Question

Why 600nm of light use for concentration of sample?

Answer 1

600nm is an easy to obtain wavelenght of the optical light, you avoid possible strong UV absorbances of dissolved molecules and the defined wavelenght helps to standard your measurings. 600nm is most common, sometimes you have other wavelengths like 550nm in literature The measurement is also reliable under "imperfect" optical conditions in MTPs with only one straight cuvette wall and the quality of your photometer does not need to be high-end. Sometimes you have turbidity measuring systems like orbisphere that work with the full optically visible spectra, but these measurings mostly require strict calibrations and are not so easy to handle like a single filtered wavelength.