ABSTRACT
This chapter outlines the main factors requiring the use of chemical reactions prior to the spectrophotometric measurement. In the case of old, classical reactions exceptions sometimes have to be made: even the pharmacopoeias make concessions in some cases by applying spectrophotometric methods with dubious chemical background. In the majority of spectrophotometric methods the proper selection of the pH is an important factor of method optimization. Since the optimum pH of the reaction mixture and of the spectrophotometric measurement are not necessarily the same, the pH has often to be changed before the spectrophotometric measurement is carried out. In advantageous cases the solvent for the reaction and for the spectrophotometric measurement is the same. It is an important step for the optimization of a procedure to find the optimum reagent concentration which enables the reaction to run at an appropriate rate and, if possible, without side reactions and equilibria.
