NATIONAL BUREAU OF STANDARDS REPORT

1455

CRYSTALLOGRAPHIC investigation

OF TRICALCIUM PHOSPHATE HYDRATE

by

Aaron S. Posner

Samuel R* Stephenson

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U., S. DEPARTMENT OF COMMERCE

Charles Sawyer, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Acting Director

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NBS PROJECT

0703- 11-0707

NBS REPORT

1455

CRYSTALLOGRAPHIC INVESTIGATION

OF TRI CALCIUM PHOSPHATE HYDRATE

by

Aaron S. Posnerl

Samuel R. Stephenson^

''"Research Associate, American Dental Association Research

Fellowship, National Bureau of Standards*

o^Corporal, Medical Department, U. S. Army, stationed at the

National Bureau of Standards.

This work is a part of the dental research program conducted

at the National Bureau of Standards in cooperation with the

Council on Dental Research of the American Dental Association, the Army Dental Corps, the Air Force Dental Corps, the

Navy Dental Corps and the Veterans Administration.

This report is issued fo

in sny form, either in wh

fro* the Office of the Di

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

, reprinting, or reproduction

ission in writing is obtsinsd

hington 25, D. C.

CRYSTALLOGRAPHIC INVESTIGATION

OF TRICALCIUM PHOSPHATE HYDRATE

Abstract

By the use of the hydrothermal bomb technique, crystals of tricalcium phosphate hydrate

were grown from about 500 angstrom units to

approximately 3 microns in length. By this

means it was possible to obtain accurate crystallographic data which, when combined with the

chemical data, suggested that this substance

might be hydroxy apatite with phosphoric acid

occluded in the ratio of 0.6 H^PO^ to one

Ca10< PV6(0H) 2-

1 . INTRODUCTION

The existence of tricalcium phosphate hydrate, or

°£-tri calcium phosphate as it is sometimes called, is a problem which concerns workers in many fields * It has been suggested by Dallemagne and his coworkers (1), (2) that this

substance, admixed with magnesium and calcium carbonate, is

the main inorganic constituent of bone, dentin and cementum.

However, at this writing, chemists, biologists and mineralogists, as well as scientists in other branches of research,

cannot agree as to whether this substance is a real chemical

entity or is merely a defective or physically modified hydroxy

apatite. The main problem has been the inability of the experimentalist to make exact x-ray and optical measurements on

this material because of the extremely small size of the

crystals of tricalcium phosphate hydrate found in natural or

laboratory samples.

There are two main structural concepts of this precipitated calcium phosphate in which the formula Ca:P0^ ratio is

1.50 as opposed to 1.67 for hydroxy apatite. One viewpoint,

held by Dallemagne, Brasseur and Melon (3), is that since the

x-ray patterns of these substances are quite similar but their

optical and chemical properties are different, tricalcium

phosphate hydrate may be thought of as a calcium-deficient

hydroxy apatite, Ca^ ( P0^) ^HglOH) 2 , with two hydrogen ions in

place of the missing calcium. On the other hand, Bale,

- 2 - LeFevre and Hodge (4) maintain that the tricalcium phosphate

hydrate is merely hydroxy apatite with phosphate ions adsorbed

in sufficient quantities to reduce the Ca:PO^ ratio to the

1.50 value.

In order to characterize tricalcium phosphate hydrate

conclusively, it is necessary to enlarge the crystals so that

precise crystallographic measurements can be made. Then, by

combining these x-ray and optical data with the chemical data,

it may be possible to reach a reasonable conclusion.

2 . CRYSTAL GROWTH

Commercial tricalcium phosphates were -chosen as specimens

for the crystal growth experiments. These substances had an

original particle size of about 500 angstrom units. As cited

by Hodge, LeFevre and Bale (5)

,

the composition of commercial

tertiary calcium phosphates is found to vary considerably.

Therefore, precautions were taken to choose only those phosphates which had a Ca:P0^ ratio as close as possible to 1.50,

the theoretical ratio in o<: -tricalcium phosphate.

