/l) for moist storage of pH electrodes. It keeps the hydrated layer and junction functional - distilled water is unsuitable for this. Learn more →
Suspension effect: Measurement deviation in suspensions such as soil slurries: the displayed pH depends on whether the junction sits in the sediment or in the supernatant solution.

T

Temperature compensation (ATC): Automatic correction of the temperature-dependent electrode voltage by the meter (ATC = Automatic Temperature Compensation). Necessary because the voltage per pH unit varies with temperature. Learn more →
Temperature dependence: The dependence of the electrode signal on temperature. It has two components: the physical change of the Nernst factor and the temperature-dependent behaviour of the respective sample itself. Learn more →
Temperature sensor (NTC/Pt1000): Sensor integrated into many electrodes (often NTC 30 kΩ or Pt1000) that supplies the temperature to the meter for automatic compensation. Learn more →
Transmitter: Device that converts the sensitive electrode signal into a robust standard signal (e.g. 4-20 mA) close to the measuring point - indispensable for long cable runs in process plants. Learn more →
Two-point calibration: The standard procedure: calibration with two buffer solutions (usually pH 7 and pH 4), from which the meter calculates the electrode's zero point and slope. Learn more →

Z

Zero point: The voltage a pH electrode delivers at pH 7 - ideally close to 0 mV. A zero point outside ±30 mV indicates ageing or contamination. Learn more →