  The composition of a forming planet is set by the material it accretes from
its parent protoplanetary disk. Therefore, it is crucial to map the chemical
make-up of the gas in disks to understand the chemical environment of planet
formation. This paper presents molecular line observations taken with the
Atacama Large Millimeter/submillimeter Array of the planet-hosting disk around
the young star HD 169142. We detect N2H+, CH3OH, [CI], DCN, CS, C34S, 13CS,
H2CS, H2CO, HC3N and c-C3H2 in this system for the first time. Combining these
data with the recent detection of SO and previously published DCO+ data, we
estimate the location of H2O and CO snowlines and investigate radial variations
in the gas phase C/O ratio. We find that the HD 169142 disk has a relatively
low N2H+ flux compared to the disks around Herbig stars HD 163296 and MWC 480
indicating less CO freeze-out and place the CO snowline beyond the millimetre
disk at ~150 au. The detection of CH3OH from the inner disk is consistent with
the H2O snowline being located at the edge of the central dust cavity at ~20
au. The radially varying CS/SO ratio across the proposed H2O snowline location
is consistent with this interpretation. Additionally, the detection of CH3OH in
such a warm disk adds to the growing evidence supporting the inheritance of
complex ices in disks from the earlier, colder stages of star formation.
Finally, we propose that the giant HD 169142 b located at 37 au is forming
between the CO2 and H2O snowlines where the local elemental make of the gas is
expected to have C/O=1.0.
