REACTIONS OF FLUORIDE ION WITH HYDROXYAPATITE

BY HAROLD G. MCCANN (From the National Institute of Dental Research, National Institutes of Health,

Bethesda, Maryland)

(Received for publication, September 22, 1952)

A problem of long standing in the study of dentin and enamel is the exact nature and structure of the inorganic material in these tissues. A review of these studies from the time of Berzelius in 1845 up to 1949 has been published by Armstrong (1). In general the mineral component of teeth as well as bones consists chiefly of a basic calcium phosphate which is most probably hydroxyapatite (2, 3). Hydroxyapatites may be syn- thesized in the laboratory and their basic chemistry and reactions studied directly. The use of a synthetic hydroxyapatite instead of the natural hydroxyapatites, as may be found in teeth and bones, avoids possible complications introduced by extraneous chemicals present. in the natural materials. Chemical methods devised to study the synthetic apatites may then be adapted for study of natural tooth substance. At the same time information is obtained concerning basic chemical reactions which take place and the effects of varying conditions on these basic reactions. In these experiments we have been concerned for obvious reasons (4, 5) pri- marily with reactions between the fluoride ion and synthetic hydroxyapa- tite.

EXPERIMENTAL

Preparation of Hydroxyapatite-This study was made on a series of syn- thesized compounds ranging in empirical composition from Ca9.g(P04)6- (OH)1 having a Ca:P mole ratio of 1.593 to that corresponding to pure hydroxyapatite, Calo(PO&(OH)z, having a Ca:P ratio of 1.667. These hydroxyapatites, listed with their analyses in Table I, were prepared by a modification of the acidimetric precipitation method of Rathje (6, 7), later employed by Egan et al. (8).

To 6 liters of continuously boiling solution (approximately 1 M in respect to an ammonium salt) in a 12 liter flask were added 2 liters each of a solution of 0.12 M KH2P0, and 0.20 M Ca(NO& at an equal rate of about 60 ml. per hour for 8 hour periods during the day. The slurry was left boiling overnight and addition continued the following day. Samples K and I, were allowed to cool overnight and absorbed CO2 as shown in Table I. The pH was kept, just alkaline to phenolphthalein, brom cresol purple, or brom phenol blue by the periodic addition of concentrated NH,OH.

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248 HYDROXYAP4TITE-FLUORIDE ION REACTION

The NH&l first used caused the presence of 0.3 to 0.5 per cent Cl as an impurity. This was avoided by the use of ammonium nitrate, but am- monium acetate or borate made it easier to maintain the desired pH. The final products were filtered on a sintered glass funnel, washed with copious quantities of distilled water, and dried at 110”. All chemicals used were reagent grade. The Ca(NO& was prepared from CaC03 treated with a slight excess of HNOS, the COP was boiled off, and the excess HNOJ neu- tralized.

A hydroxyapatite radioactive in respect to both calcium and phosphorus was prepared from 10 gm. of CaC03 containing 1.5 mc. of Ca45 and 8.15 gm. of KH2P04 plus 2 ml. of a solution containing 2 mc. of P32 as phosphate in dilute HCl. This sample was analyzed gravimetrically after it had decayed for 2; months.

TABLE I

Analyses of Hydroxyapaiites Prepared and Used in This Study

Apatite sample No. CaO PZOS

per cent ger cent

I 53.32 42.40 K 53.54 41.62

ii 53.65 53.65 41.53 40.83 II 53.10 40.39 Radioactive 51.85 40.01

* Microscopical observation

Ratio Ca:P

?tSOkS

1.593

1.628 1.645 1.663 1.665 1.641

_ P

Cl

er ceni

0.35 0.51 0.39 Nil

“ I‘

CO2

rer c&

0.31 0.64

-

-

Particle size’ ~-_____

Some particles 3 to 4 p Small; few crystals Needles; some 6 to 7 p Few very small crystals Mostly amorphous; smaller

than Sample Q

Although these preparations varied considerably in composition, it has been suggested by Bale et al. (a), Hendricks and Hill (3), and Posner and Stephenson (9) that all such materials are hydroxyapatite with entrapped acid phosphate ions on the surfaces.

