NATIONAL BUREAU OF STANDARDS REPORT

10 155

Progress Report

PREPARATION OF HYDROXYAPATITE BY

HYDROLYSIS OF a - TRICALCIUM PHOSPHATE

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards ' was established by an act of Congress March 3. 1901 . Today,

in addition to serving as the Nation’s central measurement laboratory, the Bureau is a principal

focal point in the Federal Government for assuring maximum application of the physical and

engineering sciences to the advancement of technology in industry and commerce. To this end

the Bureau conducts research and provides central national services in four broad program

areas. These are: (1) basic measurements and standards, (2) materials measurements and

standards, (3) technological measurements and standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials Research, the

Institute for Applied Technology, the Center for Radiation Research, the Center for Computer

Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and

uniform physical measurements throughout the Nation’s scientific community, industry, and commerce. The Institute consists of an Office of Measurement Services and the following technical

divisions:Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molecular Physics—Radio Physics -—Radio Engineering -—Time and Frequency -—Astrophysics -—Cryogenics.

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to im- proved methods of measurement standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; develops,

produces, and distributes standard reference materials; relates the physical and chemical properties of materials to their behavior and their interaction with their environments; and provides

advisory and research services to other Government agencies. The Institute consists of an Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations in the development of technological standards, and test methodologies; and provides advisory and research services for Federal, state, and local government agencies. The Institute consists of the following technical divisions and

offices: Engineering Standards—Weights and Measures — Invention and Innovation — Vehicle Systems Research—Product Evaluation —Building Research—Instrument Shops—Measurement Engineering—Electronic Technology—Technical Analysis.

THE CENTER FOR RADIATION RESEARCH engages in research, measurement, and application of radiation to the solution of Bureau mission problems and the problems of other agencies and institutions. The Center consists of the following divisions: Reactor Radiation—Linac Radiation—Nuclear Radiation—Applied Radiation. THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and

provides technical services designed to aid Government agencies in the selection, acquisition, and effective use of automatic data processing equipment; and serves as the principal focus

for the development of Federal standards for automatic data processing equipment, techniques,

and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measure- ment System, and provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following

organizational units:

Office of Standard Reference Data—Clearinghouse for Federal Scientific and Technical Information —Office of Technical Information and Publications—Library—Office of Public Information—Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg. Maryland, unless otherwise noted: mailing address Washington, DC. 20234.

- Located at Boulder. Colorado 80302.

:i Located at 5285 Port Royal Road, Springfield. VirRinia 22161.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

31 1.05- 1 1-31 1056 1 December 31, 1969

NBS REPORT

10 155

Progress Report

on PREPARATION OF HYDROXYAPATITE BY

HYDROLYSIS OF « - TRICALCIUM PHOSPHATE

by

Elmer T. Carlson* and Edgard C. Moreno**

* Research Associate from the American Dental Association at the

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

** Research Associate from the American Dental Association at the

Dental Research Section, National Bureau of Standards, Washington,

D. C., currently at the Forsyth Dental Center.

This investigation was supported in part by research grant DE02659

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 U. S. Army

Medical Research and Development Command, the Dental Sciences Division

of the School of Aerospace Medicine, USAF; the National Institute of Dental Research; and the Veterans Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government. I and review. For this reason, the whole or in part, is not authori; Bureau of Standards, Washingtoi

the Report has been specifically |

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

; accounting documents intended

ubjected to additional evaluation

listing of this Report, either in

Office of the Director, National

the Government agency for which

pies for its own use. <NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Preparation of Hydroxyapatite by

Hydrolysis of a - Tricalcium Phosphate

Elmer T. Carlson* and Edgard C. Moreno**

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

This investigation was supported in part by research

grant DE02659 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 United States Army Medical Research

and Development Command; the Dental Sciences Division

of the School of Aerospace Medicine, USAF; the

National Institute of Dental Research; and the Veterans

Administration.

* Research Associate from the American Dental Association

at the National Bureau of Standards

** Research Associate from the American Dental Association

at the National Bureau of Standards, currently at

Forsyth Dental Center.

1 Introduction

Hydroxyapatite (HA), Ca 5 0H(P04 ) 3 , prepared in the

laboratory by precipitation methods, is always an extremely

fine-grained material, even after prolonged boiling. There

have been numerous attempts at synthesis of HA crystals

sufficiently large to be clearly resolved by the optical

microscope, and to permit their crystal structure to be

studied by single-crystal methods. W. Rathje, in 1941,

1

published a report of an "acidimetric precipitation" process

whereby he prepared microscopically visible crystals of

hydroxyapatite. Solutions of calcium nitrate and monobasic

potassium phosphate were mixed dropwise in the correct

stoichiometric ratio for hydroxyapatite, and the solution

was kept neutral by simultaneous addition of sodium hydroxide

solution. By working with very dilute solutions near the

boiling point and adding the reagents at the rate of 5 drops

per day, he succeeded in obtaining polycrystalline aggregates with

individual crystals apparently 30 pm or more in length.

In the production of quantities of HA adequate for

various experimental studies it has been necessary to sacrifice

crystal size to increase the yield and to keep within a
- 2 -

reasonable time schedule. For example, the preparation used

2 by Moreno, Gregory and Brown in their solubility studies

was reported to contain "a minor fraction of thin needles

up to 10pm in length, " the rest consisting of smaller,

irregular equant grains.

Hydrothermal methods have been more successful in the

production of larger crystals of hydroxyapatite. The presence

of sodium hydroxide appears to promote crystal growth. Thus

3 Hayek and co-workers prepared crystals up to 2 mm in length

by hydrothermal treatment of a previously prepared finegrained specimen of hydroxyapatite, in the presence of NaOH.

It should be noted, however, that approximately one atompercent of sodium was found by analysis to be present in the

crystals. More recently, Mosebach^ reported production of

crystals up to 4 mm long by hydrothermal treatment of a

mixture of KH2 P04 , Ca(N03 ) ? and NaOH for 10 days at 500 °C

and 360 atm. No analysis was reported.

5 Perloff and Posner synthesized crystals of hydroxyapatite

up to 0.3 mm in length by hydrolysis of dicalcium phosphate

under hydrothermal conditions, thus avoiding the presence

of extraneous ions. With the equipment available to them.
-3-

the quantity obtainable in one batch was limited to about 0.1 g.

