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How Colorimetry Determines an Analyte (such as P) in a Water Sample

Colorimetry is a widely used analytical technique that relies on converting the substance to be measured (the analyte) into a coloured compound that absorbs visible light. The concentration of the substance (analyte) is determined by passing a particular wavelength (colour) of light through the solution and measuring how much of this light of is absorbed. Light absorption is governed by the Beer-Lambert law, which states that

Aλ = ελcl = log10(I0/I)

In the above equation, the symbols have the following definitions (and units) :

Aλ is the (dimensionless) absorbance of the sample at a wavelength λ
ελ is the molar extinction coefficient of the sample at the wavelength λ , expressed in M-1cm-1
c is the sample's concentration, in M
l is the sample path length in cm; 1 cm cuvettes are commonly used
I0 is the incident light intensity
I is the transmitted light intensity

The above definition of absorbance relates things that can be measured (intensities) to properties of the sample. The key point here is that a sample's abosrbance will be proportional to its concentration. In order to determine the absorbance we need a means to measure the intensity of the light first using a "blank", giving I0 and then with our sample, giving I.

An inexpensive instrument to do this is called a colorimeter which, though simple in design, can give excellent results. Most commonly, a light emitting diode is used for the light source and an optoelectronic device (photodiode) serves as the detector.

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Colorimeter components exposed : left - LED source, centre - sample cuvette, right - optical detector
By adding chemical reagents to a solution containing phosphate ions the phosphorus (the analyte) can be converted into a pigment called “phosphomolybdenum blue”. Colour development takes about 15 minutes and forms the basis for the measurement of phosphate.

A colorimeter for sensitive phosphomolybdenum blue measurement has a near-infrared light emitting diode (LED) that emits light in a narrow band of wavelengths centred at 880 nm. This light source is similar to the LED used in a TV/DVD remote control and its output is just invisible to the human eye. Light of this wavelength is strongly absorbed by the blue solution.

A series of solutions each having a precisely known phosphate concentration (our phosphate standards) are prepared and the colorimeter is used to measure the absorbance of each of these standards. A plot of the absorbance readings versus phosphate concentrations gives us a working “standard curve” that can be used to determine the concentration of an unknown sample(s).

Similar procedures to the one we have just described are available for almost every possible chemical species that one might wish to analyze for. For example, the nutrient nitrogen (N), in the form of nitrate and nitrite ions, can be measured colorimetrically by converting the nitrogen into a pink azo dye that can be measured using green light.
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A series of standard solutions being used in a colorimetric nitrite determination