Context : Aims : Our goal is to study the chemical composition of the outflows of active galactic nuclei and starburst galaxies . Methods : We obtained high-resolution interferometric observations of HCN and HCO ^ { + } J = 1 \rightarrow 0 and J = 2 \rightarrow 1 of the ultraluminous infrared galaxy Mrk 231 with the IRAM Plateau de Bure Interferometer . We also use previously published observations of HCN and HCO ^ { + } J = 1 \rightarrow 0 and J = 3 \rightarrow 2 , and HNC J = 1 \rightarrow 0 in the same source . Results : In the line wings of the HCN , HCO ^ { + } , and HNC emission , we find that these three molecular species exhibit features at distinct velocities which differ between the species . The features are not consistent with emission lines of other molecular species . Through radiative transfer modelling of the HCN and HCO ^ { + } outflow emission we find an average abundance ratio X ( \mathrm { HCN } ) / X ( \mathrm { HCO } ^ { + } ) \gtrsim 1000 . Assuming a clumpy outflow , modelling of the HCN and HCO ^ { + } emission produces strongly inconsistent outflow masses . Conclusions : Both the anti-correlated outflow features of HCN and HCO ^ { + } and the different outflow masses calculated from the radiative transfer models of the HCN and HCO ^ { + } emission suggest that the outflow is chemically differentiated . The separation between HCN and HCO ^ { + } could be an indicator of shock fronts present in the outflow , since the HCN/HCO ^ { + } ratio is expected to be elevated in shocked regions . Our result shows that studies of the chemistry in large-scale galactic outflows can be used to better understand the physical properties of these outflows and their effects on the interstellar medium ( ISM ) in the galaxy .