It is clear that a limiting factor here is the increase in

acidity of the solution as the acid phosphate dissolves and

the basic hydroxyapatite is precipitated. Recent attempts

in this laboratory to repeat the synthesis by the method of

Perloff and Posner produced crystals only slightly longer

than 0.1 mm. Recrystallized dicalcium phosphate was also

present

The fact that the methods used for preparing hydroxyapatite crystals of desired size and purity for single-crystal

work have yielded only minute quantities points to the need

for additional crystallization effort.

Although the processes of nucleation and growth of

hydroxyapatite crystals are not understood, it seems evident

that, at the boiling temperature and below, the rate of

growth on the surface of crystals already present is slow,

relative to the rate of formation of new crystals. Attention

has, therefore, been given to methods of controlling the

degree of supersaturation, in the expectation that a very low

degree of supersaturation might inhibit nucleation while still

permitting growth on crystal faces. One obvious method of
-4-

increasing the concentration slowly is to start with a solid

substance which dissolves slowly, yielding calcium and phosphate ions, and preferably no others aside from H

+ or OH"

.

Hydroxyapatite itself, in poorly crystalline form, would appear

to be a logical starting material, but experience has shown

that only a slight degree of crystal growth is achieved even

after prolonged digestion at the boiling temperature.

Dicalcium phosphate is another possible source of Ca+

+

and PO4 ions, and many attempts have been made to prepare

hydroxyapatite crystals by hydrolysis of this compound or its

hydrate, with appropriate control of pH. However, it dissolves rather rapidly, thus affording little advantage over

precipitation methods

.

Tricalcium phosphate (TCP) is much less readily soluble

than CaHP04 , and for this reason it seemed that it might serve

as a suitable source of Ca++ and P04 ions. Tricalcium phosphate occurs in three crystalline forms. The most common form,

P-Ca3 (P04 ) 2 or whitlockite, is stable up to about 1100°C. At

higher temperatures the °cform is stable, but it can also exist

at room temperature if it has been cooled rapidly enough.

A third modification apparently exists only above 1430 °C^, and

hence need not be of concern here.
-5-

The scientific literature relative to TCP is contradictory in some respects. For example, temperatures reported

by different authors for the cc-g inversion temperature span

several hundred degrees. Nurse, Welch, and Gutt6 placed

this inversion point at 1125 °C on the basis of heating

curves, but stated that the curve dropped steeply in the

presence of pyrophosphate. The latter phase, of course,

would be expected only if the composition departed from TCP

stoichiometry in the acid direction. In a later paper , Welch and Gutt did not mention the lowering of the inversion

temperature in this range; on the other hand, they reported

finding 3-TCP in samples quenched from above 1125 °C. It

appears that the inversion may be sluggish in both directions,

which would explain, at least in part, the uncertainty regarding the inversion temperature. From a practical standpoint,

whatever the reason, the preparation of «-TCP free from the

|3-phase is difficult.

There appears to have been no systematic study of the

effects of heat treatment and stoichiometry on the properties

of the product, including solubility.
- 6-

2. Experimental Procedures

2.1 Preparation of Tricalcium Phosphate

Several lots of (3-TCP were on hand from an earlier study.

Three of these (designated lots 1, 2 and 3) were reheated at

a higher temperature to prepare the cc-TCP needed for the

present work. Additional batches of «-TCP (lots 4 , 5 and 6 ) were later prepared directly from the raw materials. Because

of differences in behavior among the various preparations, it

is appropriate to describe the methods in some detail.

With one exception, all the lots of 3~TCP were made

from mixtures of dicalcium phosphate (CaHP04 ) and calcium

carbonate. The exception was made by adding calcium carbonate

to a solution of monocalcium phosphate (Catij (P04 ) 2 H2 0) in

water. After filtration, drying, and heating, it was treated

like the others. This preparation was not used in making

the oc-phase; however, it will be referred to later.

The CaHP04 and CaC03 were mixed in various ways, but

all involved suspension in alcohol. The methods included

wet sieving, grinding a thick suspension in a mortar, shaking

in a flask with Teflon beads, and stirring in a Waring Blendor.

The last two methods in particular might have been expected

to effect very thorough mixing, yet evidence of non-uniformity
-7-

was found in all cases. It is assumed that segregation occurred

during filtration and drying. The dried mixture was then

heated for a day or longer, usually between 950 and 1100°C.

The cooled product was examined microscopically and analyzed.

In all cases some adjustment of composition was needed, and

the process was repeated after appropriate addition of phosphate

or CaC03 , or, in some cases, Ca (OH) 2 in solution. Even after

the composition was satisfactory and after reheating for

several days, inhomogeneities were noted microscopically. The

batches were therefore heated to 1150-1200°C, converting the

TCP to the oc-form, and then reconverted to the (3-form by

prolonged heating at 800-950 °C.

Microscopically, the (3-TCP preparations consisted of

aggregates of rounded grains ranging from 7 to 50pm in diameter.

Except for some variation in average particle size, the different

lots (including that prepared from monocalcium phosphate)

appeared identical. The Ca/P ratio (molar) was 1.50, within

the limits of analytical error, and the percentage of phosphorus

present as pyrophosphate was 0.30 or less, which is within the

uncertainty of the analytical method. X-ray powder patterns

showed 3-TCP, with a few barely detectable peaks that could

be attributed to «-TCP.
- 8 -

As mentioned above, o=-TCP was prepared from the 3-form

by heat treatment at 1150-1200 °C. Microscopic examination

revealed the presence of some (3-TCP in all such preparations,

ranging from less than 1 % to as much as 10%. It was not

always clear whether this was residual or whether it had

formed during cooling, but in many cases the particle size

and the distribution indicated the latter case. For unexplained

reasons, the proportion of the 3-phase was often much higher

in the layer in contact with the platinum container than in

the bulk of the sintered mass. Efforts to reduce the amount

of 3-TCP by speeding up the cooling were not consistently

successful

.

For the direct preparation of °=-TCP the method of mixing

the raw materials in alcohol was abandoned in favor of dry

mixing. The procedure ultimately adopted as standard was

to mix the dry ingredients briefly by shaking, then to sieve

3 times through a No. 200 (74 pm) stainless steel sieve, and

to heat the well compacted batch at 1200 °C for 24 hours or

longer. After cooling in air, the material was ground to

pass the No. 200 sieve, examined, analyzed, and given such

further heat treatment as was desired. Some 3-TCP was

observed in these preparations also.
-9-

Further details as to heating and cooling treatment

will be given below, whenever pertinent.

