Photometric and fluorimetric study of the acid-base behavior of 2,2'-diquinolyl and 2,2',2″-terpyridyl.

A photometric and fluorimetric study of the acid-base behavior of 2,2'-diquinolyl and 2,2',2″-terpyridyl was performed. In sulfuric acid medium, the doubly charged 2,2'-diquinolynium ion undergoes the first dissociation atH 0=0.20±0.09, as determined by fluorimetry (λex=336 nm, λem=424 nm). Photometric titration is less accurate because of the overlapping of the absorption spectra. The second dissociation constant of 2,2'-diquinolyl was determined by fluorimetric titration (λex=336 nm, λem=420 nm), obtaining a value of 3.67±0.03. The triply charged 2,2',2″-terpyridyl molecule was found to undergo the first dissociation atH 0=-7.17±0.04, as determined by fluorimetric titration (λex=316 nm, λem=350 nm), in aqueous sulfuric acid medium. Photometric titration (λ=335 nm) was performed in the presence of 6.5% ethanol because of the low solubility of the compound in water. In this ethanolic∶water medium, a value of the dissociation constant atH 0=-7.39±0.03 was calculated. The second dissociation constant was determined to be 2.81±0.12 by photometric titration at 285 nm, and values of 4.03±0.26 and 4.16±0.20 were found for the third dissociation constant by photometric titrations at 320 and 295 nm, in 10% ethanol, in close agreement with previously reported values. The fluorimetric titration profile obtained by exciting at 274 nm and measuring the fluorescence emission at 350 nm, in the zone betweenH 0=-3 and pH=10, is complicated by the several equilibria involved.