Determination of Colour of Chemical Reagents: Hazen (Platinum–Cobalt) Colour Testing Methods and Sta

Colour determination on the Hazen (platinum–cobalt, Pt-Co) scale is a fundamental purity and quality test for chemical reagents and clear liquid chemical products, verifying conformity with standards such as GB/T 605, ISO 2211, and ASTM D1209. This technical article explains the measurement principle, applicable sample types, tested parameters, visual and instrumental procedures, and typical industrial applications of chemical reagent colour testing. It also answers frequently asked questions relevant to laboratory analysts and quality-control professionals.

Technical Overview

Colour is one of the most immediate and sensitive indicators of the purity and stability of a chemical reagent. Discoloration of a nominally colourless or water-white liquid often signals oxidation, thermal decomposition, residual process impurities, contamination by transition metals, or degradation during storage and transport. For this reason, colour is specified as a routine release parameter in nearly all reagent, solvent, and fine-chemical specifications.

For clear liquid reagents, colour is quantified on the platinum–cobalt (Pt-Co) scale and expressed in Hazen units, which are numerically identical to APHA colour numbers. The method is based on comparing the sample, either visually or photometrically, against a graded series of platinum–cobalt colour standard solutions. The result is reported as the Hazen (Pt-Co) number of the matching standard, or as a conformity statement such as "colour not deeper than 10 Hazen units".

The determination of colour of chemical reagents is governed by a well-established family of national and international standards, including:

• GB/T 605 — Chemical reagent — General method for the determination of colour (Hazen units, platinum–cobalt scale)
• GB/T 3143 — Liquid chemical products — Determination of colour (Hazen units, platinum–cobalt scale)
• ISO 2211 — Liquid chemical products — Measurement of colour in Hazen units (platinum–cobalt scale)
• ISO 6271 — Clear liquids — Estimation of colour by the platinum–cobalt scale (visual and instrumental methods)
• ASTM D1209 — Standard Test Method for Color of Clear Liquids (Platinum-Cobalt Scale)
• ASTM D5386 — Standard Test Method for Color of Clear Liquids by Instrumental Method (Platinum-Cobalt Scale)
• JIS K 0071 — General rules for colour determination of chemical products

Both the classical visual comparison method and modern instrumental (spectrophotometric) methods are accepted by these standards, and both are routinely performed in accredited third-party laboratories and in-house quality-control laboratories.

Testing Samples

Colour determination on the Hazen scale applies to clear, transparent liquid chemical reagents that are free from suspended matter. Typical sample categories submitted for testing include:

Acids and Inorganic Reagents

• Mineral acids: sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid
• Organic acids: acetic acid, formic acid, citric acid solutions
• Hydrogen peroxide, ammonia solutions, and salt stock solutions

Alcohols and Polyols

• Methanol, ethanol, isopropanol, n-butanol
• Ethylene glycol, propylene glycol, glycerol, diethylene glycol

Ketones, Esters, Ethers and Hydrocarbon Solvents

• Acetone, methyl ethyl ketone, cyclohexanone
• Ethyl acetate, butyl acetate, dimethyl carbonate
• Tetrahydrofuran, dioxane, toluene, xylene, cyclohexane

Fine Chemicals, Intermediates and Pharmaceutical Raw Materials

• Pharmaceutical excipients and intermediates (for example, molten phenol tested at a specified temperature)
• Electronic-grade and HPLC-grade solvents
• Water treatment chemicals, catalysts precursors, and additive solutions

Sampling should be representative, and samples must be collected in clean, colourless borosilicate glass containers, protected from light and heat, and tested promptly. A minimum sample volume of approximately 100 to 200 mL is normally sufficient for a duplicate determination.

Testing Items

The following colour parameters are commonly measured for chemical reagents:

• Hazen colour (Pt-Co colour number), over the range 0 to 500 Hazen units
• APHA colour number — numerically equivalent to the Pt-Co scale
• Gardner colour (ASTM D1544) for yellow-to-brown liquids darker than 500 Hazen units
• Saybolt colour (ASTM D156) for water-white petroleum-derived solvents
• Specification conformity assessment, for example "colour not more than 10 Hazen units (Pt-Co)"

Typical reagent specifications require colour values of 10 to 20 Hazen units for high-purity solvents such as ethanol, acetone, and glycerol, while electronic-grade and HPLC-grade materials are frequently limited to 10 Hazen units or lower. The visual method resolves differences of about 5 Hazen units in the low range and 10 to 25 units at higher values, while instrumental methods achieve repeatability within a few units across the range.

Testing Methods

Preparation of Platinum–Cobalt Colour Standards

The platinum–cobalt stock solution (500 Hazen units) is prepared by dissolving 1.245 g of potassium hexachloroplatinate(IV) (K2PtCl6) and 1.000 g of cobalt(II) chloride hexahydrate (CoCl2·6H2O) in water, adding 100 mL of hydrochloric acid (density approximately 1.18 g/mL), and diluting to exactly 1000 mL in a volumetric flask. The stock solution is stored in amber borosilicate glass, protected from light and contamination.

The stock is verified spectrophotometrically in a 10 mm cell: the absorbance at 430 nm should fall within approximately 0.115 to 0.120, with additional verification checks at 455, 480, and 510 nm as specified in ASTM D1209. A 500 Hazen solution corresponds to 500 mg of platinum and 250 mg of cobalt per litre; one Hazen unit therefore equals 1 mg Pt and 0.5 mg Co per litre in the same acid medium.