2.2 Hydrolysis Procedures

The general procedure followed was very simple. A

weighed quantity of the pulverized Ca 3 (P04 ) g was added to

a measured quantity of distilled water, (usually 100 or

200ml) which had been previously boiled to expel C02 , in a

300-ml Erlenmeyer flask. A few drops of bromthymol blue

were added to serve as an approximate measure of pH. The

flask was fitted with a rubber stopper through which a glass

tube about 20 inches long extended, serving as an air condenser.

The contents were heated quickly to boiling and the flask

placed on a hotplate, where boiling was continued as long as

warranted by the results. Progress of the reaction was checked

occasionally by microscopic examination of a drop of the suspension. Minor modifications of the procedure were introduced from time to time; these are indicated in the discussion

of the results below.

Following a long series of experiments in flasks, operations were continued on a larger scale. The vessel was a

stainless steel kettle. Teflon-lined, with a capacity of

about 13 liters, fitted with a lid of the same material which
- 10-

could be clamped on. There were several holes in the lid,

fitted with Teflon bushings drilled to accept standard taper

glass joints. Contents were stirred by a motor-driven propeller, 9 inches from tip to tip, moving at about 160 r.p.m.

The propeller and shaft were gold plated. A glass condenser

was fitted on top, and a slow current of nitrogen was bubbled

through the suspension. The kettle was heated by a hotplate.

3 . Experimental Results

3.1 Preliminary

One-gram samples of cc_TCP and 3-TCP were placed in

bottles with 100 ml distilled water and kept for several

months. No hydrolysis was apparent. Similar samples were

placed in flasks with 200 ml water and boiled. The 3-TCP was

unaltered after 18 hours boiling, but the cc_TCP was appreciably

changed in a few hours . The greater part of the subsequent

experimental work has been devoted to «-TCP at boiling

temperature

.

3.2 Hydrolysis of « - Tricalcium Phosphate

The sequence of changes observed during the hydrolysis

of cc-TCP is the same, except for rate and degree of completion,

for most of the experiments described below, and will therefore

be discussed in some detail . When «_tcp (prepared as described

above) is added to hot water in the ratio of, for example,
- 11 -

0.2 g to 100 ml, the pH rises rapidly. If the solution

contains bromthymol blue, an indicator which changes from

yellow to blue over the pH range 6. 0-7. 6, the change is

observed as a deepening blue. The liquid remains clear, and

at first there is no change in the microscopic appearance of

the TCP. From these observations it is inferred that the

first reaction is a direct dissolution of TCP. After a few

minutes or hours, depending on the solid-liquid ratio and

other factors, the color slowly fades and eventually becomes

yellow. At the same time there is a tendency of the TCP

grains to agglomerate and to stick to the flask. Under the

microscope the TCP grains are seen to be more or less covered

with an overgrowth of rods, prisms, or needles radiating outward. This type of crystal growth is sometimes referred to

as"drusy". In the early stages of the reaction, certain

crystals or polycrystalline particles of TCP will be found

entirely coated with the new growth, while other grains will

be completely clean. In exceptional cases, particles have

been observed in an incipient stage of alteration, with the

new growth appearing as a well defined patch on an otherwise

smooth surface. In other cases, formation of the rod-shaped

crystals seems to occur more quickly at surface irregularities.
- 12 -

Under favorable conditions, the reaction will progress to

the interior of the particles until no TCP remains. However,

the reaction product in the interior apparently is unable to

develop well formed crystals, and appears as irregular fragments after the aggregates are crushed. In several instances

it was possible to examine fully hydrolyzed grains without

crushing and several of these showed fairly well defined

cores of material with refractive index lower than that of

the radiating HA crystals. The cores were isotropic, showing

that they were not simply unaltered °=-TCP.

Figure 1 is a photomicrograph of a few dispersed crystals

broken from the drusy aggregates. Several of the typical

acicular crystals ("needles") are visible, as well as some

of the smaller irregular fragments and uncrushed aggregates.

Typically, the needles are 30 pm or less in length and seldom

over 2 pm in diameter, too small to permit any prism faces

to be discerned. It is possible, by careful manipulation of

the cover glass, to rotate the crystals on the microscope

slide, whereby it can be seen that they are rod-shaped rather

than flat. Individual, slender needles up to 100 pm long
-13-

have been observed, and occasionally there are thicker

crystals, much shorter but as much as 5 pm in diameter. A

few of these have been observed microscopically, 'oriented

parallel to the microscope axis, and some of them appear

hexagonal in shape. The indices of refraction of the needles,

with unfiltered light, range from 1.630 to 1.632 and from

1.632 to 1.635 (±0.003) for the low and high indices,

respectively. These figures are appreciably lower than the

accepted values (1.640 and 1.647) for mineral hydroxyapatite,

but are close to the average index for typical precipitated

HA. Most of the irregularly shaped fragments show a somewhat

lower refractive index, difficult to determine precisely

because of the small particle size, but apparently close to

1.62. Minute particles with index appreciably below 1.62 are

also observed in some of the preparations.

XRD patterns have been obtained for several of the

hydrolysis products. In each case a strong pattern,

characteristic of HA, was obtained. In general, extraneous

peaks were either absent or so small as to be barely distinguishable from the background. In one pattern, however, two
-14-

peaks of appreciable size were observed, corresponding in

position to two of the strongest lines of 3-TCP. The stronger

was about 1/7 the height of the strongest HA peak, giving a

rough indication of the relative amounts present. It should

be noted here that the rest of the strong (3-TCP lines are in

positions where they are masked by nearly coincident HA lines.

The pattern of one of the other hydrolysis products showed

the same two 3-TCP lines, though in a considerably lower intensity. It is considered significant that both of these

preparations showed unusually high amounts of the phase having

an index of 1.62. On the basis of this evidence the phase is

tentatively identified as 3-TCP.

No other constituents have been even tentatively identified in the hydrolysis products, though there is indirect

evidence (presented later) that one or more of the acid phosphates may be present.

Chemical analyses of the reaction products have been

made only in relatively few cases, but the results are fairly

consistent in showing a Ca/P ratio well below the theoretical

value for HA (1.67). The average found is about 1.57. The

discrepancy can be explained, at least in part, if it is
-15-

assumed that other phosphates are formed along with HA,

possibly when the hydrolysis has progressed to the interior

of the TCP grains. This can perhaps be shown more clearly

by consideration of a hypothetical initial reaction

5Ca 3 (P04 ) e + 3H2 0 = 3Ca 5 0H(P04 ) 3 + H3 P0 4 . The formation of

Ha P04 would result in a gradual lowering of pH, in accordance

with observations

The increasing acidity might reach a point at which 3-TCP

or an acid phosphate could precipitate, with the result that

the overall composition of the residue would be more acid than

that of HA.