The material was then treated in a hydrothermal bomb

•according to the method of Morey and Ingerson (6). A charge

of about two grams of the original, poorly crystallized material was introduced into the platinum-lined chamber of the

bomb (roughly 20 ml in volume). This was then covered with

about 4 ml of distilled water, the bomb was sealed, using a

platinum gasket, and was inserted into a furnace held at the

desired temperature for a given time. The temperatures used

ranged from 250 to 450° C (4^2 to 342° F), resulting in

hydrothermal pressures that attained approximately 2,000 psi

.

The experiments lasted from a few hours to a few weeks.

The results of the crystal growth were followed by x-ray

diffraction and electron and optical microscopy. In Figure 1

the improvement of crystallinity is shown by the sharpening

of the x-ray pattern of the sample treated in a bomb at 300° C

(572° F) for two days. The same samples are shown in Figures

2 and 3, as seen under the electron microscope, the former

being the untreated, the latter the treated sample. Figure 4

shows the same sample treated at 300° C for one week. It is

of interest to note that the crystallite size of the original

unimproved sample, as calculated by the x-ray line broadening

*For a more complete review of the theories concerning tricalcium phosphate hydrate, see the article by Dallemagne

and coworkers mentioned above (3)*

CD

CO

LlI

LlJ <rCD

LUQ

&o

•H

&!

X-ray

powder

diagrams

of

untreated

and

treated

tricalcium

phosphate

hydrate.

-3-

( Figure 1) according to the method outlined by Warren and

Biscoe (7), came to an average value of 500 angstroms. This

checks very well with the finest particles seen in the electron

micrograph of this substance (Figure 2). Since the particle

size and the crystallite size are of about the same dimensions,

this leads to the conclusion that the ultimate particles of the

tricalcium phosphate hydrate observed are single crystals.

There has been no attempt in this paper to report on a

systematic study of the hydrothermal growth of the tricalcium

phosphate crystals. The experimental conditions mentioned

herein were chosen arbitrarily since they s'uited the available

equipment and furnished us with crystals large enough for

making accurate x-ray and optical measurements.

The results of a few representative crystallization experiments can be seen in Column 5 of Table 1. This table

contains data on only those samples made from Baker & Adamson

C.P. tribasic calcium phosphate (Serial Number 3772). It was

found that the minimum treatment necessary to sharpen the

x-ray diffraction pattern (i.e., to grow the crystals larger

than about 0.1 micron) was a two-hour digestion in the hydrothermal bomb at 300° C (572° F) with a resulting internal

pressure of approximately 2,000 psi. As the same treatment

was continued for a week, the crystals grew to a length of

about two to three microns (Figure 4). This size seems to be

the practical maximum that can be attained under these experimental conditions, since doubling the time of digestion produced no measurable increase in particle size.

3. ANALYSIS

The original, poorly crystallized samples and a few of

the improved samples were analyzed for calcium and phosphate,

and the Ca:P0^ ratios were calculated. The methods used are

described on pages 358 and 396 respectively of Kolthoff and

Sandell (8). The calcium was precipitated as the oxalate and

ignited to constant weight as CaO, while the phosphate was

determined by precipitating it as magnesium ammonium phosphate

and igniting to constant weight as Nl^P-pOy. The results are

given as the Ca:P0^ ratios in Column 3 of Table 1.

In every hydrothermal experiment the residual bomb water

was analyzed by microchemical qualitative methods for the

presence of calcium and/or phosphate ions. In general these

methods depend upon the addition of a suitable reagent to a

drop of the unknown solution which has been placed on a slide

mounted on the stage of a petrographic microscope. The separation of a solid phase which possesses crystalline properties

peculiar to the ion being sought is verified by microscopy.
v

'

I
Typical

Results

of

Chemical

and

Physical

Analyses

of

Samples

of

Tricalcium

Phosphate

Hydrate

and

Hydroxy

Apatite

*Tricalcium

phosphate

f

This

density

calculated

from

x-ray

data;

others

observed.
1

^ -9 « «

I
Fig. 2. Electron micrograph of untreated tricalcium phosphate

hydrate. Magnification 25,000 diameters. The < spherical polystyrene particle in center of picture

is about 1/4- micron.

