Strong and Weak Acids

In earlier posts, I discussed the concept of pH and acids & bases. If you remember, H+ ion concentration is often measured as pH as pH = -log [H+]. When dissolved in water, all acids undergo a dissociation reaction:

HA ⇌H⁺ + A ^–

Or more accurately, as the water is acting as a base:

HA + H₂O ⇌ H₃O⁺ + A ^–

Strong acids, such as HCl are almost totally dissociated are are said to have a high K_a value. Where K_a is the acidity dissociation constant and is the equilibrium constant for the reaction of the acid in water. Thus we can write the equation as:

HCl —> H⁺ + Cl ^–

Weak acids such as acetic acid, are not totally dissociated in solution:

CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO^–

In this reaction, if hydrogen or acetate [CH₃COO^–] ions are removed from the system, the reaction moves to the right producing more hydrogen and acetate ions. If hydrogen (or acetate) are added to the system, then the reaction moves to the left producing more acetic acid.

Whether an acid is weak or strong can be determined by carrying out a titration with a strong base, such as sodium hydroxide (NaOH). When NaOH is added to a strong acid drop by drop, the pH increases linearly with volume of base. When NaOH is added to a weak acid, at certain values of pH, more base is required to change the pH.

Most organic acids are weak acids. Examples include citric acid, acetic acid, ascorbic acid, malic acid, tartaric acid. This is important as weak acids can act as buffers absorbing hydrogen ions without change the pH. I’ll write more on buffers later.

In food, most acids present are weak, so the total titratable acidity is measured as well as the pH. Even citrus fruits contain others acids, not just citric acid. These all have a role in controlling the pH of the systems. In certain food products the predominate acid changes with ripeness, while the total titratable acidity may remain constant.

—
References
Wikipedia: Strong Acids, weak acid
Miller, D.D. (1998) Food Chemistry: A Laboratory Manual, John Wiley & Sons