Treatment of Hydroxyapatites with Fluoride-These apatites were treated in various ways in order to determine the effect of concentration of the fluoride ion, temperature, and Ca:P ratio on reactions with the fluoride ion. Concentration of fluoride, usually as NaF, was varied from 1 part per million such as occurs naturally in water up to 2 per cent NaF as used in the topical application of fluoride to the teeth. Temperatures of 20”, 37”, and 100” were employed.

Samples in the more concentrated solutions were treated by placing 2.5 gm. of the solid apatite into a container and adding 100 ml. of a solution containing the desired fluoride concentration. At 37” and below, poly- ethylene bottles were used; at loo”, Pyrex flasks. The more dilute solu-

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H. G. MCCANN 249

tions were made up in 12 liter flasks, as a volume of 10 liters of solution was employed to obtain sufficient fluoride to react with 2.5 gm. of apatite. Smaller amounts were sometimes used when insufficient material was avail- able; a less complete analysis was then obtained. Samples were boiled continuously to maintain 100’ temperature; at lower temperatures samples were placed in a constant temperature oven or room and shaken occasion- ally. After a period of time varying from several days for the 100” samples to 1 to 4 months for the lower temperature samples, they were filtered, washed, and dried at 110”. The filtrates from the concentrated samples included the washings and were made up to 500 ml. 1 liter of filtrate from the dilute solutions was reserved.

Analytical-The original apatites and the treated solids were anilyzed gravimetrically for calcium by double precipitation of the oxalate from acid solution by slow addition of NHIOH to avoid contamination with phosphate, with final weighing as the fluoride, and for phosphate by double precipitation as magnesium ammonium phosphate to assure the proper stoichiometric relationship (10). Fluoride was determined calorimetrically by visual matching or spectrophotometrically; both methods are modifica- tions of the Willard and Winter method (11). In the liquid filtrates cal- cium was determined microvolumetrically on a concentrated aliquot by a modification of the ceric sulfate procedure of Kirk (12), and phosphorus spectrophotometrically by the molybdivanadophosphoric acid procedure as modified by Simonsen et al. (13).

Radioactivity ABeasurements-Counts were made with a thin mica window Geiger tube and a Nuclear Instrument scaling unit model No. 163. The mixed Ca4j and P32 were determined in the same sample by the use of an aluminum absorber according to the method of Comar et al. (14). Samples of 0.100 gm. were equilibrated with a series of 500 ml. of 1 p.p.m. to 1 per cent F for 4 days at 20”; then they were filtered and aliquots counted.

Electrolytic Conductivity Apparatus-The Wheatstone bridge was made up of a Campbell-Shackelton shielded ratio box, Leeds and Northrup No. 1553, and a six dial shielded alternating current resistance box from 0.01 to 11,111.l ohms, No. 4764. The 500, 1000, and 2000 cycle oscillator and amplifier were standard Leeds and Northrup instruments, Nos. 9842 and 9847. A 5 inch DuMont type 274-A oscillograph served as null-point indicator. Cell capacity was compensated by an adjustable air capacitor, No. 4914. Leeds and Northrup conductivity cells were employed and platinized according to the directions of Jones and Bollinger (15). During measurement, cells were immersed in a constant temperature bath held to f0.01”. Cells were standardized with KC1 solutions by using the specific conductance values and directions given by Jones and Bollinger (16).

The conductivity of solutions containing 1 to 1000 p.p.m. of fluoride ion

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250 HYDROXYAPATITE-FLUORIDE ION REACTION

as NaF mixed with hydroxyapatite Sample II and equilibrated for 4 to 8 days at 37” was measured.

TABLE II

Treatment of Hydroxyapatites with Concentrated Fluoride Solutions

2.5 gm. of hydroxyapatite treated at 37” with 100 ml. of the indicated sodium fluoride solution.

