Acid Base Theory Flashcards
Why is the ionisation state important?
It is important for the absorption of drugs.
How does the pH in different body fluids influence the ionisation state of drugs?
- Drugs with basic side chains may be protonated and become positively charged
- Drugs with acidic side chains may be deprotonated and become negatively charged.
Based on the Arrhenius theory define an acid.
Any substance which produces a hydrogen ion in aqueous solution i.e. a proton donor.
Based on the Arrhenius theory define base.
Any substance which delivers a hydroxide ion in an aqueous solution i.e. a hydroxyl donor.
What are the limitations of the Arrhenius theory?
- No consideration of solvent effects
- Ammonia is a base with no hydroxyl group
Based on the bronsted-lowry theory define an acid.
Any substance that donates a H+
Based on the bronsted-lowry theory define base.
Any substance that accepts a H+
Why is water described as amphoteric?
It can act as both acid and base.
Define acidity
A measure of the tendency of a compound to give up H+
Define basicity
A measure of a compound’s affinity for a H+
What does bronsted-lowry theory fail to explain?
- Acid-base behaviour in other solvents
- Proton-less compounds like AlCl3
Based on the Lewis theory define acid.
Electron pair acceptor
Based on the Lewis theory define base.
Electron pair donor
Give the equation to calculate pH.
pH = -log[H3O+]
Give the equation to calculate pOH.
pOH = -log[OH-]
Describe aqueous solutions of weak acids.
- Weak acids only partially ionise in aqueous solutions
- Equilibrium is setup favouring reactants
Give the equation for pKa
pKa = -logKa
What is meant by the common ion effect?
The shift in equilibrium caused by the addition of a compound having an ion in common with the dissolved substance.
Describe the presence of a common ion.
Suppresses the ionisation of a weak acid or a weak base.
What is the effect of having a weak acid HA and its salt in solution?
- Ionisation of HA is suppressed by the presence of A-
- Hydrolysis of A- is suppressed by the presence of HA
- The acid and conjugate base may react with one another
What happens when a strong base is added to a buffer?
- The weak acid HA will give up its H+ to transform the base into water and the conjugate base
- OH- is being consumed so pH only changes slightly.
What happens if a strong acid is added to a buffer?
- The weak base will react with the H+ from the strong acid to form the weak acid.
- The H+ gets absorbed by the A- instead instead of reacting with water to form H3O+ so the pH changes only slightly
What is a buffer solution?
A solution which has the ability to resist changes inn pH upon the addition of small amounts of either acid or base.
What do buffers contain?
Either a weak acid and its salt or a weak base and its salt.
How does a buffer work?
- At the pKa HA = A- so the system is able to absorb the addition of OH- or H+
- Adding OH- near the pH where HA = A- then HA releases H+ to offset the OH- added so ratio of HA to A- does not change much
- Adding H+ the A- can absorb H+ to form HA
State the Henderson-Hasselbalch equation.
pH = pKa + log([base]/[acid])
What is the goal of a buffer?
- To keep the pH of a solution within a narrow range.
What is the importance of the ratio of [A-]/[HA]?
It influences the pH of a solution.
What is the importance of the concentrations of [A-] [HA]?
Controls the effectiveness of a buffer.
Describe the effect of more A- and HA molecules.
- The less of an effect addition of a strong acid or base will have on the pH of a system
The amount of strong base required to change the pH by one unit depends on what?
- The concentration of the components.
- Higher concentration gives a higher beta.
What happens to blood pH when you hyperventilate?
If blood pH drops due to metabolic production of H+ then [H2CO3] increases by protonation of HCO3-, H2CO3 rapidly loses water to form CO2(aq) which is expelled as CO2(g).
What happens to blood pH when you hypoventilate?
If the blood pH rises [HCO3-] increases by deprotonation of H2CO3 then breathing changes and CO2(g) is converted to CO2(aq) and then to H2CO3 in the capillaries in the lungs.
What is meant by the ionisation of drugs?
Protonation or deprotonation resulting in charged molecules.
The acidity or basicity of a compound plays a major role in controlling what?
- Absorption and transport to site of action
- Solubility, bioavailability, absorption and cell penetration, plasma binding, volume of distribution
- Binding of a compound at its site of action
- Un-ionised form involved in hydrogen bonding
- Ionised form influences strength of salt bridges or H bonds
- Elimination of compound
- Biliary and renal excretion
- CYP P450 metabolism
Which form of the drug diffuses into the bloodstream?
The unionised form
What is the strongest acid you can have in an aqueous solution?
H3O+
What is the strongest base you can have in aqueous solution?
OH-
What is meant by levelling effect?
- The effect of solvent on the property of acids and bases
- Strength of strong acid is levelled by the basicity of the solvent
- Strength of a strong base is levelled by the acidity of the solvent
Give the equation to find pKa.
pKa = pH - log[A-]/[HA]
therefore, [base]/[acid] = 10^(pH-pKa)
