# pKa and pH behaviour of a drug lignocain

Lignocain is a weak base with pKa of 7.9. if injected into an acidic ( low pH medium or tissue ) it does not act adequately. Reason probably is that it becomes more dissociated and ionized drugs donot penetrate tissues adequately. But according to relationship between pH and pKa in this forum it is given to understand that substances with high pKa in low pH solution get protonated or undissociated ? why the difference in interpretation?

An "uncharged" base with a high pKa is protonated to form an ionic species: $$\ce{B + HX <=> HB^+ + X^-}$$ $$Ex:\ \ce{NH3 + HCl <=>> NH4^+ + Cl^-}$$
A negatively-charged base with a high pKa is protonated to form an uncharged species: $$\ce{B^- + HX <=> HB + X^-}$$ $$Ex:\ \ce{OH^- + HCl <=>> H2O + Cl^-}$$