EFFECT OF CONCENTRATED SALT SOLUTIONS ON A MEROCYANINE DYE A VINYLOGOUS AMIDE

The spectral changes which occur in aqueous solutions of the merocyanine dye 1-methyl-4-[(oxocyclo-hexadienylidene)ethylidene]-1, 4-dihydropyridine (MOED) in the presence of 13 salts exhibit isosbestic points. The increases and decreases in absorbance at the wavelengths at which maximum changes of absorption take place are linearly related to each other. This suggests that the spectral changes of this vinylogous amide are caused by the appearance of a new absorbing species in the presence of salt, rather than by a continuous, nonspecific solvent effect. The spectral changes show a leveling off with increasing salt concentration and, for most of the salts examined, follow closely the behavior expected for the formation of a 1:1 complex between the dye and a component of the salt. Equilibrium constants near 1.0 M−1 were calculated for a series of uniunivalent salts of widely varying character. Complex formation can account for the salting in observed with sodium and potassium chloride and contributes ignificantly to the observed effects of other salts on the solubility of the dye. However, there remain very large salting in and salting out effects of some salts, such as sodium perchlorate, potassium iodide, sodium fluoride, and sodium sulfate, which are not accounted for by complex formation. © 1969, American Chemical Society. All rights reserved.