The results of the preliminary experiments were sufficiently

promising to warrant further study under more closely controlled

conditions. The effects of several variables were investigated,

and these will now be considered in some detail.

3.2.1 Variation Among Different Lots of =c-TCP

It will be useful first to consider the variability in

behavior among the several lots of cc-TCP used, and the

reproducibility of results with a given preparation. Since

the primary aim of the study was to produce large crystals of

HA, the size of the crystals and the rate at which they are
-16-

formed are the prime considerations. As an approximate

measure of the latter, the time required for the reaction to

proceed to the point where the pH had fallen to 6, as shown

by the indicator, was recorded. This, of course, is not a

point that can be determined with precision, but it is adequate

for comparative purposes. No attempt was made to determine the

average crystal size of the product; instead, a search was made

for the largest HA needle on the microscope slide, and this was

taken as a measure of relative particle size. This method also

is lacking in precision, but it has the advantage of being

rapid

.

In Table 1 are given the results of hydrolysis experiments

with several different preparations of «-TCP at the same solidliquid ratio (expressed as grams of solid per 100 ml of water)

and with other conditions held constant. Experiments 1-5A

and 1-5B are duplicates, done concurrently, to obtain an idea

of the reproducibility of results. The same is true of 1-6A

and 1-6B and 1-7A and 1-7B. There was a slight difference in

reaction rate between 1-7A and 1-7B, as measured by the time

required for the pH to drop to 6, whereas in the other two

pairs there was no observable difference in rate. Slight

differences in crystal size were noted for all three pairs of

duplicates

.
-17-

Among the different TCP preparations there is a fairly

large spread in the time required to reach pH 6, ranging from

1§ to 6-§ hours

In column 3 of Table 1, is given the "age" of the TCP

at the time of the hydrolysis experiment; that is, the elapsed

time since the final heat treatment of the preparation. The

data in Table 1 were selected for comparison because all the

TCP preparations included were several months old at the time

of hydrolysis. Evidence at hand indicated that aging might

affect the behavior of the TCP. Some additional experiments

were run as a check on this effect, and the data are presented

in Table 2. There appears to be a trend toward decreased

reactivity with aging, especially for periods beyond 4 months.

Because of the variability in behavior among the various

lots of cc-TCP it appeared desirable to make a brief study of

the effects of minor variations in procedure during preparation

of the TCP on its behavior in contact with water.

The three most obvious variables to consider are temperature, duration of heating, and rate of cooling. With the

furnaces readily available, the upper limit of temperature

was about 1230 °C while the lower limit was fixed at about

1125 °C because of formation of the |3-phase at lower temperatures.
-18-

Within this limited range, no consistent relation was found

between either temperature or duration of heating and the

manner in which the product reacted with water. (This

applies to all the ^-TCP preparations referred to herein

except lots 4b and 4c.)

Subsequently, the use of a high-temperature furnace in

another division of the National Bureau of Standards was

obtained. It was then possible to study the variation in

behavior of a preparation of ^-TCP that had been heated

over a wider range of temperature. The data are given in

Table 3. It may be seen that the TCP heated at 1200°

C

(lot 4a) is slightly more reactive than that heated at

1300°C and much more reactive than that heated at 1395°C.

The rate of cooling conceivably could control both the

amount of the 3-phase present and the amount of strain remaining in the crystals, either of which would be expected to

affect the rate of solution. However, it is difficult to

achieve very rapid cooling of quantities of the size needed

in these studies. When the container was removed from the
-19-

hot furnace and allowed to cool in air, the contents glowed

visibly for one to four minutes, depending on the quantity

of material. (Duration of glow is of some significance, since

it coincides roughly with the temperature range in which the

inversion proceeds most readily.) Setting the containers

in water had little effect. Some attempts were made to quench

the hot oc_TCP by immersing it or spraying it with water . The

products obtained after drying were very rapid reacting, and

the hydrolysis products greatly inferior. Further efforts

along this line were abandoned. Minor differences in rate of

cooling were effected by using smaller quantitites of TCP, and

by spreading it in a thin layer in a large dish prior to heating. The results of hydrolysis experiments on these variously

cooled lots of TCP were conflicting, so that no conclusions

as to the effect of cooling rate can be offered.

With the possible exception of Lot 4c, heated at 1395 °C,

all the cc_TCP preparations contained some 3-TCP, the amount

usually ranging up to 10 percent. Within this range, no

correlation was apparent between the amount of the 3-phase

and the reactivity. In some specially prepared batches with

much higher 3-TCP content, the hydrolysis was definitely

retarded
- 20-

3.2.2 Effect of Fineness

The fineness, or more correctly the surface area, of a

crystalline solid affects its rate of dissolution. In the

present case there are, strictly speaking, two effects to

consider: the size of the grains and the formation of

fresh broken surfaces by grinding. In practice, these

were inseparable. The «-TCP prepared at 1200°C or lower is

a soft sinter, readily crushed to polycrystalline grains

that will pass a No. 100 sieve. The material prepared at

1395 °C required vigorous grinding to reduce it to pass the

same sieve, and the resultant grains were usually fragments

of larger crystals.

Hydrolysis data are given in Table 4 for three lots of

oc-TCP, each separated into three fractions by dry sieving.

The particle size ranges given are the nominal openings of

the three standard sieves used. Nos. 100, 200, and 325. Lot

No. 3a, typical of the cc_TCP prepared by heating at 1200 °C,

was relatively unaffected by sieving, all three fractions

reacting at about the same rate. On the other hand. Lot 4c

(heated at 1395 °C) shows extreme differences between the

three fineness fractions. Lot 4b (1300°C) is intermediate
- 21 -

in behavior, the two coarse fractions being about equal in

reactivity while the finest fraction is appreciably quicker

in its reaction. The difference is clearly related to the

actual crystal size, which is not always the same as the

gross particle size. In the TCP prepared at 1200°C, the

larger grains consist of relatively loosely bound aggregates

of smaller crystals, most of which are smaller than 30 pm.

The aggregate grains are sometimes chainlike structures,

sometimes rather porous clusters. Crushing these to <44 pm

has relatively little effect on the surface area. In specimen 4b the actual crystals range up to 60 pm or more in

size, so that crushing to <44 pm results in an increase in

surface area. In the material prepared at 1395 °C the crystal

size was much larger and the porosity very low, so that even

coarse grinding resulted in formation of new surfaces.

