Two acid hydrazones incorporating xanthene as a metal chelating centre (B-XAN and N-XAN) have been synthesized, characterized and evaluated for the optical sensing of Mg²⁺ and Al³⁺. The replacement of the naphthyl moiety in N-XAN by the phenyl moiety in B-XAN leads to total specificity of the latter one towards Mg²⁺. The B-XAN exhibited a highly sensitive and selective ‘turn-on’ fluorescent response towards Mg²⁺ through intense blue fluorescence enhancement. This measurement was free from any interference by other cationic analytes including the Ca²⁺ and Al³⁺ also. The binding constant, stoichiometry as well as the mechanistic aspect of the above sensing phenomenon were also studied through various spectroscopic methods.