Lecture 4: pH and pKa

pKa

tells you how acidic or basic a functional group of drug is

pH

tells you how acidic or basic the environment is

difference b/t pKa and pH

pKa is drugs tendency to gain/lose H and pH is how acidic/basic the environment is

high pH

low H+ concentration, bc there isn’t a lot of H+ the molecule tends to give up it’s H+ → deprotonation

low pH

high H+ concentration, bc a lot of H+ the molecule will hold onto H+ → protonated

pKa is more than pH (pH < pKa)

more protonated

pH is more than pKa (pH > pKa)

more deprotonated

pH formula

= -log[H+]

Higher H⁺ concentration

lower pH = more acidic

Lower H⁺ concentration

higher pH = less acidic

acid equation (HA ⇌ H⁺ + A⁻)

HA = acid before losing H⁺

H⁺ = proton released

A⁻ = conjugate base after losing H⁺

Ka (acid dissociation constant)

measures how much the acid wants to dissociate (give away H⁺)

Large Ka acid

strong acid

Small Ka acid

weak acid

Ka into pKa (acid)

large Ka → low pKa → strong acid

small Ka → high pKa → weak acid

pKa​=−log(Ka​)

pKa formula acid

low pKa base

weak base

high pKa base

strong base

Henderson–Hasselbalch formula acid

addition

Henderson–Hasselbalch formula base

subtraction

pH=pKa

1:1 ratio 50% ionized + 50% unionized

Nonelectrolytes

such as dexamethasone, are 100% unionized regardless of the physiologic pH

quaternary ammonium groups

always 100% ionized regardless of the environmental pH