Working standards spanning 0 to 500 Hazen units (for example 0, 5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, and 500) are prepared by accurately diluting the stock with water. Working standards should be freshly prepared at defined intervals and kept in sealed, matched comparison tubes.

Visual Comparison Method

The visual method described in GB/T 605, GB/T 3143, ISO 2211, and ISO 6271-1 proceeds as follows:

• Bring the sample and the standards to the specified temperature, commonly 20 °C to 25 °C or as defined in the product specification.
• Fill matched 50 mL (or 100 mL) Nessler-type colour comparison tubes with the sample and with bracketing standards to the graduation mark.
• Compare the tubes under diffuse north-sky daylight or an equivalent standardised light source, viewed against a matte white background, looking vertically down the axis of the tubes or horizontally through a calibrated light box as the method requires.
• Record the Hazen number of the standard that most closely matches the sample. If the colour lies between two standards, report the darker value, or report the result as "colour not deeper than X Hazen units".
• If the sample is darker than 500 Hazen units, either dilute it with a specified solvent and report the diluted result together with the dilution factor, or apply a more appropriate colour scale such as the Gardner scale.

Instrumental (Spectrophotometric) Method

Instrumental colour measurement, covered by ISO 6271-2 and ASTM D5386, uses a spectrophotometer or tristimulus colorimeter to measure the light absorbed by the sample in a 50 mm path-length cuvette at the characteristic wavelengths of the Pt-Co scale, or across the visible spectrum with conversion to Pt-Co equivalents. The instrument is calibrated with certified platinum–cobalt standards, and the Hazen value is computed directly from the absorbance or tristimulus data.

Instrumental determination offers objective, traceable, and recordable results, superior repeatability at low colour levels, and eliminates inter-operator variability, making it the preferred approach for automated quality-control laboratories.

Interferences, Quality Control and Precautions

• Turbidity or suspended matter scatters light and mimics colour; only visually clear samples are suitable, and filtration should not be used unless the specification permits, since it may alter the colour.
• Air bubbles, fingerprints, scratches, and mismatched tube dimensions cause systematic errors; use clean, dry, optically matched tubes and cuvettes.
• Illumination must be diffuse and consistent; avoid direct sunlight and coloured reflections from walls or clothing.
• Verify the absorbance of the stock standard periodically and prepare working standards fresh according to the controlled schedule.
• Observe the specified test temperature, particularly for solid or viscous reagents that must be liquefied before measurement.
• Perform duplicate determinations and ensure that visual observers have passed colour-vision screening.

Applications

Colour determination of chemical reagents supports quality, safety, and regulatory objectives across many industries:

• Reagent and solvent manufacturing: batch release testing against GB/T 605, ISO 2211, or ASTM D1209 acceptance limits.
• Incoming inspection of raw materials in pharmaceutical, electronic, agrochemical, and fine-chemical production.
• Stability and shelf-life studies, where a drifting colour number provides early evidence of oxidation or decomposition.
• Process optimisation and troubleshooting: monitoring distillation, hydrogenation, adsorption, carbon treatment, and purification trains.
• Conformity demonstration for pharmacopoeial and industrial monographs that specify colour limits for products such as ethanol, glycerol, and propylene glycol.
• Support of certification and regulatory submissions through testing performed in accredited third-party laboratories.

FAQ

What is a Hazen unit?

A Hazen unit is the colour of a solution containing 1 mg of platinum (as chloroplatinate) and 0.5 mg of cobalt (as cobalt chloride) per litre in a hydrochloric acid medium. The scale runs from 0 (water-white) to 500 (deep yellow) and is named after the chemist Allen Hazen.

Are APHA and Pt-Co colour numbers the same?

Yes. APHA (from the American Public Health Association), Pt-Co, and Hazen designations all refer to the same platinum–cobalt colour scale, and the numbers are numerically equivalent up to the maximum of 500.

What if the sample is darker than 500 Hazen units?

Liquids darker than the top of the Pt-Co range are not quantifiable on this scale. In such cases the Gardner colour scale (ASTM D1544) or another appropriate scale is used, or the sample is quantitatively diluted with a specified solvent and the diluted colour is reported together with the dilution factor.

Can turbid or coloured reagents be tested by this method?

No. The platinum–cobalt method applies only to clear, transparent liquids. Turbidity interferes because light scattering is indistinguishable from true colour, and intrinsically dark products require a different colour scale adapted to their appearance.

How are results reported?

Results are typically reported as "colour X Hazen units (Pt-Co)" or as a conformity statement such as "not more than 10 Hazen units". Visual results are reported to the nearest available standard, while instrumental results are reported to the validated resolution of the measuring system.

How often should platinum–cobalt standards be renewed?

The 500 Hazen stock solution is stable for about one year when stored in amber glass away from light and heat, subject to periodic absorbance verification. Working standards should be prepared freshly on a monthly basis or at the interval defined by the laboratory's quality system.

Which is better, visual or instrumental measurement?

Visual comparison remains the reference method and is simple and inexpensive, but it depends on observer acuity and lighting conditions. Instrumental measurement provides objective, repeatable, and digitally recorded results and is recommended when low-level colour values, statistical process control, or data integrity requirements are involved.