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Finally, using the Henderson-Hasselbalch equation, and knowing the drug's (pH at which there is an equilibrium between its ionized and non-ionized molecules), it is possible to calculate the non-ionized concentration of the drug and therefore the concentration that will be subject to absorption:
The Henderson–Hasselbalch equation relates the pH of a solution containing a mixture of the two components to the acid dissociation constant, K a of the acid, and the concentrations of the species in solution. [2] Simulated titration of an acidified solution of a weak acid (pK a = 4.7) with alkali
This is why weak acids are better absorbed from the stomach and weak bases from intestine where the pH is alkaline. When pH of a solution is equal to pKa of dissolved drug, then 50% of the drug is ionized, another 50% is unionized. This is described by the Henderson-Hasselbalch equation. [citation needed]
The Henderson-Hasselbalch equation offers a way to determine the proportion of a substance that is ionized at a given pH. In the stomach, drugs that are weak acids (such as aspirin ) will be present mainly in their non-ionic form, and weak bases will be in their ionic form.
As calculated by the Henderson–Hasselbalch equation, in order to maintain a normal pH of 7.4 in the blood (whereby the pK a of carbonic acid is 6.1 at physiological temperature), a 20:1 ratio of bicarbonate to carbonic acid must constantly be maintained; this homeostasis is mainly mediated by pH sensors in the medulla oblongata of the brain ...
The Henderson–Hasselbalch equation is used to calculate pKa. ... Absorption (pharmacokinetics) ADME; Bioequivalence; Distribution (pharmacology) Elimination ...
Lawrence Joseph Henderson (June 3, 1878 – February 10, 1942) was an American physiologist, chemist, biologist, philosopher, and sociologist. He became one of the leading biochemists of the early 20th century. His work contributed to the Henderson–Hasselbalch equation, used to calculate pH as a measure of acidity.
Hence, the pK of each buffer will dictate the ratio of the concentrations of its base and weak acid forms at the given pH, in accordance with the Henderson-Hasselbalch equation. Any condition that changes the balance of one of the buffer systems, also changes the balance of all the others because the buffer systems actually buffer one another ...