Quantum chemical calculations based on DFT are employed to study the electronic structure and binding affinity of chelators used in the removal of iron sulphide scales. Three chelating agents, EDTA, HEDTA, and DTPA, are considered in this work. The complexes showed a coordination number of 5, 6, and 7 for Fe 2þ and Fe 3þ ions with HEDTA, EDTA, and DTPA, respectively. However, regarding EDTA, Fe 3þ could coordinate with an additional water molecule and form a seven-coordinate complex. The calculated binding energies agreed with the experimental stability constants of the chelators in the order DTPA > EDTA > HEDTA for both Fe 2þ /Fe 3þ complexes. The binding free energies showed a spontaneous reaction with Fe 3þ having a stronger binding affinity than Fe 2þ due to electrostatic forces. This investigation provides insights regarding how chelators that are applied in iron sulphide scale removal may be designed by increasing the number of nitrogen atoms to above the number of carboxylate groups.