eLFH - Acid and Bases Flashcards
At which pH do most body enzymes function optimally
At normal pH of intracellular and extracellular compartments
Intracellular pH
7.0
Extracellular pH
7.4
Examples of enzymes which work optimally at different pH to intra/extracellular pH
Proteases in stomach
Acid definition examples
A substance which:
- dissociates in water to form H+ ions
- donates a proton to a solution
- is a potential acceptor of an electron pair
Base definition examples
A substance which:
- accepts a H+ ion
- dissociates in water to form hydroxide ions
pH equation
pH indicator definition
Chemicals which change colour when put into acids or bases
Change in H+ ion concentration changes their spectral properties
Examples of pH indicators
Litmus
Phenolphthalein
Methyl orange
Methyl red
Complete dissociation definition
Characteristic of compounds with ionic bonds - e.g NaCl
#
Dissociates completely in aqueous solution into its component sodium ions and chloride ions
Strong ion definition
Ions that are completely dissociated in aquesous solution
Examples of strong ions in biological solution
Cations:
Na+
K+
Ca2+
Mg2+
Anions:
Cl-
SO4 (sulphate 2-)
Lactate
Weak ion definition
Compounds which dissociate incompletely
Example of weak ion
Carbonic acid with incompletely dissociates into bicarbonate and H+ ions
Dissociation constant (K)
Rate of forward reaction when partial dissociation occurs
Governs proportion of molecular and ionic compound i.e carbonic acid molecules and bicarbonate ions
Strong acid definition
Acid which completely dissociates into H+ ions and the anion of the acid
Strong acid example
Hydrochloric acid
HCl -> H+ + Cl-
Lactic acid also behaves as a strong acid
Weak acid definition
Does not completely dissociate in solution
Acid and ions are in equilibrium
Equation for dissociation constant of an acid A
Strong base definition
Fully dissociates in aqueous solution into component cation and hydroxyl ion
Example of strong base
Sodium hydroxide
NaOH -> Na+ + OH-
Weak base definition
Only partially dissociates in aqueous solution
Reacts with water molecules to generate hydroxyl ions in equilibrium
BH+ is cation of the base B
Examples of weak bases
Ammonia
Many local anaesthetics including lidocaine
Equation for weak base dissociation constant Kb
pK definition
pK of a substance is the pH at which a weak acid or base exists in its ionised and unionised forms to an equal degree
pKa = pK for a weak acid
pKb = pK for a weak base
Henderson-Hasselbalch equation
Henderson-Hasselbalch equation for carbon dioxide and bicarbonate equilibrium
Substitute HA for CO2
Substitute A- for HCO3-
Then replace CO2 as:
[CO2] = PCO2 x solubility CO2
pKa for reaction CO2 + H2O <-> H+ + HCO3-
6.1
Buffer definition
Solutions able to resist changes in the concentration of H+ and OH-
Commonly solutions of weak acids (and less commonly weak bases)
How do weak acids act as buffers
Addition of H+ to the solution shifts dissociation equilibrium to the left ‘mopping up’ some additional H+
Main buffer systems in human body to maintain normal pH
Bicarbonate buffer system
Phosphate buffer system
Proteins
Bicarbonate buffer system
Phosphate buffer system
Protein buffer system
The pH glass electrode
Hydrogen ion sensitive glass
Solutions containing different concentrations of H+ and therefore pH are placed either side of this glass creating a potential difference
pH on one side kept constant using buffer solution
Electrical circuit used to measure pH of test solution on other side e.g blood in gas machine
The pH glass electrode circuit components
Why is body temperature important for accurate blood pH measurement
pH electrode is calibrated and maintained at 37 degrees Celsius
Dissociation of acids and bases increases as temperature rises
Therefore if body temp lower than 37 degrees a correction must be made for an accurate pH at patient’s body temp
