ISO 23157:2021

Overview

ISO 23157:2021 specifies a laboratory method for the determination of silanol group content on the surface of fumed silica using a reaction gas chromatographic method. The standard details reagents, apparatus, sample pre‑treatment, reaction with a Grignard reagent, headspace sampling and gas chromatography with flame ionization detection (GC‑FID) to quantify methane produced from surface hydroxyl (silanol) groups. This method is optimized for sensitivity and reproducibility in measuring surface silanol concentration on pyrogenic (fumed) silica.

Key topics and technical requirements

• Principle: Surface silanol (–OH) groups react with a methyl Grignard reagent to produce methane. Methane is separated and detected by GC‑FID; peak area is quantified by an external methane calibration.
• Major reagents:
  • Toluene (dispersion solvent; trace water removed)
  • Methyl magnesium iodide (Grignard reagent, 3.0 mol·l−1; typically diluted 10× with toluene immediately before use)
  • Methane (calibration gas, ≥99.9% purity)
  • High‑purity carrier/diluent gases (nitrogen or helium)
• Apparatus:
  • Gas chromatograph with capillary column and FID
  • Headspace system, sealed reaction vials, syringes and gas collecting bag for calibration
• Procedure elements:
  • Sample drying and pre‑treatment (including determination of volatiles at 105 °C)
  • Blank tests and controlled reaction in airtight vials
  • Headspace sampling and GC‑FID analysis
  • External calibration using prepared methane standards and calculation of silanol content from methane yield
• Quality and precision: The standard includes instructions for calibration, repeatability and reproducibility assessment, and required reporting of results.

Applications and users

ISO 23157:2021 is directly applicable to:

• Manufacturers of fumed silica for quality control and product specification of surface chemistry.
• Suppliers and formulators in industries using fumed silica as a reinforcing filler or rheology modifier (silicone rubber, coatings, adhesives, sealants).
• Analytical and R&D laboratories assessing surface modification efficacy, silanization treatments, or comparative material characterization.
• Regulatory and procurement teams requiring standardized reporting of silanol content for material acceptance and supplier communication.

Practical benefits include improved sensitivity and reproducibility compared with titration or some spectroscopic methods, allowing better control of surface modification and end‑use performance.

Related standards

• ISO 18451‑1 (terminology for pigments, dyestuffs and extenders) is referenced for terms and definitions.
• The introduction notes alternative methods (titration, TG, IR) for silanol determination; ISO 23157:2021 is preferred where higher sensitivity and reproducibility are required.

Keywords: ISO 23157:2021, silanol group content, fumed silica, reaction gas chromatographic method, Grignard reagent, methane GC‑FID, surface hydroxyl analysis, quality control.