40—THE EFFECTS OF SODIUM CHLORIDE, PYRIDINE AND ETHYL ALCOHOL ON THE ABSORPTION SPECTRA OF CERTAIN AZO DYES IN AQUEOUS SOLUTION

Abstract

Sodium chloride has a considerable effect upon the absorption spectra of solutions of Benzopurpurine ₄B, its meta isomer and Direct Fast Orange SE in water. These dyes are known from other evidence to form aggregates in the solution when sodium chloride is added to it. The spectra of Chrysophenine G and Naphthalene Orange 2G in aqueous solution are unaffected by the addition of salt; it is known that these dyes are not appreciably aggregated by salt. The spectral effect of sodium chloride is therefore probably due to aggregation of the dye. When dissolved in aqueous pyridine containing 10 per cent, or more of pyridine, none of the dyes shows any considerable change in its absorption spectrum on the addition of sodium chloride to the solution. The presence of pyridine in the dye solution therefore probably prevents the aggregation of the dye by the salt. The spectrum of each dye in aqueous pyridine depends upon the percentage of pyridine in the solution, but even when the pyridine concentration is only 10 per cent., the spectrum of the dye differs significantly from that of the dye in water. The absorption spectrum of each dye in aqueous ethyl alcohol is independent of the concentration of sodium chloride in the solution when the alcohol concentration is 50 per cent, but not when it is 10 per cent. The spectrum of each dye in aqueous alcohol (free from salt) depends on the percentage of alcohol in the solution; the effect of 10 per cent, alcohol on the spectrum of the dye is small, however, compared with the effect of 10 per cent, pyridine. It is suggested that the spectral effect of ethyl alcohol is due to the effect of alcohol on the dielectric constant of the solution whereas the spectral effect of pyridine is due in part to complex formation between the pyridine and the dye.