Absorption spectra of sodium and potassium in liquid ammonia

The absorption spectra of dilute solutions of sodium and potassium in liquid ammonia were obtained as a function of concentration and temperature. The shapes of the absorption curves were found to be identical for the two metals and independent of concentration. With an increase in temperature, the absorption curve is shifted to lower energies. These results indicate that the absorption process in the near infrared (6800 cm. (-1)) involves the excitation of an electron in a cavity to a higher energy state and that this process is the same for dilute solutions of both metals. Sodium-ammonia solutions obeyed Beer's law within experimental error over the concentration range covered (3 X 10(-4) to 4 X10(-3) M). Potassium solutions (1 X 10(-3) to 1 X 10(-2) M) showed a negative deviation from Beer's law. The molar absorbancy index of the sodium solutions and the extrapolated value for potassium (to zero concentration) were found to be 4.5 +/- 0.3 X 10(4) l. mole(-1) cm. (-1). The results are explained in the light of current models for these solutions.