| Primary Function | Measures the hydrogen-ion activity of an aqueous solution and expresses it as pH. | A glass or ion-sensitive electrode generates an electrical potential related to the solution’s acidity or alkalinity. The meter converts this signal into a pH reading. | Choose an analyser according to the liquid composition, measurement frequency, required accuracy, and installation method. |
| Measurement Scale | Common laboratory and process instruments cover approximately pH 0 to 14. | pH 7 is approximately neutral at 25°C; values below 7 are acidic and values above 7 are alkaline. | Strongly acidic, strongly alkaline, concentrated, or non-aqueous samples may require a specially selected electrode. |
| Measurement Principle | Electrochemical potentiometric measurement using a measuring electrode and a reference electrode. | The potential difference between the electrodes changes with hydrogen-ion activity. The analyser applies the Nernst relationship to calculate pH. | Stable reference junctions and suitable electrode materials are important for repeatable results. |
| Typical Accuracy | General portable and laboratory instruments commonly provide about ±0.01 to ±0.02 pH, while some basic meters provide about ±0.1 pH. | Accuracy depends on electrode condition, calibration quality, temperature, sample handling, and electrical stability. | Specify the required accuracy rather than selecting only by displayed resolution. |
| Resolution | Common display resolutions are 0.1, 0.01, or 0.001 pH. | Resolution indicates the smallest displayed increment; it does not by itself represent measurement accuracy. | For routine water and wastewater work, 0.01 pH resolution is commonly suitable. |
| Calibration | One-, two-, or three-point calibration is commonly available using certified buffer solutions. | The instrument compares the measured electrode signal with known buffer values and adjusts slope and offset. | Use fresh, uncontaminated buffers and calibrate at intervals appropriate to the application. |
| Common Buffer Points | pH 4.01, 6.86 or 7.00, and 9.18 or 10.01 are widely used reference values at specified temperatures. | Acidic and alkaline buffers help verify electrode response across the intended measurement range. | Select buffers that bracket the expected sample pH and follow the temperature information on the buffer certificate. |
| Temperature Compensation | Manual or automatic temperature compensation is commonly provided; temperature sensors may be integrated or external. | Temperature affects electrode response and the actual pH of many solutions. The analyser corrects the electrode response when temperature data is available. | Temperature compensation does not remove all temperature-related changes in the sample’s chemistry. |
| Sensor Types | Combination glass electrodes, refillable electrodes, maintenance-free electrodes, and application-specific sensors are common. | A combination electrode houses the measuring and reference elements in one probe to simplify measurement. | Consider chemical compatibility, clogging risk, pressure, temperature, cleaning requirements, and expected service life. |
| Sample Types | Drinking water, process water, wastewater, beverages, laboratory solutions, soil extracts, and industrial liquids can be measured with suitable sensors. | The electrode contacts the sample and responds to its hydrogen-ion activity. | Viscous, dirty, low-conductivity, oily, or suspended-solid samples may require specialised junctions and cleaning procedures. |
| Response Time | Many clean aqueous samples stabilise within several seconds to approximately one minute. | The response depends on electrode condition, sample temperature, mixing, conductivity, and the difference between calibration and sample conditions. | Allow the reading to stabilise before recording data, especially in low-conductivity or complex samples. |
| Installation Formats | Portable meters, benchtop meters, panel-mounted analysers, and inline or submersible systems are widely used. | Portable and benchtop units are used for grab samples, while process analysers continuously monitor a sample stream or tank. | For continuous monitoring, evaluate sensor mounting, flow conditions, cleaning access, and environmental protection. |
| Signal and Connectivity | Depending on configuration, outputs may include display-only operation, USB, serial communication, analogue current output, relay contacts, or digital industrial communication. | The analyser converts the electrode signal into a digital reading and may transmit measurement and alarm data to a control system. | Confirm output type, electrical isolation, communication protocol, cable length, and control-system compatibility. |
| Process Control Features | Process instruments may provide high and low alarms, configurable limits, data logging, password protection, and automatic cleaning control. | The analyser compares the measured value with configured limits and activates alarms or connected equipment when required. | Define alarm logic, fail-safe behaviour, data retention, and integration requirements before purchase. |
| Operating Temperature | Many general-purpose electrodes operate near 0°C to 100°C, but the permissible range varies significantly by sensor construction. | Temperature changes affect the glass membrane, reference system, seals, and sample chemistry. | Always verify the complete sensor-and-process assembly rating rather than relying only on the meter rating. |
| Maintenance Requirements | Routine tasks include calibration, rinsing, inspection, appropriate storage, cleaning, and replacement of damaged or aged electrodes. | Contamination, dehydration, clogged junctions, and ageing can cause slow response, unstable readings, or calibration errors. | Include buffer solutions, cleaning agents, storage solution, spare electrodes, and operator training in the maintenance plan. |
| Storage Practice | Most conventional glass pH electrodes should be kept hydrated in a suitable electrode storage solution rather than stored dry. | Hydration preserves the glass membrane and helps maintain the reference junction’s performance. | Follow the electrode manufacturer’s storage instructions; do not routinely store the sensing tip in pure distilled or deionised water. |
| Quality Verification | Verification can include buffer checks, repeatability tests, slope checks, offset checks, and comparison with a traceable reference method. | Verification identifies sensor drift and instrument problems before measurement data is used for process or quality decisions. | Document calibration dates, buffer batch details, temperature, slope, offset, and corrective actions. |
| Selection Priority | Key selection factors include pH range, accuracy, sample matrix, temperature, pressure, sensor material, installation, outputs, and service support. | The meter and electrode operate as one measurement system; an advanced meter cannot compensate for an unsuitable or poorly maintained sensor. | Request application data, technical drawings, calibration information, replacement-part availability, and test documentation before final approval. |