Acids and Bases Flashcards
What are the 3 ways to define an acid?
Dissociates into water to form H+ ions
Proton (H+) donor
Electron pair acceptor
What are the 2 ways to define a base?
Dissociates into water to form OH- ions
Proton acceptor
What is the formula for pH?
pH = -log10 [H+]
pH: Is the negative logarithm to the base of 10 of
the H+ concentration
As it is a logarthmic scale a small change in pH e.g 7.4 to 7 represents a significant difference in H+ ion concentration.
7.4 represents a H+ concentration of 40nmol/L
7 represents a H+ concentration of 100nmol/L
What happens to the pH of water as the temperature is increased and why?
As temperature is increased molecules are more likely to dissociate.
As H2O is polar it will dissociate into OH- and H+. Therefore as the temperature increases there is greater dissociation and therefore a higher H+ concentration therefore pH will decrease.
However the OH- (hydroxyl) concentration is equal to the H+ concentration so although the pH is decreasing in reality the water is effectively neutral as it contains equal quantities of acid and base.
What is the difference between strong acids/bases and weak acids/bases?
Strong acids/bases completely dissociate to form H+ and X- (acids) or OH- and X+
Weak acids/bases do not completely dissociate and instead are in equlibrium with the acid one side and the ions on the other.
What is the pKa?
pKa is the pH at which a weak acid/base exists in its ionised and unionised form to an equal degree.
Write a generic formula for Ka (acid dissociation constant) and Kb (base dissociation constant)?
[H+] x [A-] / [HA] =Ka
[OH-] x [BH+] /B =Kb
What is the Henderson Hasslebach equation and what is it used for?
Henderson-Hasselbalch equation relates the concentration of a weak acid to that of the dissociation constant and the ratio of concentrations of the acid and its conjugate base.
pH = pKa + log 10 [HCO3-]/[CO2]
The Henderson–Hasselbalch equation can be used to calculate the amount of acid and conjugate base to be combined for the preparation of a buffer solution having a particular pH.
In the human body the henderson hasslebach equation relates the concentration of bicarbonate to that of carbon dioxide in the blood as the pH of the blood changes.
What is a buffer?
A buffer is a solution that resists changes in H+ or OH- concentrations. They are usually weak acids but can also be weak bases.
Explain how carbonic acid acts a a buffer?
Carbonic acid/HCO3-
H2CO3 —> (H+) + (HCO3-)
As H+ is added to the solution the dissociation constant moves to favour the unionised form (in essence mopping up the excess H+) in doing so it is resisting the change in H+ concentration and therefore pH.
What are the 3 main physiological buffering systems?
Carbonic acid/HCO3
Phosphate
Proteins
How does the pH glass electrode measure pH? (draw a diagram)
Relies on the presence of H+ sensitive glass.
With a concentration gradient of H+ ions either side of the glass, a potential difference develops.
If one side is kept constant using a buffer with no net movement of H+ ions across the glass occurring, the change in potential difference can be used to determine the pH using an electrical circuit.
The circuit consists of:
A silver/silver chloride electrode in a buffer solution
On the inside of the hydrogen ion sensitive glass.
The test solution is on the other side of the hydrogen ion sensitive glass.
This is connected to a second ‘reference’ electrode which sits in a electrolyte solution such as KCl
What is the significance in having a temperature when checking a blood gas?
As temperature increases there is greater dissociation therefore if the machine is calibrated to 37 degrees and the test solution (e.g. blood) is a different temperature you will have an inaccurate reading.
Also the amount of gas dissolved in a solution increases as the temperature drops.
