Buffer Systems in the Human Oral Environment
- Page ID
- 418920
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)This Exemplar will teach the following concept(s) from the ACS Examinations Institute General Chemistry ACCM:
- VIII. G. 1. a. Acid–base chemistry, particularly in water, forms an important example of equilibrium systems. Conceptual and quantitative understanding of this form of equilibrium system is important.
- VIII. G. 1. c. pH is used in quantitative descriptions of acid–base chemistry.
- VIII. G. 2. a. Weak acid–base systems are capable of forming buffer systems that tend to resist changes in the pH of the system.
- VIII. G. 2. b. Conceptual and quantitative understanding of buffers is important
From the sugary coffee you drank at breakfast to the pizza you snacked on while studying, every single type of food and drink we consume has acidic or basic components and properties. The acids and bases found in our food can change our oral environments, where too much of either can negatively impact our dental health. But what exactly do these consumed acids and bases affect in the human mouth? And how do our bodies respond to these changes? To answer these questions, we can investigate the chemistry at play in our favorite foods and drinks, as well as how human saliva maintains our dental health.
What are acids and bases?
There are three main frameworks used to identify and define acids and bases:
Table 1: Frameworks for Identifying Acids and Bases
|
Definition: |
Example: |
Important Fact: |
|
|
Arrhenius |
Acid: H3O+ production (abbrev. to H+) Base: OH- production |
Acid: HCl (aq) + H2O → H+ (aq) + Cl- (aq)
Base: NaOH (aq) + H2O → Na+ (aq) + OH- (aq) |
Precondition: The substance must be dissolved in an aqueous solution. |
|
Bronsted-Lowry |
Acid: Proton (H+) Donor Base: Proton (H+) Acceptor |
HCl (aq) + NH3 (aq) ⇌ NH3+ (aq) + Cl- (aq) |
Introduces the idea of conjugate acids and bases, which is integral to understanding the oral environment. |
|
Lewis |
Acid: Electron Pair Acceptor Base: Electron Pair Donor |
A+ + B- → AB A and B bond due to the electron pair exchange. *Very simplified example |
Currently the most inclusive classification of acids and bases as it discusses electron transfer instead of hydrogen atom behavior. |
Identify the bolded substance as an acid or a base. Explain your reasoning using the appropriate framework from Table 1.
H2SO4 can be used to preserve pickles.
1. H2SO4 (aq) + H2O (l) → HSO4- (aq) + H3O+ (aq)
You go foraging and find some wild carrots. Wild carrots are known to have high concentrations of CH3NH2.
2. 2CH3NH2 (aq) + 2HBr (aq) → 2CH3NH3+ (aq) + Br2
BF3 is highly toxic and we do not recommend ingesting BF3.
3. F- (aq) + BF3 (aq) → BF4 (aq)
Solution
- Acid; Arrhenius framework states that if H3O+ is produced, then the substance is an acid
- Base; Bronsted-Lowry Framework states that if a substance accepts a proton, then the substance is a base. CH3NH2 accepted a proton, gaining a positive charge and changed into CH3NH3+
- Acid; Lewis Framework states that if a substance accepts a pair of electrons, then the substance is an acid. By accepting the electron pair from F- (Lewis Base), a bond between the two species is formed, creating BF4
Regardless of which framework is chosen to classify acids and bases, in order to assess acid and base strength, the pH scale is used.
The pH Scale: Quantifying Acidity/Basicity in the Mouth
The pH Scale quantifies the acidity or basicity of an aqueous solution on a scale of 0 to 14. Specifically, a pH less than 7 indicates that the given solution is relatively acidic, while a pH greater than 7 indicates that the given solution is relatively basic. A pH equal to 7 indicates neutrality.
Figure 1. The pH Scale
The exact pH of a solution is calculated using the concentration of H+ ions (denoted as [H+]) in solution:
pH = -log10[H+]
[H+] and pH are inversely related. As [H+] increases, the pH decreases, and the given solution becomes relatively acidic. As [H+] decreases, the pH increases, and the given solution becomes relatively basic.
In humans, a healthy mouth has a pH range of 6.0 to 7.5. However, external stressors can affect the concentration of H+ and change the acidity/basicity of the oral environment. For instance, acidic foods and drinks (i.e. dairy products, soft drinks, lemonade, etc.) that enter the mouth lower the pH of an oral environment. Additionally, acidogenic bacteria – bacteria that exist naturally in the mouth – ferment carbohydrates (from food) to produce lactic acid, which further decreases oral pH. This is important because the calcium and phosphate ions within tooth enamel tend to dissolve in acids, so an oral pH that is approximately 5.5 or below will lead to enamel demineralization and cause cavities.11 Luckily, human saliva has three buffer systems that help resist changes in pH.
