ph hypothesis for medical,nursing and pharmacy student to identify the drug absorption and its all kinetics related to ph-pka.

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08/05/18BHARDWAJ CLASS

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 pH is the logarithm of hydrogen and hydroxyl ion
concentration.
 pH scale is present between 0 to 14.
ACIDIC BASIC
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 pKa is the negative logarithm of the dissociation constant of a drug.
 According to Henderson -Hasselbalch equation –
pH = pKa – log ( ionized/unionized)………..for a weak acid.
pH = pKa – log ( unionized/ionized)………...for a weak base.
pH = pKa – log ( ionized/unionized)………… .1
when ionized = unionized
equation change into
pH = pKa – log ( 1/1)………………….2
Or pH = pKa – log 1 (log 1 = 0)
pH = pKa ………………………………3
From the equation 3 the pKa is Ph where the ionized part is equal to
unionized part.
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MOST DRUG MOLECULES ARE EITHER WEAK ACIDS OR WEAK BASES.
Very weak acidic drug (pKa>8) e.g. is pentobarbitol ,hexobarbitol.
Strong acidic drug (pKa <2.5) e.g. is disodium cromoglycate.
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This theory given by Brodie et.al.
This is based on the assumption that the gastrointestinal /blood barrier acts as a
lipid Barrier toward weak electrolyte drugs which are absorbed by passive
diffusion. The gastrointestinal /blood barrier is permeable to the non-ionized
drug form and impermeable to ionized form.
Thus process of absorption is depend on THE IONIZAION AND
UNIONIZATION STATE OF DRUG.
BHARDWAJ CLASS

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According to Henderson -Hasselbalch equation –
pH = pKa – log ( ionized/unionized)………..for a weak acid.
pH = pKa – log ( unionized/ionized)………...for a weak base.
Example – calculate the part of drug absorption of drug having pKa
4.
Solution – if the drug have pKa = 4
In gastric fluid (where ph is in the range of 2 to 4.) and in blood
(Ph=7.4)
In gastric fluid (assume that pH is 1.2)
pH = pKa – log ( ionized/unionized)………..for a weak acid
1.2 = 4- log (ionized/unionized)
1.2-4 = log (ionized/unionized)
-2.8 = log (ionized/unionized)
Antilog (-2.8) = (ionized/unionized)
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Example – calculate the part of drug absorption of drug having pKa 4.
Solution – if the drug have pKa = 4
In gastric fluid (where ph is in the range of 2 to 4.) and in blood (Ph=7.4)
pH = pKa – log ( ionized/unionized)………..for a weak acid
STOMACH pH=1.2 BLOOD (Ph=7.4)
ionized ionizedunionized unionized
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