NATIONAL BUREAU OF STANDARDS REPORT

9920

Progress Report

on

STOICHIOMETRIC COMPOSITION OF WHITLOCKITE

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

311.05-11-5110561 June 30, 1968 9920

Progress Report

on

STOICHIOMETRIC COMPOSITION OF WHITLOCKITE

By

•ft -Vr -Hr Betty M, Wallace and Walter E. Brown

* Research Associate, American Dental Association Research

Div., National Bureau of Standards, Washington, D.C. 20234

** Director, Research Associate Program of the American

Dental Association, Dental Research Section, National

Bureau of Standards, Washington, D. C. 20234

This investigation was supported in part by Research Grants

DE- 00572 to the American Dental Association from the National Institute of Dental Research, and is part of the dental

research program conducted by the National Bureau of Standards

in cooperation with the Council on Dental Research of the

American Dental Association; the Army Dental Corps; the Dental

Sciences Division of the School of Aerospace Medicine, USAF;

and the Veterans Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF ST/

for use within the Government. Approved for public release by the

end review. For this reason, the Dj rector of the National Institute of

whole or in part, is not authori Bureau of standards, Washington Standards and Technology (NIST)

the Report has been specifically on October 9, 2015

> accounting documents intendad

subjected to additional evaluation

listing of this Report, either in Office of the Oirector, National

' the Government agency for which

pies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

STOICHIOMETRIC COMPOSITION OF WHITLOCKITE

ABSTRACT

X-ray studies have shown that the mineral

whitlockite is identical with the salt known as

3-tricalcium phosphate (Ca/P = 3/2) . The exact

stoichiometry of whitlockite is in doubt because

arguments have also been presented for a molar Ca/P

ratio of 10/7. Samples of dicalcium phosphate dihydrate, octacalcium phosphate and mixtures of

anhydrous dicalcium phosphate with CaC03 were ignited

to give the products calcium pyrophosphate and whitlockite. The empirical Ca/P ratio and the % phosphorus

present as pyrophosphate in the ignited material were

used to calculate the molar Ca/P ratio of the whitlockite product, giving a mean value of 1.506 ± 0.015.

This value differs from 3/2 by less than the standard

error of the mean and indicates that the formula for

whitlockite is Ca 3 (P04 ) 2

.

INTRODUCTION

X-ray studies have shown that the mineral whitlockite

is identical with the salt known as 3-tricalcium phosphate

(Ca/P=3/2) . It is a material of biological significance1 as

a constituent of dental calculi and urinary tract concretions

However, the exact stoichiometry of whitlockite is now in

doubt because arguments have been presented for molar calcium

to-phosphorus ratio of 10/7. Frondel 2 first described the

mineral whitlockite as anhydrous tricalcium phosphate containing small amounts of magnesium and iron (both Fe + + and

Fe +++

). Its apparent unit-cell contents, Ca 2l P 1 4 0 5S , agree

reasonably well with the measured density 3.12; however its

space group r3c would not permit an odd number of cations,

and Frondel suggested that some calcium ions are missing and

are partly replaced by ferric ions. Mackay3 concluded that

the space group of whitlockite is R3c and investigated the

inconsistency between cell contents and space groups without

coming to a definite conclusion.
- 2 -

Keppler 4 noted that the space groups for Sr 3 (P04 ) s and

Ba3 (P04 )g, R3m, indicate the presence of a three-fold inversion axis so that there is no inconsistency between the unitcell contents and the indicated space group. He then proposed

the formula 3Caa (P04 ) 3 •M-J- IHP04 for whitlockite, with M

11

representing a divalent cation with smaller ionic radius than

calcium (e.g., Fe or Mg), which has a 10/7 cation/anion ratio.

Keppler8 mentioned the isotypy of whitlockite to that of the

mineral cerite6 (a rare earth silicate of cerium and lanthanum

(RE) 8 (Ca, Mg, Fe) 2 (SiCU ) • 3H2 0) which also has a 10/7 ratio.

Welch and Gutt7 report a phase diagram showing a region

of solid solution between 2CaOP2 05 and 3Ca0*P s 05 . Their

data points indicate ratios between 1.39 and 1.42 at 1,000°

and 1,050°C, respectively (i.e., Ca/P= 10/7).

The work of Fowler, Moreno and Brown8 on the infrared

spectra of the products of pyrolysis of octacalcium phosphate

showed that whitlockite and calcium pyrophosphate are the

products formed between 650 and 900°C. Their work suggested

a means of determining the stoichiometric composition of

whitlockite by measuring the per cent conversion of orthophosphate to pyrophosphate; a Ca/P ratio of 10/7 would yield

a smaller amount of pyrophosphate than if the ratio were 3/2.
-3-

EXPERIMENTAL SECTION

Materials—S amp 1 e s used were: (a) dicalcium phosphate

dihydrate (DCPD) , (CaHP04

• 2H2 0) ; (b) three preparations of

octacalcium phosphate (OCP) 10

, (Ca8 H2 (P04 ) 6

• 5H2 0) ; (c) mixtures

of DCPD with CaC03 ; and (d) mixtures of commercial anhydrous

dicalcium phosphate (DCPA) , CaHP04 , with CaC03

. A sample of

the compound or mixture was slurried in ethyl alcohol, ground

in an agate mortar, and air dried before ignition.

Ignition conditions-—Loosely covered platinum foil

boats containing 0.2 g samples with Ca/P ratios in the range

1.0 to 1.5 were heated in an electric muffle furnace at

700° and 900°C for either 24 or 96 hours.

