What Causes Bad Smells?

On 720 ABC today the topic of discussion was “what’s the worst thing you have spilled in your car?”

It led to a catalogue of calamities and catastrophes involving aftershave, butter, steak, and even a dead body.

So what causes bad smells and what can we do about them?

Essentially, bad smells are caused by three classes of chemicals – volatile fatty acids (VFA), amines and mercaptans.

The VFAs are any dairy product, body odour, or vomit.

Amines are urine and fishy smells.

Mercaptans are anything that is rotten, although these can also contain amines, like the elegantly named putrescine and cadaverine.

As it happens the VFAs and amines are relatively easy to deal with, but the mercaptans are a whole different ball game.

With the VFA’s the relevant word is “acid”. Because they are acids, their volatility, and therefore smell, may be counteracted by simply neutralising them with an alkaline product of some sort. In this case, ammonia would be the product of choice. The neutralised fatty acid (for for example, ammonium butyrate) could then simply be blotted up with a paper towel. Make up a solution of ammonia, about one part in 20, spray it on, then just blot it up. You will find after this that the smell has pretty much disappeared.

With amines, exactly the opposite approach is required. Because amines are alkaline, the choice of a cleaning agent is an acid. The vinegar is an excellent choice because it has a smell of its own, and no one mines a residual smell of vinegar after you have cleaned something.

So if your cat has weed on your carpet, spray some vinegar on it, leave it for a little while, then blot it up. Same with a fishy smell.

The effectiveness of these two approaches is the result of simply working with the chemistry of the products in question.

But with mercaptans, however, this approach is not available to us, as mercaptans have three properties that all conspire against us:

1. They are chemically stable and are not easily reacted with anything to bring about a change in their chemistry which would change their smell.

2. Being sulphur based compounds they are very chemically sticky. That is, the non-bonding electron groups in the sulphur will bind strongly to many substrates (this is the reason that sulphur-containing natural fibres like wool and cotton stain more easily than synthetic fibres).

3. Mercaptans also have the property of having are very intense smell. That is, a lot less mercaptan is required to create a bad smell than is required for the VFA’s or amines. Typically, a mercaptan only has to be present in parts per billion quantities to produce a noticeable smell. An example here is rotten egg gas (H2S).

H2S is actually more toxic than cyanide gas (the stuff used by the Nazis in their gas chambers). This may surprise you, as we have all smelled it and are still alive. The reason is simple – it has an incredibly intense smell and is detectable by our noses in tiny, tiny amounts.

This property can, however, be put to good use. Mercaptans are added to natural gas and LPG (which are otherwise odourless) in tiny amounts so that if there is a leak we can smell it.

So the lesson is this – VFAs or amines are relatively easily to remove, but if you’ve had a rotten steak or dead rat in your car, it may well be impossible to remove the smell. If you can get to the surface you could try dislodging it with either a caustic cleaner (Easy Off Oven Cleaner) or a solvent (acetone), but given that most things in your car are porous (including plastic) it may be a lost cause.

Mythbusters discovered this with a dead pig smell in a car. No one could remove the smell even with the interior completely stripped. Some things just can’t be done….



Shannon Lush Gets it Wrong #2

Last week on 6PR Shannon Lush gave us her tips on cleaning in response to listener questions.
Are her tips right? Will they work?

Let’s look at it.
The first thing that needs to be understood is that Shannon is not a chemist. She doesn’t know much chemistry, and has little understanding of the chemistry and principles behind off-the-shelf products. The result of this is that whether her home-spun ideas work or not, there is usually an off-the-shelf product that will do the job just as well, or better, saving you a lot of time, and often money. She is not able to recommend these products, as she does not know what is in them, nor how they work.
Let’s look at the questions one by one.

The first caller wanted to know how to get sunscreen stains out of a white t-shirt.
Shannon’s answer was to use a dishwashing detergent as a spot cleaner. Put some detergent on the stain, close your eyes, and rub it in. When you feel the detergent thicken, this is caused by the detergent emulsifying the stain. She was not able to name a particular brand to use.

Is that why it thickens? Possibly, but the thickening will also be caused by the water in the detergent evaporating, and the thickening agent therefore being present at a higher concentration. She is clearly not aware that most dishwashing detergents are about 80% water, and the thick consistency is caused by a thickening agent to disguise this fact.
So will it work? Probably – but use Morning Fresh, as it has the highest concentration of surfactant (40% last time I looked).
Alternatively you could use acetone, which would probably work instantly.

If neither of these work, try Easy Off oven cleaner. Spray on, rub in and rinse. If that doesn’t work, try a powder laundry detergent, preferably one with enzymes. Make the powder into a paste and rub it on and rinse.

The next question involved gravy stains in microwave cookware. Shannon was not able to answer this, but merely advised the listener how to protect the cookware from being stained in the first place. Fortunately, I can answer it. Put some washing soda in there, make it into a paste by adding a minimum amount of water to it, and rub it into the gravy stains and it will come out.

The next question was about cleaning and maintaining leather car seats. In particular, the listener said that the product he was using left the car seats a little sticky. Shannon’s answer involved a concoction of lavender oil and a couple of other things. I haven’t tried this myself, because I don’t need to, as there is an off-the-shelf product that is a terrific leather cleaner and conditioner – Mr Sheen.

Originally marketed as a furniture polish, the people at Reckitts finally realized what I have known for years – that it is s terrific general purpose cleaner, and particularly for leather. It has just enough white spirit to clean grime off seats, and leaves a silky smooth silicone coating that leaves your seats smooth and shiny. It’s also terrific for sprucing up your dashboard, paintwork and engine. Spray it on the outside of your windscreen and it’ll help the raindrops run off and your wipers glide smoothly. But don’t spray it on the inside of the windscreen as it’ll promote fogging.

The final question involved a brown stain on a carpet. Shannon told us that it was caused by tannins that leached out of the carpet lining. Unfortunately, it doesn’t require a whole lot of analysis to see that this is completely wrong. She would have us believe that carpets contain a water soluble, brown substance in the lining, and all that has to happen for the stain to appear is for the carpets to get wet. Somehow I think that the people that make carpets are not quite that dumb.

I don’t know what the stain is, but I’d hit it with an enzyme cleaner (WhiteKing Stainlift) and if needs be, an oxidiser (Preen OxyAction). There are other more aggressive approaches if required – check the Q&A section on my website.