Overview
ISO 21822:2019 specifies a laboratory test method to determine the iso-electric point (IEP) of fine ceramic powders measured in the state of suspension. The IEP is the pH at which dispersed ceramic particles have zero zeta potential, a critical condition affecting colloidal stability, agglomeration and processing behavior. This standard guides sample preparation, dispersion, measurement and reporting to ensure reproducible IEP determination for advanced/technical ceramics.
Key topics and technical requirements
- Measurement principle: Zeta potential may be measured by electrophoretic light scattering (ELS), streaming potential or electroacoustic methods in accordance with ISO 13099 parts.
- Sample preparation: Use ion-exchanged distilled water as dispersion medium and prepare a ceramic slurry; recommended mass fraction typically ranges from 1 % to 10 % depending on particle size. Specific concentrations are provided for different measurement methods (e.g., streaming potential, electroacoustic, ELS).
- Dispersion and sonication: Tip sonicator is preferred (1–5 min sonication with stabilization) or ultrasonic bath (≥3 min under specified power). Stabilize suspensions (recommended 30 min) and sample the supernatant.
- Chemicals and electrolyte: pH adjustment using HCl or HNO3 (acids) and NaOH or NH4OH (bases) is allowed; electrolytes such as NaCl or NH4NO3 are recommended. The standard specifies appropriate electrolyte concentrations for reliable measurements.
- Instrumentation and calibration: Use a calibrated zeta potential analyser and pH meter (pH measuring range recommended within pH 2–10). Record instrument model, measuring principle and calibration evidence.
- Measurement procedure: Measure zeta potential and pH across a pH range (commonly pH 2–10), plot pH versus zeta potential and read the IEP where the curve crosses zero zeta potential (to one decimal place). Note that zeta potential values near ±10 mV can have low reproducibility.
- Reporting: Test report must include sample ID, reference to ISO 21822:2019, instrument details, electrolyte and pH-adjusting agents, dispersion conditions, stabilization time, IEP data and zeta potential–pH graph.
Applications and users
ISO 21822:2019 is practical for:
- Ceramic materials scientists and R&D teams characterizing surface charge and dispersion behavior.
- Quality control laboratories for powders, slurries and coatings in ceramics, pigments, catalysts and functional materials.
- Process engineers optimizing slurry stability, milling, casting, and formulation of ceramic suspensions.
- Instrument manufacturers and service labs validating zeta potential analysers and measurement workflows.
Keywords: ISO 21822:2019, iso-electric point, zeta potential, fine ceramics, ceramic powder, suspension measurement, zeta potential analyser.
Related standards
- ISO 13099-1 / -2 / -3 - Methods for zeta-potential determination (electroacoustic, optical, streaming)
- ISO 4316 - Potentiometric pH determination
- ISO 20507 - Fine ceramics vocabulary