A diprotic acid is an acid that can donate two hydrogen ions (H+) or protons per molecule in an aqueous solution. Another name for a diprotic acid is a dibasic acid. A diprotic acid is a type of polyprotic acid, which is an acid able to donate more than one proton per molecule. In contrast, a monoprotic acid only donates one proton or hydrogen in water.
Diprotic Acid Examples
Examples of diprotic acids include sulfuric acid (H2SO4), carbonic acid (H2CO3), chromic acid (H2CrO4), hydrogen sulfide (H2S), and oxalic acid (H2C2O4).
How Diprotic Acids Work
Students generally assume a diprotic acid always loses both of its protons or hydrogen ions. However, this is not the case because the ease of losing the first and second protons usually is very different. The first acid dissociation constant Ka is always larger than the second one. In other words, it’s always easier for a diprotic acid to lose its first proton than to lose its second proton.
For example, sulfuric acid loses its first proton so easily (Ka > 1) that it acts as a strong acid, forming the hydrogen sulfate anion, HSO4–.
H2SO4(aq) + H2O(l) → H3O+(aq) + HSO4–(aq) [Ka1 = 1 x 103]
The Ka for losing the second proton is much lower, so only around 10% of sulfuric acid molecules in a 1M solution go on to form the sulfate anion (SO42-).
HSO4–(aq) + H2O(l) ↔ H3O+(aq) + SO42-(aq) [Ka2 = 1.2 x 10-2]
In practice, sulfuric acid fully deprotonates or loses both hydrogen atoms when it reacts with a base, like ammonia.
Diprotic Acid Titration Curves
Titration is used to calculate acid dissociation constants. For a monoprotic acid, the pH of the point of a titration curve halfway between the start of the curve and the equivalence point is used to find the pKa value using the Henderson-Hasselbalch equation:
pH = pKa + log ([base]/[acid]
pH = pKa + log(1)
pH = pKa
For a diprotic acid, you can find the first acid dissociation constant the same way as for a monoprotic acid. The second acid dissociation constant is the point halfway between the first equivalence point and the second equivalence point.
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