How to measure the colour of water

How to measure the colour of water

Measuring the colour of water

When we talk about water relevant to the everyday members of the public, the three main categories that come to mind are; tap water, waste water and bottled water and, although each category has its own treatment process and quality testing, they use similar filtration and water treatment techniques.

Why measure the appearance of water?

Appearance can tell us a lot about the quality of a product and when it comes to water, it’s no different. Let’s take drinking water; it should be clear with no visible colour, cloudiness or bubbles (unless it’s sparkling, of course!) Having a way of ensuring that the water quality is as expected is a must for any treatment plant.

What colour measurement scales should be used?

Commonly used colour scales for water quality measurement are the APHA, ADMI and Gardner colour scales.

APHA stands for American Public Health Association. This colour scale is also known as the PtCo Colour scale or Hazen Colour Scale and acts as a yellowness index, allowing for accurate measurements of transparent liquids. The APHA colour scale ranges from 0 (clean water) to 500 (distorted waste water). The Hazen Colour Scale can sometimes be referred to as Hazen Colour or 1500 Hazen Colour. If this is the case then the range is often goes above the 500 units associated with APHA.

Both the ADMI (American Dye Manufacturers’ Institute) Colour Scale and Gardner Colour Scale are similar to the APHA Scale but the Gardner Colour Scale is more used to differentiate samples of a higher yellow chroma that vary in lightness and hue. The Gardner Scale ranges from 0 (distilled water) to 19 (dark and murky).

When should the appearance be measured?

In most cases, before water is deemed fit for public consumption it will have been subjected to many treatment and filtration processes. After it has been through these stages is a good point to analyse the appearance to make sure it is suitable to be sent out.

In addition, appearance measurement is a good way to measure effectiveness of any filtration process. Water can be analysed before and after treatment to make sure the desired results are being achieved. If there does happen to be any problems with any of the treatments, it is a lot more cost effective to find out at each stage, rather than at the end.

How do we measure the quality of water?

To accurately gauge the quality and cleanliness of a water source, an instrument is needed that is capable on measuring a number of different aspects such as the aforementioned colour, haze and turbidity.

For an idea on how an instrument can aid a quality control process, the ColorQuest XE and ColorQuest XT can be used as examples.
The ColorQuest XE is capable of measuring water using ADMI, APHA and Gardner colour scales. It has two measurement areas; a reflectance port and a transmission compartment. The transmission compartment would be commonly used however, for a translucent sample, it may be possible to use a thick enough sample that would become more opaque, therefore allowing for a reflected colour measurement as extra data for quality control. This could be used more for waste water or similar.

For the majority of water samples, however, all measurements can be completed using a transmission instrument, such as the ColorQuest XT. A sample of the water can be placed in a sample cell and set for analysis. A sample of water should be very carefully poured into a cell so that no bubbles are added to the sample as this can alter the results. It is also important that no other external impurities contaminate the sample being tested. When using a spectrophotometer to analyse appearance, a method should be devised that compliments the quality control and treatment processes whilst being simple to follow and eliminates any external contaminants that could affect data.

Water analysis should be a simple process that can accommodate the vast quantities of water being treated easily and be able to give quantifiable data that can keep water quality as good as it should be.

Content Written by Rachael Stothard

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How to measure the colour of water

HunterLab ColourQuest XT Spectrophotometer

The ColorQuest XT is a self-contained transmittance measurement spectrophotometer designed to measure the transmitted color of transparent liquids. Products such as resins, solvents, acids, perfumes and varnishes can be measured using the APHA/PtCo/Hazen and Gardner color scales.

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