Reading Colour in Water: How Platinum-Cobalt Degrees and Dilution Multiples Are Reported

Colour looks like the simplest thing in the world to judge until you have to record it as a number. Human visual perception of colour is highly complex, so describing it quantitatively required the International Commission on Illumination (CIE) to build a bridge between physics and sensation. From experiments, CIE recorded the sensations triggered in the human visual system by radiant energy of different wavelengths across the visible spectrum, using the color-matching functions of the three primary colours red, green and blue. Those functions are what make colour describeable and, more usefully, comparable between laboratories.

Two methods, and they do not agree with each other

The standard specifies two ways of determining colour, both applied to samples after 15 minutes of clarification. The pH value has a large influence on colour, so pH is measured at the same time as colour is determined, and the two are reported together.

The platinum-cobalt colorimetric method references the international standard ISO 7887-1985, "Examination and determination of the color of water". It suits clean water, lightly polluted water with a slight yellow tint, and relatively clean surface water, groundwater and drinking water. The dilution multiple method is aimed at the other end of the range: heavily polluted surface water and industrial wastewater. The two are used independently and are generally not comparable, and the standard is not applicable when the hue of the sample and that of the standard solution are inconsistent.

Definitions worth getting straight

The definitions come from CIE Publication No. 17. Colour is an optical property that changes the spectral composition of transmitted visible light. True colour is the colour produced by dissolved substances and undissolved suspended matter, determined on the original, unfiltered or uncentrifuged sample. Apparent colour is the colour produced only by dissolved substances, determined on a sample filtered through a 0.45 μm membrane filter. Confusing the two is one of the more common sources of disagreement between a plant laboratory and a regulator.

The standard unit of colority, the degree, is the colour produced when each litre of solution contains 2 mg of cobalt(II) chloride hexahydrate and 1 mg of platinum in the form of hexachloroplatinic(IV) acid. A colour standard solution is prepared from potassium chloroplatinate and cobalt chloride and compared visually with the sample to establish colority. This degree is sometimes called the Hazen unit or the Pt–Co scale, or expressed as milligrams of platinum per litre.

Making the standards

Optically pure water comes first: a 0.2 μm membrane filter of the type used in bacteriological studies is soaked in 100 mL of distilled or deionized water for 1 hour, then 250 mL is filtered through it and the first 250 mL discarded.

The stock solution, equivalent to 500 degrees, is made by dissolving 1.245 ± 0.001 g of potassium hexachloroplatinate(IV) (K2PtCl6) and 1.000 ± 0.001 g of cobalt(II) chloride hexahydrate (CoCl2·6H2O) in about 500 mL of water. The 6H in that formula is water of crystallization, not free hydrogen, and it is part of the mass being weighed. Add 100 ± 1 mL of hydrochloric acid (ρ = 1.18 g/mL) and dilute to the mark in a 1000 mL volumetric flask. Stored in a sealed glass bottle in the dark at no more than 30 °C, it is stable for at least 6 months. Unless otherwise stated, only optically pure water and analytical reagent grade chemicals are used.

Working standards are prepared by pipetting 2.50, 5.00, 7.50, 10.00, 12.50, 15.00, 17.50, 20.00, 30.00 and 35.00 mL of stock solution into a series of 250 mL volumetric flasks and diluting to the mark, giving 5, 10, 15, 20, 25, 30, 35, 40, 50, 60 and 70 degrees. Kept tightly closed in glass in the dark at no more than 30 °C, they hold for at least 1 month.

Running the comparison

Instrumentation is modest: common laboratory apparatus, 50 mL stoppered color-comparison tubes of consistent specification and optically clear glass with no shadow at the bottom, and a pH meter. The accuracy needed is ±0.1 pH, which is why 3.3.3 pH meter, accuracy ±0.1 pH unit, appears as a listed requirement alongside the 250 mL volumetric flasks. Every piece of glassware touching the sample is cleaned with hydrochloric acid or surfactant solution, then rinsed with distilled or deionized water and drained.

Samples are collected in glass and analysed as early as possible; if storage is unavoidable they are kept in the dark, sometimes with air contact avoided and temperature held steady. The sample is poured into a 250 mL or larger measuring cylinder, left to stand for 15 minutes, and the upper layer decanted as the test portion. One set of stoppered tubes is filled with the standards, another with the test portion. Tubes go on a white surface at an angle that sends light upward through the liquid column, and the observer looks down vertically to find the closest match. If colority is ≥70 degrees, the test portion is diluted with optically pure water into the standard range first. Below 40 degrees results are reported to the nearest 5 degrees; from 40 to 70 degrees, to the nearest 10.

When dilution is the only sensible answer

For the dilution multiple method, the test portion is diluted stepwise with optically pure water until it is just indistinguishable from the water itself, and that dilution multiple expresses colour intensity. Above 50 multiples, aliquots are diluted to the mark using a large ratio each time; below 50 multiples, 25 mL is diluted with a multiple of 2 each time; at very low colority the multiple drops below 2. Successive multiples are multiplied together and the integer product is the colority. Alongside it, the analyst records depth, hue and, where possible, transparency in words, and reports the pH value with the result.

Colority is ultimately a sensory indicator. Pure natural water is clear and transparent, in other words colourless; wastewater carrying metal compounds or organic compounds shows colour in endless variety. Diluting that coloured wastewater with distilled water until it matches a reference is still, after all the CIE machinery, what the number means.