solutions. The major differences between solutions were the concentration
of hydrogen ions and the concentration of protonated
acid species. The pH was about 7.1 for the sodium acetate solution,
and the pH values were 3.77, 3.59 and 3.49 for 50, 100 and 150 mg/
l acetic acid solutions, respectively. In an aqueous solution there is
an equilibrium between different acetic acid species, and the
extent of the equilibrium is determined by the pKa value. The
ratios of different acetic acid species can be calculated by the pH
value. For acetic acid, its pKa value is 4.75 at 25 C. Thus, it can
be calculated that the anionic acetic acid species is more than
99% when the pH is over 6.75, and that the protonated acetic acid
species accounts for more than 99% when the pH is less than 2.75.
Therefore the sodium acetate solution just contained the anionic
acetic acid species as its pH value was about 7.1.
Therefore it can be concluded that the anionic acid species
makes no contribution to sour taste, and that hydrogen ions and
protonated acid species could be the chemical substances producing
sour taste.