Experiments with other lots of «-TCP prepared at 1200 °C

are consistent with those shown for Lot 3a in Table 4. In

general, the quality of the reaction product, from the standpoint of crystal size, was inferior for the finest size

fractions
- 22-

3.2.3 Solid - Liquid Ratio

In the foregoing we have considered variability introduced during preparation and subsequent treatment of the

oc-TCP. Variations in the hydrolysis procedure will now be

considered

.

The rate of solution of a solid in a given volume of

liquid is related to the surface area in contact with the

liquid which varies directly as the mass of the solid

(assuming uniform size distribution) . Thus the solid-liquid

ratio, expressed here as grams of solid per 100 ml of water,

was expected to control the rate of solution, and this presumably would affect the rate of the overall hydrolysis

process. This proved to be the case. Table 5 gives the

pertinent data for a series of hydrolysis experiments on a

single lot of «-TCP, designated Lot 3a. The time required

to reach pH6 increases as the solid-liquid ratio decreases.

The relative reaction rate (reciprocal of time) is, very

roughly, proportional to the solid-liquid ratio. Crystal

size in the product increases as the solid-liquid ratio

decreases. In the low-ratio suspensions, the hydrolysis

was still incomplete after the indicated periods. Those

with greater amounts of the solid present were essentially
-23acompletely hydrolyzed, but contained larger amounts of lowindex material in relation to the HA needles present.

The molar ratio, Ca/P in the hydrolysis product was

determined by chemical analysis for several of the experiments

in this series. There is a significant increase in this

ratio as the solid-liquid ratio is lowered, approaching the

theoretical value for HA (1.667). Along with this there is

a decrease in the relative amount of the contaminating phase

or phases ("low-index material") at the lower solid-liquid

ratios

For experiments 5-7 and 5-8 a larger flask was used, in

order to have enough solid material for examination and

analysis. The flask had standard-taper glass fittings, thus

avoiding the possible undersirable effect of using rubber

stoppers for the relatively long reaction period required.

Experiment 5-8 was an attempt to determine whether the solidliquid ratio could be reduced to a practical minimum without inhibiting the hydrolysis completely. This was shown to

be the case. The reaction proceeded in the normal fashion

but at a slower rate.
2 3bComparable data were obtained for the other lots of

oc-TCP , though over a less extended range. Within a given

batch, the same dependence of reaction rate on solid-liquid

ratio was observed.

The IR spectra for several samples having a Ca/P ratio

in the range 1.56 to 1.59 were consistent with the presence

of acid phosphate groups in the samples. Likewise, after

pyrolysis of these samples at around 460°C, the IR spectra

showed peaks corresponding to the presence of pyrophosphate

g groups, as could have been anticipated.

In further reference to Table 5, it was expected that

the Ca/P ratio found in experiment 5-8 might be higher than

in 5-6, but the results proved to be identical. The filtrate

from 5-8 was evaporated to dryness and the nonvolatile

residue was weighed. The weight of the residue was found

to be considerably greater than that of the TCP initially

present, and qualitative tests indicated the presence of

silica. It was concluded that the flask had been attacked

by the solution, which is not surprising when it is recalled

that the pH was on the alkaline side during much of the

14-day period of boiling. It is not known whether the solid

hydrolysis product also was contaminated with Si0 2 , but this
-23cis considered a possibility. The presence of Si0 2 would

interfere with the analytical determination of phosphorus,

resulting in a high value which in turn would make the

calculated Ca/P ratio too low.

It was decided to repeat one of the experiments in

Table 5, making use of the Teflon-lined steel vessel

described in Section 2.2. For practical reasons, the

solid-liquid ratio selected was that of experiment 5-6

rather than 5-8. The volume of water used was 4 liters.

After 4 days boiling, hydrolysis appeared to be complete,

and analysis indicated a Ca/P ratio of 1.67, the theoretical

value for HA. This is to be compared with the value 1.63

obtained for the same materials in the same ratio (but smaller

volume) in glass (experiment 5-6)
-24-

3.2.4 Effect of Temperature During Hydrolysis

The hydrolysis experiments described in the preceding

sections were carried out at boiling temperature. Table 6

gives some data on a series of similar experiments at lower

temperatures. The rate of the reaction falls off at lower

temperatures, as would be expected. The appearance of the

hydrolysis product also changes, though between 90 and 100 °C

there is no great difference. Below 90 °C the needles become

smaller and the amount of irregularly shaped grains proportionately greater. Even at 25 °C there was evidence of hydrolysis, though the reaction was incomplete during the period

indicated

.

3.2.5 Effect of Agitation

As mentioned earlier, one of the phenomena observed

during the hydrolysis is a tendency of the solid residue

to stick together and to the bottom of the flask. At first,

attempts were made to prevent this by frequent shaking of

the flask. Later a hotplate with magnetic stirrer was used.

It was soon observed, however, that the frequent or continuous

agitation usually slowed the hydrolysis reaction and invariably

resulted in smaller HA crystals . The reasons for this are not

clear, but for practical reasons the practice of agitating
25-

the suspension was abandoned. Even vigorous boiling was

found to be detrimental, better crystals being obtained

with gentle boiling or even two or three degrees below the

boiling point. It is possible that the formation of

larger crystals is related in some way to localized concentration gradients that may be disturbed by agitation

of the solution.

3.2.6 Effects of Addition of Ca (OH) 2

In some of the preliminary experiments, Ca(0H) 2 was

added to the reaction mixture to bring the stoichiometry

up to that of HA. This was found to inhibit the reaction

completely. The next step was to start with <=c-TCP in water

and add Ca(0H) 2 dropwise to the boiling mixture to keep

the pH at approximately 7. In this way, the conversion

of TCP was apparently complete, but there was an amount

of gelatinous or finely crystalline material, presumably

a precipitate of calcium phosphate of undetermined composition.

After it had been proved by chemical analysis that

the overall composition of the hydrolysis products was low

in calcium, in comparison to HA, further attempts were made

to correct the composition by adding Ca (OH) 2 to the solution.

