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1 3 01 Clhow. LC, Eanes, ED (eds): Octacalcium Phosplrate. M o n o g l O r a l S c i . B a s e l . K a r g e r , 2 0 0 1 , v o l l 1 3 .p p 9 4 - 1 1 I
Solubility of Calcium Phosphates L.C. Chow Amcrican
Dcntal Association Health Foundalion. Paffenbargel Rcscarch Cenlcr.
National Inslitute o[ Standards and Tcchnology, Gaithersburg. Md.. USA
Solubility is one of the most important properties of calcium phosphate salts.It is the solubility that determinesthe direction of many reactionsthat involve calcium phosphates such as dissolution, precipitation, hydrolysis, and phase transformation. Calcium phosphate solubility also plays a maior role in biological processesthat involve formation and resorption ol' hard tissuesas well as pathological calcifications.This chapter will focus on concepts that would aid in gaining a better understandingof the solubility behavior of calcium phosphzrtes, rather than provide a survey of the literature, w h i c h h a s b e e n g i v e n e l s e w h e r e[ E l l i o t . 1 9 9 4 ] .S o l u b i l i t y i s c o n v e n t i o n a l l y d e s c r i b c dn s t h e n m o u n t o f a s o l i d t h a t c a n d i s s t > l vien t o a u n i t v o l u m e o f s o lution. Fclr calcium phosphates,this quantity often changesby several orders of magnituclewith changesin the pH and concentrationsof acids and bases, such as HCI and NaOH. Thus, on the surface.the solubility may appear to be a complex function of these solution parameters.However, a clearer picture o1 the solubility properties for calcium phosphatescan be gaineclby understandingthe basic principlesgoverning the solid-solutionequilibrium.
Gibbs' Components The composition of a solution may be describedin terms ol the quantities of inclependentcomponents defined in Gibbs'phase rule [Gibbs, 1876]. The componentsmay be understood as the minimum number of compounds that are neecledto form all of the phasesthat may be present in a system. In the simplest case,a solution that contains calciurn and phosphate ions is
comprised of three such components,e.g., Ca(OH)2-H3PO1-H2O [Moreno et al., 1966]. Because the components in a given phase must be homogencously distributed, the Ca(OH)2 and H3POa components here refer to materials dissolvedin water, which is also a component. The selection of components is quite arbitrary and is usually based on the ease of expressingthe concentrations and chemical potentials of the components. For example. CaO-P2O.5-H2Ois another set of componentsthat have been used in many studies [Brown arndLehr, 19-59;Brown, 1992].While considerablefreedom exists in choosing components,these components must be independent in that zrcombination of any ol the componentsdoes not form another component. 'component' and It is important to distinguishbetween the concept of a 'species'. that of a The Ca(OH)2 and HjPO4 speciesrefer to the undissoc i a t e d C a ( O H ) 2 a n d H 3 P O 4p r e s e n ti n t h e s o l u t i o n .A n a c i d i c s o l u t i o n w i t h a p H o l ' a p p r o x i m a t e l y4 . 4 c a n b e p r e p a r e db y c o m b i n i n g0 . 0 1m o l e s o f t h e c o m p o u n d C a ( O H ) 2 , 0 . 0 2m o l e s o f t h e c o m p o u n d H j P O 4 , a n d a s u f f i c i e n t amount of water to make I liter of solution. While the concentration of the C a ( O H ) 2 c o m p o n e n t i n t h e s o l u t i o n i s 0 . 0 1m o l / 1 ,t h e c o n c e n t r a t i o no f t h c C a ( O H ) 2 s p e c i e si s n e g l i g i b l ea t t h i s p H b e c a u s et h e b u l k o f t h e s o l u t i o n calcium is in the form of free Ca2* ions ancl a small amount is in the fortr cll' . s i m i l a r a n a l y s i sc a n b e m a d e o [ a b a s i cs o l u t i o n i o n p a i r s w i t h p h o s p h a t eA c o n t a i n i n ga s i g n i f i c a n ta n l o u n t o f t h c H 3 P O 1 c o m p o n e n t ,b u t a n c g l i g i b l y s m a l l a m o u n t o f t h e H 3 P O 4 s p e c i e sb c c a u s ei n a l k a l i n e s o l u t i o n st h e s o l u t i o n p h o s p h a t ca r c p r c s c n tp r i m a r i l y i n t h e f o r m s t l l ' H P O a r a r r c lP O 1 ' ' i o r r s . It can be seen l'rom the above example that the concentrations of the C a ( O H ) 2 a n c l H . 1 P O ac o m p o n e n t sa r e t h e s a m e a s t h e t o l a l c a l c i u r nc o n c e n t r a t i o n , I C a ] , a n c lt o t a l p h o s p h a t ec o n c e n t r a t i o n I, P ] , r e s p e c t i v e l yo, l ' t h c s o l u t i o n . A s a r e s u l t ,t h e s et e r m s a r e e q u i v a l e n ta n d c a n b e u s e c li n t e r c h a n g c ably. For practical purposes,[Ca]. IP], and pH are the three most commonly used parameters I'or describing the solution composition because all three are directly measurzrblequantities. The reason for expressingthe solt-ttion c o m p o s i t i o ni n t e r n s o f G i b b s ' c o m p o n e n t si n s t e a do f I C a ] , I P ] a n d p H i n t h i s c h a p t e ri s b e c a u s eo f t h e e a s eo f d e f i n i n g t h e s o l i d - s o l u t i o ne q u i l i b r i u m describednext.
Solubility - Theoretical Considerations The clissolutionreaction of a calcium phosphatesolid, such as hydroxyapatite (OHAp), may be expressedin two equivalent ways in terms of the i o n i c s p e c i e s( e q 1 ) a n d t h e c o m p o n e n t s( e q . 2 ) .
S o l u h i l i t rvr l ( r r l c i u r P r rh t r s p h : r t e s
Cas(PO+):OH (solid)
