This page offers an IDL routine that manages the H2SO4 refractive indices from Neidziela et al. [1998].  The refractive indices are given as a functino of temperature,  composition,  and wavelength.  The routine provided simply performs interpolations on these data.  Note that many combinations of temperature and composition are out of bounds for the Neidziela data.  Get the two routines (the 1st one is the main routine) and the data file below. The data file is a zip archive that you will need to unpack. You will need to provide the path to the data files in the routine (see routine for comments).

routine 1

routine 2