LaMotte 1200 Instruction Manual

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Contents

LaMotte 1200 Instruction Manual Manual pdf 10 page image

AN INTRODUCTION TOCOLORIMETRIC ANALYSISMost test substances in water are colorless and undetectable to the human eye. Totest for their presence we must find a way to “see” them. The LaMottecolorimeter can be used to measure any test substance that is itself colored or canbe reacted to produce a color. In fact a simple definition of colorimetry is “themeasurement of color” and a colorimetric method is “any technique used toevaluate an unknown color in reference to known colors”. In a colorimetricchemical test, the intensity of the color from the reaction must be proportional tothe concentration of the substance being tested. Some reactions have limitationsor variances inherent to them that may give misleading results. Many suchinterferences are discussed with each particular test instruction. In the most basiccolorimetric method the reacted test sample is visually compared to a knowncolor standard. However, accurate and reproducible results are limited by theeyesight of the analyst, inconsistencies in the light sources, and the fading ofcolor standards.To avoid these sources of error, a colorimeter can be used to photoelectricallymeasure the amount of colored light absorbed by a colored sample in reference toa colorless sample (blank).White light is made up of many different colors or wavelengths of light. Acolored sample typically absorbs only one color or one band of wavelengths fromthe white light. Only a small difference would be measured between white lightbefore it passes through a colored sample versus after it passes through a coloredsample. The reason for this is that the one color absorbed by the sample is only asmall portion of the total amount of light passing through the sample. However, ifwe could select only that one color or band of wavelengths of light to which thetest sample is most sensitive, we would see a large difference between the lightbefore it passes through the sample and after it passes through the sample.A colorimeter passes a white light beam through an optical filter which transmitsonly one particular color or band of wavelengths of light to the photodetectorwhere it is measured. The difference in the amount of colored light transmittedby a colorless sample (blank) and the amount of colored light transmitted by acolored sample is a measurement of the amount of colored light absorbed by thesample. In most colorimetric tests the amount of colored light absorbed is directlyproportional to the concentration of the test factor producing the color and thepath length through the sample. However, for some tests the amount of coloredlight absorbed is inversely proportional to the concentration.The choice of the correct optical filter and therefore the correct color orwavelength of light is important. It is interesting to note that the filter that givesthe most sensitive calibration for a test factor is the complementary color of thetest sample. For example, the Nitrate-Nitrogen test produces a pink colorproportional to the nitrate concentration in the sample (the greater the nitrateconcentration, the darker the pink color). A green filter is used since apinkish-red solution absorbs mostly green light.10