ApatiE;ample

I (1.593)*

K (1.628) L (1.645)

Q (1.663)

II (1.665)

11 WWt

II (NH,F)t

T Original solution

F

per cent

0.09 0.95 0.09 0.095 0.95 0.095 0.19 0.475 0.950 0.095 0.19 0.95 1.90 0.095 9.5 0.095

Solid

53.56 42.45 0.10 1.598 1.593 41 53.60 41.89 0.52 1.620 1.596 250 53.70 41.48 0.21 1.639 1.629 94 53.77 41.24 0.04 1.651 1.648 16 54.01 41.08 0.38 1.665 1.647 173 53.89 41.05 0.07 1.662 1.659 20 53.96 41.08 0.09 1.663 1.659 27 54.39 40.77 0.39 1.689 1.671 167 54.60 40.47 0.74 1.708 1.674 323 52.96 40.31 0.19 1.663 1.655 40 52.89 40.19 0.24 1.666 1.655 48 53.24 39.89 0.80 1.690 1.652 290 53.36 38.84 1.74 1.739 1.656 833 53.10 40.51 0.21 1.660 1.650 51 57.08 25.4917.4 2.835 1.56 5980 53.20 40.43 0.52 1.666 1.642 142

idjusted Ca:Pt

moles

P

Filtrate 1 Solid

Ca _

PM B

22 9

27 22 22 14 14 23 19 28 28 42

xx3 PM

123 131 750 690 282 270

48 46 519 500

60 92 81 118

501 510 969 970 120 250 144 320 8701 1,050

5 2,499 2,290 13 153 280

Low 17,940 22,900 9 426 680

* The ratio of the original apatite is given in parentheses for convenience in re- ferring to changes occurring through reaction with fluoride.

f KF and NH,F substituted for NaF used in previous samples. The KF permits a higher fluoride concentration due to much greater solubility. The increased reaction with the NH,F is due to its greater acidity.

$ Actual Ca:P ratio adjusted by subtracting the CaO equivalent of the F from the actual per cent CaO.

5 Actual moles of phosphorus X 3, as approximately 1 mole of P would be released for each 3 moles of F in a direct replacement reaction. This calculation allows a ready comparison with the amount of fluoride reacted.

Results

Treatment with Concentrated Fluoride Solutions at 9” (Table II)-The results of treating the various hydroxyapatite samples with fluoride con- centrations in and above the range used in topical application to teeth are shown in Table II. By comparing the analysis of the solid before and after treatment, together with the concentration of calcium and phospho-

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H. G. MCCANN 251

rus released into the liquid phase during reaction, it was possible to deter- mine in what manner the fluoride had reacted with the solid: by double decomposition to form CaFz or by exchange with the hydroxyl to form fluorapatite. If reaction with phosphate occurs somewhat according to the reaction

(A) Ca&POJc(OHz) + 20F- -+ 1OCaFz + 6P0,’ + 20H-

the ratio of Ca to P in the solid will be increased, since the calcium will remain in the solid as CaF2, while the phosphate will be removed into the solution. If this reaction occurs exclusively, the original ratio will be ob- tained by adjusting the experimental ratio for the calcium combined with fluoride. This is the case with Samples I, K, and L, indicating the formation of calcium fluoride, and very little or no fluorapatite.

If fluorapatite forms according to the reaction

(B) Calo(POa)6(OH)s + 2F- + Calo(P04)6F2 + 20H-

or hydroxyfluorapatite according to the reaction as suggested by Giesecke

CC) Calo(P04),(OH)~ + F- -+ CadPO&FOH + OH-

and Rathje (17), the Ca: P ratio of the solid remains unchanged, as the hy- droxyl group is the only portion of the molecule affected. If the ratio is adjusted to calcium equivalent to fluoride in this case, the ratio will be lower than that in the original apatite. Samples Q and II give results indicating considerable fluorapatite formation at fluoride concentrations up to 0.2 per cent, and probably some fluorapatite along with the CaFz at higher fluoride concentrations with Sample II.

