Acids and bases 3 Flashcards
(14 cards)
What is the acid-base phenomenon: ion ‘trapping’ (2)
- Unionised species can pass from blood to other fluids.
- If fluid pH doesn’t equal 7.45, %ionisation may increase in fluid: ions effectively ‘trapped’ in fluid.
What does the formation of salts do (3)
- This can lead to drugs with more desirable properties (solubility, pH, stability, & bioavailability)
- Care is needed as it can be detrimental to the therapeutic effect.
- Once the patent of a drug/salt has expired, generic versions are produced by changing the metal ion: different inactive ingredient, same active ingredient.
What can affect acid strength (4)
- EWG increases acid strength
- EDG decrease acid strength
- COOH = strong (due to delocalisation)
- More electronegative species = increased acid strength
What can affect base strength (3)
- Available lone pairs
- EDG increase base strength
- EWG decrease base strength
What are buffers (2)
- a solution that can resist pH change upon the addition of an acidic or basic components.
- The rate at which a drug degrades can be related to the pH of system and so buffering can aid stability.
How do you calculate the pH of acidic buffers (2)
pH = pKa + log[A-] / [HA]
used to determine the concentrations of salt and acid needed to obtain a desired pH, [A-] (conjugate base) being assumed equal to [salt].
Calculating buffer pH changes (4)
- Determine the moles of HA and A- before reaction.
- Determine the number of H+ or HO- added. (convert mL to L → /1000)
- Determine the moles of HA and A- after reaction.
- Recalculate the pH using moles after addition.
What is Basic buffer preparation
Basic buffers are made by using a weak base (B) and its salt.
What does adding H+ and HO- do to basic buffer action (3)
- Adding H+ shifts equilibrium to right, favouring BH+.
- Adding HO- shifts equilibrium to left, favouring B.
- In each case, the effect of adding H+/HO- is mitigated.
How do you calculate basic buffer pH (2)
pH = pKw - pKb + log[B] / [BH+]
Like acidic buffers, pH depends on concentration ratio
When is acidic buffer capacity high (4)
- [HA], [A-] are high so can cope with lots of H+ or HO-
- [HA] = [A-] so can respond to addition of H+ or HO-
- capacity is highest when pH = pKa (pKa of the weak acid) i.e. [HA] = [A-].
In practice, prepare buffer using a weak acid with a pKa as close as possible to the desired pH.
How do you calculate buffer capacity when acid is added
beta = d[B] / d[pH]
How do you calculate buffer capacity when base is added
beta = - (d[HA] / d[pH])
Van Slyke’s capacity (2)
beta = 2.303 x C x Ka x [H3O+] / (Ka + H3O+)^2
