  The acoustic excitations of the expanded metal solutions Li-NH$_3$ have been
measured by inelastic X-ray scattering as a function of the electron density by
changing the Li concentration. The dielectric functions of these model metals
with very low electron density have been derived from the high frequency sound
velocity using the one component plasma approach corrected for screening. Their
values, when combined with those from other metals, suggest that the electron
correlation induced departure of the dielectric function from the random phase
approximation follows a universal behavior whose main parameter is the electron
density.
