Overview
ISO 15715:2003 specifies an instrumental method for determining the turbidity of clear liquids used as binders for paints and varnishes. The method quantifies light scattering by insoluble matter and reports results in formazine turbidity units (FTU). It is applicable to resins and resin solutions, solvents, clear coating materials, monomers and other liquids where clarity is specified. The instrumental approach provides an objective, reproducible measurement that is largely insensitive to dissolved colour.
Key Topics
- Principle: Turbidity is measured by the ratio of light intensity at 0° and 90°; a factor converts this ratio to FTU. Measurements capture scattering from particles, droplets or gels that reduce transparency.
- Apparatus requirements: Turbidimeters used should comply with ISO 15715 optical requirements, including a near-infrared light source (nominally 860 nm with allowed bandwidth) and a defined 90° scattering geometry. Aperture and beam geometry criteria ensure consistent results between instruments.
- Calibration: Instruments are calibrated with formazine standards prepared from hexamethylenetetramine and hydrazine sulfate as described in the standard. A calibration curve should include at least five points across the measurement range and be repeated frequently.
- Sample preparation and procedure: Samples are placed in optical cells or specified disposable glass jars, filled carefully to within 1 cm of the top to avoid bubbles, capped and conditioned to room temperature. Bubbles must be removed (for example by ultrasonics or centrifuging) only after verifying such treatment does not alter turbidity. Wiping the outside of the cell and using the lowest instrument range improve accuracy.
- Expression of results: Results are reported in FTU with specified rounding rules depending on magnitude (e.g., to 0.01 FTU for