Diprotic and polyprotic acids contain multiple acidic protons that
dissociate in distinct, sequential steps.
LEARNING OBJECTIVE [ edit ]
Identify the key features that distinguish polyprotic acids from monoprotic acids.
KEY POINTS [ edit ]
Polyprotic acids can lose two or more acidic protons; diprotic acids and triprotic acids are specific
types of polyprotic acidsthat can lose two and three protons, respectively.
Polyprotic acids display as many equivalence points intitration curves as the number of acidic
protons they have; for instance, a diprotic acid would have two equivalence points, while a
triprotic acid would have three equivalence points.
For polyprotic acids, the first Ka is always the largest, followed by the second, etc.; this indicates
that the protons become successively less acidic as they are lost.
Although the tendency to lose each acidic proton decreases as subsequent ones are lost, all
possible ionic species do exist insolution; to calculate their fractional concentration, one can use
equations that rely on equilibrium constants and the concentration of protons in solution.
TERMS [ edit ]
triprotic acid
one that can donate three hydrogen ions per molecule duringdissociation
diprotic acid
one that contains within its molecular structure two hydrogen atoms per molecule capable of
dissociating
equivalence point
the point at which an added titrant is stoichiometrically equal to the number of moles in a
sample's substance; the smallest amount of titrant necessary to fully neutralize or react with the
analyte
titration
determining a substance's concentration in a solution by slowly adding measured amounts of
another substance (often with a burette) until a reaction is shown complete
Give us feedback on this content: FULL TEXT [ edit ]
As their name suggests, polyprotic acids contain more than one acidic proton. Two common
examples are carbonic acid (H2CO3, which has two acidic protons and is therefore
adiprotic acid) and phosphoric acid (H3PO4, which has three acidic protons and is therefore
a triprotic acid).
Diprotic and polyprotic acids show unique profiles in titration experiments, where
a pH versus titrant volumecurve clearly shows two equivalence points for the acid; this is
because the two ionizing hydrogens do not dissociate from the acid at the same time. With
any polyprotic acid, the first amd most strongly acidic proton dissociates completely before
the second­most acidic proton even begins to dissociate.
Titration curve of carbonic acid
The titration curve of a polyprotic acid has multiple equivalence points, one for each proton. In carbonic
acid's case, the two ionizing protons each have a unique equivalence point.
Diprotic Acids
A diprotic acid (here symbolized by H2A) can undergo one or two dissociations depending on
the pH. Dissociation does not happen all at once; each dissociation step has its own Kavalue,
designated Ka1 and Ka2:
H 2 A(aq) ⇌ H + (aq) + H A − (aq)
−
H A (aq) ⇌ H + (aq) + A
2−
K a1
K a2
(aq)
The first dissociation constant is necessarily greater than the second ( i.e. Ka1 > Ka2); this is
because the first proton to dissociate is always the most strongly acidic, followed in order by
the next most strongly acidic proton. For example, sulfuric acid (H2SO4) can donate two
protons in solution:
−
H 2 S O 4 (aq) → H + (aq) + HS O 4 (aq)
−
−
HS O 4 (aq) ⇌ H + (aq) + S O 4 (aq)
K a1 = large
K a2 = small
This first dissociation step of sulfuric acid will occur completely, which is why sulfuric acid is
considered a strong acid; the second dissociation step is only weakly dissociating, however.
Triprotic Acids
A triprotic acid (H3A) can undergo three dissociations and will therefore have three
dissociation constants: Ka1 > Ka2 > Ka3. Take, for example the three dissociation steps of the
common triprotic acid phosphoric acid:
−
H 3 P O 4 (aq) → H + (aq) + H 2 P O 4 (aq)
K a1 = large
H 2 P O −4 (aq) ⇌ H + (aq) + HP O 2−
4 (aq)
K a2 = small
2−
3−
HP O 4 ⇌ H + (aq) + P O 4 (aq)
K a3 = smallest
Fractional Concentration of Conjugate BaseSpecies
Although the subsequent loss of each hydrogen ion is less favorable, all of a polyprotic
acid's conjugate bases are present to some extent in solution. Each species' relative level is
dependent on the pH of the solution. Given the pH and the values of Ka for each dissociation
step, we can calculate each species' fractional concentration, α (alpha). The fractional
concentration is defined as the concentration of a particular conjugate base of interest,
divided by the sum of all species' concentrations. For example, a generic diprotic acid will
generate three species in solution: H2A, HA­, and A2­, and the fractional concentration of HA­
, which is given by:
α=
[H A − ]
[H 2 A]+[H A − ]+[A 2− ]
The following formula shows how to find this fractional concentration of HA­, in which pH
and the acid dissociation constants for each dissociation step are known:
Fractional ion calculations for polyprotic acids
The above complex equations can determine the fractional concentration of various ions from polyprotic
acids.