*3.2.1 Ion-pair complex*

Ion-pair formation results from electrostatic according to Coulomb attraction law without the formation of a covalent bond [49]. The formation of an ion-pair complex between the drug and the choice dye followed by its extraction into an organic solvent for absorbance measurement is a widely used reaction as the basis of spectrophotometric assays for pharmaceutical formulations. These are the simplest of the spectrophotometric methods ever developed since they involve mere mixing of drug and dye solutions in an organic solvent before measuring the absorbance of the colored species. However, these methods require the drug to be present in the base or acid forms for complexation. For example, typical, some methods based on extractive spectrophotometric are used for the determination of some formulations after selecting the optimum conditions by preliminary experiments, such as pH, buffer, and solvent; at the wavelength of maximum absorption; (a) bromocresol purple used for terbinafine [21, 24], nifedipine [50], sulfadimidine, sulfaguanidine, sulfametrole, sulfaquinoxaline, and sulfamethoxazole [51], hydroxyzine dihydrochloride [52], lercanidipine [53], oxomemazine hydrochloride [54], and atorvastatin [55].

The widely used spectrophotometric methods in trace analysis are based on the possibility of converting the constituent to be determined into a substance whose solution is strongly colored. Such a solution shows differential absorption to light of different wavelengths (**Table 3**).
