Lecture 2 (pH)

Question 1

Bronsted acid

Answer 1

Proton donor
HCl + NH3 ↔ NH4+ + Cl-

Question 2

Bronsted base

Answer 2

Proton acceptor
PO43-(aq) + H2O(l) ↔ HPO42-(aq)+ OH-(aq)

Question 3

:NH3(aq) + H2O(l) –> NH4+(aq) + :OH-

Answer 3

Forward reaction - Water is a BrØnsted acid & Lewis acid,
Reverse reaction - Hydroxide ion is a BrØnsted & Lewis base,

Question 4

How to measure the pH using an indicator?

Answer 4

substance that changes colour in specific pH ranges. Indicators (HIn) exhibit pH dependent colour changes because they are weak acids and have different colours in their acid & conjugate base forms. For a wide pH range a universal indicator can be used.

Question 5

How to measure pH by using a pH meter

Answer 5

is sensitive to the hydrogen ion activity and has a potential proportional to pH. It contains phosphate buffer with chloride ions and the E value is zero when the external pH is 7. This system MUST be calibrated with solutions of known pH prior to use.

Question 6

What is a strong acid?

Answer 6

almost completely dissociated into ions, e.g. HCl,

Question 7

What is a weak acid?

Answer 7

Weak acids – partially dissociated into ions, e.g. acetic acid,

Question 8

What makes an acid strong?

Answer 8

1) Solvent effects,
2) Energetic stability,
3) The nature of the parent molecule,
4) The strength of the HA bond.

Question 9

What is pH?

Answer 9

pH is the negative logarithm of the hydrogen ion concentration in the solution.
pH = -log10[H+]

Question 10

What is a Monoprotic acid?

Answer 10

It is only possible to lose one proton per molecule of acid, e.g. HCl, HCN, HF & CH3COOH.

Question 11

What is a diprotic acid?

Answer 11

Diprotic – It is possible to lose two protons per molecule of acid, e.g. H2SO4

Question 12

What is a logarithm?

Answer 12

Converts a large range of values into a small range

Question 13

What is polyprotic?

Answer 13

As many pKa values as protons lost per molecule, e.g.oxytetracycline with three pKa values.

Question 14

What is a buffer?

Answer 14

keeps the pH approximately constant when small amounts of acid or alkali are added. With enough dilution the effects are suppressed. The buffer capacity is a measure of the resistance to pH change.

Question 15

When is a buffer capacity at its maximum?

Answer 15

when the pH of the buffer is equal to the pKa of the weak acid. It decreases appreciably as the pH extends more than one unit either side of this value.

Question 16

Ka expression

Answer 16

Ka = [H+ ] [A-]/[HA]

Question 17

What is the pH of blood?

Answer 17

7.40

Question 18

What is the pH Partition theory

Answer 18

A drug must cross the plasma membrane therefore it must be lipid soluble. As most drugs are weak electrolytes it is to be expected that the unionised form of acids and bases i.e. the lipid soluble species, will diffuse across while the ionised form will be rejected.
The extent of ionisation of a drug has an important effect on its absorption, distribution and elimination

Question 19

What are the limitations of the pH partition theory

Answer 19

In practice, such an equilibrium will rarely (if ever) be achieved since the stomach and blood are not closed, static compartments.