STEWART APPROACH TO ACID-BASE BALANCE
What Determines the Acidity or Alkalinity of a Solution?
The extracellular fluid (ECF) is a solution containing a number
of different molecules that directly influence the dissociation of water. It is
inconvenient to use the molar concentration of hydrogen and hydroxide to reflect
the relative acidity and alkalinity of a solution. The pH scale, developed by Sorenson
in the 1920s, is a simple device for acid-base quantification. By using the negative
logarithm (p) of the hydrogen ion concentration (H), the pH scale was developed.
Neutral pH for pure water is 7.0 (1.0 × 10−7
mmol/L).
Physiological pH for the ECF is 7.4, which is alkaline. Conventionally, we refer
to relative acidity and alkalinity from this starting point. If the pH is more than
7.4, the ECF is alkaline (relative to normal), and if the pH is less than 7.4, the
ECF is acidic (relative to normal). The pH of the intracellular space is 6.8 to
7.0, and remains constant despite dramatic changes in extracellular pH. The reason
for the difference in acidity between the spaces is unclear.[8]
[9]
To determine the hydrogen ion concentration of ECF, the investigator
must look at the dissociation equilibria for all fully dissociated and partially
dissociated ionic compounds and apply three simple rules:
- Electrical neutrality. In aqueous solutions
in any compartment, the sum of all of the positive charged ions must equal the sum
of all of the negative charged ions.
- Dissociation equilibria. The dissociation
equilibria of all incompletely dissociated substances, as derived from the law of
mass action, must be satisified at all times.
- Mass conservation. The amount of a substance
remains constant unless it is added, removed, generated, or destroyed. The total
concentration of an incompletely dissociated substance is the sum of concentrations
of its dissociated and undissociated forms.
To determine the acid-base status of a fluid, all substances to
which these rules may be applied must be accounted for. A discussion of key groups
follows.
