In the German chemistry forum "chemie online", which I frequent, a question arose recently whether the old myth that a human tooth placed in a glass of coke would dissolve over night was true or not.
Being a scientist, there was only one way to find the answer: I procured a human tooth (Third molar, carious spot [cavity] on the side of the crown, packed air-tight since its removal, see Fig. 1. The reddish-brown substance on the root is blood) and started my experiment.
An ordinary glass was filled with coke (Diet Pepsi, to be exact, I didn't have anything else) and the tooth was immersed in it. (yielding a "one-molar solution", so to say, see Fig. 2) After five hours, it was taken out for inspection and immediately put back. It showed little to no degradation at this point, however, it had developed a brownish discolouration. (see Fig. 3)
After twenty-six hours after the start of the experiment, the tooth was removed from the glass again. It now showed strong brown discolouration, some of it could be removed by scratching, but the tooth still remained brown. (see Fig. 4) The carious spot showed strong degradation due to the acids contained in coke. (see Fig. 5) Other than this, the enamel appeared largely intact. (see Fig. 6) The cementum's colour also turned brown, but its colour is more regular and a lighter shade than the enamel.
No change in the hardness of the tooth could be observed. All parts of the tooth were still as hard as another third molar used for comparison (see Fig. 7), they could only be scratched on the surface, and only when great force was applied. The tips of the root were unstable and could be broken off easily, but this was also the case with the non-immersed tooth, although to a lesser extent.
So, in conclusion, it can be said with certainty that this myth is "busted". Calcium hydroxyl apatite, the material the outer parts of human teeth are mainly made of, is indeed very sensitive to acids, but the amount of acids in coke (mainly carbonic acid, phosphoric acid and citric acid) is not enough to dissolve a tooth, as long as its structure has not been significantly weakened before. (Such as the carious spot on the tooth in this test, which is still remarkably intact)
However, I would not recommend storing your teeth in coke, and several studies suggest that phosphoric acid in coke and other soft drinks can have other harmful effects on the body when consumed in large quantities, such as osteoporosis due to the phosphate binding calcium, especially in children. (one study found that risk of bone fracture in teenage girls who consume large amounts of coke was three times as high as in those who do not, I can't find this particular study anymore, but this blog entry shows similar results - although I am not completely sure about the quality of this study)
Addendum: the experiment was repeated with a deciduous tooth. (molar, no caries, roots completely resorbed, age approximately 10 years, no air-tight storage, see Fig. 8) The results were the same, but due to the lack of caries, the deciduous tooth showed no signs of degradation at all. (See Fig. 9)
Now, of course, these results were a bit dissatisfying. None of the teeth dissolved in coke at all - so I recalled a statement of my (more professional) colleagues at MythBusters: "If it's worth doing, it's worth overdoing."
It certainly is. So I repeated the experiment with concentrated hydrochloric acid (20 %) and the results were interesting. Bubbles formed at the teeth immediately (See Fig. 10) and they reverted to their original, bright white colour while brown streaks formed in the solution. (See Fig. 11) The colour change was visible first at the enamel, then also at the roots. Bubbles stopped forming after about half an hour and no further reaction was visible. The experiment was continued over night and at the next morning, both teeth had completely dissolved and the solution had adapted a greenish-brown colour. (See Fig.12)
This proves that acid can dissolve teeth, but the acidity of coke is far too low. It may weaken the stability of a tooth and will lead to discolouration, but it can not dissolve a tooth by its own.
Fig. 1: Before the experiment (carious spot marked with red circle)
Fig. 2: The tooth in the glass of coke
Fig. 3: After 5 hours
Fig. 4: After 26 hours (end of experiment)
Fig. 5: The carious spot showing signs of degradation
Fig. 6: The largely intact side of the tooth
Fig. 7: The tooth used in the experiment and a clean tooth for comparison
Fig. 8: The deciduous tooth before the experiment
Fig. 9: The deciduous tooth after 16 hours
Fig. 10: The teeth in hydrochloric acid
Fig. 11: White teeth and brown streaks in the solution
Fig. 12: The solution after both teeth had dissolved completely.
