Chelator Denticity Controls Iron Recoveries in the 1,10-Phenanthroline Method via an Enveloping Effect

The 1,10-phenanthroline (Phen) method is commonly used for iron determination because of its high sensitivity and simplicity. However, its accuracy is affected by chelating ligands, while the influence mechanisms remain unclear. This study investigated the impact of 14 chelating ligands with different coordination numbers on the determination of Fe(II) and total iron (T-Fe) by the Phen method. Highly coordinated ligands, particularly hexadentate and octadentate ligands, significantly reduced the detection accuracy, even at low ligand-to-iron ratios. Electrochemical tests, spectroscopic characterizations, and density functional theory (DFT) calculations collectively demonstrated that highly coordinated ligands strongly coordinated with iron ions, decreasing the accessibility and lability of the metal center for Phen complexation. This enveloping effect was accompanied by an increase in the d orbital splitting energy of Fe, indicating a modified electronic environment that was less favorable for electron transfer from hydroxylamine hydrochloride to Fe(III), thus hindering Fe(III) reduction prior to color development. DFT results further showed that the sequential coordination of Phen to Fe(II) became progressively less favorable, with the binding of the third Phen molecule constituting the highest energetic barrier. This study reveals the reliability of the Phen method in quantitatively determining iron when chelating ligands are present, providing new insights for iron determination.