Colorimetric methods are used todetermine the concentration of dissolved species, relying on the
ability of many ionic species to absorb light of one or more specific wavelengths following the
Beer-Lambert law shown in Equation (1):
(1)
where A is absorbance (absorbance units), I is the intensity of light passing through unknown, and I0is
the intensity of light passing through the blank. The absorbance is related to concentration as shown in
Equation (2):
(2)
where aλ is the molar absorptivity of the species of interest at a certain wavelength, b is the path length
of light through solution, and c is the concentration ofanalyte. The wavelength of light absorbed is an
intrinsic property of the analyte and instruments typically measure absorbance of a specific wavelength
that is a property of the species of interest.
The closed reflux COD method [38] was selectedfor evaluating the open-source colorimeter
because COD vials were readily available from other research and also due to the availability of LED
light sources having peak emission near the 606 nm chromic absorbance peak. The closed reflux
colorimetric method used for COD employs potassium dichromate (K2Cr2O7) as oxidant, requires only