The calcium and phosphate content of the liquid phase also indicates in most cases a similar type of reaction. Since the calcium content is low, it was assumed that if calcium fluoride were formed most of the phosphate in solution would be due to replacement by fluoride. According to Reaction A above, 20 fluoride ions replace 6 phosphate ions so that moles of P X 3.33 = moles of F. This gives results which are somewhat high since some of the phosphate is due to the apatite solubility. However, if moles of P are multiplied by 3, results are obtained which indicate somewhat quantita- tively the amount of phosphate replaced, although, when the amount of fluoride which reacted is low, a greater proportion of the phosphate is due to apatite solubility.

Treatment with Concentrated Fluoride Solutions at 100’ (Table III)-By a similar calculation it is seen that Sample I has reacted mostly according to Reaction A as before, but Sample L has reacted t.o form fluorapatite ac- cording to Reaction B, while Sample K, although forming mostly CaF2, has reacted partially as in Reaction B.

Dilute and Intermediate Values of Fluoride Concentrations at 37” (Tables

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252 HYDROXYAPATITE-FLUORIDE ION REACTION

I‘Ii and 8)-The results of treating the hydroxyapatite samples with the fluoride concentrations found in natural waters and with intermediate val- ues up to those used in topical application to teeth are shown in Table IV. The first four values of the Ca: P ratio in Table IV are somewhat lower than the original hydroxyapatite due to reaction with the large volume of water. As the fluoride concentration goes up, this effect is overcome. However, since the formation of calcium fluoride would increase this ratio considerably, the assumption that fluorapatite has formed is believed to be valid. Up through 100 p.p.m. of fluoride, only fluorapatite is formed; above this, considerable amounts of CaF, are formed. Although the fluo-

TABLE III

Treatment with Concentrated Fluoride Solutions

2.5 gm. of hydroxyapatite treated at 100” with 100 ml. of the indicated sodium fluoride concentration.

I (1.593)*

K (1.628)

L (1.645) Q (1.663)

Original solution

F

9er cent

0.09 0.18 0.91 0.09 0.18 0.18 0.095 0.95 I

cao PtOs

per cent per cent 53.27 42.07 53.22 42.02 53.57 41.15 53.20 41.06 53.33 41.14 53.58 41.33 53.90 41.03 52.89 38.24

t

Solid

F Ca:P ratio

djusted Ca:PI

w cent moles moles

0.17 1.603 1.594 0.18 1.604 1.595 1.29 1.648 1.589 0.31 1.641 1.626 0.39 1.641 1.623 0.19 1.641 1.632 0.16 1.663 1.656 1.81 1.751 1.662

_ I

Filtrate

PI F

ILM LM x 3 P-V

11 165 223 10 162 236 20 1650 1690 11 340 408 16 340 510 11 147 250

8 99 210 13 2220 2380

Solid

See foot-notes in Table II.

ride concentration in the 1000 p.p.m. solution is about the same as that in the 0.095 per cent solution in Table II, where mostly fluorapatite was formed, the formation of CaFz in this case (Table IV) may possibly be ac- counted for by the loo-fold difference in the ratio of solid to fluoride ion.

At this dilution considerably more apatite dissolves compared to the amount of fluoride which reacted, so that the phosphate is not considered in excess unless the concentration is greater than the calcium concentration. Thus the liquid phase analyses indicate the occurrence of the same reactions as those indicated by the solid Ca:P ratios. In the first half of Table V are given liquid phase values of some of the apatites in which insufficient material was available for complete solid analyses. Apparently at this concentration fluorapatite has formed in all cases.

Dilute and Intermediate Values of Fluoride Concentrations at 100” (Tables

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H. G. MCCANN 253

TABLE IV

Dilute and Intermediate Values of Fluoride Concentrations

2.5 gm. of hydroxyapatite treated at 37” with 10 liters of the indicated sodium fluoride concentration.

Original rolution Solid Liquid’ jolid*

F

IIM

7 26 29 42 42 49 41 87

174 320

Apatiti;ample - P Ca

PM PM

29 30 27 55 16 28 16 22 26 36 22 30 35 39 48 8

123 3 145 2

I I

Sxcess P

wf

0 0 0

0 0 0

0 40

120 140

Adjusted Ca:Pt Ca:P

moles

1.655 1.656 1.655 1.656 1.663 1.657 1.662 1.676 1.702 1.763

_ -

- cao

fier cent

53.84 52.97 53.08 53.13 53.26 53.11 53.15 53.27 53.40 53.70

PaOs

per cent I 41.18 40.50 40.60 40.61 40.52 40.58 40.49 40.24 39.71 38.56 I -

F

,er CL%

0.05 0.20 0.22 0.32 0.32 0.37 0.31 0.66 1.32 2.43

i! F

y *er ml.