Coarsely crystalline Ca(0H) 2 , prepared by slowly heating a
-26-

saturated solution to boiling, was added to the reaction

mixture, either at the start or in increments after the

reaction commenced. It was hoped that the rate of dissolution

of the Ca(OH) 2 would be slow enough to prevent precipitation

of an amorphous or fine-grained phosphate. When added at

the start, the Ca(0H) 2 dissolved too quickly, and inhibited

the hydrolysis reaction. When added during the hydrolysis,

it served to keep the solution neutral; nevertheless, the

hydrolysis product, on analysis, proved to have the Ca/P

ratio 1.57, approximately the same as the average found in

the absence of added Ca(OH) 2 for suspensions of the same

solid-liquid ratio.

3.2.7 Effects of other Alkalizing Agents

In one series of experiments, NH4 0H was added to the

suspension during boiling, to keep the solution neutral or

alkaline. Again the Ca/P ratio in the residue came out at

1.57.

In another series, urea was substituted for NH4 0H.

Urea is hydrolyzed more rapidly in acidic than in neutral

solution, and the overall effect is to maintain a moderate

degree of alkalinity. At concentrations up to about 0.25%

there was a small improvement in the Ca/P ratio, but this
-27-

effect was sharply reversed when the amount of urea was

doubled. The matter was not pursued further.

Sodium hydroxide was also tried on one occasion.

The product appeared unsatisfactory, and no analysis was

made

The addition of calcium carbonate, in amount sufficient

to over come the deficiency in calcium, inhibited the hydrolysis, though not completely. After prolonged boiling,

only a few needle crystals were observed.

Tetracalcium phosphate, added to ^-TCP in amount

sufficient to give the overall composition of HA, accelerated the hydrolysis, but the product was distinctly

inferior

3.2.8 Effects of the Presence of Other Substances

In a few of the earliest experiments the water was not

boiled to expel CO2 before use. There was no apparent

difference in the reaction or in the product after the

practice of boiling out the C02 was adopted.
-28-

In one experiment, calcium pyrophosphate was added in

the amount of 1.5% of the weight of the TCP used (this

being in the range of the amount that might be present as a

contaminant in the TCP) . The course of the reaction was

unaffected, and the pyrophosphate was observed microscopically in the product, apparently unaltered.

3.2.9 Production in Larger Quantity

Following the determination of the conditions favoring

good crystal formation on the basis of experience gained

in the work described above, two larger batches of HA were

prepared in the stainless steel kettle described in the section on Procedures. For the first, 25 grams of ^-TCP, Lot

2a, was ground to pass a No. 200 sieve, added to 5 liters of

water, and the mixture boiled 21 hours with constant stirring.

The product was essentially all HA, but the crystals were

disappointingly small—about 10 Pm and smaller. Analysis

indicated a Ca/P ratio of 1.56. The second batch used 29

grams of the same TCP in 9.9 liters of water. After 20 hours
-29-

boiling it was filtered off, dried in air at about 70°C

and examined. There was some residual TCP, so the product

was returned to the kettle with part of the solution and

boiling was resumed for 45 hours. After this treatment, the

TCP had been reduced to less than 0.1%. Maximum crystal

length observed was 35 pm. The Ca/P ratio was 1.60.

3.3 Attempts at Further Crystal Growth

When it became apparent that varying the experimental

conditions was unlikely to produce HA crystals larger than

60 pm, or exceptionally 100 pm, in length, a number of

attempts were made to enlarge the crystals already produced,

by keeping them in contact with calcium phosphate solutions

supersaturated with respect to HA. These were completely unsuccessful. At room temperature, nothing happened. At

45 °C, rosettes of octacalcium phosphate slowly formed. At

higher temperatures, there was a gelatinous precipitate.

In other experiments, the hydrolysis reaction was

permitted to proceed to varying stages of completion, then

the bulk of the residue was removed, leaving only a few

crystals to serve as seeds for further precipitation.

Subsequent evaporation to a small volume was also tried.

These experiments were equally unsuccessful.
-30-

3.4 Attempts at Removal of Acid Phosphates

It was pointed out above that the Ca/P ratio found

by analysis of the hydrolysis product is always lower than

the theoretical ratio for HA, and that the product is contaminated with some material, presumably one or more other

phosphates, having an average refractive index slightly

lower than that of HA. Efforts were made to remove this

material by extraction with water and with 10-3 M H3 P04 .

Two successive extractions of 1 . 5g of residue with 1 liter

of boiling water, dissolved only 0.1% of the residue.

Extraction with the acid removed over 11%, but the increase

in Ca/P ratio was so slight as to be within the limits of

precision of the analytical method.

An attempt was also made at increasing the Ca/P ratio

by heating the residue with Ca (OH) 2 solution. Again the

slight increase found was within the limits of analytical

precision

.

3.5 Reaction of other Phosphates with Water

3.5.1 3-TCP

As mentioned earlier, the initial experiments with

(3-TCP were almost completely negative, only a few needle

crystals being observed after many hours boiling.
-31-

Later the experiments were repeated with all of the

four lots of 3-TCP available, plus one freshly prepared.

Four of these yielded the same negative results, but one

reacted very much as did some of the less reactive lots of

oc— TCP. For example, at a solid-liquid ratio of 1 gram/100 ml,

the solid was largely but not completely converted to HA

needles and irregular grains after 5 hours boiling. The

only difference between this lot of 3-TCP and the others is

that it was prepared from monocalcium phosphate and calcium

carbonate rather than dicalcium phosphate and CaC03 . Subsequent heat treatment was essentially the same, and there

was no difference in analysis or in microscopic appearance.

3.5.2 Other Calcium Phosphates

A few experiments were run with calcium phosphate

preparations richer in calcium than TCP (up to HA composition)

They probably consisted of TCP and HA in different proportions

These reacted rather rapidly with boiling water, but yielded

only small needles and much irregular material.

Tetracalcium phosphate, prepared at 1500 °C, produced

an alkaline solution with boiling water, but apparently was

not hydrolyzed after several hours boiling.
-32-

4. Discussion

Preparation of hydroxyapatite by hydrolysis of

a-tricalcium phosphate in water at or near the boiling

point appears to be a promising method if moderate quantities of small but clearly defined crystals are desired.

It has the advantage of avoiding the presence of foreign

ions, and the process can be completed in a day, or in a

few days at most.

Unfortunately, the method also has certain limitations,

at least in the light of present information. Most serious

of these is contamination with other phases. Part of this

material has been identified as 3-TCP on the basis of XRD

and microscopic evidence. The presence of this phase in

the hydrolysis products might be assumed to result from the

|3-TCP present as a contaminant in the oc-TCP before hydrolysis.

