Weak Acids/Bases 1 Flashcards

1
Q

What is a weak acid?

A

An acid that is partially deprotonated/dissociates

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2
Q

What is a weak base?

A

A base that is partially protonated

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3
Q

What is an acid?

A

A proton donor

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4
Q

What is a base?

A

A proton acceptor

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5
Q

What are most drugs?

A

Weak acids/bases

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6
Q

Equation for the acid constant (Ka)

A

Ka = [A− ][H+] / [HA]

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7
Q

Equation for the base constant (Kb)

A

Kb = [OH− ][BH+ ] / [B]

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8
Q

What are the acid/base contants a measure of?

A

A measure of how much of a drug and its conjugate exist in solution.

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9
Q

How can acid constant be used?

A

By comparison to pKa

pKa = -log(Ka)

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10
Q

What is pKa?

A

The point at which 50% of the drug is ionised.

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11
Q

What are the general rules of pKa?

A

Stronger acid (lower pH) will have a smaller pKa

Stronger base (higher pH) will have a larger pKa

Value of pKa is NOT an indication of whether a drug is an acid or a base - consider functional groups - which will ionise?

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12
Q

What does Ka tell us?

A
  • pH of solution
  • Ionisation of drug at given pH
  • Solubility of drug at given pH
  • pH of buffer
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13
Q

For a drug to be soluble in aqueous conditions it must be…

A

Polar/charged/ionised

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14
Q

Weak acid/base solubility equations (S)

A

WEAK ACID: pH − pKa = log((S−So)/So)

WEAK BASE: pH − pKa = log⁡(So/(S −So))

S = solubility 
So = Solubility of uncharged species

To be insolution, MUST be ionised

S is measured - does not indicate whether charged/uncharged

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15
Q

What is an amphoteric drug?

A

Drug molecule which has positive and negative charges depending on the pH

They can be zwitterionic

E.g. Doxycycline, Sulfadiazine, Acyclovir

At the isoelectric point, the overall charge is 0

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16
Q

What is the relevance of pH : pI (isoelectric point)?

A

pH < pI (drug is behaving as a base) - a more acidic environment

pH > pI (drug is behaving as an acid) - a more basic environment

17
Q

Discuss salts and solubility.

A

Drugs are often formulated as salts to improve solubility

Must be chosen carefully as it influences physicochemical properties

18
Q

Commonly used anions (-ve) for basic drugs?

A

hydrochloride, sulphate, bromide, chloride, phosphate, maleate, citrate, acetate

19
Q

Commonly used cations (+ve) for acidic drugs?

A

sodium, potassium, calcium, zinc

20
Q

What is the salting in/out effect?

A

The presence of a high amount of salt forces some of the drug out of solution (precipitation)

Observed for inorganic salts e.g. AgCl

Ksp = [Ag+][Cl-]

  • Ag is poorly soluble in water
  • NaCl is very soluble and dissociates to [Na+][Cl-]
  • Concentration of [Cl-]/Chloride increases
  • To maintain equilibrium/ksp constant, [Ag+] must reduce
  • Ag+ is removed from solution and appears as a solid

This is used in purification