ISO 720:2020 - Glass: Hydrolytic resistance of glass grains at 121 °C (method and classification)
Overview
ISO 720:2020 specifies a laboratory method and classification system for measuring the hydrolytic resistance of glass grains at 121 °C. The standard quantifies the alkaline species released from glass by autoclave extraction and expresses results as the volume of acid required to titrate the extracted alkali or as the sodium oxide equivalent. ISO 720 is intended for more chemically resistant glasses (for example, borosilicate glass) and harmonizes key test steps with pharmacopeial practice to support consistent quality control.
Key topics and technical requirements
- Test principle: Extraction of glass grains and titration of released alkali to measure hydrolytic attack.
- Sample and particle size: Test uses 10 g of glass grains in the particle size range 300 µm to 425 µm.
- Extraction conditions: Autoclave extraction with Grade 2 test water for 30 minutes at 121 °C (autoclave preferably ±1 °C).
- Result expression: Volume of 0.02 mol/L HCl consumed (titration) and corresponding Na2O equivalent.
- Apparatus & consumables: Class A burettes/pipettes, sieves (300 µm / 425 µm), ball mill or manual crushing tools, autoclave, drying oven, ultrasonic cleaner, methyl red indicator, and strictly controlled test water (ISO 3696 Grade 2 / pharmacopeial R1).
- Sample preparation: Detailed crushing, sieving and cleaning procedures (manual and mechanical options), magnetic removal of iron, and pre-treatment requirements for glassware (autoclaving).
- Limitations: Not suitable for glasses with extremely low alkaline content; ISO 719 is recommended for less resistant glasses (e.g., soda-lime).
- Quality controls: Fresh acetone usage, specific sieve mesh and sample mass, and harmonized autoclaving cycle ensure reproducible results.
Applications
- Quality control and batch verification for borosilicate labware, pharmaceutical containers, vials, syringes and other medical glass components.
- Assessing chemical durability of glass intended for sterile processing or prolonged contact with aqueous formulations.
- Supporting material selection in medical, pharmaceutical, and laboratory equipment manufacturing where resistance to hydrolytic attack at sterilization temperatures is critical.
- Comparative testing during glass formulation development or supplier qualification.
Who uses this standard
- Glass manufacturers and formulators (R&D and QC)
- Pharmaceutical and medical-device manufacturers
- Analytical and materials testing laboratories
- Regulatory and compliance teams assessing container-closure systems
Related standards
- ISO 719 (hydrolytic resistance - method for less resistant glasses)
- ISO 3696 (water for analytical laboratory use)
- ISO 385, ISO 648, ISO 565 (laboratory glassware and sieves)
- European Pharmacopoeia (Ch. 3.2.1) and USP (Ch. 660) - harmonized elements referenced in ISO 720
ISO 720:2020 is a practical, reproducible test method for laboratories needing standardized assessment and classification of glass hydrolytic resistance at sterilization conditions.
