This study describes the development of vortex-assisted low density solvent and surfactant based dispersive liquid-liquid microextraction (VALS-DLLME) for Co(II) prior to its spectrophotometric detection. The method consisted of the complexation of Co(II) with pyrocatechol violet (PV) followed by the preconcentration of the Co(II)-PV complex using VALS-DLLME and then an absorption measurement at 600 nm. The optimum conditions for complex formation were a 1 : 3 mole ratio of Co(II) and PV at pH 7.5, while the conditions for VALS-DLLME were 300 mL 1-dodecanol as extraction solvent, and 300 mL acetonitrile as dispersive solvent under a vortex for 20 s with the addition of cationic surfactant (0.02 mmol L À1 CTAB). Under the optimum conditions, good linearity was in the range of 0.1-10 mg L À1 , the enrichment factor (EF) was 13.5 and the low limit of detection (LOD) was 0.04 mg L À1 . The method was applied to the analysis of Co(II) in water, green leaf vegetable and vitamin B 12 samples. The proposed method provided good recoveries in the range of 86-104%, which were comparable to those obtained from flame atomic absorption spectrophotometry. Results and discussionOptimization for the formation of Co(II)-PV complex Absorption spectra. The Co(II)-PV complex has blue color with the maximum absorption wavelength at 590 nm, while PV is yellow color with the maximum absorption wavelength at 7244 | RSC Adv., 2018, 8, 7243-7251 This journal is Fig. 1 (a) Absorption spectra of Co(II)-PV complex at different concentrations of Co(II) obtained from direct analysis (left) and with VALS-DLLME (right). Conditions of complexation: molar ratio (Co(II) : PV) of 1 : 3, phosphate buffer pH 7.5. Condition of VALS-DLLME: 300 mL 1-dodecanol, 0.02 mmol L À1 CTAB, 300 mL acetonitrile, 0.2 g Na 2 SO 4 , vortex at 3200 rpm for 40 s. (b) Effect of the PV concentration. Conditions: 1.00 mg L À1 Co(II) and phosphate buffer pH 7.5. (c) Effect of pH. Conditions: 1.00 mg L À1 Co(II) and 0.06 mmol L À1 PV in phosphate buffer.This journal is

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