It has been suggested that the dc conductivity of polar ice sheets could be due to the presence of liquid layers at the grain boundaries. These layers would consist of acids with water. We show that it is plausible that acids will be at three-grain boundaries in polar ice. Using reliable data for H2SO4, HNO3, and HCl concentrations in ice at South Pole, we derive the correct the magnitude and temperature dependence for its conductivity. The model explains the narrow range of ice conductivities found in polar regions.