Acids and Bases

Acids and bases

Acid: An "acid" is defined, according to (Lowry and Bronsted), as a substance  which, in solution, tends to liberate hydrogen ions (protons).

These hydrogen ions may

1.   remain as such

2.   may combine with the solvent forming, e.g., hydroxonium ions, H30⁺

3.   may combine with negatively charged ions.

 Base: A “base " is defined as a substance which tends to accept hydrogen ions.

Thus

H⁺ donor ↔ H⁺ + H⁺ acceptor

                                  (Acid)     (Base)

·        Acids, may not always be  undissociated molecules.

·        Bases, may be molecules, though the class includes all anions.

Thus the cation NH₄⁺ liberated from ammonium salts is an acid because it is capable of producing hydrogen ions according to the

equation:

NH₄⁺ ↔ H⁺ + NH₃

                                                (Acid)            (Base)

Similarly the anion HPO₄^-2 is a base because it can accept H⁺ to form H₂PO₄^- and the acid molecule H₃P0₄.

The anions H₂PO₄^- and HPO₄^-2 can also act as acids because they can lose H⁺ to form the base PO₄^-3 (" amphoteric").

Strength of acids

The " strength " of an acid is measured by its tendency to donate hydrogen ions ( by its degree of dissociation)

·        The freely dissociated mineral acids (HCI, etc.) are strong acids; slightly dissociated acids such as carbonic acid are weak.

·        A strong base has a great affinity for hydrogen ions whereas a weak base is a poor acceptor.

·        The anion of a strong acid is a weak base, e.g., Cl-, and the anion of a weak acid is a strong base, e.g., HCO3 -, OH-.

As I was searching literature for a short  and crisp note on acids and bases for the students of medicine, I came across a brilliant paper by 

Frazer and Stewart (J. clin. Path. (1959), 12, 195). I would advise the readers to go through the paper once if time permits. On my part I would definitely strive to elaborate upon all the subtopics explained in the paper and of course all the related material as well. Subsequent posts will lay stress on the physiological buffers and acidosis/ alkalosis.

I wish to have suggestions from the readers in order to improve my blog. Please feel free to ask any doubts related to these topics. As far as the explanation is concerned I think these topics can act as ready-made notes to be covered before each professional examination.

Poorvi