  [Abridged] Chemical evolution in the protoplanetary disk midplane can modify
the composition of ices and gases. We have investigated if and how chemical
evolution affects the abundances and distributions of key volatile species in
the midplane of a protoplanetary disk in the 0.2-30 AU range. A full chemical
network including gas-phase, gas-grain interactions and grain-surface chemistry
is used to evolve chemistry in time, for 1 Myr. Great diversity is observed in
the relative abundance ratios of the main considered species: H2O, CO, CO2,
CH4, O2, NH3 and N2. The choice of ionisation level, the choice of initial
abundances, as well as the extent of chemical reaction types included are all
factors that affect the chemical evolution. The only exception is the
inheritance scenario with a low ionisation level, which results in negligible
changes compared with the initial abundances, regardless of whether
grain-surface chemistry is included. The chemical processing changes the C/O
ratios for gas and ice significantly with radius and with model assumptions.
For high ionisation levels, chemical processing becomes significant after a few
times $10^{5}$~yrs. It is concluded that chemical evolution in the disk
midplane needs to be considered in the determination of the volatile
composition of planetesimals.
