Alizarin complexone–lanthanide(III)–fluoride system: Revised speciation and the origin of the analytical signal

Potentiometric titrations of alizarin complexone (ALC) in the presence of La(III), Eu(III) and Y(III) in 50% vol. DMSO reveal formation of mostly binuclear complexes with monoprotonated (LH) and completely deprotonated (L) forms of the ligand of the type M2L2H2, M2L2H and M2L2. Only for La(III) a single mononuclear complex LaLH is observed in acidic media. Titrations of the same mixtures with added fluoride show that with Eu(III) and Y(III), fluoride forms ternary complexes without shifting the distribution profile of complexes with differently protonated forms of bound ALC, but with La(III) a strong shift in distribution in favor of complexes with the deprotonated ligand occurs, which explains the appearance of blue colored species in acidic medium giving rise to the analytical signal employed for quantitative determination of fluoride. Detailed analysis of the spectral properties of La(III)–ALC–fluoride system on the basis of the established species distribution diagrams demonstrate the following factors to be involved: (1) fluoride-induced conversion of LaLH to La2L2HF around pH 3; (2) anomalously high affinity of La(III)–ALC complexes to fluoride and (3) some sort of mutual effect of fluoride and ALC ligands in ternary La(III) complexes, probably involving change in the metal coordination number, leading to enhanced absorptivity of La2L2HF as compared to La2L2H.

Graphical abstractAddition of fluoride to alizarin complexone complexes of La(III) shifts the species distribution in favor of complexes with the deprotonated ligand, which explains the appearance of blue colored species in acidic medium giving rise to the analytical signal employed for quantitative determination of fluoride. Such effect is not observed for Eu(III) and Y(III) complexes.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Fluoride stabilizes less protonated forms of alizarin complexone complexes with La(III), but not with Eu(III) and Y(III). ► There is a significant mutual ligand influence in ternary complexes of alizarine complexone–La(III)–fluoride. ► La(III)–alizarin complexone complexes have anomalously high affinity to fluoride and hydroxide anions.