  Supernova remnants (SNRs) exert strong influence on the physics and chemistry
of the nearby molecular clouds (MCs) through shock waves and the cosmic rays
(CRs) they accelerate. To investigate the SNR-cloud interaction in the
prototype interacting SNR W28 (G6.4$-$0.1), we present new observations of $\rm
HCO^+$, HCN and HNC $J=1\text{--}0$ lines, supplemented by archival data of CO
isotopes, $\rm N_2H^+$ and $\rm H^{13}CO^+$. We compare the spatial
distribution and spectral line profiles of different molecular species. Using
local thermodynatic equilibrium (LTE) assumption, we obtain an abundance ratio
$N({\rm HCO^+})/N({\rm CO})\sim10^{-4}$ in the northeastern shocked cloud,
which is higher by an order of magnitude than the values in unshocked clouds.
This can be accounted for by the chemistry jointly induced by shock and CRs,
with the physical parameters previously obtained from observations: preshock
density $n_{\rm H}\sim 2\times 10^{5}\rm \ cm^{-3}$, CR ionization rate
$\zeta=2.5\times 10^{-15} \rm \ s^{-1}$ and shock velocity $V_{\rm
s}=15\text{--}20\rm \ km\ s^{-1}$. Towards a point outside the northeastern
boundary of W28 with known high CR ionization rate, we estimate the abundance
ratio $ N({\rm HCO^+})/N({\rm N_2H^+}) \approx 0.6\text{--}3.3$, which can be
reproduced by a chemical simulation if a high density $n_{\rm H}\sim 2\times
10^5 \ \rm cm^{-3}$ is adopted.
