McGill Alert / Alerte de McGill

Updated: Thu, 07/18/2024 - 18:12

Gradual reopening continues on downtown campus. See Campus Public Safety website for details.

La réouverture graduelle du campus du centre-ville se poursuit. Complément d'information : Direction de la protection et de la prévention.

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From “Iron Fallout” to Hair Permanents

I had never heard of iron fallout until I was asked a question about its removal. I’m glad that question came up because it leads us into some interesting chemistry that ranges from beautifying car wheels to beautifying hair.

For most people a car is just a vehicle that gets them from one place to another, and as far as cleanliness is concerned, occasionally driving through a car wash is good enough. But some individuals are so reviled by a speck of dirt that they will wash, wax and polish until the car has the appearance of one sitting in a dealer’s showroom. The wheels, be they steel or alloys of magnesium or aluminum, receive much attention because they must shine! And this is where focus falls on iron fallout.

Fallout just refers to any substance that literally falls out of the air, in this case tiny particles of iron. Every time brakes are applied, microscopic particles of iron flake off. This also happens when trains speed along iron rails or gears rotate in various machines. Although the surface of wheel rims may appear to be smooth, on a microscopic level that surface can be seen as a series of peaks and valleys. Iron particles can be embedded in the valleys, roughening the surface and reducing shine. Worse, the iron particles can lose electrons to oxygen in the air in a process known as oxidation, resulting in the formation of ferrous and ferric hydroxides. Rust! To form the ferrous ion, iron loses two electrons and three to form the ferric ion.

Wheels are not the only problem. Iron particles can also embed in the car’s paint and alter the way it reflects light, interfering with the sparkle that car lovers seek. Chemistry to the rescue! There are numerous ways to remove rust, but the general idea is always the same. Convert insoluble iron oxides to soluble substances. For example, hydrochloric acid reacts with iron oxides to form soluble iron chlorides. But HCl is also corrosive and can eat away metal. Weaker acids like lactic or gluconic acid are less corrosive and also form soluble salts. These are the ingredients in CLR, a popular rust and lime remover. Lime is a deposit of calcium carbonate that forms when hard water evaporates. Acids convert it into carbon dioxide, water and soluble calcium salts.

Oxalic, tartaric and citric acids can also remove rust by forming soluble complexes with the iron ions but are not very effective against rust that has wedged into paint or the metal in wheel rims. But another chemical that forms a soluble complex with iron ions is effective. Sodium thioglycolate, HSCH2COO-Na+, is the active ingredient in most iron fallout removers.

Interestingly this chemical did not originate as a rust remover. Back in the 1930s, it was found to be an effective chemical for breaking sulfur-sulfur bonds in keratin, the protein that is the main component of nails and hair. That finding led to the formulation of “hair permanents,” products that would permanently change the shape of hair from curly to straight, or straight to curly. When a solution of thioglycolate is applied to hair, the sulfur-sulfur bonds between adjacent keratin molecules are broken, allowing free movement of the proteins. The hair can then be placed on curlers, or combed until straight, followed by the application of hydrogen peroxide that reforms the sulfur-sulfur bonds holding the hair in its new shape. In higher concentrations, thioglycolates can break not only sulfur-sulfur bonds but also the bonds between the amino acids that make up keratin. This breaks keratin down into soluble amino acids meaning that the thioglycolate can be used as a depilatory, or hair remover.

When it comes to iron fallout, thioglycolate forms a soluble complex with ferrous iron that can be washed away. As an added bonus, it also reduces ferric iron to ferrous, so it too can be removed. And there is even proof that it is working! Ferrous thioglycolate is purple, so when a thioglycolate solution is sprayed on a surface that contains iron oxides the purple colour that forms indicates that the iron oxides are being solubilized.

There is one drawback to this operation. Like many sulfur compounds, thioglycolates have a less than pleasant smell. This can be noted when using a fallout remover or when getting a permanent. They are in the same family, although far less potent, as the compounds in skunk “fragrance.” But that is no reason to fall out with fallout removers. Just spray and hold your nose.


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