One of the main functions of saliva is to buff er against acid from food and pla

Question

One of the main functions of saliva is to buff er against acid from food and plaque, which contributes significantly to the formation of cavities. While there are several buffers in saliva, carbonic acid (H2CO3) has the highest concentration and has the greatest effect on pH.

(a) While the salivary concentration of carbonic acid stays at a fairly constant 1.3 mM, the level of bicarbonate (HCO3-) can vary wit h the rate that saliva flows from salivary glands. For low flow rates, the bicarbonat e concentration is around 2 mM; for medium flow rates, it is 30 mM; and for high flow rates, around 60 mM. The pKa of carbonic acid at body temperature is 6.1. Assuming that the pH of Saliva is determined primarily by carbonic acid and bicarbonate, determine the pH of saliva for each of the three flow rates. The normal pH of saliva is about 6.3.

(b) The most prevalent bacterium in the mouth, streptococcus mutans, breaks down sugar and releases lactic acid (pKa = 3.86). If S. mutans has produced 10^-8 moles of lactic acid since your last swallow, what is the pH of you saliva? What would the pH be without the bicarbonate buffer? Assume that your mouth contains about 1 mL of saliva and that your saliva is flowing at a low rate.

(c) You take a drink of orange juice, and after you swallow, 0.5 mL remains in your mouth. What is the pH of your saliva if your mouth contains 1 mL of pure saliva, and if you model orange juice as 1.0 mM citric acid (pKa = 3.13; assume only one dissociation).

(d) Why do you think some toothpastes contain baking soda (sodium bicarbonate)?

Explanation / Answer

(a) Using Hendersen-Hasselbalc equation for buffer,

pH = pKa + log([base]/[acid])

at low flow rate,

[HCO3-] = 2 mM

[H2CO3] = 1.3 mM

pH = 6.1 + log(2/1.3) = 6.29

at medium flow rate,

[HCO3-] = 30 mM

[H2CO3] = 1.3 mM

pH = 6.1 + log(30/1.3) = 7.46

at high flow rate,

[HCO3-] = 60 mM

[H2CO3] = 1.3 mM

pH = 6.1 + log(60/1.3) = 7.76

(b) [citric acid] = 1 x 10^-8 mols/0.001 L = 1 x 10^-5 M

lactic acid dissociates to H+ and lactate, let x amount has dissociated then,

Ka = 1.38 x 10^-4 = x^2/1 x 10^-5

x = [H+] = 3.71 x 10^-5

pH = -log[H+] = 4.43

(c) moles of citric acid left in mouth = 0.001 M x 0.0005 L = 5 x 10^-7 mols

[citric acid] = 5 x 10^-7/0.0015 = 3.33 x 10^-4

citric acid dissociates to form H+ and citrate, let x be the amount of dissoication then,

Ka = 7.41 x 10^-4 = x^2/3.33 x 10^-4

x = [H+] = 4.97 x 10^-4 M

pH = -log[H+] = 3.30

(d) Baking soda in toothpaste acts as a antistaining agent and teeth whitening agent. It also reacts with the acid present and helps reduce or inhibits formation of plaque.

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