Multi-analytical approach based on Schubert ion-exchange, polarography, and UV–Vis spectrophotometry to study cobalt(II) complexation with Jordanian humic acid from Azraq

The interaction of cobalt (Ⅱ) with Jordanian humic acid from Azraq in aqueous solutions was studied across different pH levels and temperatures (25-65°C) using Schubert’s ion exchange method, along with a modified version of this method at a constant ionic strength of 0.05 mol L-1. The cobalt-humate complex formed in solution was characterized by polarography and UV-Vis spectroscopy. The linear range and LOD were 10-70 µg L-1 and 0.0625 µg L-1, respectively. The conditional stability constant (logβn) was determined to assess the strength of the binding between Co (Ⅱ) and humic acid. The relationship between logβn and temperature facilitated the calculation of thermodynamic parameters (∆G, ∆H, and ∆S), indicating that all reactions were spontaneous. The results obtained from both methods were comparable. The number of metal ions (j values) stated the formation of a mononuclear cobalt-humate complex, while the number of ligands-Co (Ⅱ) (i value) suggested the presence of 1:1 and 1:2 cobalt-humate complexes. It was observed that the stability constants increased with increasing pH and temperature, confirming that all reactions occur spontaneously in accordance with thermodynamic principles. Additionally, the results demonstrated that (∆G < 0) for both approaches at all pH levels, indicating a spontaneous nature. At the same time, Schubert's method's ∆H and ∆S values reveal endothermic, favorable reactions at pH 4 and 5. The E4/E6 ratio (humification index) indicated a high organic matter content in the humic acid; it was determined by analyzing a humic acid solution with a UV–vis spectrophotometer.