Colour measurements are the basis of several quantitative estimations and are based on light absorbance by the coloured medium. In addition to absorption several methods make use of colour filters for visual matching of colours.
The basis of colour of different materials was discussed earlier. Now a question arises can colour be measured and what are the units of colour measurement. The answer is yes-colour shades can be measured and quantified as per the specific scales. The different colour measurements are based on industry specific requirements. Numerous colour scales have emerged over the years and ASTM is the basis of most of the adopted scales.
American Society for Testing Materials (ASTM) is well known universally. ASTM D1500 -12(2017) standard defines colour on the basis of glass transmission standards. ASTM standards are popular in specifying colour of petroleum products such as diesel, lubricating oils, heating oils, gasoline, petroleum waxes, etc.
APHA/Platinum- cobalt/ Hazen
American Public Health Association (APHA), a widely recognized body, published reference methods for tests on water and waste waters covering both chemical and microbiological parameters. Colour shades are based on Platinum- cobalt colour standards, also referred to as Hazen units. The Hazen scale ranges from 0 for distilled water using a calibration stock solution strength upto 500ppm.This scale is also accepted for colour measurement for other liquids.
The calibration mixture is prepared by dissolving 1.245 gm potassium hexachloroplatinate and 1.0gm of Cobalt(II) chloride hexahydrate in 100ml of Hydrochloric acid and making up the volume to 1 litre with distilled water. As per the ASTM D 1209 standard the calibration standard thus prepared has 500 mg/l concentration which is equivalent to 500 Hazen units.
Saybolt scale is typically used to specify lighter shade petroleum products, such as mineral oils, kerosene, gasoline, naphtha, aviation fuels, petroleum waxes, etc whose values typically lie below 0.6 units. The shades range between dark(-16) to light(+30).A standard colour disk is matched with adjustable volume of liquid in terms of height of the sample column.
Rosin is a solid resin obtained from pine conifers. It is a semi transparent material varying in colour shades ranging from yellow to reddish-orange. Glass filters are assigned different codes following the sequence XC(brightest) to D(darkest). The coloured glass standards are sealed in cubical frames for direct visual comparison with the resin materials.
The European Brewery Convention (EBC) is common in Brewery industry. The colour scale is used to classify brews in terms of product quality covering colour, bitterness and turbidity. Light absorbance is measured at 430nm wavelength.
The Lovibond scale is based on colour comparison with range of 84 coloured glass filters covering Neutral(N), Red(R),Yellow(Y) and Blue(B).RYBN units are adopted to define different shades. The scale finds major application in edible oils and fats industries.
The colour measurement scale listing is even more exhaustive and is generally industry specific covering products such as oils, fats, breweries, synthetic polymers, textiles, paints and coatings, etc. In many cases several colour scales are also employed simultaneously in specifying colour characteristics of manufactured products.