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PMP 102 Drugs & Medicines > Solubility of weak acids and bases > Flashcards

Solubility of weak acids and bases Flashcards

1
Q

Weak acid

Henderson hasselbalch equation

A

pH = pKa + log([A-]/[HA])

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2
Q

How is solubility effected by pH

A
  • Ionised species are more soluable compared to unionised species
  • Decreased solubility could lead to precipitation
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3
Q

Weak baic drugs

A
  • Almost 100% of the drug is ionised at low pH (accept proton)
  • Rapid decrease in ionisation as there is an increase in pH
  • Backwards s shaped curve do none of the drug is ionised at a high pH
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4
Q

Ionisation pH graph

Pka

A

50% ionisation

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5
Q

Weakly Acidic drug

A
  • Initially at a low pH none of the drugs are ionised an increases in ph causes increase in ionisation
  • Almost 100% of the drug is ionised at high pH (accept proton)
  • S shaped curve do none of the drug is ionised at a low pH
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6
Q

ionised-pH graph (acidic)

Pka - 1 unit

A
  • 10% of the drug is ionised
  • 90% of the drug is unionised
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7
Q

ionised-pH graph (acidic)

Pka + 1 unit

A
  • 90% of the drug ionised
  • 10% of the drug unionised
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8
Q

ionised-pH graph (acidic)

Pka - 2 unit

A
  • 1% of the drug ionised
  • 99% of the drug unionised
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9
Q

ionised-pH graph (acidic)

Pka + 2 units

A
  • 99% of the drug is ionised
  • 1% of the drug is ionised
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10
Q

Weakly acidic drug properties

A
  • More polar therefore more aqueous solubility at higher pH values
  • [HA] - undissociated unionised and free acid form
  • [A-] - Ionised form
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11
Q

Weak acid saturation solubility equation

A

pH = Pka + log (S - S0)/S0
S = Saturation solubility of weak acids at given pH
S0 = Solubility of undissociated drug

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12
Q

Weakly basic drugs properties

A
  • More polar so have a greater aqueous solubility at lower pH values
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13
Q

Henderson hasselbalch equation for weak bases

A
  • pH = Pka - log ([B]/[BH+])
  • [B] = Concentration of undissociated base
  • [BH+] = Concentration of ionised molecule
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14
Q

Weak base saturation solubility equation

A

pH = Pka + log S0/(S - S0)
S = Solubility of weak basic drugs at a given pH
S0 = Solubility of undissociated drug

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15
Q

Amphoteric drugs

A
  • Contain acidic and basic groups
  • ionised at all pH values
  • Depends on if the acid or base is predominently ionised
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16
Q

Solubility graph for amphoteric drugs

A
  • Has a minimum solubility point where there is no net charge and the molecule is neutral
  • This is the isoelectric point:
  • (Pka1 + Pka2)/2

PMP 102 Drugs & Medicines (56 decks)

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