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Updated: Dec 20, 2025

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Published on: July 23, 2020
A primer on tissue pH and local anesthetic potency
Rebecca L E Pope1, Angus M Brown1,2
1School of Life Sciences, University of Nottingham, Nottingham, United Kingdom.
Abstract:
The relationship between pH, pKa, and degree of local anesthetic ionization is quantified by the Henderson-Hasselbalch equation. As presented in standard textbooks, the effect of pH on the degree of ionization of any particular local anesthetic is not immediately clear due to the x-axis displaying pH - pKa, which requires conversion to pH, based on the pKa for each local anesthetic, a complex process. We present a graphical solution that clarifies the interrelationships between pH, pKa, and degree of ionization by plotting pKa on the x-axis versus the percentage of unionized local anesthetic on the y-axis. The vertical intercept from the x-axis to the pH curves allows rapid and accurate estimation of the degree of ionization of any local anesthetic of known pKa.
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