(6-7) Acid Base Theory, pH, pKa [I-II]

What is an acid according to Brønsted-Lowry theory

A substance that donates a proton

What is a base according to Brønsted-Lowry theory

A substance that accepts a proton

Why is water considered amphoteric

Water can act as both an acid and a base depending on the chemical environment

What is a conjugate acid-base pair

Acids and bases that differ from each other through the transfer of a single proton

What is the relationship between acid strength and conjugate base strength

The strength of an acid is inversely related to the strength of its conjugate base

What does the acid dissociation constant Ka measure

The equilibrium constant that quantifies the extent of dissociation of a weak acid in solution

What is pKa

The negative logarithm of Ka lower pKa indicates a stronger acid

How do you interpret Ka values

Higher Ka means stronger acid with greater dissociation lower Ka means weaker acid with less dissociation

What is the relationship between pKa and pKb for conjugate pairs at 25 degrees Celsius

pKa plus pKb equals 14

Define a strong acid

An acid that dissociates completely in aqueous solution

Define a weak acid

An acid that dissociates only partially in aqueous solution reaching an equilibrium

Example of weak acid in pharmacy

Aspirin or ibuprofen

Example of weak base in pharmacy

Morphine or procaine

What is the difference between strong and weak acid dissociation representation

Strong acids use single-headed arrow showing complete reaction weak acids use double-headed arrow showing reversible equilibrium

What is a polyprotic acid

An acid capable of donating more than one proton in a stepwise manner

Example of a triprotic acid

Phosphoric acid which can donate three protons

In polyprotic acid dissociation which proton is lost first

The proton with the lowest pKa value

What is an amphoteric electrolyte

An electrolyte that can function as either an acid or a base depending on the chemical environment

What is a zwitterion

A molecule carrying both positive and negative charges simultaneously on the same structure

Example of zwitterion

Glycine at physiological pH with NH3 plus and COO minus groups

What are the three categories of pharmaceutical drugs based on acid-base properties

Weak organic acids weak organic bases and salts of these compounds

Why is understanding drug ionisation important in pharmacy

The extent of ionisation affects drug absorption distribution and elimination

What is the effect of pH on weak acid dissociation

Lower pH favours the un-ionised form higher pH favours the ionised form

What does the Henderson-Hasselbalch equation predict

The relationship between pKa pH and the ratio of ionised to un-ionised drug

What is the clinical significance of un-ionised drugs

Un-ionised drugs have greater lipophilicity and cross biological membranes more easily

What is the clinical significance of ionised drugs

Ionised drugs are more water-soluble and have reduced membrane permeability affecting distribution and elimination

What is Kw

The ion product of water equals 1 times 10 to the minus 14 at 25 degrees Celsius

How does autoionisation of water occur

Water plus water yields hydronium ion H3O plus plus hydroxide ion OH minus

What is the role of water in weak acid dissociation

Water acts as a base accepting the proton from the weak acid HA

What is the role of water in weak base dissociation

Water acts as an acid donating the proton to the weak base B

What is a hydronium ion

A hydrated proton H3O plus formed when an acid donates a proton to water

For the reaction HA plus H2O in equilibrium with A minus plus H3O plus identify the conjugate pairs

First pair is HA and A minus second pair is H3O plus and H2O

Why is water concentration not included in equilibrium expressions

Water is the solvent and its concentration remains essentially constant

What is the difference between Ka and Kb

Ka measures acid dissociation strength while Kb measures base dissociation strength

Interpretation of pKa value of 4

pKa 4 means Ka equals 1 times 10 to the minus 4 making it a weak acid

Interpretation of pKa value of minus 1

pKa minus 1 means Ka equals 10 indicating a very strong acid

Why is the pKa scale more practical than Ka

pKa converts very large or very small numbers into a manageable scale

What are the three ionisable groups in amino acids

Amino group carboxyl group and in some cases ionisable side chains

pKa values for aspartic acid side chain

pKa 3.71 making it negatively charged at physiological pH

pKa values for lysine side chain

pKa 10.67 making it positively charged at physiological pH

pKa values for arginine side chain

pKa 12.10 making it positively charged at physiological pH

pKa values for glutamic acid side chain

pKa 4.15 making it negatively charged at physiological pH

pKa values for histidine side chain

pKa 6.04 giving variable charge around physiological pH

Clinical relevance of amino acid side chain pKa values

They determine how amino acids and proteins behave in different physiological fluids and interact with drugs

Why do pharmaceutical salts exist

Salt forms of weak acids and bases can improve solubility and bioavailability

Examples of pharmaceutical salts

Ephedrine hydrochloride and potassium diclofenac

What determines the ability of an acid to donate a proton

The stability of the conjugate base and the stability of the parent acid

What does the higher Ka value for first dissociation compared to second dissociation of polyprotic acid indicate

The first proton is easier to remove making it a stronger acid than subsequent dissociations

In phosphoric acid what is the correct order of proton loss

H3PO4 loses first proton to form H2PO4 minus then HPO4 2 minus then PO4 3 minus

Why are three key processes absorption distribution and elimination affected by ionisation

Un-ionised drugs cross membranes differently than ionised drugs affecting entry into tissues circulation and elimination

How pH in stomach affects weak acid drugs

Acidic stomach pH shifts equilibrium towards un-ionised form which can be readily absorbed
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