Acid-Base and pH III Flashcards
1 acids and bases
two or more acidic or basic functional groups
Dibasic acid
2 ionisable groups
H2SO4 strongest acid:
two S=O
Tribasic acid
3 ionisable groups
Loss of a further H+ from an ion is ____ ____ __________than loss of the first H+ from the non-ionised acid (stating acid)
much less favourable
It is more _______ to lose a H+ from an anion than from an uncharged molecule
difficult
Charge density;
Extra COOH: electron-withdrawing stabilises conjugate base
Difference between pKa1 and pKa2 diminishes as -CH2- separating COOH _______
increases
After removing the first H from ne COOH: pKa2 < pKa1 COO- electron-donating _______ the dianion
destabilise
what is an organic molecule?
When it includes C and H
Extra COOH: increases _____
acidity
What causes weakness second ionisation
COO-
electron withdrawing:
-NH2 and -NH3+
Extra -NH2
not very marked effect in pka1 reduction
-NH3+
reducing a lot the pka2
Oxoacids (HmXOn)
Examples?
HNO3, H2SO4, H3PO4
a) HNO3
b) H2SO4
c) H3PO4
a) monobasic
b) dibasic
c) tribasic
↑doubly bonded O
drawing electrons from O-H, H easily removed, more acidic
↑doubly bonded O
more resonance forms, greater e- delocalisation, more stable conjugate base
Acidic and basic oxides: not obvious H+ _____ or _____
donors or acceptors
Example of Basic oxide:
NaOH
Example of Acidic oxide:
H2SO3
What does oxides react with?
Basic oxides react with acids
Acid oxides react with bases
Amphoteric:
- react with acids and bases
- have properties of acid and bases
What is a Equivalence point?
acid-base → equal stoichiometric amounts
(reaction completed: all acid has been converted to its conjugated base)
Titration curve dibasic acid-strong base
2 moles base and 1 mol acid
How to know what is the best range for an indicator?
At equivalence point = where the curve is vertical
Acid–base interactions:
formation of the weaker acid and the weaker base (most stable species being favoured at equilibrium)
Large pKa difference means…
reaction irreversible
In aqueous solution = H2O ______ H+ to any base ______ than OH-
donate
stronger
To use bases that are stronger than OH-
solvent weaker acid than H2O
Hydrocarbons or ethers (pKa 50), ammonia (pKa 38): Strong or weak acid?
Very weak acid
Amphoteric compounds
amino acids = can react with acid and bases (depending on conditions)
Amphiprotic compounds
can act as a proton donor and as a proton acceptor
What do Amino acids contain?
amphoteric compounds contain separate acidic and basic groups
What is a Zwitterion?
doubly charged form
Are amino acids ionic compounds?
Yes
ammonium cation
pH high or low?
ACIDIC - low
aminocarboxylate anion
pH high or low?
BASIC - high
pI=pH = pka1 + pka2 / 2
Ka1 = [H+] [zwitterion] / [cation]
Ka2 = [H+] [anion] / [zwitterion]
Amino acids: positively charged at low (acidic) pH, negatively charged at high (basic) pH and zwitterionic at neutral pH
Implications for the oral absorption and bioavailability of amino acids from the diet
Proteins:
polyelectrolyte whose properties depend on the balance of acidic and basic groups on the side-chains
1) 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
pH = pKa +log [ionised]/ [non-ionised]
pH = pKa + 1 [ionised] / [non-ionised] = 10
10/1
[ionised] 10 times
2) pH = pKa - 1 [ionised]/ [non-ionised] = 0.1
1/10
for a weak base
pH = pKa + log - [B] / [BH+]
pH = pKa + log [base]/[acid]
log [base] [acid] = log [non-ionised-[ionised]
For drugs that are weak acids : calculate fraction of the total dose that is ionised for any pH if the pKa is known
Fraction [ionised] = 1 / 1+ antilog (pKa - pH)
% ionised = 100/ 1+ antilog (pka –pH)
For drugs that are weak bases: calculate fraction of the total dose that is ionised for any pH if the pKa is known
Fraction [ionised] = 1/ 1 + antilog (pH - pKa)
% ionised = 100/ 1 + antilog (pKa - pH)
pH of blood plasma:
7.4
gastric juice is strongly acidic (pH from about 1 to 7), and urine can vary from about 4.8 to 7.5.
Very weak acids:
pKa >7.5, non-ionised at pH 1–8, absorption will be largely independent of pH.
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: 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.
Basic drugs, absorbed?
not absorbed from the stomach, where the pH is strongly acidic.
