An acid is that substance that dissociates, or donates protons, or hydrogen ions in an aqueous solution or water-based solution in an acid-base reaction. These protons donated to the aqueous solution will affect the pH (potential of hydrogen) of said solution, making it more acidic. At lower pH values, there will be higher acidity in the solution. Over the years, this definition was polished, as there were different definitions formulated by different researchers such as those of Arrhenius, Bronsted-Lowry and Lewis.
It should be noted that the pH scale has values between 1 to 14, being pH=1 “very acid” or “strong acid”. Therefore, the higher the amount of hydrogen ions, the lower the pH value, which translates into a more acidic solution.
Types of acids by their acid strength:
All acids release protons in solution, and the number of ions released per molecule will determine the strength of the acid.
A weak acid is one that partially releases the hydrogen atoms or all the protons contained in its molecule. These acids have a small effect on the pH of the solutions in which they are dissolved. Some examples of weak acids are:
- Acetic acid (vinegar)
- Citric acid (found in oranges and lemons)
- Hydrochloric acid (can be found in the stomach)
- Sulfuric acid (a very corrosive acid)
Types of acids by their number of exchangeable protons
By definition an acid is the substance capable of donating protons, however not all acids have the same number of protons. There are acids that have a greater or lesser ability to donate protons and this is directly related to how many protons the acid molecule contains to be released into the aqueous solution.
A monoprotic acid can donate only one proton of those that make up its molecule, to the aqueous solution. An example of a monoprotic acid is also one of the strongest existing, “HCl” or hydrochloric acid.
It is easy to guess what polyprotic acids are, once you know what monoprotic and diprotic acids are. That’s right, polyprotic acids are those that contain in their molecular structure, more than two interchangeable protons, ready to be released in an aqueous solution.
Both diprotic acids and polyprotic acids do not release their protons at the same time, they do so through dissociation steps.
Some of these acids are used as pH buffering substances, since by not releasing their protons all at once, the change in pH is gradual and dependent on the concentration of protons.
Even some acids, after having released the first proton, have the ability to recover it, causing no change in pH.
Types of acids by their chemical nature
As is known, an organic compound is made up of carbon and hydrogen, as its main structure, therefore an organic acid is nothing more than an organic compound that also has and fulfills the characteristics of an acid.
Example: Formic acid.
On the other hand, an inorganic acid lacks the main characteristic of organic compounds, containing within the structure of its molecule, hydrogen and carbon atoms. They are also called mineral acids.
Example: hydrogen sulfide.
In practice, it is easy to determine whether an acid is organic or inorganic, one only has to pay attention to how its conjugate base is formed. The conjugate base is formed when the acid is dissolved in water. Since the hydrogen dissociates from the rest of the acid molecule. This part of the remaining acid molecule is called the conjugate base.
Therefore, if the conjugate base, after dissociation, meets the definition of an organic compound, it means that it is an organic acid, and if the conjugate base is a mineral compound, it is an inorganic acid.