Q: Cola drinks do a beautiful job of cleaning the corrosion from brass and similar alloys. How does this work? And what effect do the chemicals that cause this have on your stomach when you drink cola?
A: The tarnish that forms on alloys is caused by a reaction between oxygen and one of the metals contained in the alloy. This forms an oxide. Cola drinks are weakly acidic and so, when a tarnished alloy such as a used coin is placed in them, the acid reacts with the oxide. This removes the oxygen from it and leaves a coating of metal behind, providing a clean and shiny alloy. The acid has no adverse affect on the stomach which is more acidic than the drink. However consuming large quantities of cola damages your teeth, which can be destroyed by acid.
A: The reason why cola drinks clean the corrosion from brass and other alloys is that cola contains quite a large amount of phosphoric acid. This acid eats away the corrosion. Phosphoric acid will also eat away your teeth, so you must always brush them after drinking cola.
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There is a similar amount of phosphoric acid in both cola and diet cola, so don鈥檛 think that the diet version is any better for your teeth than standard cola. There is very little difference.
A: I shouldn鈥檛 worry too much about the effects of cola on one鈥檚 stomach. In my experience the acids in the stomach have a far greater effect on metalwork than cola does on your innards.
My anecdotal evidence for this is a childhood memory of cycling to Cub Scouts with an upset stomach. I had just leant my bike against the hut when, to my horror, I threw up over it. Because I was late I dashed inside. On emerging 90 minutes later, I found the areas of the bicycle鈥檚 frame that had come into contact with the vomit had completely lost their paintwork.