ISO 787-4:1981

Overview

ISO 787-4:1981 is Part 4 of the ISO 787 series of general test methods for pigments and extenders. It defines two accepted procedures to determine the acidity or alkalinity of the aqueous extract obtained from pigments or extenders: an indicator titration method (Method A) and a potentiometric/pH method (Method B). The standard is intended to support consistent quality control and specification cross-references for paint raw materials.

Key Topics

• Scope and relation to other parts: The aqueous extract is prepared by the hot extraction procedure described in ISO 787-3; the cold extraction procedure of ISO 787-8 may be used by agreement (with reduced stirring time).
• Sampling: Representative sampling shall follow ISO 842.
• Apparatus:
  • Burette, 50 ml (ISO/R 385) for titrations.
  • pH measuring device with 0.1 pH unit resolution, calibrated at test temperature for the potentiometric method.
• Reagents (analytical grade):
  • Hydrochloric acid standard solution c(HCl) = 0.05 mol/L.
  • Sodium hydroxide or potassium hydroxide standard solution c(NaOH) or c(KOH) = 0.05 mol/L.
  • Methyl red indicator, 1 g/L in 60% (w/v) ethanol (or another agreed indicator).
• Method A - Indicator titration:
  • Add 5 drops of methyl red to 100 ml of test solution.
  • If the solution is yellow (alkaline), titrate with HCl to an orange end‑point.
  • If the solution is red (acid), titrate with NaOH/KOH to an orange end‑point.
  • If the solution is orange initially, consider it neutral.
• Method B - Potentiometric determination:
  • Take 100 ml of test solution and read pH with electrodes.
  • If pH is between 4 and 8, report as neutral.
  • If pH > 8, titrate to just below pH 8 with HCl; if pH < 4, titrate to just above pH 4 with NaOH/KOH.
• Repeatability and reporting:
  • Perform determinations in duplicate; if duplicates differ by more than 5% of the higher value, repeat the test.
  • Report mean result, product identification, reference to ISO 787-4, any deviations, and test date.

Applications

• Quality control of pigments and extenders during raw material receipt and batch release.
• Specification compliance and cross-referencing in product standards for paints and varnishes.
• Screening for residual acidic or alkaline components that can affect formulation stability, corrosion, or pigment dispersibility.

Related Standards

• ISO 787-1 to -24 (series of general methods for pigments and extenders)
• ISO 787-3 (hot extraction: matter soluble in water)
• ISO 787-8 (cold extraction alternative)
• ISO 787-9 (pH of aqueous suspension)
• ISO 842 (sampling of raw materials for paints and varnishes)
• ISO/R 385 (burette requirements)

This standard provides practical, repeatable procedures for acidity/alkalinity assessment of aqueous extracts and supports robust quality assurance in paint raw material testing.