  We present a determination of chemical freeze-out conditions in heavy ion
collisions based on ratios of cumulants of net electric charge fluctuations.
These ratios can reliably be calculated in lattice QCD for a wide range of
chemical potential values by using a next-to-leading order Taylor series
expansion around the limit of vanishing baryon, electric charge and strangeness
chemical potentials. From a computation of up to fourth order cumulants and
charge correlations we first determine the strangeness and electric charge
chemical potentials that characterize freeze-out conditions in a heavy ion
collision and confirm that in the temperature range 150 MeV < T < 170 MeV the
hadron resonance gas model provides good approximations for these parameters
that agree with QCD calculations on the (5-15)% level. We then show that a
comparison of lattice QCD results for ratios of up to third order cumulants of
electric charge fluctuations with experimental results allows to extract the
freeze-out baryon chemical potential and the freeze-out temperature.
