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Turbidity Interference

What is Turbidity?

Turbidity is haze or cloudiness in water caused by suspended and colloidal particles or matter. Turbid water is commonly described as hazy, milky, cloudy or dirty. High levels of turbidity usually indicate low water quality. Turbidity can vary from a muddy river or pool (high turbidity) to a clear stream (low turbidity). Suspended/colloidal particles are typically picked up in streams and rivers, and are composed of clay, silt, dirt, organic debris, lime scale and corrosion/oxidation products (rust, lead, copper oxides etc).

Although turbidity is not intrinsically harmful, it can shield harmful contaminants (e.g. viri or bacteria) from disinfectants such as chlorine. Turbidity particles can also absorb and carry other dissolved contaminants throughout a water supply system. High turbidity can block filters and fill pipes and tanks with mud and silt and cause damage to valves and taps. Turbidity also is undesirable for aesthetic reasons and texture in potable water and consumer products.

One the primary applications for turbidity reduction and monitoring is in drinking water supplies. Cross flow membranes, including reverse osmosis, nano-filtration or ultra-filtration, are commonly used to remove or reduce turbidity.

Measurement

A common field test to check for turbidity is to hold a torch up to a water sample. If the light makes the sample look cloudy or the beam highlights particles in the water, your sample is turbid.

Turbidity levels can be more accurately quantified using either a turbidimeter or nephelometer. Turbidimetry measures the loss of intensity of light transmitted through the sample through the scattering effect of suspended particles and nephelometry measures the intensity of light scattered at 90° relative to the light source, compared to a reference solution. Both methods employ Nephelometric Turbidity Units (NTU).

Colourimetric Water Analysis

Turbidity interferes with instrumental colourimetric water analysis by causing discolouration and light scattering in the sample when measured with a photometer or spectrophotometer, resulting in elevated readings. For visual test kits, any turbidity present with affect the developed colour and make it appear darker, thus giving a false positive reading, or making it harder to match the developed colour to the correct increment on the colour comparator. In order to reduce inaccuracies in sample measurement due to light scattering and discolouration, the following methods are available.

1. Filtration

One option is to reduce the sample turbidity using filtration, in particular removing the component of turbidity unrelated to the analyte to be measured. However this may not always be appropriate depending on the testing environment or the analyte being measured. See the test instructions for the relevant test for more information on the parameter pages.

Decanting may also be an option if there are suspended particles that can readily sink to the bottom of the sample container within an acceptable time period (refrigeration may be necessary), depending on the analyte.

2. Dilution

Another option is to perform a sample dilution to the lower end of the range of the available test kit, or use another lower range test kit.

The CHEMetrics MiniPet® is pictured left, which is intended for use with selected CHEMets visual test kits to support a wide range of dilution requirements.

However, besides the potential to introduce inaccuracies through improper dilution procedures, the effective kit accuracy is reduced by the factor of the dilution. It also not appropriate for all analytes.

3. Sample Zeroing

If sample filtration or dilution are not practical or appropriate for your analyte and you do not want to introduce inaccuracies from sample dilution, then the Sample Zeroing Accessory Pack may be your answer.

The Sample Zeroing Accessory Packs are designed for use with CHEMetrics Vacu-vials® instrumental test kits and allow the operator to zero the photometer or spectrophotometer with a zero blank from the same source as the water sample. This corrects for the turbidity in the sample allowing for a more accurate measurement of the analyte being tested. Available for a wide range of visual Vacu-vials test kits.

Do not hesistate to contact us to discuss your exact application requirements.

Patrik Askert, Galgo, July 2021

References