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  Colorimeters and Turbidimeters
General Information

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There are several methods for testing water and waste water. These methods include colorimetric, turbidimetric, electrometric, titimetric, nephlometric, demonstrative. More than one method can be used to test for most parameters but the methods can vary as to accuracy and ease of use. Three of the most common, easiest to use, and accurate methods are measurements using a Colorimeter (colorimetric and turbidimetric methods) and a Turbidimeter (nephlometric methods).

Colorimetric Method

The colorimetric method is measurement of the concentration or intensity of the color that develops when a specific reagent is introduced to a solution containing a parameter (the substance or chemical quantity you wish to measure).    This process can take up to ten minutes for the color to fully develop or it can be nearly immediate, depending on the specific parameter being tested. Most tests evaluate how intense the color becomes- the more intense the color, the higher the concentration of the parameter. However, with some tests,  the opposite can be true. In these special cases, the greater the color, the lower the concentration of the parameter.

To test for concentrations, the color can be visually compared to standards on a chart (color comparator) or, where greater accuracy is required, be inserted into a colorimeter for direct readout or output to an outside device like a printer or a computer. Test results can be expressed as parts per million (ppm), milligrams per liter (mg/l), grains per gallon (gpg) or other appropriate scales. Visually comparing the color concentration to a color comparator can be very inaccurate because of individual abilities to discern color, ambient lighting, color blindness, and the inability to see variations in certain colors (yellows and some shades of blue specifically). While this method is inexpensive, it is quite inaccurate and can not be verifiable. The use of a colorimeter is very handy. They are battery operated for portability and quite accurate. Basically, a light is shined though a sample. This light is detected by a photodiode which displays the concentration of the parameter directly. The colorimeter eliminates visual interpretation, concern about lighting, and variations in an operator’s ability to discern color. The colorimeter offers on-the-spot results and reagent tests are available for a very wide variety of parameters. These include: Chlorine, Iron, Manganese, Copper, Zinc, Aluminum, Fluoride, Ozone, Nitrates, Phosphates, Sulfides, and many more.

Turbidimetric methods

Turbidity is defined as "cloudiness or opacity of a normally clear liquid due to a suspension of solid particles or colloidal droplets". Unlike colorimetric methods where color is used to determine the concentrations of a parameter in a liquid, turbidimetric method is the process of measuring  the cloudiness or opacity of the liquid. A reagent is combined with a sample which causes cloudiness or turbidity in the sample. Concentration of the parameter being measure determines how cloudy the sample becomes. Turbidimetric method is the process of measuring this cloudiness and translating it into concentration. As in the use of the colorimetric method, this can be done with either a visual comparator or a colorimeter. Tests using this method are Potassiums, Sulfates, and others.

Nephlometric Methods

Water turbidity is another method of measurement. Within the sample, a focused light beam is passed to a meter which measures suspended matter within the water sample. The silt, dirt, and solids, suspended in the water scatter the light. This is measure by a photodiode at 90 angle, incident to the light source. Results are expressed in NTU units or Nephlometric Turbidity units. Turbidimeters can be either handheld portable or benchtop units. They are used extensively in water treatment systems, lakes, streams, and testing the performance of sand filters and settling basins. There are also units for continuous monitoring. 


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