Aconjugate acid, within theBrønsted–Lowry acid–base theory, is achemical compound formed when an acidgives a proton (H+) to abase—in other words, it is a base with ahydrogen ion added to it, as it loses a hydrogen ion in the reverse reaction. On the other hand, aconjugate base is what remains after an acid has donated a proton during a chemical reaction. Hence, a conjugate base is a substance formed by theremoval of a proton from an acid, as it can gain a hydrogen ion in the reverse reaction.[1] Becausesome acids can give multiple protons, the conjugate base of an acid may itself be acidic.
In summary, this can be represented as the followingchemical reaction:
Johannes Nicolaus Brønsted andMartin Lowry introduced the Brønsted–Lowry theory, which said that any compound that can give a proton to another compound is an acid, and the compound that receives the proton is a base. A proton is a subatomic particle in the nucleus with a unit positive electrical charge. It is represented by the symbolH+ because it has thenucleus of a hydrogenatom,[2] that is, ahydrogen cation.
Acation can be a conjugate acid, and ananion can be a conjugate base, depending on whichsubstance is involved and whichacid–base theory is used. The simplest anion which can be a conjugate base is thefree electron in a solution whose conjugate acid is the atomic hydrogen.
In anacid–base reaction, an acid and a base react to form a conjugate base and a conjugate acid respectively. The acid loses a proton and the base gains a proton. In diagrams which indicate this, the new bond formed between the base and the proton is shown by an arrow that starts on anelectron pair from the base and ends at the hydrogen ion (proton) that will be transferred:
In this case, the water molecule is the conjugate acid of the basic hydroxide ion after the latter received the hydrogen ion fromammonium. On the other hand,ammonia is the conjugate base for the acidic ammonium after ammonium has donated a hydrogen ion to produce the water molecule. Also, OH− can be considered as the conjugate base ofH
2O, since the water molecule donates a proton to giveNH+
4 in the reverse reaction. The terms "acid", "base", "conjugate acid", and "conjugate base" are not fixed for a certain chemical substance but can be swapped if the reaction taking place is reversed.[citation needed]
The strength of a conjugate acid is proportional to itssplitting constant. A stronger conjugate acid will split more easily into its products, "push" hydrogen protons away and have a higherequilibrium constant. The strength of a conjugate base can be seen as its tendency to "pull" hydrogen protons towards itself. If a conjugate base is classified as strong, it will "hold on" to the hydrogen proton when dissolved and its acid will not split.[citation needed]
If a chemical is a strong acid, its conjugate base will be weak.[3] An example of this case would be the splitting ofhydrochloric acidHCl in water. SinceHCl is a strong acid (it splits up to a large extent), its conjugate base (Cl−
) will be weak. Therefore, in this system, mostH+
will behydronium ionsH
3O+
instead of attached to Cl− anions and the conjugate bases will be weaker than water molecules.[citation needed]
On the other hand, if a chemical is a weak acid its conjugate base will not necessarily be strong. Consider that ethanoate, the conjugate base of ethanoic acid, has abase splitting constant (Kb) of about5.6×10−10, making it a weak base.In order for a species to have a strong conjugate base it has to be a very weak acid, like water.[citation needed]
To identify the conjugate acid, look for the pair of compounds that are related. Theacid–base reaction can be viewed in a before and after sense. The before is the reactant side of the equation, the after is the product side of the equation. The conjugate acid in the after side of an equation gains a hydrogen ion, so in the before side of the equation the compound that has one less hydrogen ion of the conjugate acid is the base. The conjugate base in the after side of the equation lost a hydrogen ion, so in the before side of the equation, the compound that has one more hydrogen ion of the conjugate base is the acid.
Consider the following acid–base reaction:
Nitric acid (HNO
3) is anacid because it donates a proton to the water molecule and itsconjugate base isnitrate (NO−
3). The water molecule acts as a base because it receives the hydrogen cation (proton) and its conjugate acid is thehydronium ion (H
3O+
).
| Equation | Acid | Base | Conjugate base | Conjugate acid |
|---|---|---|---|---|
| HClO 2 +H 2O →ClO− 2 +H 3O+ | HClO 2 | H 2O | ClO− 2 | H 3O+ |
| ClO− +H 2O →HClO +OH− | H 2O | ClO− | OH− | HClO |
| HCl +H 2PO− 4 →Cl− +H 3PO 4 | HCl | H 2PO− 4 | Cl− | H 3PO 4 |
One use of conjugate acids and bases lies in buffering systems, which include abuffer solution. In a buffer, a weak acid and its conjugate base (in the form of a salt), or a weak base and its conjugate acid, are used in order to limit the pH change during a titration process. Buffers have both organic and non-organic chemical applications. For example, besides buffers being used in lab processes, human blood acts as a buffer to maintain pH. The most important buffer in our bloodstream is thecarbonic acid-bicarbonate buffer, which prevents drastic pH changes whenCO
2 is introduced. This functions as such:[citation needed]
Furthermore, here is a table of common buffers.