This may indeed be the origin of part of it, but there is

some evidence that it is formed, at least in part, in the

hydrolysis product. The material observed microscopically

is often very finely divided and irregular in outline,

whereas the original 3-TCP grains are larger, nearly round,

and well defined. Furthermore, the amount revealed by XRD
-33

in the case referred to earlier appears to be greater than

that present in the cc_TCP , whereas most of the other hydrolysis products for which XRD patterns were obtained show no

p—TCP at all.

Acid phosphates have not been clearly identified in

the hydrolysis products by either XRD or microscopy, but

the formation of pyrophosphate on heating strongly suggests

that an acid phosphate is present. This could be either

octacalcium phosphate or dicalcium phosphate, or a mixture

of the two. So far, no practical means of eliminating these

contaminants has been found. Apparently they can be minimized,

however, by decreasing the solid-liquid ratio to the lowest

practical point. Under such conditions, the composition of

the acicular crystals is close to the theoretical value of

HA. Obviously, lowering the solid-liquid ratio has the

serious disadvantage of giving a low yield.

The use of glass vessels should also be avoided, because

the persistent alkalinity and the long reaction period

accompanying the use of a low solid-liquid ratio are conducive

to attack on the glass and possible contamination of the

product
-34-

There is need for further information on the crystallography of the product, but no crystals large enough for

single-crystal x-ray study have been obtained so far. The

relatively low refractive index of the crystals (averaging

about 1.633) is closer to that of the familiar precipitated

HA than to the natural or the hydrothermally prepared

materials. The x-ray powder pattern is typical of HA, but

this in itself is not a clear indication that the composition

is strictly stoichiometric. The presence of other phosphates

in small amounts would not necessarily be revealed by x-ray

powder patterns.

Much remains to be learned about the mechanism of the

precipitation of the HA crystals. Typically, the needles

or rods project radially from the surface of the TCP grains,

which generally are relatively much larger. Some nucleation

sites are evidently more active than others, because a

partially altered suspension of TCP will usually show some

TCP grains heavily coated with HA while others appear unaltered. As long as the HA crystals can develop freely into

the surrounding liquid they are well formed. As the reaction
-35-

progresses into the interior of a large TCP grain, the

development of HA presumably is impeded by the crust of

crystals already formed, with the result that poorly

formed or fragmented crystals are produced in the interior.

Restricted diffusion of ions in this area may lead to coprecipitation of 3 -TCP or acid phosphates.

In summary, it has been shown that the hydrolysis of

“-TCP at, or near, the boiling point will result in

acicular crystals of HA ranging up to 100 pm in length by

1-2 pm in diameter, but that the problem of preventing or

removing other coprecipitated phases, while still obtaining

a reasonable yield, has not been solved.

Acknowledgments. Several members of the National

Bureau of Standards staff and American Dental Association

research associates cooperated in various phases of this

study. Special thanks are due to H. E. Swanson for the

XRD patterns; to D. H. Blackburn for heating specimens at

high temperatures; to C. P. Mabie, Jr. for IR absorption

analyses and photomicrographs; and to B. M. Wallace and

J. M. Palcic Patel for chemical analyses.

USC OMM-N BS-D C

References

1. Rathje, W., Ber . d. deut. chem . Gesell

74, 342-349 (1941)

2. Moreno, E. C., Gregory, T. M. , and Brown, W. E.,

NBS Report 9631, Oct. 25, 1967.

3. Hayek, E., Bohler, W., Lechleitner, J., and

Petter, H., Z. anorg . Chem. 295 , 241-246 (1958).

4. Mosebach, R., Bull. Group. Int . Rech. Sc. Stomat.

9, 45-49 (1966) .

5. Perloff, A., and Posner, A. S., Science 124 , No. 3222, 583-584 (1956).

6. Nurse, R. W., Welch, J. H., and Gutt, W. ( J.

Chem. Soc. (London) 1959 , 1077-83.

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

1961 , 4442-4444.

Fowler, B. O., Moreno, E. C., and Brown, W. E.,

Arch, oral Biol. 1±, 477-492 (1966).

8 .

Table 1

Hydrolysis of a-TCP in boiling water;

variation among several lots of a-TCP

Experiment

No . TCP

Lot No.

Age of

TCP

SolidLiquid

Ratio

g/100 ml

Time taken

to reach

pH 6

hours

Max . crystal

length

um

1-1 1 c 7 mo 0.20 6£ 40

1-2 2 b 6 mo 0.20 4-§ 40

1-3 3 a 4 mo 0.20 2 35

1-4 3 b 4 mo 0.20 4 40

1-5A 4 a 6 mo 0.20 3 18

1-5B 4 a 6 mo 0.20 3 22

1-6A 4 b 6 mo 0.20 3 15

1-6B 4 b 6 mo 0.20 3 20

1-7A 5 4 mo 0.20 35

1-7B 5 4 mo 0.20 If 40
Table 2

Hydrolysis of a-TCP in boiling water

after aging for various periods

Experiment

No.

TCP

Lot No.

Age of

TCP

SolidLiquid

Ratio

g/100 ml

Time taken

to reach

pH 6

hours

Max.

crysta

length

pm

2-1 3 a 1 day 0.20 1| 30

2-2 3 a 4 mo 0.20 2 35

2-3 3 a 5 mo 0.20 3 30

2-4 3 a 6 mo 0.20 4 45

2-5 3 b 1 day 0.20 2 50

2-6 3 b 4 mo 0.20 4 40

2-7 4 a 1 day 0.20 if 25

2-8 4 a 6 mo 0.20 3 18

2-9 4 b 1 day 0.20 2 30

2-10 4 b 6 mo 0.20 3 20

2-11 5 1 day 0.15 If 30

2-12 5 4 mo 0.15 Ifc 35

2-13 6 1 day 0.15 2 25

2-14 6 4 mo 0.15 2- 20
Table 3

Hydrolysis of a-TCP in boiling water;

effect of temperature of preparation

of a-TCP on rate of hydrolysis

Experiment

No

TCP

Lot No

Max . Temp

TCP heated

°C

SolidLiquid

Ratio

g/100 ml

Time taken

to reach

pH 6

3-1 4 a 1200 0.20 1^ hr

3-2 4 b 1300 0.20 2 hr

3-3 4 c 1395 0.20 9 days*

*pH still above 7 when run was terminated.
Table 4

Hydrolysis of a-TCP in boiling water;

effect of fineness

er iment

No.

TCP

Lot No.

Max. Temp.

