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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?

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If I understand your question correctly, this should answer it:

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: $$\ce{B^- + HX <=> HB + X^-}$$ $$Ex:\ \ce{OH^- + HCl <=>> H2O + Cl^-}$$

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  • $\begingroup$ Thanks for explaining . Actually it does not clarify what I asked. Pharmacokinetics says that substances( read drug) which are weak bases having higher pKa in low pH get associated or undissociated or unionized, but clinical book says the substance ( read drug) which are weak bases , if put in low pH get ionized or dissociated. $\endgroup$ – kamal fotedar Mar 20 '16 at 2:41
  • $\begingroup$ Thanks for answering . Does this mean that the drug ( a Local anaesthetic) , usually a weak base is existing as negatively charged when manufactured). Are we injecting a charged drug or uncharged drug. I thought that drugs do not have charges on them when they are in bottles. $\endgroup$ – kamal fotedar Mar 20 '16 at 3:31
  • $\begingroup$ @kamalfotedar You might want to reread what you wrote in your question and make it more obvious what you're asking. $\endgroup$ – SendersReagent Mar 20 '16 at 3:45
  • $\begingroup$ So, specific to lidocaine, when it is in a solution with a pH lower than 7.9, it will be protonated to form a positively-charged ammonium ion. $\endgroup$ – SendersReagent Mar 20 '16 at 3:48

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