Review water-soluble salts, aqueous-extract conductivity and ash as three different measurements. A water-extraction test measures material recovered under specified conditions; conductivity describes the extract’s electrical response; ash measures residue after a defined combustion procedure. None independently identifies every contaminant or proves pigment suitability. For incoming quality control, first align the pigment grade, extraction conditions, reporting basis and test method. Then use the pattern of results to decide whether the lot meets its specification, needs targeted analysis or should remain on hold. A low result in one test does not override a failure in another required test.

Establish the Pigment and the Sample Being Evaluated

This guide primarily addresses dry, water-insoluble organic pigments and pigment-grade carbon black used in aqueous coatings and printing inks. Examples include quinacridone, phthalocyanine and carbon black grades.

Inorganic pigments require composition-specific interpretation, particularly for ash. Soluble dyes fall outside the aqueous-extract conductivity method discussed here. Ready-made pigment dispersions also need separate treatment because their water, binder, dispersant and neutralizer contribute to the analytical response.

Identify the exact commercial grade, Colour Index designation, delivery form and declared surface treatment. Compare a received lot with the approved specification and a suitable reference of that grade.

These distinctions determine whether a difference represents unexpected material, an intended grade component or an incompatible test basis.

Separate Water-Soluble Matter From Identified Salts

A certificate may use the term “water-soluble salts” while referencing a method for matter soluble in water. Confirm what the procedure actually measures.

ISO 787-3:2000 addresses hot extraction; ISO 787-8:2000 addresses cold extraction. The latter explicitly notes that the methods can give different results and requires the selected method to be identified in the specification and report.

The cold-extraction procedure separates the liquid from the pigment and determines residue from an aliquot of clear extract. That gravimetric result does not identify the residue’s chemical composition. Material contributing to the recovered mass should not automatically be assigned to sodium chloride, sulfate or another specific salt.

Check three details before comparing supplier and receiving results:

• Whether both laboratories used hot or cold extraction.
• Whether sample mass, separation and reporting basis match.
• Whether any wetting-agent contribution was handled as required.

ISO 787-8 includes a blank correction where a nonvolatile wetting agent is used. An unaccounted preparation aid can therefore become part of the apparent recovered mass.

Use a specific-ion determination when the decision concerns a particular contaminant. ISO 787-13:2019 separately addresses water-soluble sulfates, chlorides and nitrates in pigments. A general extract-residue result cannot replace that identity-specific evidence.

Interpret Conductivity as an Ionic Response

ISO 787-14:2019 addresses the resistivity or conductivity of a pigment’s aqueous extract, excluding pigments soluble in water. It treats extract resistivity as a property independent of the amount of water-soluble matter.

The distinction is physical. Conductivity responds to dissolved ions and their mobility. Temperature also affects the response. Metrohm’s conductometry guidance describes conductivity as a combined ionic measurement that cannot generally identify one ion in a mixed solution.

Consequently:

• Similar extract-residue masses can produce different conductivities.
• Similar conductivities can arise from different ionic compositions.
• A conductivity increase does not quantify a specific salt.
• A low conductivity does not demonstrate that all water-soluble material is absent.

Do not apply a generic conductivity-to-salt conversion factor to pigment extracts. Such a conversion would require evidence for the relevant composition and measurement conditions.

Also distinguish aqueous-extract conductivity from the electrical conductivity of the pigment powder or a conductive coating. These concern different test objects.

Preserve the Extraction and Measurement Conditions

For meaningful lot comparisons, align pigment concentration, extraction route, wetting treatment, separation, measurement temperature and calibration.

The ISO 787-14 procedure addresses these controls, including water loss during warm extraction and the contribution of water passed through the filter. A change in extract concentration or background can otherwise resemble a pigment-related change.

Record conductivity and resistivity with their units. Their reciprocal relationship only supports conversion when the units and measurement conditions are consistent; it does not convert either result into a mass percentage.

Interpret Ash Against the Grade’s Composition

Ash addresses what remains after the specified combustion treatment. It does not identify which part was water-soluble before combustion.

For carbon black, ASTM D1506-25 defines ash in terms of the non-carbon components remaining after combustion. Its scope is carbon black; it should not be adopted automatically for every organic or inorganic pigment.

For other pigment families, establish the applicable method and expected residue before interpreting a value as contamination.

Declared inorganic components are particularly important. Heubach’s technical sheet for VANADUR PLUS Yellow 9010 (C), for example, identifies a bismuth vanadate pigment encapsulated with silica and alumina. This illustrates why grade composition must precede any assumption that inorganic material is unwanted. It does not provide an ash conversion for other grades.

For an ash comparison, confirm the combustion conditions, endpoint, sample basis and crucible preparation. A residue obtained under a different procedure may not be comparable.

Do not subtract water-soluble matter from ash to calculate “insoluble contamination.” The measurements arise from different treatments, and that subtraction requires a validated relationship that routine certificates rarely establish.

Use Disagreement to Select the Next Test

The following patterns are investigation guides, not reported experimental results.

A single pattern cannot identify the responsible material or prove that washing was inadequate. Confirm the observation before assigning a manufacturing cause.

Connect Ionic Changes to the Intended Formulation Carefully

For aqueous systems relying on electrostatic pigment stabilization, ionic composition can matter beyond its contribution to conductivity.

BYK’s technical guidance explains that ion concentration and ion valency influence the electrical double layer around dispersed particles. High ionic concentrations or multivalent ions can impair that stabilization mechanism.

This supports a conditional investigation: when a lot shows a confirmed extract change and the formulation develops instability, examine whether the changed ionic contribution is relevant to the actual stabilization system.

It does not establish a universal pigment-conductivity failure threshold.

Where application testing is needed, compare the suspect and reference lots at equal dry-pigment loading with the same vehicle, dispersant, water quality, pH, preparation and conditioning. Measure the relevant response, such as viscosity drift, color change or particle growth.

A difference supports further investigation of the tested combination. Identifying the responsible ion or mechanism requires additional evidence. ChemicalCell’s waterborne-ink pigment selection guide provides a separate framework for formulation compatibility assessment.

Make the Incoming Sample Representative

Record the lot, container, sampling location and condition of the powder. Use a sampling plan that represents the delivery rather than relying on an unidentified surface scoop.

For supplier–buyer comparisons, retain traceable split samples where practical. Keep the test-portion mass and preparation consistent. Record any wetting aid, filter change or unusual extract appearance.

Control the measurement background:

• Check extraction water and the relevant preparation blank.
• Verify conductivity-cell calibration and temperature handling.
• Examine whether fine pigment remains in a gravimetric extract.
• Check crucible preparation, residue handling and repeatability for ash.

Near an acceptance limit, confirm that reporting resolution and method capability support the decision. A value reported only as “below detection” cannot demonstrate compliance if the applicable reporting capability is inadequate for the specified limit.

Decide Lot Conformity Without Substituting Tests

Retesting should follow the documented investigation procedure. Preserve the original result and explain the reason for additional testing. Do not select a passing result from incompatible methods.

ChemicalCell’s Pigments directory supports material-family browsing. Confirm the exact grade and analytical specification before applying incoming limits.

Complete the review by recording what each result measures, whether the methods are comparable and which evidence supports release or hold. Water-soluble matter, conductivity and ash become useful together when their distinct meanings are preserved.