Defining selective chelation windows for Ca²⁺-Mg²⁺ discrimination in saline matrices: comparative performance of EDTA...

Selective discrimination of Ca²⁺ from Mg²⁺ remains challenging in saline matrices because both ions exhibit similar coordination behaviour and their chelation responses can be altered by solution conditions. EDTA and EGTA were systematically compared for Ca²⁺ and Mg²⁺ complexometric titration over 0-1000 ppm. The effects of pH (3-12), temperature (25-45 °C), NaCl (0-30 mM), KCl (0-19 000 mg L^-1), SO₄^2- (0-2700 mg L^-1), and K⁺ (0-600 mg L^-1) were evaluated in individual and equimolar Ca²⁺ : Mg²⁺ systems. Both chelators produced highly linear calibrations, but their responses differed markedly. EGTA increased Ca²⁺ calibration sensitivity by 32.9% relative to EDTA and showed substantially greater thermal stability. EDTA responded to both metals across pH 3-12, whereas EGTA produced no measurable Mg²⁺ endpoint at pH 3-7. Crucially, this Ca²⁺-selective response persisted in mixed Ca²⁺ : Mg²⁺ solutions containing chloride, sulphate, and potassium at the investigated concentrations. The study identifies an experimentally observed operational selectivity interval at pH 3-7, defined by differential endpoint detectability, in which Ca²⁺ remains measurable with EGTA while no stable visual Mg²⁺ endpoint is obtained under the investigated conditions. This matrix-resilient behaviour provides a practical basis for selective calcium analysis and future Ca²⁺/Mg²⁺ separation strategies.