Moisture and Colour in Formulated Materials

Two measurements decide more release decisions in formulated materials than any other pair, and neither is expensive: how much volatile the material still holds, and whether its colour matches the reference. Both are simple to perform and easy to perform inconsistently, which is why they cause a disproportionate share of disputes.

Two cheap measurements, one release decision

A batch that is inside its moisture limit and outside its colour tolerance is not released. A batch that is inside both is usually released without further testing. That makes the pair a gate, and a gate has to be defined in numbers that two operators would interpret the same way.

Moisture: what the method actually measures

Loss on drying measures everything that evaporates at the test temperature — water, residual solvent, and in some materials a fraction that is neither. Karl Fischer measures water specifically. The two agree only when water is the only volatile present, and choosing between them is a statement about what the specification means.

If the specification says “water content”, use Karl Fischer. If it says “loss on drying”, use the moisture analyser and accept that the number includes other volatiles. Using one method to demonstrate the other is a common and avoidable finding in an audit.

Colour: from visual to instrumental

Visual comparison against a reference sample is fast, needs no instrument, and fails as soon as the lighting or the observer changes. Instrumental measurement — transmittance at a fixed wavelength, or a platinum-cobalt scale for clear liquids — removes the observer but introduces its own requirements: a fixed cell path, a clean cell and a defined sample temperature.

Making the numbers comparable

Setting a specification you can hold

A specification is only useful if the method can resolve it. Set the limit from the process capability first, then check that the method’s repeatability is comfortably smaller than the tolerance. A tolerance of 0.1 percent measured by a method that scatters by 0.15 percent generates rejections that are really measurement noise.

Where the two interact

Moisture and colour are not independent. Drying can concentrate colour bodies, heat can darken a heat-sensitive material, and a moist powder scatters light differently from a dry one. When both measurements sit near their limits at once, the useful next step is to measure them on the same portion rather than on two samples taken minutes apart.