Forming an Absorbing Species:

There are just a few analytes which have a strong absorption in the UV or Visible region and could be subjected to direct determinations.  Therefore, for most of the analytes we have to form an absorbing species through reacting them along with an appropriate reagent. In any case, the absorbance of the solution should be stable and should not modify with minor variations in the pH, temperature and the ionic strength of the solution.

In addition, the reagent should be selective towards the analyte and its reaction along with the analyte should be quantitative i.e. 100%.