Abstract
Abstract The mechanism of the fluorescence quenching of aqueous thionine by various metal ions (Ce3+, Fe2+, Mn2+, Mg2+, Cs+ and Na+) was studied at pH = 2.5. The following Stern-Volmer constants (Ksv) were determined: KSV(Fe2+) == 1.13 dm3 mol-1, KSV(Mn2+) = 0.24 dm3 mol-1, KSV(Ce3+) = 0.24 dm3 mol-1, KSV(Cs+) - 0.03 dm3 mol-1. At concentrations below 1 mol.dm-3 Na2SO4 and MgSO4 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 2- 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.
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