Complex formation and liquid-liquid extraction were studied in systems containing Co(II), 4-(2-pyridylazo)resorcinol (PAR), tetrazolium salt (TZS) {2,3,5-triphenyl-2H-tetrazolium chloride (TTC) or 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride (INT)}, water and chloroform. The optimum conditions for the extraction of Co were found: pH, extraction time, concentration of the reagents PAR and TZS. The results show that the extracted species could be represented with the general formula (TZ⁺)[Co(PAR)₂], where Co is in its +3 oxidation state and PAR is in a doubly deprotonated form. The following key constants and characteristics were established for both systems: constants of extraction (K_ex), constants of association (β), constants of distribution (K_D), recovery factors (R%), wavelengths of maximum absorption (λ_max), molar absorptivities, Sandell’s sensitivities, intervals of adherence to Beer’s Law, limits of detection and limits of quantification. The ion-associate with INT is more stable and extractable (Log β = 5.0, Log K_ex = 6.1, Log K_D = 1.12, R = 92.8%). Beer’s law for the system with this reagent is observed in a wider concentration interval (0.26–2.6 μg mL^–1 Co) with a higher molar absorptivity coefficient (ε’ = 5.2 × 104 L mol^–1 cm^–1 at λ_max = 515 nm).

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