Thionine Fluorescence Quenching by Metal Cations

The mechanism of the fluorescence quenching of aqueous thionine by various metal ions (Ce3+_, Fe2+, Mn2+, Mg2+, Cs+ and Na+) was studied at pn = 2.5. The following Stern-Volmer constants (K s v) were determined: XSv(Fe2+) == 1.13 dm3 mol -1 , Ksw(Mn2+) = 0.24 dm3 mol"1, Ksv(Ce3+) = 0.24 dm3 mol -1, i£sv(Cs~) — 0.03 dm3 mol -1 . At concentrations below 1 mol.dm -3 Na2S04 and MgS04 are very weak quenchers (Ksv(Na+ or Mg2+) = 0.007), but the influence on the fluorescence yield increases considerably at higher cation concentrations. This was attributed to the formation of ground state complexes between thionine and SO4 ions. The salt concentration affects considerably the quenching rate of Fe2+ ions, but much less so that of the other cations. Fluorescence quenching mechanisms are discussed.