NBS



:

Elg. Electron micrograph of trlcalclum phosphate hydrate

treated for seven days at 3°°° C in a hydrothermal <^NBS

bomb. Magnification 25,000 diameters,

In the case of the calcium an attempt was made to precipitate

Ca(C20^)« 3 H0 and, in the case of the phosphorus, to precipitate

the complex compound formed between the phosphate ion and

silver nitrate.

There was no trace of calcium or phosphate found in all of

the microanalyses. It was then decided to abandon the complete

chemical analyses since it could be assumed that there was no

change in the original Ca:PO^ ratio if there was no appearance

of these ions in the water phase. In general it can be concluded from the analytical results that the original substance

had a Ca:PO^ ratio equivalent to the theoretical ratio of tricalcium phosphate hydrate and that no change occurred during

the hydrothermal treatment of this sample.

4. IGNITION OF PHOSPHATES

It is well established that calcium phosphates which display the x-ray diffraction pattern of hydroxy apatite and have

a Ca:P0^ ratio of 1.50 will give an x-ray diffraction pattern

of /3 -tricalcium phosphate when ignited at 900° C (1652° F)

(5), (10). On the other hand, it is also known that a calcium

phosphate giving the hydroxy apatite x-ray pattern and having

a ratio of 1.67 will not change its x-ray pattern upon ignition

at the same temperature (5), (10). This has been used as a

method of differentiating between tri calcium phosphate hydrate

and a hydroxy apatite. Some samples of tricalcium phosphate

hydrate and a hydroxy apatite were subjected to such an ignition at 900° C for at least eight hours, and the resulting

x-ray diffraction patterns are tabulated in Column 9 of Table 1.

5. X-RAY DIFFRACTION AND OPTICAL STUDIES

All of the calcium phosphates studied were examined as

powders in a geiger counter x-ray spectrometer using CuK

radiation. A few of the typical results can be seen in Figures

1 and 5* The principal observation made on the patterns of

tricalcium phosphate hydrate is that they are very similar to

one another regardless of the conditions under which they were

prepared. In fact, with the exception of very slight differences in cell dimensions (see Table 1, Columns 6 and 7), the

various tricalcium phosphate patterns are identical. The x-ray

powder pattern of a typical sample of V; improvedi? tricalcium

phosphate hydrate (S-o) was carefully compared x^ith the powder

*For details of these methods refer to pages 114 and 336

respectively of Chamot and Mason’s Handbook of Chemical

Microscopy, Volume II (9).

Comparison

of

the

x-ray

powder

diagrams

of

hydroxy

apatite

and

a

typical

tricalcium

phosphate

hydrate.

-5-

pattern of hydroxy apatite* (S-15). These patterns are shown

in Figure 5 and Table 2.

All samples were examined with the petrographic microscope. The average indices of refraction obtained by the Becke

line method are listed in Column 4 of Table 1.

6. DENSITY MEASUREMENTS

The densities of some of the samples were determined by

a modification of the ordinary picnometer method usually appi ied to liquids. After the picnometer was calibrated with

distilled water, a quantity of the powdered sample to be

studied was added to the dry weighed picnometer and the entire

system was weighed again. The picnometer was then half filled

with water and subjected to a vacuum treatment for five to ten

minutes in order to rid the sample of any air which might lead

to erroneous density measurements. The picnometer was then

filled to the mark and weighed at 20° C (63° F) . The density

was calculated according to the method outlined by Hidnert and

Peffer (11) using the following equation:

D

S + P + 'Vd-W t

‘

where V = internal volume of picnometer

D = density of the powder

d = density of water at 20° C

S = weight of the powder sample

P = weight of the dry picnometer

and W T = total weight of the picnometer, powder and water.

