December 2015 - January 2016

# Critical pH and Le Châtelier: How everyday substances and habits can dissolve your teeth away

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Enrico Uva, Lauren Hill Academy, St. Laurent QC

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We know that acids dissolve our pearly whites, but where does this acid come from and what chemistry is really going on in our mouths to make this happen? If your mouth hosts biofilms of certain bacteria, especially Streptococcus mutans that are feeding off sugars, the teeth will be in contact with acid. Such bacteria even store polysaccharides and continue to lower the pH of their environment long after food has been swallowed. If this persists, teeth could eventually decay. But are there other sources of acids that could also inflict damage?

Juice, soft drinks and vinegar-rich foods easily come to mind. Gastric juice from either bulimia or a gastro-disorder can also take its toll. Less familiar hazards include professional wine tasting, which involves keeping wine in the mouth for up to a minute, dozens of times a day. Frequent swimming in pools that are not pH-balanced also leads to tooth decay. Stabilizers in chlorine “pucks” are acidic, and the direct application of chlorine forms not only hypochlorite but hydrochloric acid.

All of this begs the question, how exactly does acid damage teeth? And why are there individual variations? This is a perfect application question for a high school chemistry class as it brings together topics, such as pH, Ksp, equilibrium and Le Châtelier’s principle.

The first part of the discussion involves what dentists refer to as critical pH. This is the pH of a solution when it is just saturated with respect to one of the minerals in enamel. If the solution’s acid-level is above the critical pH, then things are safe for teeth. If the solution is supersaturated relative to that mineral, more of it will tend to precipitate out. The worry is when the solution’s pH is below the critical value — then the solution is unsaturated — and teeth will start to dissolve.

The mineral I’ve been referring to is calcium hydroxyapatite, one of the enamel’s components. In aqueous solution it creates the following equilibrium:

Ca10(PO4)6(OH)2(s) ⇄ 10 Ca2+(aq) + 6 PO43– (aq) + 2 OH–(aq)

Normally, the mineral is highly insoluble; its Ksp is extremely small, on the order of 10-117. But of course the solubility of the enamel can increase if hydroxide ion is consumed, hampering the reverse reaction and favouring the forward reaction — Le Châtelier never rests, not even while you eat! The concentration of phosphate also decreases with lower pH as the presence of H+ forms H3PO4, H2PO4–, HPO42– in saliva. If phosphate levels decrease, the forward reaction is again favored — increasing the solubility of hydroxyapatite. For these two reasons, acidic conditions lead to tooth erosion.

The critical pH is around 5.5 but it’s not a fixed value and can vary from one individual to the next.

Here’s why:

1. The amount of fluoroapatite, another mineral present in enamel, reduces the critical pH because fluoroapatite (Ca5(PO4)3F) is free of hydroxide. It is less soluble than calcium hydroxyapatite in acidic conditions. Fluoridating teeth protects teeth against acid-erosion by displacing hydroxide with a fluoride ion.
2. Impurities in enamel such as carbonate and fluoride affect enamel solubility and those ions vary in different people. If concentrations of phosphate and calcium ions in an individual’s saliva are unusually low, the critical pH may increase by a factor of 10 to a pH of 6.5.

## References

1. C. Dawes, What Is the Critical pH and Why Does a Tooth Dissolve in Acid? http://www.cda-adc.ca/jcda/vol-69/issue-11/722.pdf
2. Microbiology of Dental Decay and Periodontal Disease https://www.ncbi.nlm.nih.gov/books/NBK8259/
3. C. Dawes and C.L. Boroditsky, Rapid and Severe Tooth Erosion from Swimming in an Improperly Chlorinated Pool: Case Report, May 2008, Vol. 74, No. 4 http://www.cda-adc.ca/jcda/vol-74/issue-4/359.pdf
4. K. H-K. Yip, R. J. Smales and J. Kaidonis, The diagnosis and control of extrinsic acid erosion of tooth substance, 2002 http://www.saudident.com/album/data/media/17/The_diagnosis_and_control_of_extrinsic_acid_erosion_of_tooth_substance.pdf.

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