Dissociation Constant.

Physical Pharmaceutics 1st
3rd Sem B Pharmacy
Dissociation Constants

1. • DISSOCIATION CONSTANT • Definition: tendency of a particular substance in solution is to reversibly dissociate into ions. • Also know as ionization constant. • It is equal to the product of respective ion concentration non ionic molecule. • The greater the dissociation constant of the acid, the stronger is the is the acid • Ka or acid dissociation constant is a quantitative measurement

2. • Let us consider the dissociation of the compound ‘HA’ • The Ka for this reaction will be given by: rr- ia-hh-1 IHAI • Expressing acidity in terms of Ka can be inconvenient for practical purposes, therefore pKa is used

3. • Degree of ionization depends on pH • Henderson-Hasselbalch equation: • For basic compounds: pH = pKa + [Ionized] \un-ionized] • For acidic compounds: pH = pKa + [un-ionized] [ionizeds]

4. □ Determination 1. Conductivity method. 2. Solubility method. 3. Potentiometric method.

5. 1. Conductivity method • The law is based on the fact that only a portion of electrolyte is dissociated into ions at ordinary dilution and completely at infinite dilution. • Weak electrolytes are partially dissociate in solution. Hence for such electrolytes the dissociation constant (Ka) is given by Ostwald’s dilution law as follows: • Where C= the molar concentration, a = degree of dissociation. • The value of a is given as the ratio of the equivalent conductivity of the electrolyte at a particular concentration to that at infinite dilution, i.e. a = yv

6. 2. Solubility method • A derivation of the Henderson-Hasselbalch equation allows us to determine the pKa from solubility • log S = logSo + log(10 -pKa_pW + 1) • Here SO is equal to the intrinsic solubility. • When pH» pKa or pH« pKa assumptions can be made and linear logS/pH function are obtained. • By extrapolating these two fimetion and calculating the intercept, the pKa • log S = (logSo- pKa) + pH loss

7. 3. Potentiometric method • In potentiometric titration, a sample is titrated with acid or base using pH electrode to monitor the course of titration. • The pKa value is calculated from the change in shape of the titration curve compared with that of blank titration without sample is present. • Relationship between pH and pKa: pH = pKa + log.o^

8. □ Application 1. Dissociation constant is incorporated in Henderson-Hasselbalch equation to calculate the extent of ionization (or dissociation), i.e per cent unionized and ionized fonns of drug. 2. The absorption of drugs in gastrointestinal tract can be predicted using dissociation constant of the drug and pH at different sites of GIT. 3. The concentration of preservatives such as benzoic acid required to preserve solution and emulsion can be prcdictcs.

9. REFERENCES Physical Pharmaceutics Author - Edword Showtton Publisher - Oxford University Press(oup) Edition - 2008 • https:" staticscience.com • http://www.researchgate.net THANKYOU