I was wondering. Does anybody know if aspirin has gone bad when it starts smelling of vinegar? I have noticed that when I open a new bottle of aspirin it has little odor. After a few month it starts to pickup the vinegar odor.
Thanks
Jim
Per a Pharmacist friend at work (I work in a hospital pharmacy): That means it has gone off, and do not use it. Discard it immediately so you don’t forget it’s off, and accidentally use it.
Are the two chemically similar? Aspirin = acetylsalicylic acid, Vinegar = acetic acid.
Does that account for the similar smell?
What specifically does “gone off” entail? Has it lost potency, or are there harmful effects?
I’ve always used aspirin even when it has “gone vinegary”, and haven’t had any noticeable problems. It still seems to work just fine also.
In the presence of excessive moisture, aspirin breaks down to acetic acid and salicylic acid. A faint smell of vinegar is OK for aspirin. But the stronger the smell, the more the aspirin has broken down.
It won’t poison you, but it will lose effectiveness as it breaks down.
I have heard moisture and medicine are a bad combination. I keep all my meds in the linen closet. Still, my aspirin suffers premature failure. I wonder if removing the desiccant from the bottle accelerated the process? 
Jim
It sure would. The hydrolysis of aspirin (AKA “going bad”) involves the addition of a water molecule, converting the acetyl group into a free acetate i.e. vinegar.
The dessicant is there to suck up the water and prevent that reaction.
I don’t have a cite for this and could be seriously misremembering, but I recall reading somewhere that salicylic acid is a painkiller like acetylsalicylic acid, but is a much stronger stomach irritant. Assuming that, you’ll trade trade your headache for a gut ache if you take Aspirin that’s gone “off”.
Making aspirin is a fairly common experiment in introductory organic chemistry courses, and I suspect that “Explain why an old bottle of aspirin might smell of vinegar” is a very common question for those experiments. When I made aspirin, the lab instructor was very excited, because we were making a compound that nearly everyone has used – a pharmaceutical drug. I was less impressed, because making aspirin is very easy, provided that you start with a bottle of salicylic acid:
2 salicylic acid + acetic anhydride --sulfuric acid–> acetylsalicylic acid + water
(Acetic anhydride is two molecules of acetic acid stuck together at the acid part.)
This is called a Fischer esterification. The product is an ester, which is the product of an alcohol’s -OH group joining with an acid’s -COOH group. (In this case, salicylic acid, which also has an -OH group, is the alcohol.) The reaction is reversible, so it is not a great way of producing esters – each time you make a molecule of the product, there’s a chance it’ll go back to the starting materials, so the yields are often low.
There are many different ways that an ester can be broken down into its acid and alcohol components by the addition of a molecule of water. This is called hydrolysis, ‘water-splitting’. Esters break down readily in the presence of a trace of acid or base. But the reaction also proceeds with water under neutral conditions:
aspirin + water --> salicylic acid + acetic acid
Water vapor from the air is the source that makes old aspirin hydrolyze and start to smell like vinegar. IIRC, salicylic acid is more toxic than aspirin, so it’s unwise to take aspirin that has substantially decomposed. However, acetic acid has such a strong smell that even a small amount may be noticable. If a new, sealed bottle of aspirin smells of vinegar, it’s still safe. The decomposition really starts once the pills are exposed to the atmosphere.
The rate of degredation of aspirin (in part into the acetic acid, which you smell), has been factored into the expiration date (with a large safety margin added). The expiration date will be the best indicator of having gone bad.
You are correct. This is why salicylic acid which has been steeped from willow bark [“salyx” is Latin for “willow”] going back to the Druids(?) was not very widely used as a analgesic until a chemist at Bayer Pharmaceuticals came up with the trick of attaching a acetyl group to it. The Romans knew that willow bark extracts were effective analgesics, but warned against it because strong solutions or repeated use could cause melena (a foul-smelling tarry stool caused by stomach bleeding into the GI tract; considered a medical emergency even today) and often death. Today we know that salicylic acid also impedes blood clotting, prolonging the stomach bleeding.
The reason aspirin is relatively safe is that the acetyl group greatly reduces (but does not entirely eliminate) the stomach irritation. The acetyl group is stable in stomach acid – in fact, the more acid the environment, the stronger it binds. When the chyme (stomach contents) containing the aspirin leaves the stomach, it is immediately doused with pancreatic fluid containing bicarbonate (to prevent stomach acid from burning through the “duodenum”, the first part of the small intesting). The pancreatic bicarbonate raises the pH of the chyme from strongly acid to slightly alkaline, and the acetyl group, no longer so tightly bound, breaks off – leaving salicylic acid, which enters the bloodstream to do its job.
Acetic acid (vinegar) is very pungent, so a small amount of hydrolysis can impart a strong aroma to a bottle of aspirin. Still, even fresh aspirin is a stomach irritant, and since it is so cheap, it’s foolhardy to risk your health or even life by using aspirin that’s already begun to degrade.