Acid/Base Balance Notes
Equilibrium Constants
All equilibrium reactions have different amounts of products and reactants present at
equilibrium. An equilibrium constant expresses this as
Keq = [products]/[reactants]
So—lots of product at equilibrium gives a high Keq but lots of reactants at equilibrium
gives a low Keq.
The equilibrium constant for an acid is called a Ka.
Thus strong acids (which donate lots of H+) have lots of product and a high Ka.
Weak acids (which donate little H+) have small amounts of product and so a low Ka.
The Keq for water (or Kw) self ionization is 1 x 10-14 --and—since water has equal
amounts of H+ and OH- the concentration of H+ of pure water = 1 x 10-7 and the
concentration of OH- of pure water is also 1 x 10-7. Adding extra acid or extra base will
increase the H+ or OH- but the Kw is a constant—and if [H+] goes up, [OH-] goes down
and vice versa.
These constants or concentrations can be expressed as the –log of the constant or
concentration, in which case a small p is added ahead of this value.
So:
- log of Ka = pKa
and –log of [H+] = pH and –log of [OH] = pOH
Buffers
A buffer is a chemical added to a water solution to resist pH change.
A buffer consists of a weak acid (donates H+) and its conjugate base (accepts H+).
The major extracellular buffer in the human body is H2CO3 and HCO3 – in the form of
NaHCO3. A buffer works by chemically reacting with a strong acid (and converting it to
the weak acid of the buffer) or chemically reacting with a strong base (and converting it
to the weak base of the buffer). (Please see Figure)
Thus the concentration of the buffer determines its buffering capacity—how much it
can buffer before being used up.
The pH of a buffer (or its [H+]) depends upon the nature of the weak acid (thus its Keq
or Ka or pKa) and the ratio of the base part of the buffer to the acid part of the buffer
[base]/[acid]. A log equation would say
pH = pKa + log [base] / [acid]
Thus—since pH is determined by mixing the acid and the base in different ratios-- the pH
of a buffer can never be lower than the pure acid of the buffer nor higher than the pure
base of the buffer.
The pH of a buffer is determined by the ratio of the base part to the acid part.
The range of a buffer is between the pH of the pure acid and the pH of the pure base.
The human body’s main extracellular buffer is H2CO3 (carbonic acid) at a concentration
of 1.3 meq/L and NaHCO3 (sodium bicarbonate) at a concentration of 26 meq/L. Thus
the ratio is 26 meq/1.3 meq = 20/1 and this ratio gives a pH of 7.4 at 370C.
Actually—in deoxygenated blood –there is more CO2. This carbon dioxide reacts with
water producing carbonic acid. So, the pH of deoxygenated blood (which has high CO2)
is 7.36 and the pH of oxygenated blood ( which has low CO2 ) is 7.46.
In the body then, four acid / base disturbances are possible:
1. Too much acid -- ratio lowers --pH drops = acidosis
2. Too little acid -- ratio increases --pH rises = alkalosis
3. Too much base –ratio increases -- pH rises = alkalosis
4. Too little base -- ratio decreases ---pH drops = acidosis
If the pH is not correct, body proteins will be denatured, body enzymes will not work
well and death may result. Thus (the lungs adjust CO2 –kidneys adjust NaHCO3) the
concentrations are adjusted (lose buffering capacity) to restore pH (keep body alive).
The buffer pH is maintained at the expense of buffering capacity.
Bonus
You will learn in physiology (or you may learn it here as a bonus) that a wrong pH
caused by the wrong acid concentration is a respiratory imbalance and the wrong pH
caused by the wrong base concentration is a metabolic disturbance.
Thus in the four disturbances above:
1 = respiratory acidosis (may be caused by poor breathing-as in asthma)
2 = respiratory alkalosis (may be caused by over breathing-as hysteria)
3 = metabolic alkalosis (as in taking too much antacid)
4 = metabolic acidosis (as in using up the sodium bicarbonate by too many ketoacids in
diabetes mellitus)