Question

why do you use a specific wavelength of light to record absorbance measurements for the dye?

Answer 1

why do you use a specific wavelength of light to record absorbance measurements for the dye.

Need of wavelength solution:

i)Various absorbing species have specific absorbance wavelength i.e. the wavelength at which it shows maximum absorbance than any other species there in solution.

ii) If we use a band of wavelength (non-specific wavelength) other component associated with analyte (dye here) may contribute to the absorption of radiation and this will make quantitative analysis of an analyte difficult and faulty.

iii) We cannot isolate a single wavelength of radiation from continuum source but we can use a monochromator for this purpose.

iv) A dye may have more than one absorbing functional group in there molecular structure like olefinic C=C, aromatic C=C, -OH, -C=O, -NH₂

v) At particular absorbance wavelength only, Beer's Law can be used to relate absorbance and concentration. Since A = c L, a plot of absorbance vs. concentration, for fixed  and L, will be a straight line with slope L  and intercept zero. And that particular wavelength is the one at which absorbance for the given dye is maximum (λmax) and this value is generally very high than any other λmax for other substances associated with dye and also quite different from any other dye’s λmax. This minimize the experimental error in calculation of dye concentration.

Hence we use specific wavelength to record absorbance measurments for the dye.