W6 Acid and pH lll Flashcards
Sulfuric acid is a ….ibasic acid?
What does this mean?
Dibasic
Has 2 pKa values
2 ionisable groups
Phosphoric acid is a ….basic acid?
What does this mean?
tribasic acid
3 ionisable groups
3 pKa values
Loss of a further H+ is much less … than the loss of the first H+ from the non-ionised acid
Favourable
It is more … to lose a H+ from an anion than an uncharged molecule
difficult
Anion
Negatively charged
Adding a COOH increases acidity but adding a second COOH does what?
Decreases the acidity of the compound
(it is already ionised)
So second pKa is bigger
-NH2 and -NH3 are electron..?
Electron withdrawing
Examples of oxyacids (HmXOn) formula
HNO3 H2SO4 and H3PO4
Doubly bonded O
Drawing e- from O-H, easily removed, more acidic
More resonance forms, greater delocalised e-, more stable conjugate base
Amphiprotic
Can act as proton donor or proton acceptor
All amphiprotic substances are also amphoteric? true or false.
True, but the reverse isn’t true.
Amphoteric compounds
Reacts with acids and bases (can behave as both) depending on conditions
e.g. amino acids
Equivalence point
- Acid-base have equal stoichiometric amounts
- Reaction completed- all acid has been converted to its conjugate base
Acid-base interactions result in formation of…
The weaker acid and the weaker base (most stable species being favoured at equilibrium)
Large pKa difference between two acids in acid-base reaction means..?
Reaction is irreversible
Amphiprotic
Can act as proton donor or proton acceptor
All amphiprotic substances are also amphoteric? true or false.
True, but the reverse isn’t true.
What type of compounds are amino acids?
Amphoteric compounds that contain separate acid and basic groups
They are ionic compounds
What is the Isoelectric point pI?
pH at which concentration of the zwitterion is maximum.
pH at which the concentrations of cationic and anionic forms are equal
What is a zwitterion?
A dipolar ion/inner salt. Has separate positively and negatively charged groups
Amino acids are positive/negative/neutral at varying pH?
Amino acids are positively charged at low (acidic) pH and negatively charged at high pH and zwitterionic at neutral pH.
Implications for the oral absorption and bioavailability of amino acids from the diet
Proteins charge?
Polyelectrolyte whose properties depend on the balance of acidic and basic groups on the side-chains
Many drugs molecules exist as ionised species (weak acids and bases) What are ionised species?
Very low lipid solubility, and are unable to permeate through membranes. Only the non-ionised drug is usually able to cross membranes
What does a shift in pH by one unit mean?
Shift in pH by one unit to either side of the pKa must change the ratio of ionised to non-ionised forms by a factor of 10 (conc is 10x higher or lower)
For drugs that are weak acids : calculate fraction of the total dose that is ionised for any pH if the pKa is known
Blood plasma has a rigorously controlled pH of 7.4; gastric juice is strongly acidic (pH from about 1 to 7), and urine can vary from about 4.8 to 7.5.
What do very weak acids have a pKa of?
*Very weak acids: pKa >7.5, non-ionised at pH 1–8, absorption will be largely independent of pH.
Acids
*Acids: 2.5 < pKa >7.5 significant changes in the proportion of non-ionised drug
according to the pH. As the pH rises, % of non- ionised drug decreases, and
absorption therefore also decreases.
Stronger acids
*Stronger acids: 2.5 <pKa , depend upon pH, fraction non-ionised very low except
under the most acidic conditions in the stomach. Absorption is typically low, even
under acidic conditions.
Are Basic drugs absorbed from the stomach? Why?
Basic drugs: not absorbed from the stomach, where the pH is strongly acidic.
It is easier to remove a proton from a neutral species than an ionised species
What does a large PKa difference mean?
Reaction is irreversible
What is the isoelectric point?
pH at which concentration of the zwitterion is maximum.
pH at which the concentrations of cationic and anionic forms are equal.
How do you calculate isoelectric point?
pI= (pka1 + pka2)/ 2
For info
If pH> pKa, the functional group has no H attached
e.g. COOH will be Coo-
NH2 will be NH2
If pH< pKa The functional group has H attached
e.g. COOH= COOH
NH2= NH3+
What is a zwitterion?
Doubly charged form, (dipolar) net charge zero/neutral