TCP heated

°C

Particle

size range

pm*

Solidliquid

ratio

g/100 ml

Time taken

to reach

pH 6

Max.

crystal

length

pm

4-1 3 a 1200 74-149 0.20 2 hr

30

4-2 3 a 1200 44-74 0.20 1| hr. 35

4-3 3 a 1200 <44 0.20 1| hr. 30

4-4 4 b 1300 74-149 0.10 10 hr

30

1 LD 4 b 1300 44-74 0.10 12 hr. 30

4-6 4 b 1300 <44 0.10 3 hr

40

4-7 4 c 1395 74-149 0.20 >3 days** 18

4-8 4 c 1395 44-74 0.20 10 hr. 20

4-9 4 c 1395 <44 0.20 50 min. 12

* Nominal sieve opening

** pH above 7 when experiment was terminated after 3 days.

Hydrolysis incomplete.
13 •—

CD

O

O 3 Q > 3 c •P b P O P

Ik CD 3 Is IS CD

CO 1—

1—

-P

•'

b Is rd rd 3 i—I o rH P •P g

id rd 13 P 13 P 13 P O-i P 0) CD CD CD CD

X

b P u p > -P > -P > CD U c (D Eh CD P id P rd p 13 Eh •p -P 1 -P <D £ CD g CD C 1 rd y rd CO CO CO •H y

CO

g £ bX

b

X

b i 0) 0 0 CD O CD O 3: ip o X a X CD 13 13 O 0 c <u CD CD b £ 3 CP 3 b rH

id 3 13 • 13

U rd •P

u •HU co -P 13 3 b 3 Eh co 1 Eh i Eh CD i—

CO -P P U •P 1 5 1 3: 1 1 1 3 b to 1 Eh 1 H o y O y -P -P 3 CD 3: 1 3: rH i—

-P CO CO P 0 0D O •H >i

pH l—

CD CD i—

P p W 1 1 1 1 U 0 b 5 b -P P >i b o o u -P -P p £

+) 3 b 3 •H •H CD CD o a co a b b > MP • «. P O rH

CL) -P rd

b

-P -P P -P 3 3 • co £ o o O o o O

3: P X >i 3 P- CM in in CTr rd p <d tn 13 C -P a O i—

•p

p

H & c

•H *H •p O '—

CD rQ 1 b — P CM CO

13 P 13

m ! I in I CD 1 in C -P rd id *p • I ! • « • 1 •H i—

O rH CO i—

1 S i—

l 1—

1 CD o 0 CD i—

b to S- ^ b u o Eh tm

Eh 1 3 • • • • • • 8 -P i—

• P P p P p P

>1 rd CD P b

b

b

bbb

ip

p -P £ b 0 rd 0 -P i

i d1 d* CO > Eh Eh d1 CM CM CM CM 'd1

ysis of c i—1 -P (D

b • o u b U c « P 0) rd id co •p P • • • • • 13 M-i -P CD £ b P p p p p

>i 4-1

u tn

bb

bbb

K CD CL) Cu o CM £ 0 in CO P|W 00 o o

•H -P d*

—

CO in

Eh i—

1 13 £ in in 13 •P 0 o o o o m in CM •H 3 -P O 00 •d1 CM Ip o o o

pH cr -p o • • • • • • • O •P rd i—

o

CO i—1 P

CP • 0 b S u

rd rd id id rd rd 3

ment T

Lot co co co ro co co co i—

CM co

d1 in ID rExper i

No. 1 m 1 LO 1 in 1 in 1 in 1 m 1 in 8

3a

.0020

**

14

days

1.63

75

Few

minute

grains

with

low-index

few

crystals

of

<x-TCP

*Experiments

5-7

and

5-8

were

run

ina

round

bottomed

flask,

with

heating

mantle,

fitted

with a

water

condenser

through

glass

joints.

Volume

of

water

1500

ml.

**pH

still

above 6

after

14

days.
Hydrolysis

of

a-TCP

in

water;

effect

of

temperature

during

hydrolysis ' to

CO CD co CO CO TO c rH c c c CD •H TO •H •H H N CO

(D (0 CD CO CO (0 >i

C £ P CD P P p 1 1 •rH CD 0 to C Co co CO 0 CO O CO P p G CO p MH P p p T3 Co

CD - c (0 0 cO ro CO >1 P CO • H l 1 i—i i—1 1—1 1—1

rG P

r0 CD rO 1 3 CO 3 3 3 CO

CD 1 1 p cD CO P CO Co Co >1 1—1

a, to Cn CD CD CD CD CD 1—1 3

Oh CD 0 P -P P P P CD CO

CO CD

p -p P CO P P P -P CD

G CO •H 3 H •H P CD P

CD i—i p i—1 i—1 P 3 >1 3 CO o >i >P >i a •H

TO 1—1 Cn 1—1 r—1 i—1 i—1 rH £ • H p CD •H -P CD •P +> p 0 CD

co CO P £ co £ CO CO COu

G

(D 0 P •H 0 0 0 0 0 G -H

CX £ H

m S co £ £ £ H 4H

ro rG

-p -p • Id £ x >1 c 3.

(0 P CD § O rP o

CO

LT) Ln

CN CM

in oo co i CM |

CO CO • • • • • >1 >1 1 1 P P P P p CO (0

CO CD H £

rG rG rG rG rG TO TO 0 -H CD P|W H|W H|« CO in n~

£n -P in cD CD CM i—i G

CD rG

rX o • • CO CO CD P P

-p CD rG rG P m

CD Oh rHP £ 0 1—1

CM

H -P

Eh

CO co • • • >1 Do P p P (0 ro rG rG rG TO TO

H|CU ro in CO CD CM i—i A I ro Co P CD CN G

CD P +1 •P 1—1

CN CN CN CN in

Ct 3 —1 i—1

CO CD O' cD in st £ -P u •P

CD o 0

Eh i—l

PQ | TO £ H 0 O o O o O o o

•H 3 -H o CM CN CN CN CN CM CN i—1

tt -P o • • • • • • • 0 P CO i—i o o O o o o o

in rP P Xco 0

Oh £ o LO in in in

mm in

Eh -PO

PI

-PG

0)£ • 1—1 CM co in CD CD

rH

O 1 1 1 | l 1 l 1 p cD cD CD cD CD CD CD CD

CDOhXH

.20

Room

temp.

60

days

77

days—

Incompletely

hydrolyzed;

product

"amorphous"
Figure 1.

HA crystals obtained on hydrolysis of oc-TCP

water

in boiling