The condensation fronts (snow lines) of H2O, CO, and other abundant volatiles in the midplane of a protoplanetary disk affect several aspects of planet formation. Locating the CO snow line, where the CO gas column density is expected to drop substantially, based solely on CO emission profiles, is challenging. This has prompted an exploration of chemical signatures of CO freeze-out. We present ALMA Cycle 1 observations of the N2H J = 3-2 and DCO J = 4-3 emission lines toward the disk around the Herbig Ae star HD 163296 at ˜0.″5 (60 AU) resolution, and evaluate their utility as tracers of the CO snow line location. The N2H emission is distributed in a ring with an inner radius at 90 AU, corresponding to a midplane temperature of 25 K. This result is consistent with a new analysis of optically thin C18O data, which implies a sharp drop in CO abundance at 90 AU. Thus N2H appears to be a robust tracer of the midplane CO snow line. The DCO emission also has a ring morphology, but neither the inner nor the outer radius coincide with the