Most of the food and drinks we consume in everyday life are acidic or basic. Below are two examples of common beverages that have different pH values:
- Soda: pH = 3.3
- Milk: pH = 6.7
Identify which beverage is more acidic. What can be inferred about the more acidic beverage in terms of [H+]? Then, calculate the exact [H+] for each beverage.
Solution
Soda is the relatively more acidic beverage. The lower the pH, the more acidic a solution is. Therefore, since the pH of soda (pH = 3.3) is relatively lower than the pH of milk (pH = 6.7), it is more acidic.
Additionally, since pH and [H+] are inversely related, it can be inferred that soda has a relatively higher concentration of H+ than milk (assuming that the milk has not soured).
First, recall the formula that mathematically relates pH and [H+]:
(i) pH = -log10[H+]
Next, we can rewrite (i) to specifically calculate [H+]:
(ii) [H+] = 10-pH
Finally, we can use (ii) to calculate the exact [H+] of each beverage:
Soda: [H+] = 10-3.3 ≈ 5.01 x 10-4
Milk: [H+] = 10-6.7 ≈ 2.00 x 10-7
This confirms the previous conclusion that soda has a relatively concentration of H+ than milk.
Buffer Systems in Saliva
Buffer systems help resist changes in pH when small amounts of an acid or base is added to the environment. Buffer solutions are made by mixing an acid and its conjugate base or a base and its conjugate acid. Therefore, buffers can neutralize small amounts of added acid or base by reacting its basic species with the added acid or its acidic species with the added base.
Human saliva has three main buffer systems: the carbonic acid/bicarbonate system, the phosphate system, and proteins.7
Whenever we eat or drink, the flow rate of saliva increases in response to the taste, consistency, and concentration of the consumed substance. In such stimulated saliva, the carbonic acid/bicarbonate system becomes significant to maintaining the pH of an oral environment.7
The carbonic acid/bicarbonate buffer system can be represented by the following equilibrium equation:
H+ + HCO3- ⇄ H2CO3 ⇄ CO2 + H2O
Let's say a person takes a sip from a bottle of lemonade, an acidic beverage. Explain how the carbonic acid/bicarbonate buffer system helps resist changes in their oral pH.
Solution
Le Chatelier’s Principle states that a system at equilibrium will shift its equilibrium to counteract the effect of any change in one of the system’s components.
Since lemonade is acidic, it would cause an increase in [H+]. This change in the system affects the left side of the above equilibrium equation, as shown below:
H+ + HCO3- ⇄ H2CO3
According to Le Chatelier’s Principle, the system would shift its equilibrium towards the right in response and produce H2CO3. Since H+ must react with HCO3- in order to produce H2CO3, the concentration of H+ decreases as the concentration of H2O3 increases. In other words, the carbonic acid/bicarbonate buffer system neutralizes the added acid (H+ from the lemonade) by reacting it with its basic species (HCO3-). Thus, the overall concentration H+ in this person's mouth remains about the same, so there is no significant change in their oral pH.
The buffer systems in saliva shift their equilibrium to neutralize the oral environment whenever its pH becomes too acidic or basic. However, what happens when a change in pH is beyond the buffering capacity of saliva?
Demineralization of Tooth Enamel
When changes in oral pH cannot be managed by the buffer systems, our teeth become much more susceptible to demineralization.
Demineralization, aka tooth decay, is the removal of minerals from tooth enamel. This can result in rough, stained, and/or porous teeth that are vulnerable to cavities, infections, etc.
To combat this, dental medicine has developed several treatments to help facilitate remineralization, the natural repair of tooth enamel via the restoration of minerals in demineralized structures. For instance, fluoride treatments are used to slightly change the structure of tooth enamel so that it is less susceptible to demineralization:
Ca10(PO4)6(OH)2 + H+ + 2F- + Ca2+ + H2PO2- → Ca10(PO4)6F2 + Ca2+
The main structure in tooth enamel is hydroxyapatite (HA), denoted as Ca10(PO4)6(OH)2. Fluoride ions can replace the hydroxyl groups (OH-) in HA and form fluorapatite, denoted as Ca5(PO4)3F. Fluorapatite is relatively less soluble in acid than HA. Therefore, fluoride treatments help strengthen tooth enamel and minimize demineralization.
Although dental medicine has developed effective demineralization/remineralization treatments, it is still important to take care of your teeth through daily brushing, flossing, and rinsing. The rise of processed and junk foods in the modern-day human diet has increased our vulnerability to demineralization of tooth enamel. So, next time you eat or drink something, be mindful about how it may affect your oral pH!
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