Dental pH, Buffer Systems, and Acid-Base Balance in Oral Health

A lower pH means a higher hydrogen ion concentration

What does a lower pH indicate about hydrogen ion concentration?

A tenfold change in hydrogen ion concentration

What does a change of one whole pH unit represent?

Incomplete dissociation

What does it mean for an acid to be weak?

Near its own pKa

Where does a buffer work most effectively?

The acid and conjugate base are present in equal amounts

What is true when pH equals pKa?

50% dissociated

What percentage of an ionizable group is dissociated when pH equals pKa?

Streptococcus mutans and Lactobacillus

Which bacteria are major producers of lactic acid in dental plaque?

The principal acid involved in dental caries

Why is lactic acid important in dentistry?

Plaque fermentation and vinegar

What are common sources of acetic acid relevant to the mouth?

Citrus fruit, soft drinks, and sour candy

What are common sources of citric acid?

It chelates calcium and can remove mineral even above the critical pH

Why can citric acid damage enamel even at a pH above 5.5?

Cola beverages

What is a common dietary source of phosphoric acid?

A strong acid

How is hydrochloric acid classified?

Gastric reflux, vomiting, and rumination

What are common oral sources of hydrochloric acid exposure?

Approximately pH 1-2

What is the approximate pH of gastric acid?

Erosion rather than caries

What type of tooth damage is caused by direct gastric acid exposure?

7.35-7.45

What is the normal arterial blood pH range?

Below 7.35

What arterial blood pH defines acidemia?

Above 7.45

What arterial blood pH defines alkalemia?

Arterial vasodilation

What vascular effect can occur during acidemia?

Insulin resistance

What metabolic effect can occur during acidemia?

Approximately 6.7-7.4

What is the approximate pH range of resting saliva?

Approximately 6.8-7.0

What is the approximate resting pH of plaque fluid?

Bicarbonate

What is the dominant buffer system in arterial blood?

Phosphate and proteins

What are major buffers in resting saliva?

Proteins, peptides, and ammonia from urea

What are important buffering components of plaque fluid?

The lungs and kidneys

What systems regulate blood acid-base balance?

Salivary flow rate

What is a major regulator of oral pH?

Bacterial metabolism and diffusion

What are major influences on plaque pH?

It rises steeply with increased salivary flow

What happens to salivary bicarbonate concentration as flow rate increases?

Poorly

How well does bicarbonate penetrate plaque?

20-60 minutes after a sugar challenge

How long does plaque typically take to return to baseline pH?

Approximately 5.5

What is the critical pH for enamel demineralization?

Hydroxyapatite

What is the primary mineral in tooth enamel?

Ca10(PO4)6(OH)2

What is the chemical formula of hydroxyapatite?

It begins to dissolve

What happens to enamel when oral pH falls below approximately 5.5?

Mechanical cleansing

What protective function of saliva is reduced during xerostomia?

Buffering acids

What acid-base function of saliva is reduced during xerostomia?

Remineralization of enamel

What mineral-related function of saliva is reduced during xerostomia?

The Stephan curve

What curve shows plaque pH over time after a sugar challenge?

Near neutral, approximately pH 6.8-7.0

What is the approximate baseline plaque pH?

Streptococcus mutans and Lactobacillus ferment dietary carbohydrate

What causes the rapid fall in plaque pH after sugar intake?

Within 2-5 minutes

How quickly can plaque pH fall after a sugar ?

Hydroxyapatite dissolves

What happens to enamel mineral below the critical pH?

Salivary bicarbonate, phosphate, and proteins

What helps neutralize plaque acid during recovery?

Salivary flow rate

What major factor influences how quickly plaque pH recovers?

Time spent below the critical pH

What is more important for enamel damage than the depth of the pH trough alone?

Frequent snacking

What behavior can keep the Stephan curve below the critical pH for longer?

Calcium, phosphate, and fluoride

What substances support enamel remineralization?

Gastric acid exposure

What causes enamel erosion in patients with frequent vomiting or bulimia?

