Abundances of HCN, HCO⁺, HNC and CS versus n_H2, for two different CR ionization rates (solid lines for ζ_CR = 10⁻¹³ s⁻¹ and solid-dotted lines for 10⁻¹² s⁻¹). Our model shows that with such a high ionization rate ζ_CR = 10⁻¹³–10⁻¹² s⁻¹, the abundances of HCO⁺ is expected to be lower than HCN, HNC, and CS in the dense gas regions of n_H2 > 10⁵ cm⁻³. However, with increasing CR ionization rates, at the same H₂ densities, the abundance of HCO⁺ is rising for the dense gas while the abundance of CS is decreasing.