The reader who has sent us this question specifies that he does not understand that if a substance needs water to manifest its level of acidity why in the absence of water, acids are still dangerous. To give a clear example: why do we get burned if we get a little bit of, for example, sulfuric acid in the skin?
The answer is that there is also water there. Let's go slowly. The acid burns because it dissociates, donates a proton to water, (a Bronsted acid), and then a chemical reaction occurs with heat that causes what we call a chemical burn. It is this heat that causes irritation when it causes a denaturation of proteins, and, in the case of some acids, very serious burns. And that happens when the acid comes in contact with our skin and causes corrosion in it.
And it is so with all the acids although some are stronger and others less. For example, the acetic acid that is an organic acid is less strong, although if you put pure acetic acid your skin would be irritated but it is not the same as, for example, the sulfuric acid which is one of the strong acids.
What determines the ability of each acid to produce these consequences is the constant of dissociation or constant of acidity. This dissociation constant is the measure of the strength of an acid in solution, or what is the same its ability to donate protons to the solution with which it comes into contact. We all know that sulfuric acid can make holes and that is because it has a very high dissociation constant, however, if acetic acid falls on your skin, it only causes irritation and that is because its dissociation constant, its ability to yield protons to the medium with which it comes into contact, is much lower than sulfuric. The more protons a compound gives, the greater the chemical reaction that occurs and the greater its energy production and, therefore, the greater the capacity to produce corrosion.
You have to be very careful when handling acids for that reason. It is necessary to put protection: glasses, gloves and, in addition, to work with them in a bell. And the measurements are stricter depending on what the dissociation constant of the acid in question is. For example, when we use acetic acid to prepare mobile phases of the chromatographs in our laboratory, we do not handle it as carefully as when we handle sulfuric acid. We take the sulfuric acid with special gloves resistant to acid and always work with it in the hood because it also emits gases.
In our laboratory, we use a lot of sulfuric acid to prepare a reagent called oleum, which is basically water with sulfuric acid. For example, when we need to separate the different natural products from the plants, we put them in thin plates of Silicagel and rebel with sulfuric acid. And we also use sulfuric acid to clean glass material (chromic mixture) that there is no way to clean it in any other way.
Organic acids are everywhere. For example, there are acids that are produced by biological fermentations such as lactic or acetic acid that are mild acids and are part of many foods. They play a very important role in our daily diet. Also our body produces acids, when we exercise we generate lactic acid which is what later causes the laces in the muscles.
All our daily life is surrounded by acids. The ants secrete formic acid and that's why when they bite you and irritate you what is irritating you is the formic acid that the ants use as defense.
Azucena González Coloma She is the scientific head of the CSIC at the Institute of Agrarian Sciences.
Question done via email by Leonardo Chacón
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