19. Enamel and pH

Question 1

3 conditions that require restoration

Answer 1

• fracture
• caries
• erosion/abrasion

Question 2

Define ‘fracture’

Answer 2

loss of tooth due to sudden impact of force

Question 3

Define ‘caries’

Answer 3

loss of tooth due to acid of bacterial origin

Question 4

Define ‘erosion/abrasion’

Answer 4

tooth loss due to dietary acids and mechanical wear

Question 5

What is biological apatite?

Answer 5

• main component of enamel
• made of mainly calcium hydroxyapatite

Question 6

Chemical formula of calcium hydroxyapatite

Answer 6

Ca10(PO4)6(OH)2

Question 7

Calcium hydroxyapatite is sparingly soluble in … and solubitlity increases in …

Answer 7

• saliva
• low pH

Question 8

What does the Stephan curve have on axises?

Answer 8

• pH on y-axis
• minutes after sucrose rinse on x

Question 9

What does Stephan curve show?

Answer 9

• plaque pH drops when challenged (by food)
• oral pH drops with acidic food and drinks
• rise is much slower than drop
• enamel dissolves more quickly below critical pH (around 5.5)

Question 10

Silver chloride is a … solubility salt

Answer 10

sparingly

Question 11

What happens to equilibrium of silver and chloride ions in silver chloride at saturation?

Answer 11

• some silver ions and some chloride ones dissolve into solution
• product of conc of silver and chloride ions (moles/L) is called solubility product (Ksp)

Question 12

Why is calcium flouride’s solubility product more complex than silver chloride?

Answer 12

• number of dissolved cations is different than anions (as 2 F ions to 1 Ca)
• have to measure free calcium

Question 13

How to work out solubility product

Answer 13

• measure calcium at equilibrium (only need conc of 1 ion)
• multiply by 0.6 for phosphate (as calcium is for 10, only 6 phosphate) and by 0.2 for OH as only 2
• then times these numbers together to the power of however many molecules of each for example calcium is to power of 10
• put in moles/L (to power of 18 as 10+6+2 for number of molecules)

Question 14

What’s the ionic product?

Answer 14

• same as solubility product but in solution

Question 15

For solutions not in equilibrium, if IP is largee than Ksp, what does that mean?

Answer 15

• solution is supersaturated
  – precipitation unless ions are stabilised

Question 16

For solutions not in equilibrium, if IP is smaller than Ksp, what does that mean?

Answer 16

• solution is unsaturated
• dissolution

Question 17

In water, IP of hydroxyapatite is …

Answer 17

• 0 moles/L
• hydroxide is present but no calcium or P

Question 18

When tooth is placed in water, what happens?

Answer 18

• hydroxyapatite will dissolve
• until equilibrium is reached
• when IP equals Ksp

Question 19

Saliva is normally … for calcium and phosphate
What does this mean?

Answer 19

supersaturated
IP is larger than Ksp

Question 20

In dissociation of calcium hydroxyapatite, there is loss of … and …

Answer 20

• hydroxide ions
• phosphate ions

Question 21

Explain loss of OH- in dissociation of calcium hydroxyapatite

Answer 21

• hydrogen ions remove hydroxyl ions in water
• H+ + OH- = H2O
• in water, conc of hydrogen and hydroxide ions are 10-14
• as hydrogen increases, hydroxide must decrease

Question 22

Explain loss of PO4 3- in dissociation of calcium hydroxyapatite

Answer 22

• phosphate exists in 4 forms but only the 3- form is important for IP of HA
• as pH decreases, equilibrium is pushed away from phosphate 3-

Question 23

Different people have different calcium and phosphate in saliva. What does this mean?

Answer 23

• lower conc lead to higher critical pH (up to 6.5)
• differences in biological apatite, which affect dissolution
• measurements of pH in saliva show alkali production is more important than acid (not all about critical pH)