The acidity of your precious bodily fluids is a carefully guarded parameter.

To allow this parameter to deviate out of a very narrow range would massively impair your capacity to continue living. Basic molecular services would break down. Cellular anarchy would ensue. In order for the ICU physician to wrest a form of order from this chaos, a reasonable grasp of basic acid-base chemistry is expected. An excellent medically themed foundation for acid-base chemistry is laid by Kerry Brandis in a highly acclaimed series of online articles, to which I will constantly refer. It is not my intention to supercede this resource, as it remains canonical for all critical care trainees.

So, let us have some basic definitions.

1. acid:

A molecular entity or chemical species capable of donating a hydron (proton) (see Brønsted acid) or capable of forming a covalent bond with an electron pair (see Lewis acid).

2. base:

A chemical species or molecular entity having an available pair of electrons capable of forming a covalent bond with a hydron (proton) (see Brønsted base) or with the vacant orbital of some other species (see Lewis base).

The two coexisting models of acid-base behaviour are the Brønsted-Lowry model and the Lewis model.

These models compliment each other. The Brønsted-Lowry model is well suited to the discussions of acid-base chemistry which involve aqueous solutions and protic acids, whereas the Lewis model is more general and can be called upon to offer explanations of more exotic scenarios.

 

3. Brønsted acid:

A molecular entity capable of donating a hydron (proton) to a base, (i.e. a 'hydron donor') or the corresponding chemical species.

4. Brønsted base:

A molecular entity capable of accepting a hydron (proton) from an acid (i.e. a 'hydron acceptor') or the corresponding chemical species.

5. Lewis acid:

A molecular entity (and the corresponding chemical species ) that is an electron-pair acceptor and therefore able to react with a Lewis base to form a Lewis adduct, by sharing the electron pair furnished by the Lewis base.

6. Lewis base:

A molecular entity (and the corresponding chemical species) able to provide a pair of electrons and
thus capable of coordination to a Lewis acid, thereby producing a Lewis adduct.

The strength of an acid or a base is by convention defined as pH, which is related to the activity rather than the concentration of hydrogen ions.

pH is the negative logarithm (base 10) of the hydrogen ion activity in a solution

This is a notional definition. It is validated by an experimental method which operates according to a well-defined measurement equation in which all of the variables can be determined in terms of SI units.

References

For the history of acids and bases, apart from the lucid and comprehensive chapter from Kerry Brandis, one can turn to acid-base.com by Alan W. Grogono; the linked chapter draws heavily on the famous (in certain circles) book by Astrup and Severinghaus: The history of blood gases, acids, and bases ( Munksgaard, 1986). This opus is not available for free online, which is very sad (I'm looking at you, Google books).

 

Buck, R. P., et al. "Measurement of pH. Definition, standards, and procedures (IUPAC Recommendations 2002). " Pure and applied chemistry 74.11 (2002): 2169-2200.