The results of these measurements may be seen in Column £ of

Table 1.

7. ADSORBED PHOSPHATE IONS

Bale, LeFevre and Hodge (4) write the reaction (unbalance!)

in which tricalcium phosphate hydrate transforms to/5 -tricalcium

phosphate upon ignition at 900° C (1652° F) as follows:

3Ca

9

(PC

4

) 6Ca(OH)

2

+ < P0

4

> 2

~ 3 —* 10Ca (PC^).,

This viewpoint stems from their theory that tricalcium phosphate

hydrate is indeed hydroxy apatite with adsorbed phosphate ions.

^Sample obtained from the National Institute of Dental Research.

This sample is not pure hydroxy apatite, as shown by its analysis (Table 1); however, there were no spurious lines in the

diffraction pattern and so it was chosen as a standard.
'
Table

2

Comparison

of

X-ray

Diffraction

Data

of

Hydroxy

Apatite

and

Tricalcium

Phosphate

Hydrat

Phosphate

Relative intensity

9.95 un

• to

UH

to

-4-

16.6

UH

ON

• to

rH

00*001 O• to

UH

61.7 Z'LZ 7.1 O• ON

NO

• OH

07

4.5 NO

•

g

CD

0 un un un UH o 07 rH o UH UH UH o UH UH

O 0 to -4- ON o UH ON (—

i

UH O O- NO On 07 NO

r—

1

0 • • • • • • • • • <% • O 4

ctf £ o rH 07 -4- UH NO NO o- 0- ON ON o O

a bij rH rH rH I—

<

1

1 i—

1

rH rH rH rH rH rH 07 07

£

0Q

t>5 0 4_) > »H o 0 •H CO UH <\7 NO o O o to rH 07 07 OH ON nO -4- -P P £ • • • • • • • • • • « • • • •H cd 0 nO On UH ON O- o o- -4 NO ON 1

1 -4- 07 07

P fH P rH OH rH o UH NO (V rH 07

CTJ 0 £ 1—

1

P- -h

0

CD

X CO O -ct ON o UH o UH UH UH UH UH o UH UH

o 0 to rn to o OH to O OH ON NO UH to OH UH

£ 0 • • • • • O • • • • • • • T

3

£ o rH 07 ,4- -4 UH NO NO NO ON ON o o >- hO rH rH rH rH i—1 rH rH rH rH rH rH rH 07 07

K 0

«

rH o o i—

1

07 07 07 O O i—1 rH 07 O O 07 07 rH rH o o i—

1

OH OH i—1 rH o 07 rH o O rH 07 rH 07 rH rH O OH O 07 O OH OH rH 07 O i—

1

rH Or\

.£ 07 o rH o i—1 o rH 07 «H 07 rH ohO 07 O ohO i—1 OH 07 rH O OH i—

1

Continued

in

next

column.

- 6- A series of experiments was performed to test the above

reaction* A charge of two grams of hydroxy apatite was added

to the hydrothermal bomb and covered with 4 ml of a dilute

solution of ortho phosphoric acid, the concentration of which

was adjusted to change the total Ca:PO^ ratio of the charge

(solid plus liquid) from 1*67 to 1*50. This was then sealed

as usual and heated for seven days at 300° C (572° F) . The

resultant sample was examined by x-ray and optical methods and

part of the sample was ignited for 12 hours at 900° C (1652° F)

.

The results, as seen in Table 1 (Sample S-29) show that by

adding phosphate ion to hydroxy apatite and igniting it at

900° C, a /3 -tricalcium phosphate structure can be achieved.

It should be noted that the addition of the phosphate ion has

lowered the index of refraction from 1.64 to 1*61, the latter

figure resembling the index of refraction of the chemically

precipitated samples of tricalcium phosphate hydrate.

