You may have noticed that water around us can differ in colour – there could be clear water in a bucket, blue water in a swimming pool or reddish-brown water coming out of your kitchen tap. You would be right to be wary of the latter as it may contain unhygienic organic matter.
Red algae flows into the sea in Sydney in 2012
In 1892, a chemist and outstanding engineer by the name of Allen Hazen, who carried out research on water purification and sewage treatment, developed a colour measurement index to quantify the colour of wastewater. The Hazen Colour Scale made it possible to compare a sample of wastewater against a standard solution of diluted platinum cobalt. This enabled us to formally evaluate the purity of water and determine how much organic material or impurities are in the water.
Hazen Colour Scale:
Nowadays, the colour scale used for measuring water quality can also be referred to as the Pt-Co (Platinum-Cobalt) / APHA (from American Public Health Association) / HU (Hazen Units) colour scale. It ranges from 0 (clean or distilled water) to 500 (very dark, polluted water).
When should we Measure the Colour of Water?
The colour of drinking water is often evaluated before it is treated to see if there are contaminants and then after it has been filtered / treated, to measure the effectiveness of the treatment and as a quality assurance measure – ahead of the water being sent out and the risk of health problems.
When manufacturing liquids, a set colour standard will inform manufacturers that the liquid’s appearance conforms to the standard and will also give an indication of the quality or purity of the product. Think about this in terms of manufacturing varnishes, chemicals, petroleum or pharmaceuticals – imagine the effect impurities can have on a batch of medicine!
It is more cost effective to test liquids before they are packaged / piped than to try to remedy a colour / quality problem at later stages.
Manufacturers of dye or coloured paper can produce coloured effluent water during processing that needs to be cleaned before releasing it back into the water system.
How to measure the Colour of Water:
A colour testing instrument is used to measure the colour of water. You could use:
- A comparator and disc system – long lasting, but dependant on the human eye, so more open to inconsistencies
- Ecomparator – new, simple digital comparator that measures only the Pt-Co / Hazen scale
- Photometer – more complex digital instrument that can also be used to measure 60 other parameters, such as Chlorine levels
- Spectrophotometer – top-of-the-range benchtop instrument for highly accurate, in-depth testing of many water parameters
TOP Technical Tip:
A water sample should be carefully poured into a very clean sample cuvette, taking care that no bubbles are added. Hold the cuvette at the top, so that no fingerprints are left on the cuvette. This will help to ensure that the colour of the sample is accurately measured.
Drinking water Standards:
< 15 Pt-Co / HU / Hazen units*
Natural water can range from < 5 to 1 200 mg / litre Pt-Co
* According to SANS 241:2015 Limits
Common Colour Indicators:
- Green – could indicate copper leaching from copper plumbing, or algae
- Blue – could also indicate copper, or it could be caused be industrial toilet cleaners
- Red – signs of rust from iron pipes or bacteria from lakes; could also be harmless tannins
- Black – could indicate the growth of sulfur-reducing bacteria inside a hot water tank set at too low a temperature
WHY does the water in a swimming pool appear bluer than the water in a bucket?
When light travels through water, the red and yellow wavelengths are absorbed and we see mainly the shorter wavelength blue. The same water in a smaller bucket appears only a little blue, as less of the other wavelengths are absorbed.
Further water testing is often done if the cause of the discolouration is unknown, but testing the colour of the water can be a quick and easy test to indicate water quality.
Slow sand and coagulants are often used to remove colour from water.
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