Hydroxyapatite

Question 1

ionic solid

Answer 1

1) salt
- positive and negative charge
2) aggregation
- formation of solid state
- ions attract (electrostatic or ion-ion)
3) dispersion
- attraction between ion and solvent (ion dipole)
4) entropy of interactions with solvent

Question 2

slightly soluble in water

Answer 2

1) stable in solid state
2) ions with high charge density
- Ca2+, Al3_, PO43-
3) pack to:
- maximize separation of same charge ions
- minimize separation of opposite charge ion

Question 3

highly soluble

Answer 3

1) low charge density
2) pack relatively poorly
3) NaCl

Question 4

basic dissolution/precipitation stages

Answer 4

1) dynamic state
2) reaches equilibrium
- when dissolution and aggregation rates are the same

Question 5

solid-solution transition

Answer 5

1) ions on left side are shown hydrated (aquo ions) with the common coordination of #6 waters
M(H2O)6 + X(H2O)6
2) usually ion’s hydration state is not shown
3) moving to right: precipitation
MX + 12H2O

Question 6

equations to describe rate of dissolution and precipitation

Answer 6

1) depends on surface area of the solid
2) rate (dsln) = k(dsln)(Area of solid)
3) rate (pptn) = k(dsln)(Area of solid)*[M+][X-]
4) ion product
- [M+][X-]
5) net rate = rate (dsln) - rate (pptn)

Question 7

at equilibrium

Answer 7

1) net rate = 0
2) so rate dsln = rate pptn

Question 8

ksp

Answer 8

1) higher Ksp is higher solubility
2) equilibrium constant
Ksp = kdsln/kpptn = [M+]eq*[X-]eq
3) at equilibrium, the product of component ions is equal to a constant (Ksp)
- saturated
- no net dissolution or precipitation

Question 9

deviation from equilibrium

Answer 9

1) applicable ion product =/= Ksp
2) ion product > Ksp
- precipitation
- mineralization
- (SUPERSATURATED)
3) ion product < Ksp
- dissolution
- demineralization
(UNDERSATURATION)

Question 10

in an undersaturated solution

Answer 10

1) K(dsln) > K(pptn)[M+][X-]
2) because the rate of dissolution is more than precipitation (there is more solvent, you need more things to dissolve to reach saturation)

Question 11

hydroxyapatite composition

Answer 11

1) calcium and phosphate
- pH = 7 ( lowest solubility solid)
2) Ca10(PO4)6(OH)2 – dimer
3) in solutions containing physiological concentrations of other ions, solubility is higher
- Ksp = 0.7 mM^2
4) versus salt
- Ksp = 3*10^7 mM^2

Question 12

Ksp and ions can allow you to calculate:

Answer 12

1) tendency of a fluid to dissolve
2) it is the product of ion concentration that matters
- differing ion concentrations can lead to same Ksp if multiplying it by phosphate (Pi)

Question 13

conditions where ion product < Ksp

Answer 13

1) pH dependent for hydroxyapatite
2) the Ksp increase when pH<5
- Ksp > ion product
- dissolution

Question 14

pH dependent solubility

Answer 14

1) for many other salts, solubility pH dependent too
2) because Pi forms are acids/bases with each form being pH dependent

Question 15

hydroxyapatite crystals have an anion channel on the c-axis

Answer 15

1) inside is OH-, F-, Cl-

Question 16

ion exchange of HA

Answer 16

1) even when there is no net precipitation or dissolution, exchange of ions occurs
2) depends on location
a) hydration shell (very fast)
b) surface (slow)
c) crystal interior (very slow)

Question 17

HA ions substituted

Answer 17

1) because bulk solutions in contact with HA vary in ion content
- biological HA contains other ions
2) most substitutions increase HA solubility
- ex. carbonated apatite
3) exception
- F- (fluoridated HA)
- makes it more stable