L10 - Electrolytes And Buffers Flashcards
What are weak electrolytes?
Weak lowry-brønsted acids/bases
What are strong electrolytes?
Strong lowry-brønsted acids/bases and salts
What is the primary purpose and importance of buffers in pharmacy?
Protect pharmaceutical preparations from any sudden change in pH even when limited conc of acid/base are added
Why is it important to control the pH of a solution?
Minimise drug degradation
Improve patient comfort and compliance
Improve the efficacy of delivery
What is the conjugate base of a strong acid?
Weak base
What is the conjugate acid of a strong base?
Weak acid
What are weak brønsted acids like in aq solutions? (2)
- partially (weakly) donates its proton(s)
- remains main undissociated (unionised) in aq solution
Why do weak brønsted acids remain mainly unionised in aq solutions?
Conjugate base has a strong tendency to accept proton(s) in H2O
- shifts the eq towards formation of the acid
= remains mainly undissociated in H2O
What are weak brønstead bases like in aq solutions? (2)
- has a weak tendency to accept proton(s)
- only ‘partially’ protonates (ionised)
Why do weak brønsted bases only partially protonate?
The conjugated acid has a strong tendency to lose its proton in solution
- shifts the eq to the left
= towards formation of base1
What is an electrolyte?
Compound that is ionised in solution
When do electrolytes yield ions? And what effect does this have?
When dissolved in water
- could conduct an electric current through solution
What are strong electrolytes?
Compound that is completely ionised in aq solution
What are weak electrolytes?
Compound that is incompletely ionised in aq solutions
What are non-electrolytes?
Substances that do not yield ions when dissolved in water and therefore do not conduct an electric current through the solution
What are examples of non-electrolytes? (5)
- sucrose
- glycerin
- napthalene
- urea
- steroids
What are examples of strong salts as electrolytes?
CH3COONa (WA CH3COOH and SB NaOH)
NH4Cl (SA HCl and WB NH3)
Ephedrine hydrochloride (WB ephedrine and SA HCl)
Sodium salicylate (WA salicylic acid and SB NaOH)
What are buffers?
Usually a micture of weak acid/base and its salt (conj base/acid)
- resistant to changes in pH
Buffer resistance to changes in pH WA and its salt:
Have an ion in common
- ties up the hydrogen ion
Eqn for WA and its ionised salt:
WA (HA), ionised salt (NaA)
A- + H3O+ <=> H2O + HA(undissociated WA)
HA + OH- <=> H2O(undissociated water) + A-
Eqn for WB and it’s ionised salt:
WB (B), ionised salt (BH+)
B + H3O+ <=> H2O + BH+(salt)
BH+ + OH- <=> H2O(water) + B
The buffer eqn for WA and its salt
Ka = [H3O+][A-]/[HA]
- Ka momentarily disturbed, +salt increases [A-]
- to re-establish Ka, [H3O+] decreases, with an increase in [HA]
- eq shifts towards reactants (left)
- pH of final solution, rearrange eq expression
pH = pKa + log[salt]/[acid]
The buffer eqn for WB ans its salt:
pH = pKw - pKb + log[base]/[salt]
What is buffer capacity?
The magnitude of the resistance of a buffer to pH changes
(Buffer efficiency, buffer index, buffer value)
What is buffer capacity defined as?
The ratio of the increment of strong base (or acid) to the small change in pH brought about by this addition
B = deltam/deltapH
What is delta m?
The no of moles of a base/acid needed to change the pH of 1 liter of solution by an amount deltapH
When does B = 1?
When the addition of 1 mole of strong base/acid to 1L of the buffer solution results in a change of 1 pH unit
