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A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid protonation () to a base—in other words, it is a base with a hydrogen ion added to it, as it loses a hydrogen ion in the reverse reaction. On the other hand, a conjugate base is what remains after an acid has donated a proton during a chemical reaction. Hence, a conjugate base is a substance formed by the deprotonation from an acid, as it can gain a hydrogen ion in the reverse reaction. Zumdahl, Stephen S., & Zumdahl, Susan A. Chemistry. Houghton Mifflin, 2007, Because Polyprotic acid can give multiple protons, the conjugate base of an acid may itself be acidic.
In summary, this can be represented as the following chemical reaction:
Johannes Nicolaus Brønsted and Martin 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 symbol because it has the atomic nucleus of a hydrogen atom, that is, a hydrogen cation.
A cation can be a conjugate acid, and an anion can be a conjugate base, depending on which substance is involved and which acid–base theory is used. The simplest anion which can be a conjugate base is the free electron in a solution whose conjugate acid is the atomic hydrogen.
In this case, the water molecule is the conjugate acid of the basic hydroxide ion after the latter received the hydrogen ion from ammonium. 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 of , since the water molecule donates a proton to give 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.
If a chemical is a strong acid, its conjugate base will be weak. An example of this case would be the splitting of hydrochloric acid in water. Since is a strong acid (it splits up to a large extent), its conjugate base () will be weak. Therefore, in this system, most will be hydronium ions instead of attached to Cl− anions and the conjugate bases will be weaker than water molecules.
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 a base splitting constant (Kb) of about , 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.
Consider the following acid–base reaction:
Nitric acid () is an acid because it donates a proton to the water molecule and its conjugate base is nitrate (). The water molecule acts as a base because it receives the hydrogen cation (proton) and its conjugate acid is the hydronium ion ().
| hydronium |
| phosphoric acid |
Furthermore, here is a table of common buffers.
| 2.1 - 7.4 |
| 3.8 - 5.8 |
| 6.2 - 8.2 |
| 8.3–10.3 |
| 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 formula , was made into a buffer solution, it would need to be combined with its conjugate base in the form of a salt. The resulting mixture is called an acetate buffer, consisting of aqueous and aqueous . 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.
Ringer's lactate solution is an example where the conjugate base of an organic acid, lactic acid, is combined with sodium, calcium and potassium cations and chloride anions in distilled water (2025). 9780857111562, British Medical Association. ISBN 9780857111562 which together form a fluid which is isotonic in relation to human blood and is used for fluid resuscitation after blood loss due to Physical trauma, surgery, or a burn injury. (2020). 9781506254340, Kaplan Medical Test Prep. ISBN 9781506254340
| Fluoronium ion | HF Hydrogen fluoride |
| HCl Hydrochloric acid | Cl− Chloride ion |
| H2SO4 Sulfuric acid | HSO Hydrogen sulfate ion ( bisulfate ion) |
| HNO3 Nitric acid | NO Nitrate ion |
| H3O+ Hydronium ion | H2O Water |
| HSO Hydrogen sulfate ion | SO Sulfate ion |
| H3PO4 Phosphoric acid | H2PO Dihydrogen phosphate ion |
| CH3COOH Acetic acid | CH3COO− Acetate ion |
| HF Hydrofluoric acid | F− Fluoride ion |
| H2CO3 Carbonic acid | HCO Hydrogen carbonate ion |
| H2S Hydrosulfuric acid | HS− Hydrosulfide ion |
| H2PO Dihydrogen phosphate ion | HPO Hydrogen phosphate ion |
| NH Ammonium ion | NH3 Ammonia |
| H2O Water (pH=7) | OH− Hydroxide ion |
| HCO Hydrogencarbonate (bicarbonate) ion | CO Carbonate ion |
| Ethylamine | Ethylammonium ion |
| Methylamine | Methylammonium ion |
| Ammonia | Ammonium ion |
| Pyridine | Pyridinium |
| Aniline | Phenylammonium ion |
| Benzoate ion | Benzoic acid |
| Fluoride ion | Hydrogen fluoride |
| PO Phosphate ion | HPO Hydrogen phosphate ion |
| OH− Hydroxide ion | H2O Water (neutral, pH 7) |
| Bicarbonate | Carbonic acid |
| Carbonate ion | Bicarbonate |
| Bromide ion | Hydrogen bromide |
| Hydrogen phosphate | Dihydrogen phosphate ion |
| Chloride ion | Hydrogen chloride |
| Water | Hydronium ion |
| Nitrite ion | Nitrous acid |
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