P460
Dental carries (cavities) are caused by bacteria that reside in the oral cavity, most often Streptococcus mutans. Bacteria ingest sugars such as sucrose and metabolically convert them into lactic acid in a process similar to that which occurs in muscle. The acidic environment in turn causes demineralization of tooth enamel, predominantly composed of crystals of hydroxylapatite [Ca10(PO4)6(OH)2, MM = 500g/mol], the same substance that makes up bone. In fact, it is many of the ionic properties of hydroxylapatite generate the strength and rigidity of bone and teeth. The reaction of hydroxylapatite with lactic acid (CH3CH(OH)COOH, MM = 90g/mol) is shown below:
Although modern toothpastes are able to target bacteria and provide supplemental hydroxylapatite, the key ingredient in original toothpastes was baking soda (sodium bicarbonate, NaHCO3, MM = 85 g/mol). A chemistry student was interested in determining the pKa of lactic acid and decided to create an acid-base titration using sodium bicarbonate. He first added a lactic acid solution to an Erlenmeyer flask. He then used a buret to add sodium bicarbonate, drop by drop, into the flask and measured the pH and volume of base (titrant) added at regular intervals. He obtained the following graph:
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