The effect of H+, Na+, Ca2+, Mg2+ on merocyanine 540 (MC 540) absorpti
on and fluorescence spectra was determined. While MC 540 showed two ab
sorption peaks in water (at 535 nm, the monomer peak; at 503 nm, the d
imer peak) the addition of salts to MC 540 aqueous solutions resulted
in the formation of a new absorption peak at 517 nm and the disappeara
nce of both MC 540 ''water'' bands. MC 540 fluorescence spectra showed
only one peak in pure water (at 573 nm) as well as in salt and HCl aq
ueous solutions (at 571 nm). The addition of salts did not change the
shape of the fluorescence spectra but substantially quenched the fluor
escence intensity. The effect of salts reflected principally differenc
es in their cationic charge. These spectral changes were discussed in
terms of interaction between the dye anion MC 540(-) and the aqueous p
roton H+ followed by dimerization of the protonated merocyanine dye. T
his assumption was supported by the low pH form of MC 540. A pK value
of 1.6 between the neutral protonated form and the anionic form of MC
540 was determined.