acids and bases 3 Flashcards
What are the acid-base phenomena: 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 are effectively ‘trapped’ in the fluid.
What are the acid-base phenomena: salts (5)
- Drugs that are weak acids (or bases) react with a strong base (or acid) to produce a salt (and water).
- A majority of drugs are formulated as salts, e.g. sodium hydroxide with ibuprofen to give sodium ibuprofen.
- This can lead to drugs with more desirable properties, e.g. solubility, pH, stability, & bioavailability, but the care needed 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 ingredients, the same active ingredient.
- Bioequivalence testing is undertaken to check that these ‘pharmaceutical alternatives’ have equivalent therapeutic effects.
What are the factors affecting acid strength (6)
- Acids lose protons more easily (H-A → H+ + A-) when:
- Weak H-A bond (A withdraws electrons from the bond)
- Anionic (A-) charge is reduced through delocalisation and/or electron-withdrawing groups (EWG).
- Electron donating groups (EDG) decrease strength.
- Due to delocalisation, carboxylic acids XCOOH are stronger (X = CH3, pKa = 4.8).
- more electronegative species have a higher electron-withdrawing effect, increasing acid strength.
What are the factors affecting base strength (6)
- Stronger bases gain protons more easily.
- Factors that give strong bases (i.e. low pKb):
- High lone pair availability (B is electron donating)
- A stable conjugate acid cation, BH+ (B is electron-donating and can lower localised charge).
- Electron-donating groups (EDG) increase base strength.
- Electron withdrawing groups (EWG) decrease base strength.
What are buffers and their application (6)
- A buffer isa solution that can resist pH change upon the addition of acidic or basic components. **
- Acidic, neutral, and basic buffers can be prepared.
- Chemical buffers are used to maintain a system at a required pH. This may be necessary as:
- pH determines the % ionisation of a drug in solution, affecting partitioning, properties, etc
- Compounds applied to sensitive areas, such as the eye, must have near-neutral pH to avoid irritation.
- The rate at which a drug degrades can be related to the pH of the system, so buffering can aid stability.
What are acidic buffers and their preparation (7)
- buffer region is a mixture of a weak acid, and its salt-acidic buffers are made using these two compounds
- Buffer region is evident on a pH curve for a weak acid and a strong base.
- Acid + base → salt + water
- Salt gives A- (conjugate base), weak acid gives HA.
- Adding H+ shifts equilibrium to the left, favouring HA.
- Adding HO- shifts equilibrium to the right, favouring A-.
- In each case, the effect of adding H+/HO- is mitigated.
How do you calculate the pH of acidic buffers (2)
pH = pKa + log[A-] / [HA]
- This can be used to determine the concentrations of salt and acid needed to obtain a desired pH, [A-] (conjugate base) being assumed equal to [salt].
In a 100 mL solution n(CH3COOH) = 0.010 mol and n(CH3COONa) = 0.028 mol. What is the buffer pH given? pKa(HA) = 4.76 → pH = pKa + log[A-] / [HA]
n = CV
C (CH3COOH) = 0.0001
C (CH3COONa) = 0.00028
pKa = 4.76
pH = 4.76 + log(0.00028/0.0001)
pH = 4.76 + log(2.8)
pH = 5.2
Calculating buffer pH changes (4)
- Determine the moles of HA and A- before reaction.
- Determine the number of H+ or HO- added.
- 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. e.g. NH3 and NH4Cl
Ammonia NH3 + H3O+ → NH4+ + H2O
Ammonium chloride NH4Cl → NH4+ + Cl-
What does adding H+ and HO- do to basic buffer action (3)
- Adding H+ shifts equilibrium to the right, favouring BH+.
- Adding HO- shifts equilibrium to the 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 the concentration ratio.
Buffer capacity (3)
- Defined as the amount of acid or base that must be added to a buffer to give unit pH change.
- beta measures of how resistant a buffer is to pH change.
- In practice, prepare buffer using a weak acid with a pKa as close as possible to the desired pH.
When is acidic buffer capacity high (3)
- [HA], [A-] are high, so they can cope with lots of H+ or HO-
- [HA] » [A-] so can respond to the addition of H+ or HO-
- capacity is highest when pH = pKa i.e. [HA] = [A-].
How do you calculate buffer capacity when an acid is added?
beta = d[B] / d[pH]
How do you calculate buffer capacity when a base is added?
beta = - (d[HA] / d[pH])
