多质子酸

Monoprotic acids contain a single ionizable proton, like hydrofluoric acid, while polyprotic acids contain two or more ionizable protons. Sulfurous acid has two ionizable protons, and phosphoric acid has three. Each proton is lost sequentially, with each reaction having its own acid ionization constant (Ka).

Removing a proton from a neutral molecule is easier than removing one from a negatively charged ion because negative charge strengthens the bond between the proton and the anion. This is why Ka1 is larger than Ka2, which is larger than Ka3 for polyprotic acids like phosphoric acid.

You can estimate pH using only the first reaction if Ka1 is at least one thousand times larger than subsequent Ka values. For example, ascorbic acid's Ka1 of 8 × 10−5 is much larger than its Ka2, so the first ionization dominates the pH calculation using an ICE table.

Use an ICE table with the second ionization constant (Ka2) and the monoanion concentration from the first ionization. For ascorbic acid, Ka2 equals 1.6 × 10−12, and the resulting dianion concentration is negligible, confirming that the first ionization determines the solution's pH.

Sulfuric acid and carbonic acid are diprotic acids with two ionizable protons. Phosphoric acid is a triprotic acid with three ionizable protons. Each successive ionization constant decreases significantly, typically by factors of 10^5 to 10^6, allowing simplified calculations for weak acid solutions and dissociation.

Successive ionization constants decrease dramatically in polyprotic acids. For phosphoric acid, Ka1 is 7.5 × 10−3, Ka2 is 6.2 × 10−8, and Ka3 is 4.2 × 10−13. These large differences allow use of math-simplifying assumptions when calculating equilibrium concentrations in solutions.

Yes, polyprotic bases can accept more than one hydrogen ion. The carbonate ion is a diprotic base capable of accepting two protons. Like polyprotic acids, equilibrium calculations for polyprotic bases follow similar approaches using stepwise ionization constants and ICE tables.