The palatal surfaces of the maxillary anterior teeth

Where is erosion classically most severe in patients with chronic vomiting?

Chemical erosion

What type of tooth destruction occurs when gastric acid contacts enamel directly?

It is overwhelmed and recovery is slower

What happens to salivary buffering after repeated exposure to gastric acid?

Do not brush immediately

What should a patient generally avoid doing immediately after vomiting to protect enamel?

Rinse with water or a bicarbonate rinse

What can help neutralize the mouth after vomiting?

Acid directly contacts the enamel

What distinguishes gastric erosion from bacterial caries?

Carbonic anhydrase

What enzyme catalyzes the conversion between CO2 and bicarbonate?

CO2

H2O ⇌ H2CO3 ⇌ H+ + HCO3− + What is the major carbonic anhydrase reaction?

A transient intermediate

How should carbonic acid be viewed in the bicarbonate system?

Red blood cells and salivary gland duct cells

Where is carbonic anhydrase II found?

CO2 transport and bicarbonate secretion

What are major functions of carbonic anhydrase II?

The acquired pellicle

Where does carbonic anhydrase VI adsorb in the mouth?

It catalyzes bicarbonate neutralization of plaque acid

What is the role of carbonic anhydrase VI at the enamel surface?

pH = pKa + log([A−]/[HA])

What is the Henderson-Hasselbalch equation?

The conjugate base

What does A− represent in the Henderson-Hasselbalch equation?

The undissociated conjugate acid

What does HA represent in the Henderson-Hasselbalch equation?

pH = pKa

What happens mathematically when the concentrations of A− and HA are equal?

Protonated

What is the state of an ionizable group when pH is below its pKa?

Deprotonated

What is the state of an ionizable group when pH is above its pKa?

Half dissociated

What is the state of an ionizable group when pH equals pKa?

Within approximately one pH unit of its pKa

Where is buffering strongest?

−1

What charge does a deprotonated carboxyl group (COO−) have?

+1

What charge does a protonated amino group (NH3+) have?

0

What charge does a deprotonated amino group (NH2) have?

Sum the charges of all ionizable groups

How do you determine the net charge of an amino acid?

The pH at which the net charge equals zero

What is the isoelectric point (pI)?

Complete dissociation

What characterizes a strong acid?

A molecule containing both positive and negative charges

What is a zwitterion?

The net charge is zero

What is the overall charge of a zwitterion?

The two pKa values surrounding the neutral form

Which pKa values are averaged to calculate the pI?

The average of the two relevant pKa values

How is pI calculated for an amino acid with no ionizable side chain?

The average of the two pKa values that surround the zero-charge species

How is pI determined for amino acids with ionizable side chains?

Acidic

What is the overall character of an amino acid when pH is below its pI?

Basic

What is the overall character of an amino acid when pH is above its pI?

Polyprotic

How are amino acids classified based on their multiple ionizable groups?

At least two

What is the minimum number of ionizable groups in an amino acid?

Three

How many ionizable groups do amino acids with ionizable side chains have?

Plateaus

What regions of an amino acid titration curve correspond to buffering?

pKa values

What do the midpoints of buffering plateaus represent?

The pI

What does the steep midpoint between two plateaus represent?

Minimum

What is the buffering capacity at the pI?

Maximum near the pKa

When is buffering capacity greatest?

Strongly negative

What is the charge of statherin's acidic region at resting salivary pH?

Aspartate and glutamate residues

Which amino acid residues contribute to the negative charge of statherin?

Calcium ions on hydroxyapatite

What does negatively charged statherin bind?

It helps keep saliva supersaturated with calcium and phosphate

What is an important function of statherin?

The acidic residues become protonated And The negative charge begins to decrease

What happens to acidic protein residues as plaque pH falls?

A weak base

How is lidocaine classified chemically?

The uncharged free-base form

Which form of lidocaine crosses the nerve membrane?

The drug is mostly protonated and cannot cross the membrane effectively

Why does local anesthesia often fail in infected tissue?