Pharmaceutical Salts & Buffers

Salt formation

occurs when an acid reacts with a base, resulting in an ionic species held together by ionic bond

To form a salt, a weakly acidic drug must react with what?

strong base

Products from acidic drug salt formation

salt + water

Naming salts of acidic drugs

drug name + ion added

Bond formed by creating salt from weakly basic drug

ionic bond

Naming salts of basic drugs

name of acid used + “ate”

Salt former/counter ion

the strong acid/base that reacts with a basic/acidic drug to form a salt

ΔpKa

the Pka difference between the drug and the counter ion

ΔpKa for acidic drug salts

ΔpKa = (pKa of the salt former/counter ion)-(pKa of the acidic drug)

ΔpKa for basic drug salts

ΔpKa = (pKa of the basic drug)-(pKa of the salt former/counter ion)

ΔpKa >= 3

considered in pharmaceutical salt preparation

Effect of dissolution of salt of acidic drugs on pH and ionized form

produces OH-, increasing pH, driving equilibrium toward the more soluble ionized form

Effect of dissolution of salt of basic drugs on pH and ionized form

produces H+, decreasing pH, driving equilibrium toward the more soluble ionized form

Buffer

compounds or mixtures of compounds that, by their presence in solution, resist changes in pH upon the addition of small quantities of acid or base

Components of acidic buffer

weak acid and its salt with a strong base

Components of basic buffer

weak base and its salt with a strong acid

Effect of buffer on solubility

pharmaceutical formulations are adjusted to an optimum value so that the drug remains solubilized through shelf-life and not precipitated out

Effect of buffers on stability

increases stability by preventing hydrolysis

Effect of buffer on biological acitivty

optimizes activity because enzymes have maximum activity at definite pH values

Effect of buffer on the body

comforts tissues of the body and avoids irritation, hemolysis, or burning; matches pH of physiological fluid