# Polyprotic Acid Titrations

## Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule.

#### Key Points

• The first dissociation constant is typically greater than the second; i.e., Ka1 > Ka2.

• Although the subsequent loss of each sequential hydrogen ion is increasingly less favorable, all of the conjugate bases are present in solution.

• Sulfuric acid (H2SO4) can donate a proton to form the bisulfate anion (HSO4), for which Ka1 is very large. Then it can donate a second proton to form the sulfate anion (SO42-), wherein the Ka2 is intermediate .

#### Terms

• of an acid (or a base) that can donate (or accept) more than one proton; polybasic

• Containing two or more replaceable hydrogen atoms.

• the determination of the concentration of some substance in a solution by slowly adding measured amounts of some other substance (normally using a burette) until a reaction is shown to be complete -- for instance, by the color change of an indicator

• one that is able to donate one hydrogen ion per molecule during the process of dissociation

#### Figures

1. ##### Ionization of a Diprotic Acid

Oxalic acid, also called ethanedioic acid, is shown to ionize in two steps.

2. ##### Neutralization of a Diprotic Acid

Oxalic acid undergoes stepwise neutralization by sodium hydroxide solution.

3. ##### Titration Curve for Diprotic Acid

The titration of dilute oxalic acid with NaOH shows two distinct neutralization points due to the two protons.

4. ##### A Diprotic Acid Dissociation

The diprotic acid has two associated values of Ka, one for each proton.

5. ##### Triprotic Acid Dissociation

Triprotic acids can make three distinct proton donations, each with a unique Ka.

6. ##### Fractional Concentration

Expressions for fractional concentration of conjugate bases for a diprotic acid.

## Polyprotic Acids

Recall that monoprotic acids are acids able to donate one proton per molecule during the process of dissociation (sometimes called ionization) as shown below (symbolized by HA):

${ HA }_{ (aq) }\quad +\quad { H }_{ 2 }{ O }_{ (l) }\quad \rightleftharpoons \quad { H }_{ 3 }{ O }_{ (aq) }^{ + }\quad +\quad { A }_{ (aq) }$

Common examples of monoprotic acids in mineral acids include hydrochloric acid (HCl) and nitric acid (HNO3). On the other hand, for organic acids the term mainly indicates the presence of one carboxylic acid group and sometimes these acids are known as monocarboxylic acid.

Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule. Certain types of polyprotic acids have more specific names, such as diprotic acid (two potential protons to donate) and triprotic acid (three potential protons to donate).

Let's take a quick look at a diprotic acid, oxalic acid, also called ethanedioic acid. As stated, oxalic acid is diprotic, having two protons to donate Figure 1.

If a dilute solution of oxalic acid were titrated with a sodium hydroxide solution, the protons would react in a stepwise neutralization reaction Figure 2.

If the pH of this titration were recorded and plotted against the volume of NaOH added, a very clear picture of the stepwise neutralization emerges Figure 3. Note the very distinct equivalence points on the titration curves.

This oxalic acid is an example of an acid able to enter into a reaction with two available protons Figure 4.

Likewise, a triprotic, or tribasic, system can be envisioned. Each reaction proceeds with its unique value of Ka, affected by the pH environment and inductive effects of neutralization Figure 5.

An inorganic example of a triprotic acid is orthophosphoric acid (H3PO4), usually just called phosphoric acid. All three protons can be successively lost to yield H2PO4, then HPO42-, and finally PO43-, the orthophosphate ion, usually just called phosphate. An organic example of a triprotic acid is citric acid, which can successively lose three protons to finally form the citrate ion. Even though the positions of the protons on the original molecule may be equivalent, the successive Ka values will differ since it is energetically less favorable to lose a proton if the conjugate base is more negatively charged.

Although the subsequent loss of each hydrogen ion is less favorable, all of the conjugate bases are present in solution. The fractional concentration, α (alpha), for each species can be calculated. For example, a generic diprotic acid will generate three species in solution: H2A, HA-, and A2-. The fractional concentrations can be calculated as below when given either the pH (which can be converted to the [H+]) or the concentrations of the acid with all its conjugate bases Figure 6.

#### Key Term Glossary

acid
an electron pair acceptor; generally capable of donating hydrogen ions
##### Appears in these related concepts:
base
A proton acceptor, or an electron pair donor.
##### Appears in these related concepts:
carboxylic acid
Any of a class of organic compounds containing a carboxyl functional group (or a carbon with one double bond to an oxygen and another single bond to another oxygen, which is in turn bonded to a hydrogen).
##### Appears in these related concepts:
concentration
the proportion of a substance in a mixture
##### Appears in these related concepts:
conjugate base
The species that is created after the donation of a proton.
##### Appears in these related concepts:
diprotic
tending to donate two hydrogen ions (H+) in solution
##### Appears in these related concepts:
diprotic acid
one that contains within its molecular structure two hydrogen atoms per molecule capable of dissociating
##### Appears in these related concepts:
dissociation
Referring to the process by which compounds split into smaller constituent molecules, usually in a reversible manner.
##### Appears in these related concepts:
equivalence point
the point at which an added titrant is stoichiometrically equal to the number of moles of substance present in the sample: the smallest amount of titrant that is sufficient to fully neutralize or react with the analyte.
##### Appears in these related concepts:
group
A vertical column in the periodic table, which signifies the number of valence shell electrons in an element's atom.
##### Appears in these related concepts:
hydroxide
An univalent anion (OH-1) based on the hydroxyl functional group.
##### Appears in these related concepts:
ion
An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.
##### Appears in these related concepts:
ionization
Any process that leads to the dissociation of a neutral atom or molecule into charged particles (ions).
##### Appears in these related concepts:
molecule
the smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds
##### Appears in these related concepts:
monoprotic acid
one that is able to donate one hydrogen ion per molecule during the process of dissociation
neutralization
A chemical reaction in which an acid and a base react to form a salt.
##### Appears in these related concepts:
neutralization reaction
a transformation in which an acid and a base react to form a salt and another product, often water
##### Appears in these related concepts:
pH
the negative of the logarithm to base 10 of the concentration of hydrogen ions, measured in moles per liter; a measure of acidity or alkalinity of a substance, which takes numerical values from 0 (maximum acidity) through 7 (neutral) to 14 (maximum alkalinity)
##### Appears in these related concepts:
phosphate
any salt or ester of phosphoric acid
##### Appears in these related concepts:
polybasic
Containing two or more replaceable hydrogen atoms.
##### Appears in these related concepts:
polyprotic
of an acid (or a base) that can donate (or accept) more than one proton; polybasic
##### Appears in these related concepts:
polyprotic acids
Referring to an acid with multiple protons capable of dissociating from the compound.
##### Appears in these related concepts:
proton
A positively charged subatomic particle forming part of the nucleus of an atom and determining the atomic number of an element; the nucleus of the most common isotope of hydrogen; composed of two up quarks and a down quark
##### Appears in these related concepts:
solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
system
the part of the universe being studied, arbitrarily defined to any size desired
##### Appears in these related concepts:
titration
the determination of the concentration of some substance in a solution by slowly adding measured amounts of some other substance (normally using a burette) until a reaction is shown to be complete -- for instance, by the color change of an indicator
##### Appears in these related concepts:
triprotic acid
one that is able to donate three hydrogen ions per molecule during the process of dissociation
##### Appears in these related concepts:
volume
A unit of three-dimensional measure of space that comprises a length, a width, and a height. It is measured in units of cubic centimeters in metric, or cubic inches or cubic feet in English measurement.