8. CONCLUSIONS

From the analytical data obtained it might be concluded

that tricalcium phosphate hydrate is distinctly different from

hydroxy apatite. However, their x-ray diffraction patterns are

very similar, as shown in Figure 5 and Table 2. The small

difference in the a axis might be explained by the fact that

this axis in the T? hydrate ,f crystal was not developed beyond

0.1 to 0.2 micron and such maldeveloped axes have been known

to give erroneous axial lengths (12). There are also some

differences in the intensities which remain unexplained at

this point.

In general a structure must appear in some periodic arrangement whose size is of the order of 100 angstroms or greater in

order to be detected by x-ray diffraction. It is conceivable

that some substance or substances may be present in the tricalcium phosphate hydrate in large enough amounts to alter the

Ca:P0^ ratio, the index of refraction, the density and the ignition properties without being in a developed, crystalline

state which would appear in the x-ray data.

To correlate the analytical results with the above x-ray

diffraction data one may assume some sort of phosphate ion

adsorption or occlusion in the amount needed to change the

Ca:P0^ ratio of hydroxy apatite from 1.67 to 1.50. If this

apparent excess of phosphate, six percent by weight, were

present in a separate crystalline phase, it would be detected

by x-ray diffraction, assuming of course the crystallites were

larger than about 100 angstrom units.

-7-

The x-ray diffraction data, which show very little difference between hydroxy apatite and tricalcium phosphate hydrate,

can also be interpreted to show that the Dallemagne and Brasseur

(3) formulation Ca^fPO^) ^2 (OH)

2 might be true. This configuration theoretically would give a pattern quite similar to the

hydroxy apatite. However, if the density of this substance is

calculated from the unit cell volume, obtained from the x-ray

data, and the molecular weight suggested by the above formula,

it is found to be 3«03 as opposed to an observed value of 2 .

$

9 . If the average index of refraction is also calculated using

the Gladstone and Dale method (13), the theoretical index is

found to be 1*65, while the observed index is 1.60. If the

hydrate is assumed to be a monohydrate instead of the proposed

dihydrate, the theoretical index of refraction is found to be

1 , 64 , which also does not agree with the observed value. To

reconcile the density and index of refraction data with the

x-ray diffraction data it may be assumed that some sort of

diluent which fails to give a diffraction pattern is adsorbed

or occluded in the hydroxy apatite crystals, loxvering the

density and index of refraction by the amounts shown.

It is well to add that the method of Gladstone and Dale is

only approximate and does not always give exact results. However, it is felt that when we compare similar compounds as

above, the method is sufficiently accurate for our purposes.

Thus we can state that any of the suggested formulas for tricalcium phosphate hydrate would give indices of refraction

almost coincident with that of hydroxy apatite.

A complete analysis of tricalcium phosphate hydrate shows

that it contains anywhere from 1.15 to 7-30 percent of volatile

water (5)« The above evidence does not completely explain the

existence of this excess water in these tricalcium phosphate

hydrate crystals. It is strongly indicated that this water is

not contained in the structure. One can guess that it is surface-adsorbed water. There is some evidence from preliminary

work done by the authors that when enlarged tricalcium phosphate hydrate crystals are subjected to differential thermal

analysis, the curves are quite dissimilar to those obtained

from the fine particle material. This lends support to the

hypothesis of water adsorption since the enlarged particles

could not accommodate such a large excess of water on the

surface

.

A systematic study of the addition of phosphate to hydroxy

apatite is still under way. So far our experiments have shown

that by the addition of phosphoric acid to hydroxy apatite a

well crystallized product can be obtained which satisfies the

tricalcium phosphate hydrate formula. This product has a density and a refractive index different from those calculated on

the basis of an apatite of the composition Ca^H^ (P0^) ^(0H) ^

,

while the x-ray diffraction pattern is clearly very similar to

that of hydroxy apatite.
, ...

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There has been some question raised as to whether the improved crystals of tricalcium phosphate hydrate are the same

as the original material* Since they analyze the same and no

phosphate or calcium was found in the residual bomb water, and

since no amount of bomb treatment, ranging from two hours to

one week, changed the index of refraction, the density or the

x-ray pattern, it is clear that there has been no chemical or

crystallographic change during this hydrothermal growth.