Analyses—Calcium and phosphorus contents were determined

on the samples before and after ignition, and the pyrophosphate

content after ignition. Calcium was measured by atomic

absorption using 2 ppm of strontium as the chloride in both

standards and samples to suppress interference by phosphate.

Values for the amount of phosphorus in pyrophosphate form

were obtained by the difference between the amounts of orthophosphate present before and after hydrolysis in perchloric

1 x acid
“4“

RESULTS

The pyrophosphate contents in the ignited samples are

shown in Table 1®

TABLE 1

Pyrophosphate Formation

Ignition Empirical

Sample Temp

.

°C.

Ca/P :

ri Ratio0

DCPD 700 c 0.999 ± .014

900 c 0.988 ± .015

DCPA 900 a 1.150 ± .017

+CaC03

900 a 1.218 ± •

900 a 1.213 ± .018

900 a 1.336 ± .019

0CP#1 700 c 1.318 ± .019

900 c 1.337 ± .020

DCPD 900b 1.359 ± .019

+CaC03 0CP#2 700 c 1.324 ± .019

900 c 1.356 ± .020

0CP#3 700 c 1.366 ± .019

900 c 1.333 ± .019

on Ignition

Calculated

%P as Ca/P Ratio

pyrophosphate for

lOOx whitlockite, m

97.8 + 1.4 0.955 + . 910e

98.5 1.4 0.200 + 1 . 500 e

76.8 ± 1.3 1.646 ± , .127

57.2 + 1.2 1.509 ± .062

47.1 + 1.2 1.403 _L .052

38.8 + 1.2 1.549 ± .048

29.8 ± 1.3 1.453 ± .042

30.0 + 1.3 1.481 ± .043

33.1 + 1.2 1.537 ± .044

26.8 + 1.3 1.443 ± .042

26.4 ± 1.3 1.484 ± .041

29.8 ± 1.3 1.521 ± .042

29.0 1.3 1.469 + .041
-5-

Calculated

Ignition Empirical %P as Ca/P Ratio

Sample Temp

.

Ca/P Ratiod pyrophosphate for

°C. n lOOjK whitlockite

,

DCPA 9Q0b 1.399 ± .020 21.1 ± 1.3 1.506 .039

+CaC03 900b 1.478 ± .022 10.4 ± 1.4 1.533 ± .037

90Qb 1.468 ± .021 7.3 ± 1.4 1.505 ± .035

9Q0b 1.541 ± .022 1.2 ± 1.4 1.548 ± .035

Mean value 1.506 ± .015

aheated 24 hours

bheated 96 hours

caverage of 2 ignitions, 1 for 24 and 1 for 96 hours

dafter heat treatment

eomitted from calculation of the mean

^standard error of the mean

Corresponding data at 24 and 96 hours indicated that pyrophosphate formation was essentially complete at 24 hours, and

eight of the values are averages of the data for the two ignition

times. Probable errors were calculated with an estimated 1%

error in the calcium and phosphorus measurements.
-6-

DISCUSSION

The empirical composition of the heated sample may be

expressed as CanH

(

3-2n) P04 , and the pyrolysis reaction may

be written in terms of the coefficients for calcium or phosphorus : x n-x

CanH ( 3 _ 2 n) PO^ = -£ Ca2 P 2 07 + —

Ca^H ( 3 _ 2

m

) P04

"" m “

= —. Ca2 P 2 07 ± (1—x) Ca^H

(

3 - 2 m) P04

Here ri represents the empirical ratio of calcium to phosphorus

in the sample, m is the ratio of calcium to phosphorus in the

whitlockite product, and IQOx is the % phosphorus in pyrophosphate form. Equating coefficients gives:

n-x “ ~

1

^

It is assumed that the calcium pyrophosphate formed is

stoichiometric and that whitlockite is the only other product.

As shown in the table the calculated mean value of m is 1.506

± 0.015.

The above calculation of m breaks down when n approaches

one because it involves small differences between large
-7-

numbers. For this reason, the first two values were omitted

in obtaining the mean value of m. The value 1.506 differs

from 3/2 by less than the standard error of the mean. It is

concluded that the formula for whitlockite is Ca3 (P04 ) 2 under the conditions of these experiments.

USCOMM—DC— NBS
REFERENCES

1. R. W. Mooney and M. A. Aia, Chem. Rev., 61., 433 (1961).

2. C. Frondel, Am. Mineral., 26., 145 (1941).

3. A L. Mackay, Acta Cryst. , 6., 743 (1953).

4. U. Keppler, Neues Jahrb. Mineral. Monatsh. 6_, 171 (1965) .

5. U. Keppler, Naturwissenschaften, J54, 105 (1966)

.

6. J. J. Glass, H. T. Evans, Jr., M. K. Carron, and F. A.

Hildebrand, Am. Mineral., 43, 460 (1958)

„

7. J. H. Welch and W. Gutt, J. Chem. Soc. , 4442 (1961).

8. B. O. Fowler, E. C. Moreno and W. E. Brown, Arch. Oral

Biol., 11, 477 (1966).

9. DCPD prepared by ammoniation of aqueous solution saturated

with Ca (H2 P04 ) a *H2 0 and CaHP04

# 2H2 0. E 0 C. Moreno,

W. E. Brown and G. Osborn, Soil Sci. Soc. Amer. Proc.

,

24, 94 (1960).

10.

OCP prepared by slow hydrolysis of DCPD in 1M sodium acetate

at 38°C. W. E. Brown, J. R. Lehr, J. P Q Smith, and

A. W. Frazier, J. Amer. Chem. Soc., 79., 5318 (1957).

11 A. Gee and W. R. Dietz, J. Am. Chem. Soc., 77, 2961 (1955).
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