The Visible Absorption Spectrum of Bromine in Solution

Abstract

The visible absorption spectrum of bromine was measured in n-hexane (a normal solvent) and in concentrated sulphuric acid (at room temperature and at -70° C.), phosphoric acid, and ethanol (at -70° C.) as associated solvents. The absorption continuum at 4150 A. in the gas is displaced slightly to the red in n-hexane although other normal solvents may displace it to the violet. The displacement to the violet is greater in associated solvents, ranging from 650 cm.^-l in sulphuric acid to over 2000 cm.^-1 in ethanol. The displacement in sulphuric acid glass at -70° C, is nearly double the value at 18° C. The displacements in associated solvents are explained in terms of the effect of the semi-rigid cage of solvent molecules surrounding each bromine molecule, and the assumption of " solvates " is unnecessary and undesirable. The spectra in solution are between 40 and 70 per cent. more intense than in the gas, about double the increase that would be predicted by Chako's treatment of the effect of Lorentz-Lorenz forces. The differences in the shape and Emax between solutions in sulphuric acid at 18° C. and at -70° C. are due to the different statistical distribution of bromine molecules between the vibrational energy levels.