2 2 5

10 20 50

100 200 500

t .-

moles

1.653 1.646 1.646 1.642 1.650 1.639, 1.648 1.645 1.640 1.645

Q (1.663) II (1.665)

1000 -

* Concentrations are expressed in micromoles per liter in the liquid and micro- moles per 0.25 gm. of the solid, so that direct comparison may be made.

t As in Table II. f At this dilution, 100 times that of previous samples, the solubility of the apatite

in relation to thg amount of fluoride which reacted is considerably higher and phos- phate is considered to be in excess only if its concentration exceeds that of the calcium.

TABLE V

Dilute Values of Fluoride Concentrations

0.10 gm. of hydroxyapatite treated with 400 ml. of solution containing 5 p.p.m. of fluoride at the indicated temperature.

Solid Liquid* Solid* Apatite sample No. Temperature I-

“C.

37

100

F P Ca -~

per cent PM !JM

0.06 32 42 0.20 29 29 0.01 29 37 0.28 29 24 0.20 103 55 0.2 30 8 0.1 10 22 0.17 10 18

Excess P$ F

PM PM

0 8 0 26 0 1 0 37

40 26 22 26

0 13 0 43

I K L II I K

k

* See foot-notes in Table IV.

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254 HYDROXYAPATITE-FLUORIDE ION REACTION

V and VI)-Up to 20 p.p.m. of fluoride, apatite Sample II forms only fluorapatite, but, starting at about 100 p.p.m., ever increasing amounts of CaFz are formed. At low fluoride levels apatite Samples L and Q also form fluorapatite, but apatite Samples I and K release excess phosphate. Since CaFz cannot form at this fluoride concentration, it is possible that adsorbed phosphate is being released at this temperature. However, the probability is that these samples are not stable at this ‘temperature and dilution and are becoming more basic in the boiling water, thus releasing phosphate while at the same time forming some fluorapatite.

TABLE VI

Dilute and Intermediate Values of Fluoride Concentrations 2.5 gm. of hydroxyapatite treated at 100” with 10 liters of the indicated sodium

fluoride concentration.

Original solution

Apatit;;mple

F

____ y &?r ml.

I (1.593) 10 II (1.665) 10

20 100 200 500

1000

T

cao

fit?7 cm 52.30 52.20 53.21 51.19 53.02 52.41 53.11

-

-- t

-

PlOl

per cent 41.51 40.00 40.48 38.93 40.10 39.69 38.40

T

Solid

F Ca:P ratio

idjustec i ca:pt

.I- per CL% t moles ?noles

0.12 1.609 1.599 0.33 1.653 1.637 0.38 1.663 1.647 0.58 1.672 1.643 0.47 1.674 1.652 0.67 1.672 1.640

2.18 1.751 1.641 d-

T

Liquid*

P

~-__ PM PM PM

Lost 27 0 1629 0 20 25 0 84 . 7 77

129 4 125 145 2 140

1340 6 1330 ,

Solid’

F

PM

16 43 50 76 62 88

287

* See foot-notes in Table IV.

Radioactive Hydroxyapatite-This material with a calcium to phosphorus ratio of 1.641 is similar in this respect to apatite Sample L with a ratio of 1.645. The reactions with this material took place at a somewhat lower temperature (20’) and for a considerably shorter time than with other samples, so that the attainment of equilibrium is less likely. As shown in Fig. 1, there is a drop in both the calcium and phosphorus concentrations at first, showing the decreased solubility of fluorapatite as compared with hydroxyapatite. There is no indication of reabtion to form CaFz below 100 p.p.m., but at 1000 p.p.m. this type of reaction was extensive as indi- cated by the higher P32 concentration in the filtrate. Apatite Sample L showed a similar reaction at this low concentration at 37”. It is interest- ing to nobe that there is no sudden change in the type of reaction as the solubility product concentration (about 25 p.p.m. of F, based on the solu bility of the apatite in water) is passed, but that a change occurs gradually as a level considerably above this value is approached.

