pH & Buffering Flashcards
What is pH?
Measure of H+ concentration
What does acidity depend on?
FREE H+ concentration
Why is regulation of blood pH critical?
In contact with nearly every body cell
What is the range of normal blood pH?
7.35-7.45
What is it called when your blood pH is too low?
Acidosis
What is it called when your blood pH is too high?
Alkalosis
How are most acids in the body generated? (3)
Breakdown of proteins (amino acids can make acids from N or S)
Incomplete oxidation of fats or glucose
Loading and transport of carbon dioxide in the blood (equilibrium with carbonic acid)
What regulates the acid-base balance? (3)
Lungs
Kidneys
Chemical buffers
How do buffers prevent pH from increasing greatly?
Releasing H+/act as acids
How do buffers prevent pH from decreasing greatly?
Binding H+/act as bases
What is the difference between a strong and weak acid?
Strong acids fully dissociate at pH7
Weak acids partially dissociate at pH7 (dissociation can be forced in different directions)
What is the value of the equilibrium constant for the ionisation of water?
1.8 x 10^-14
What concentration is a pure water solution?
55.6M
At neutrality what is the hydrogen ion concentration in pure water?
10^-7M
[H+] x [OH-] = ?
10^-14(M^2)
What is the equation linking pKa and Ka?
pKa = -log[Ka]
What is the pKa? (2)
pH at half-neutralisation (acid is half dissociated)
pH at which the acid will buffer best
Will the pKa of a strong acid be high or low?
Low
What is the Henderson-Hasselbalch equation?
pH = pKa + log ([A-]/[HA])
What does a buffer consist of?
Mixture of an acid and its conjugate base
What is buffering?
Ability to resist a large change in pH when acid or base is added
What are physiologically important buffers present in blood, saliva and other bodily fluids?
Carbonic acid H2CO3 —> HCO3 - + H+
Phosphoric acid H2PO4- —> HPO4 2- + H+
Protein —> protein- + H+
Protein + —> protein + H+
Which is stronger in an amino acid: the carboxyl group or the amino group?
Carboxyl group (alpha)
At what pH does the alpha carboxyl group of an amino acid dissociate?
2
At what pH does the alpha amino group of an amino acid dissociate?
9
At what pH does the R carboxyl group of acidic amino acids dissociate?
4
At what pH do the R amino groups of each of the basic amino acids dissociate?
Histidine 6
Lysine 10
Arginine 12
At what pH does the R group of cysteine dissociate?
8
At what pH does the R group of tyrosine dissociate?
10
Why are the alpha carboxyl and amino groups bad physiological buffers?
pKas are outside regular blood pH range
Which amino acid is an important buffer of the blood and how does it buffer?
Histidine as R group can lose H+
Why can we use histidine as a physiological buffer?
In haemoglobin, neighbouring R groups affect the pKa of histidine’s R group
pKa is within physiological blood pH range
What is the value of the critical pH?
5.5
What is the definition of critical pH?
The highest pH at which there is a net loss of mineral from the teeth
What kind of molecules diffuse more easily across biological membranes?
Lipid-soluble
Uncharged
Why does aspirin diffuse into cells more easily at pH2 than pH7?
Less dissociated aspirin (carboxyl group) at pH 2
Uncharged molecules diffuse into cells more easily
What kind of compounds are local anaesthetics?
Weak bases
Why is lignocaine injected with bicarbonate?
Bicarbonate increase pH
More unionised lignocaine
Drug enters more readily
