  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 coincides with the CO snow line location
of 90 AU, indicative of a complex relationship between DCO+ emission and CO
freeze-out in the disk midplane. Compared to TW Hya, CO freezes out at a higher
temperature in the disk around HD 163296 (25 vs. 17 K in the TW Hya disk),
perhaps due to different ice compositions. This highlights the importance of
actually measuring the CO snow line location, rather than assuming a constant
CO freeze-out temperature for all disks.