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H. G. MCCANN 255

Electrolytic Conductivity-It is interesting to note again that there is no indication of any change in the type of reaction as the solubility product concentration is passed. A change in the type of reaction at this point would probably change the slope of the curve somewhat, due to a difference in the ions released. However, a perfectly straight line was obtained up to 50 p.p.m., as shown in Fig. 2.

E$ect of Bufering Solutions (Table VII)-The lower the pH, the greater the solubility of the hydroxyapatite and hence the greater the amount of CaFz formed from the dissolved calcium. After the acidity has been neu-

3OOL y P in filtrate,

40 60 80 IOC FLUORIDE ION IN

ORIGINAL SOLUTION-y/ml. FIG. 1

65 Oo IO 20 30 40 50

FLUORIDE ION IN ORIGINAL SOLUTION-y/ml.

FIG. 2 FIG. 1. The effect of F ion on radioactive hydroxyapatite, showing the concentra-

tion of Ca and P in the filtrates from 0.1 gm. samples of apatite treated at 20” with 500 ml. of the indicated F solutions.

FIG. 2. Electrolytic conductivity of solutions of apatite Sample II and F ion.

tralized by dissolving t.he hydroxyapatite, apparently considerable fluor- apatite also forms.

DISCUSSION

These experiments show that the most important variables in the reac- tion between fluoride ion and hydroxyapatite are the fluoride concentration and the Ca: P mole ratio of the hydroxyapatite, with the temperature playing a less important rBle. At 37” at a concentration of 5 p.p.m. of fluoride, some fluorapatite is formed with all the samples of hydroxyapatite tried. With 100 per cent hydroxyapatite, fluorapatite is formed almost exclusively up to 0.1 to 0.2 per cent fluoride when small volumes of solution are used (2.5 gm. per 100 ml. of solution), and up to 200 p.p.m. with large volumes of solution (2.5 gm. per 10,000 ml. of solution) by exchange of fluoride with hvdroxvl. At concentrations of around 0.1 or 0.2 wer cent

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256 HYDROXYAPATITE-FLUORIDE ION REACTION

fluoride at 37”, little or no fluorapatite is formed until the ratio of calcium to phosphorus is over three-fourths of the way between t;icalcium phos- phate (1.500) and hydroxyapatite (1.667). At 100” the formation of fluor- apatite starts just a little over half way between the two, as indicated in Fig. 3.

The formation of fluorapatite from hydroxyapatite (as contained in fer- tilizer or bone) at fluoride levels in which the solubility product has not been exceeded has been shown by several workers. MacIntire et al. (18)

TABLE VII

Effect of Buffering Solutions

2.5 gm. of hydroxyapatite Sample II in buffer at indicated pH; 0.09 per cent F at. 37”.

Glycine buffer I- Solid Filtrate Solid

Pa06 Ca:P ratio

- A

- P CC% PO

ger Ge%L per cent her con nzoles moles PJf w WX: / 52.99 39.56 1.36 1.696 1.631 342 12 1026 52.85 39.90 0.71 1.677 1.644 197 6 591 53.19 40.33 0.39 1.670 1.651 84 12 252 53.20 40.39 0.36 1.667 1.651 81 12 243 53.04 40.47 0.25 1.659 1.648 65 12 195

Original pH I’ ?inal pH F

I

2.95 3.93 5.97 7.97 8.95

7.40 7.38 6.90 6.99 8.54

PM

1790 930 510 470 330

5.07 6.33 53.44 40.58 1.16 1.667 1.614 1530 6.10 6.57 53.20 40.88 0.79 1.647 1.611 1040 6.98 7.23 53.18 40.99 0.61 1.643 1.615 800 8.00 8.33 53.04 40.80 0.27 1.646 1.633 360

See foot-notes in Table II.

found this to be the cause of the insolubility developed by certain phos- phatic fertilizer mixtures on storage. Neuman et al. (19) demonstrated that fluoride replaces either hydroxyl or bicarbonate ions in the surfaces of mineral phase of bone. Megirian (20) confirmed these results with glycol- ashed bone.

