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How Does Acids Corrode Skin/tissue?
I understand that the "corrosiveness" of an acid is related to the reactivity of the anion. Take HCl for example. It dissociates into H+ and Cl-. Now this is where i'm confused. If "corrosiveness" is related to the anion how does it actually corrode your skin? Cl- is very unreactive and is so safe you can eat it! Is the H+ reacting instead? If so, why does the anion have any effect on the acid's "corrosiveness"? Does the acid dissociate at all? Is it a double replacement reaction? Help me out here lol.
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Acids when dissolved in water form an equilibrium eg
HCl + H2O ---> H3O+ + Cl-
Each acid has an equilibrium constant Ka which is the degree of ionisation.
Chemists tend to use the neg log of this value or pKa. The lower the pKa the more acidic the acid is. The anion is what gives the differences in pKa.
Having said all that, in concentrated mineral acids the ionisation is low since they don't actually contain much water, but they are still corrosive.
HCl + H2O ---> H3O+ + Cl-
Each acid has an equilibrium constant Ka which is the degree of ionisation.
Chemists tend to use the neg log of this value or pKa. The lower the pKa the more acidic the acid is. The anion is what gives the differences in pKa.
Having said all that, in concentrated mineral acids the ionisation is low since they don't actually contain much water, but they are still corrosive.
Well in simple terms, acids will tear off amide bonds present in proteins and ester bonds on lipids via hydrolysis. The process is called amide/ester hydrolysis. The hydrolysis results in the destruction of connective tissue, cell walls etc. It ends with a really mushy mess that's hardly recognisable as tissue
You need to remember that acids separate readily into charged hydrogen ions and the entire process begins there.
Corrosiveness per se is not necessarily related to the anion. In the case of Oleum or concentrated sulphuric acid, the cellular damage is caused by the hygroscopic affinity of the concentrated acid. Skin contact results in instant dehydration of carbohydrate molecules resulting in severe cellular damage and blackening. You may be aware of a simple illustration of this carried out in high schools where concentrated H2SO4 is poured into a beaker of table sugar. The sugar blackens almost immediately and effectively turns into pure charcoal.
Anion reactivity does differ. Seemingly innocuous acids can cause severe damage. Perchloric acid in contact with the skin will cause the tissue to smoke as well as burn. Super Acids react in a similar manner, the typical example is antimony pentafluoride in fluorosulphonic acid, often referred to as Magic Acid. It's best avoided like the plague in practice. The fairly "weak" Hydrofluoric acid causes extensive tissue corrosion that virtually never heals but the main issue is that it causes bone tissue to dissolve irreversibly into the blood stream and consequently severe ionic disruption leading to death. Some concentrated mineral acids will hardly cause any damage. Concentrated HNO3 in general leaves tissue virtually unharmed apart from the production of xanthates resulting in a semi-permanent yellow stain.
Think of the examples I've provided above.
I note you consider Cl- as unreactive and safe. It's not always. You need to think this through a bit further.
You need to remember that acids separate readily into charged hydrogen ions and the entire process begins there.
Corrosiveness per se is not necessarily related to the anion. In the case of Oleum or concentrated sulphuric acid, the cellular damage is caused by the hygroscopic affinity of the concentrated acid. Skin contact results in instant dehydration of carbohydrate molecules resulting in severe cellular damage and blackening. You may be aware of a simple illustration of this carried out in high schools where concentrated H2SO4 is poured into a beaker of table sugar. The sugar blackens almost immediately and effectively turns into pure charcoal.
Anion reactivity does differ. Seemingly innocuous acids can cause severe damage. Perchloric acid in contact with the skin will cause the tissue to smoke as well as burn. Super Acids react in a similar manner, the typical example is antimony pentafluoride in fluorosulphonic acid, often referred to as Magic Acid. It's best avoided like the plague in practice. The fairly "weak" Hydrofluoric acid causes extensive tissue corrosion that virtually never heals but the main issue is that it causes bone tissue to dissolve irreversibly into the blood stream and consequently severe ionic disruption leading to death. Some concentrated mineral acids will hardly cause any damage. Concentrated HNO3 in general leaves tissue virtually unharmed apart from the production of xanthates resulting in a semi-permanent yellow stain.
Think of the examples I've provided above.
I note you consider Cl- as unreactive and safe. It's not always. You need to think this through a bit further.
Incidentally, not all concentrated mineral acids are as corrosive as Ianfiat seems to think. Weak mineral acids are abundant and include boric acid, sulphurous acid, phosphorous acid, hydrocyanic acid etc.
These weak mineral acids do not dissociate or ionise in water appreciably, donating only a certain amount of protons to the solution. However, they have a higher pKa than strong acids as implied by the poster.
These weak mineral acids do not dissociate or ionise in water appreciably, donating only a certain amount of protons to the solution. However, they have a higher pKa than strong acids as implied by the poster.
Tut, Tut Ianfiat. You might well have had in mind nitric, sulphuric and hydrochloric acids when you referred to mineral acids but nowadays, other acids are included including the ones I mentioned.
The old definition of a mineral acid as being an acid derived from minerals in rocks is used rarely these days. The current definition of a mineral acid is one that is derived from inorganic compounds. Collectively, they are often referred to as inorganic acids. Boric acid falls within the latter definition and although weak in "strength" compared to the acids you mention, it is still a mineral acid.
Your mistaken in thinking that bases are corrosive. Aliphatic amines are bases but are not corrosive. They are weak bases that like other weak bases are proton acceptors that do not fully ionise in aqueous solution. The aromatic amine, aniline, is an even weaker base than aliphatic amines. There are thousands of other non-corrosive bases including probably the one that most people would think of, bicarbonate of soda (sodium hydrogen carbonate). Cooks and chefs would be hard pressed to illustrate an instance of NaHCO3 corrosion.
The old definition of a mineral acid as being an acid derived from minerals in rocks is used rarely these days. The current definition of a mineral acid is one that is derived from inorganic compounds. Collectively, they are often referred to as inorganic acids. Boric acid falls within the latter definition and although weak in "strength" compared to the acids you mention, it is still a mineral acid.
Your mistaken in thinking that bases are corrosive. Aliphatic amines are bases but are not corrosive. They are weak bases that like other weak bases are proton acceptors that do not fully ionise in aqueous solution. The aromatic amine, aniline, is an even weaker base than aliphatic amines. There are thousands of other non-corrosive bases including probably the one that most people would think of, bicarbonate of soda (sodium hydrogen carbonate). Cooks and chefs would be hard pressed to illustrate an instance of NaHCO3 corrosion.
Ianfiat, sodium hydroxide corrosion to skin depends on the concentration of the sodium hydroxide that is in contact with the tissue. Pellets, flakes or pearls cause immediate localised damage and corrosion. Dilute aqueous solutions less than around 0.003M are not considered corrosive to human skin.
Ammonia is a gas. Short skin contact with ammonia gas definitely cannot be regarded as leading to skin corrosion. If you're talking about ammonium hydroxide solution, that's different. This is sometimes called "liquid ammonia" and is indeed corrosive to skin at high concentration. However it is not ammonia. Besides, once again low molar concentrations of even ammonium hydroxide are not corrosive.
Try to be clearer about what you're talking about.
Ammonia is a gas. Short skin contact with ammonia gas definitely cannot be regarded as leading to skin corrosion. If you're talking about ammonium hydroxide solution, that's different. This is sometimes called "liquid ammonia" and is indeed corrosive to skin at high concentration. However it is not ammonia. Besides, once again low molar concentrations of even ammonium hydroxide are not corrosive.
Try to be clearer about what you're talking about.
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