Indian Journal of ChemistryVol. 25A, February 1986, pp. 181-182

Solubility Behaviour of SyntheticWhitlockite Containing Magnesium in

Aqueous Medium

N S CHICKERUR*. R C LENKA.B B SABAT & G H NAYAK

Post-Graduate Department of Chemistry,Khallikote College. Berhampur 760 001

Received 9 October 1984; revised 27 May 1985;rerevised and accepted 10 September 1985

Samples of whitlockite containing varying amounts of magnesiumhave been prepared in aqueous medium and characterised bychemical analysis, X-ray and IR spectra. The shift in the majorfrequencies observed in the IR spectra of the samples with increase inmagnesium content is consistent with the change in the cation massand force field constant. The solubility of the samples in aqueousmedium is found to decrease with increase in both the pH of themedium and the magnesium content of the samples at a chosen pH.

Whitlockite is a mineral structurally related to B-tricalcium phosphate, CaiP04h(B-TCP). Mackay'suggested its idealised formula as (Ca,MghoHiP04)l4, showing that some calcium ions can besubstituted by magnesium though the extent ofpossible substitution is yet unknown. Whitlockite hasbeen reported to be associated with variouspathological conditions of caries lesions, dentalcalculus, calcified aortas and urinary concen-trations+". The extent of association was found to bedirectly dependent upon the presence of magnesium indrinking water3. It was, therefore, considered relevantto investigate the role of magnesium in the formationof stable whitlockite and the dependence of itssolubility on the magnesium content within thebiologically important pH range of 5.0 to 7.5 at 37°C.

Samples of whitlockite were prepared as follows:Two solutions (500 ml each) containing stoichiometricamounts of disodium hydrogen phosphate and amixture of calcium acetate and magnesium acetate

respectively were prepared so as to give a theoreticalyield of 109 of the sample according to thestoichiometric equation given below (Eq, I). Both thesolutions were made alkaline by the addition of NaOHsolution. The phosphate solution was added dropwiseto the mixed solutions of acetates maintained at 37°Cin CO2-free atmosphere. The gram atom ratio ofcalcium to magnesium in the solution taken forpreparation was altered as desired for the solid in eachpreparation. For preparation of the samples, one moleof whitlockite was considered to contain a total of 20cations of calcium and magnesium taken together.(20-n) (CH3COO)2Ca + nMg(CH3COO)2+ 14Na2HP04 + 12NaOH =Ca20 -nMgn(P04)l4H2 +40CH3COONa + 12H20. (1)

After completion of precipitation the samples wereseparated by filtration, washed repeatedly with CO2-free water and 0.01 M NaOH solution till the washedliquid was free from calcium and magnesium. Thesamples were then washed with ether and dried atl lfr'C for 4 hr.

The samples were analysed for Ca, Mg and P by thecomplexometric methods", IR spectra and X-raydiffraction patterns. The analytical data andcalculated lattice constants are given in Table L Agradual decrease in the values of lattice constants wasobserved with the introduction of smaller Mg2+ ion inplace ofCa 2+ ions. The characteristic phosphate peaksat 575, 875,1010, 1040and 1120cm -1 in the IR spectrashift towards higher wavelength region indicatingsubstitutional effect of calcium by magnesium in thesamples.

The solubility-studies were undertaken by themethod of equilibration. About 0.2 g of the powderedsample sieved to 200 mesh (BSS)was equilibrated with100ml of the solvent by constant shaking at regulatedspeed (using a flask shaker at 37°C. The timerequired for attainment of saturation was determined

SampleNo.

Table I - Chemical and X-ray Analyses of Samples of WhitlockiteWt% Molecular formula g atom g atom

ratio ratioMg P (Ca/Mg) (Ca + Mg)

P

Lattice constants(A)

Ca

1.

2.3.4.5.

35.26734.28533.79132.66731.286

1.5922.2852.6343.3434.403

20.57220.66620.71420.84920.954

CaI8.6Mgl.iP04)14H2CaI8Mg2(P04)14H2Cal7.7Mg2.3(P°4)'4H2CaI8Mg2.9(P04)14H2Ca'6.2Mg38(P04)'4H2

13.2979.0027.7005.8624.264

a c10.430 37.36510.420 37.38010.379 37.29010.200 37.19010.129 37.100

1.4281.4281.4281.4211.428

181

INDIAN 1. CHEM., VOL. 25A, FEBRUARY 1986

• SAMPLE. Io SAMPLE.2·•• SAMPLE.3

e SAMPLE.4II SAMPLE.5

5·5 7·06·5

!'Ii ~T"'E"'WUM

Fig. 1 - Dependence of solubility of the samples of whitlockite onthe pH of the medium at 37C.

by analysing the phosphorus content at the end of eachconvenient period of equilibration. Since the systemin vivo behaves as a reservoir of buffer combinationsmaintaining a constant pH of 7.2, acetic acid-sodiumacetate and borax-boric acid buffers were selected toinvestigate the pH-dependence of solubility ofwhitlockite in the biologically important pH range of5.0 to 7.5. Various quantities of a given buffercombination gave reproducible results indicating thatthe buffer ions had no effect.

The results of solubility investigations are showngraphically in Figs 1 and 2.

A decrease in solubility of all the samples with theincrease in pH of the medium and increase in themagnesium content at a chosen pH was observed. Theobserved pH-dependence of the samples can beexplained on the basis of proton accepting capacity ofphosphate (PO/ -) in acid-base equilibria shownbelow.

P043 -+H+¢HPO/-

In acid medium the tribasic phosphate (PO 4 3 -) ion isconverted into the dibasic phosphate ion (HPO/-)while in the basic medium the reverse occurs. The

182

5OO'J-----.- +~~-

~~~------4-~~ __~.ss~~3OOIf---------+-----

1001------t---

o 1.0 2·0 3·0 4.0

MAGNESIUM (f ATOM PER MOLE)

Fig. 2 - Dependence of solubility of the samples of whitlockite onthe magnesium content at 37' C and different values of pH of themedium (PH 5.5, 6.0, 6.5 and 7.0 represented by curves 1.2. 3 & 4

respectively).

acidic medium favours the forward reaction while thebasic medium facilitates the backward process.

The structure of whitlockite containing magnesiumin some calcium positions attains greater stability.Introduction of smaller Mg2 + in place of Ca 2+ ionsincreases the compactness of the structure" whichexplains the observed decrease in solubility of thesamples with increase in magnesium content at a givenpH.

One of the authors (RCL) thanks the CSIR, NewDelhi for the award of a senior research fellowship.

ReferencesI Mackay A L, Ph.D. Thesis, University of London. 1952.2 Bigi A. Forest E, Inurti A, Rovevi N, Borea P A & Zavagli G,

lnorg chim Acta (Lel1). 46 (1980) 271.3 Torborg Jensen A & Rowels S L. Acta ondottol Scand. 16 (1957)

121.

4 Pribil R. Analytical applications of ED TA and related compounds(Pergamon Press. New York) 1972.

5 Hannay N B. Solid-slate chcmistrv: (Prentice-Hall of India. NewDelhi) 1976.