Acid-Base Physiology

10.2 Quantitative Acid-Base Analysis - The Background

Some chemical background about the classifications of substances in solution is necessary before we proceed further.

In particular, the substances which affect acid-base balance in body fluids can all be classified into 3 groups based on their degree of dissociation. This allows certain generalisations & simplifications which are useful in understanding complex solutions.

Body fluids can be considered as aqueous solutions that contain:

 

Strong ions in solution are always fully dissociated

They exist only in the charged form.

For example: dissolving sodium chloride in water produces a solution containing Na+ and Cl-. There is no NaCl present so it is strictly incorrect to speak of ‘sodium chloride solutions’ as this species does not exist in the solution! An important practical consequence of this when analysing solutions is that the amount of the strong ion present is not affected by conversion back to the parent compound (as occurs with weak ions -see below) AND the dissociation equilibrium of this reaction does not need to be included in the analysis. The concentration of any individual strong ion in the solution is fixed unless it is transported out of the solution (eg by a cell membrane pump or transporter.)

Strong ions are mostly inorganic (eg Na+, Cl-, K+) but some are organic (eg lactate). In general, any substance which has a dissociation constant greater then 10-4 Eq/l is considered as a strong electrolyte.

 

Weak ions are those ions produced from substances that only partially dissociate in solution

Ions that are classified as 'weak ions' are produced from substances which only partly dissociate when dissolved in water. For the purposes of acid-base analysis, the weak ions in body fluids as classified into 2 groups:

Incomplete dissociation of the weak acids means that the solution contains the weak acid plus the products of its dissociation. A dissociation equilibrium equation can be written:

[H+] x [A-] = KA x [HA]

- where KA is the dissociation constant for the weak acid.

 

Non-electrolytes are those substances in solution which never dissociate into ions.

Non-electrolytes are not charged. As a consequence, non-electrolytes contribute to the osmolality of a solution but do not contribute to the charge balance in the solution.

 

How clearcut is the distinction between strong ions, weak ions & non-electrolytes?

The distinction is not completely clearcut of course BUT for practical purposes it is a sufficiently accurate & useful approximation.

Stewart uses the value of the dissociation constant (KA) to provide a clear (but still a bit arbitrary) distinction between the three groups:

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Extra Notes: