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COLORIMETRIC ANALYSIS OF WATER

water testingThe Beer-Lambert Law states that the concentration of a dissolved substance, or solute, is proportional to the amount of light that it absorbs. A common application of a colorimeter is therefore to determine the concentration of a known solute in a given solution.

Introduction/ Overview:

Most test substances in water are colorless and undetectable to the human eye. To test for their presence we must find a way to "see" them. A colorimeter can be used to measure any test substance that is itself colored or can be reacted to produce a color. In fact a simple definition of colorimetry is "the measurement of color" and a colorimetric method is "any technique used to evaluate an unknown color in reference to known colors". In a colorimetric chemical test the intensity of the color from the reaction must be proportional to the concentration of the substance being tested. Some reactions have limitations or variances inherent to them that may give misleading results. Most limitations or variances are discussed with each particular test instruction. In the most basic colorimetric method the reacted test sample is visually compared to a known color standard. However, the eyesight of the analyst, inconsistencies in the light sources, and the fading of color standards limit accurate and reproducible results.

To avoid these sources of error, a colorimeter can be used to photoelectrically measure the amount of colored light absorbed by a colored sample in reference to a colorless sample (blank). A colorimeter is generally any tool that characterizes color samples to provide an objective measure of color characteristics. In chemistry, the colorimeter is an apparatus that allows the absorbance of a solution at a particular frequency (color) of visual light to be determined. Colorimeters hence make it possible to ascertain the concentration of a known solute, since it is proportional to the absorbance.

The equipment required is a colorimeter, some cuvettes and a suitable color reagent. Mudiame’s colorimeter pass a colored light beam through an optical filter, which transmits only one particular color or band of wavelengths of light to the colorimeter's photodectector where it is measured. The difference in the amount of monochromatic light transmitted through a colorless sample (blank) and the amount of monochromatic light transmitted through a test sample is a measurement of the amount of monochromatic light absorbed by the sample. In most colorimetric tests the amount of monochromatic light absorbed is directly proportional to the concentration of the test factor producing the color and the path length through the sample. However, for a few tests the relationship is reversed and the amount of monochromatic light absorbed is inversely proportional to the concentration of the test factor.

Application:

The Beer-Lambert Law states that the concentration of a dissolved substance, or solute, is proportional to the amount of light that it absorbs. A common application of a colorimeter is therefore to determine the concentration of a known solute in a given solution.

Colorimetric analysis is a method of determining the concentration of a chemical element or compound in a solution with the aid of a colour reagent. It is applicable to both organic compounds and inorganic compounds. Besides being valuable for basic research in chemistry laboratories, colorimeters have many practical applications. For instance, they are used to test for water quality, by screening for chemicals such as chlorine, fluoride, cyanide, dissolved oxygen, iron, molybdenum, zinc and hydrazine. They are also used to determine the concentrations of plant nutrients (such as phosphorus, nitrate and ammonia) in the soil.

In biology, a colorimeter can be used to monitor the growth of a bacterial or yeast culture. As the culture grows, the medium in which it is growing becomes increasingly cloudy and absorbs more light.

By: Martins Osaze.

(CHEMICAL/CORROSION SUPERVISING ENGINEER.)

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