Selection of the Measurement Wavelength:
In the absence of interfering substances, the measurements are made at the wavelength of the maxima ( λmax) of the biggest peak in the spectrum. At this wavelength the absorbance is most sensitive to the concentration. In addition, as in several cases there is broadening of peak so inside certain interval of wavelength near the λmax, there is no appreciable change within the value of absorbance that is the measurement is not extremely sensitive to the wavelength. This is quite advantageous since even if the spectrophotometer fails to resolve λ max, the determination does not suffer.
Therefore, in systems whereas the reagent, metal and products all absorb light, it might so happen in which at λ max there is not much of difference in the absorbance value for pure reagent and the metal complex. In such cases we need to identify such a wavelength at that there is huge difference in the absorbance values of pure reagent and the complex.