SIST EN ISO 787-4:2018

Overview

EN ISO 787-4:2017 (identical to ISO 787-4:1981) specifies a general laboratory method for determining the acidity or alkalinity of the aqueous extract of pigments and extenders. The procedure relies on preparing a clear aqueous extract (typically by the hot-extraction procedure in ISO 787-3) and then determining whether the extract is acidic, neutral or alkaline by either an indicator/titration method (Method A) or a potentiometric pH measurement (Method B). Results are reported as the volume of standard titrant needed to neutralize the extract, expressed per 100 g of product.

Key topics and requirements

• Scope: Applies to pigments and extenders where a general acidity/alkalinity test is appropriate.
• Sample preparation: Follow ISO 787-3 hot extraction to obtain a perfectly clear filtrate (cold extraction per ISO 787-8 may be used by agreement).
• Method A - Indicator/titration
  • Use 100 ml of test solution; add 5 drops of methyl red indicator (1 g/L in 60% ethanol).
  • If solution is yellow (alkaline), titrate with HCl (0.05 mol/L) to an orange end‑point.
  • If solution is red (acid), titrate with NaOH or KOH (0.05 mol/L) to an orange end‑point.
• Method B - Potentiometric
  • Use a calibrated pH measuring device (0.1 unit resolution). Insert electrodes into 100 ml extract and read pH.
  • Interpret pH: between 4 and 8 ≈ neutral; >8 → titrate down to just below 8;