| Buffering agent | pKa | Useful pH range |
|---|---|---|
| Citric acid | 3.13, 4.76, 6.40 | 2.1 - 7.4 |
| Acetic acid | 4.8 | 3.8 - 5.8 |
| KH2PO4 | 7.2 | 6.2 - 8.2 |
| CHES | 9.3 | 8.3–10.3 |
| Borate | 9.24 | 8.25 - 10.25 |
A second common application with an organic compound would be the production of a buffer with acetic acid. If acetic acid, a weak acid with the formulaCH
3COOH, was made into a buffer solution, it would need to be combined with its conjugate baseCH
3COO−
in the form of a salt. The resulting mixture is called an acetate buffer, consisting of aqueousCH
3COOH and aqueousCH
3COONa. Acetic acid, along with many other weak acids, serve as useful components of buffers in different lab settings, each useful within their own pH range.[citation needed]
Ringer's lactate solution is an example where the conjugate base of an organic acid,lactic acid,CH
3CH(OH)CO−
2 is combined with sodium, calcium and potassium cations and chloride anions in distilled water[4] which together form a fluid which isisotonic in relation to human blood and is used forfluid resuscitation afterblood loss due totrauma,surgery, or aburn injury.[5]
Below are several examples of acids and their corresponding conjugate bases; note how they differ by just one proton (H+ ion). Acid strength decreases and conjugate base strength increases down the table.
| Acid | Conjugate base |
|---|---|
| H 2F+ Fluoronium ion | HFHydrogen fluoride |
| HClHydrochloric acid | Cl−Chloride ion |
| H2SO4Sulfuric acid | HSO− 4Hydrogen sulfate ion (bisulfate ion) |
| HNO3Nitric acid | NO− 3Nitrate ion |
| H3O+Hydronium ion | H2OWater |
| HSO− 4Hydrogen sulfate ion | SO2− 4Sulfate ion |
| H3PO4Phosphoric acid | H2PO− 4Dihydrogen phosphate ion |
| CH3COOHAcetic acid | CH3COO−Acetate ion |
| HFHydrofluoric acid | F−Fluoride ion |
| H2CO3Carbonic acid | HCO− 3Hydrogen carbonate ion |
| H2SHydrosulfuric acid | HS−Hydrosulfide ion |
| H2PO− 4Dihydrogen phosphate ion | HPO2− 4Hydrogen phosphate ion |
| NH+ 4Ammonium ion | NH3Ammonia |
| H2O Water (pH=7) | OH−Hydroxide ion |
| HCO− 3Hydrogencarbonate(bicarbonate) ion | CO2− 3Carbonate ion |
In contrast, here is a table of bases and their conjugate acids. Similarly, base strength decreases and conjugate acid strength increases down the table.
| Base | Conjugate acid |
|---|---|
| C 2H 5NH 2Ethylamine | C 2H 5NH+ 3Ethylammonium ion |
| CH 3NH 2Methylamine | CH 3NH+ 3Methylammonium ion |
| NH 3Ammonia | NH+ 4Ammonium ion |
| C 5H 5NPyridine | C 5H 6N+ Pyridinium |
| C 6H 5NH 2Aniline | C 6H 5NH+ 3Phenylammonium ion |
| C 6H 5CO− 2Benzoate ion | C 6H 6CO 2Benzoic acid |
| F− Fluoride ion | HFHydrogen fluoride |
| PO3− 4Phosphate ion | HPO2− 4Hydrogen phosphate ion |
| OH−Hydroxide ion | H2OWater (neutral,pH 7) |
| HCO− 3Bicarbonate | H 2CO 3Carbonic acid |
| CO2− 3Carbonate ion | HCO− 3Bicarbonate |
| Br− Bromide ion | HBrHydrogen bromide |
| HPO2− 4Hydrogen phosphate | H 2PO− 4Dihydrogen phosphate ion |
| Cl− Chloride ion | HClHydrogen chloride |
| H 2OWater | H 3O+ Hydronium ion |
| NO− 2Nitrite ion | HNO 2Nitrous acid |