All these workers used very low fluoride concentrations and the evidence for fluorapatite formation at higher concentrations has been very sparse. Reference has often been made to the importance of keeping the fluoride concentration at a level in which the solubility product is not exceeded (19, 20). Syrrist (al), in a histological study of teeth exposed to a strong NaF solution for 20 minutes in v’ivo, suggested the presence of fluorapatite rather than calcium fluoride after extraction 1 week later. Variability of

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H. G. MCCANN 257

reaction with fluoride with varying Ca: P ratio has been indicated. by MacIntire et al. (22), who noted that a number of commercially prepared calcium phosphates reacted in varying degree with CaFz to form fluorapa- tite, pure hydroxyapatite being the most reactive.

The reaction of fluoride concentrations above 0.2 per cent has been shown in the above experiments to form calcium fluoride in large amounts with small amounts of fluorapatite also forming in certain cases with 100 per cent hydroxyapatite. This formation of calcium fluoride has been

IOO-

80-

60- \

iZ 1.500 1.580 1.600 1.620 1.640 l.66d

Co: P RATIO IN ORIGINAL APATITE

FIG. 3. Effect of Ca:P ratio of apatite on the type of reaction with 0.1 to 0.2 per cent F. The ratio at Point 1 is that of tricalcium phosphate, at Point 3 that of hy- droxyapatite, and Point 2 is midway between, a point at which the empirical compo- sition 1s Ca9.s(P04)6(OH)1. The points on the 100” line represent reactions with apatite Samples I, K, and Q, and on the 37” line with apatite Samples L and Q or II, as estimated from the data in Tables II and III. Ca:P ratios have been adjusted for Cl and CO2 content.

shown by several workers to be due to the double decomposition of calcium phosphate by sodium fluoride to form calcium fluoride (23-26).

This reaction appears to take place on the surface of the crystals. Un- less the fluoride concentration is very high, forcing a considerable break- down of the hydroxyapatite structure, the amount of reaction as indicated by the fluoride content of the solid changes but slightly with fluoride con- centration in the solution. Thus the per cent of fluoride in apatite Sample II ranges from 0.2 per cent to 0.3 per cent as the fluoride concentration increases from 2 to 100 p.p.m. There is more of a change in the amount of fluoride picked up by apatites with similar Ca:P ratios but different surface areas than there is among apatites with widely different Ca: P ratios and similar surface areas. However, t’he type of reaction which takes place is dependent to a great degree on the hydroxy concentration on the surfaces.

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258 HYDROXYAPATITE-FLUORIDE ION REACTION

SUMMARY

The reactions between fluoride ion and hydroxyapatite were investigated over a fluoride range of 1 p.p.m. to 9.5 per cent, and over a calcium to phosphorus ratio in the apatites of 1.593 to 1.665, with results indicating that the type of reaction is chiefly dependent on these two variables. Fluorapatite is formed at all ratios of Ca: P at a few parts per million of fluoride, calcium fluoride or fluorapatite is formed up to 0.2 per cent fluo- ride, depending on the Ca:P ratio, and calcium fluoride is formed at all ratios at high fluoride concentrations.

We are indebted to Dr. F. J. McClure, Chief of the Division of Oral and Biological Chemistry, National Institute of Dental Research, who sug- gested the problem and contributed many helpful ideas, to Dr. R. C. Likins, who suggested and assisted in the preparation of the radioactive hydroxy- apatite and did all the counting of the samples, and to Mr. F. A. Bullock,

who performed a portion of the analytical work.

BIBLIOGRAPHY

1. Armstrong, W. D., in Reifenstein, E. C., Jr., Metabolic interrelations, Trans- actions of the 2nd Conference, Josiah Macy, Jr., Foundation, New York, 11 (1950).

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