The exact nature and placement of the physically admixed

ions, which according to this hypothesis make the hydroxy

apatite appear to be tricalcium phosphate hydrate, can only be

speculated upon. Surface adsorption of phosphate ions cannot

be the entire effect. In the 3-niicron crystals of the improved

tricalcium phosphate hydrate it would be impossible for six

percent by weight of the entire crystal to be phosphate ions

adsorbed in any reasonable number of layers on the crystal surfaces. This lends emphasis to the explanation that the bulk of

the phosphate diluent is occluded in the submicroscopic faults

of the hydroxy apatite crystal. We measure a density of 2. $9

on these tricalcium phosphate hydrate crystals, which differs

from the theoretical value of 3.13 for hydroxy apatite. Considering the six percent occlusion as phosphate ion, we cannot

completely justify this low density by this factor alone. Thus,

there must be a small amount of empty space in the crystal

which, courled with the lower density of the phosphate ions as

compared to that of hydroxy apatite, will combine to give the

observed density. Apparently the phosphate ions (and the

hydrogen ions associated with them for electrical neutrality)

would deposit imperfectly in these entrapped surfaces during

crystal growth so that there would be empty spaces remaining,

thus accounting for the drop in density mentioned above. The

results reported in this paper would not indicate whether the

occluded ions were PO^, HPO^, or some other ion involving phosphorus, but regardless of the exact nature of the ion, the

principle of occlusion would still be the same and there would

still be 0.67 H^PO^ for every hydroxy apatite molecule to form

one molecule of so-called tricalcium phosphate hydrate. Thus,

to change the view of Hendricks and Hill (15) very slightly,

the formula for tricalcium phosphate hydrate would be

C a 10(P04 )6(0H)2 * occluded 0.67 H^PO^.

There are precedents for thinking in terms of imperfections

in crystals. Addink ( 14 ) has shown that the density of certain

crystals as calculated from very accurate x-ray data is found to

be higher than the carefully measured density. It should be

noted that these imperfections were of the order of magnitude

of less than one percent, much lower than the above six percent.

The mode of occlusion of excess phosphate ions in hydroxy apatite may be similar to the carbonate occlusion suggested by

Hendricks and Hill ( 15 ) for the carbonate apatites.
.

;

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-9-

In the biological field the higher content of phosphate

in the apatite of bone, cementum and dentin as compared with

enamel indicates that a mechanism may be active which permits

the occlusion of these ions in greater amounts. Sobel, Hanok,

Kirschner and Fankuchen (16) have shown that the P0^:C0^ ratio

of blood serum bears a direct relationship to the PO^:CO^ ratio

of enamel and dentin. This may indicate that the amounts of

inclusions in the deposited apatites depend on the individual

concentration of these ions in the serum.

9 . SUMMARY

1. Crystals of so-called tricalcium phosphate hydrate

were enlarged in a hydrothermal bomb from an average length

of 500 angstrom units to about 3 microns without changing their

Ca:P0^ ratio, density, refractive index and the spacings of

their x-ray pattern.

2. The x-ray diffraction powder patterns of tricalcium

phosphate hydrate and hydroxy apatite were found to be quite

similar

.

3. The differences between hydroxy apatite and tricalcium

phosphate hydrate insofar as density, index of refraction, and

results of ignition at 900° C (1652° F) are concerned can

possibly be explained by postulating a six percent phosphate

ion inclusion in the entrapped surfaces of the hydroxy apatite

crystal

.

4. We suggest that a possible formula for tricalcium

phosphate hydrate is Ca-j^fPO^J^CH^ . occluded 0.67 H^PO^.
.

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Bibliography

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THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The National Bureau of Standards is the principal agency of

the Federal Government for fundamental and applied research in physics, mathematics, chemistry, and engineering. Its activities range

from the determination of physical constants and properties of materials, the development and maintenance of the national standards

of measurement in the physical sciences, and the development of

methods and instruments of measurement, to the development of special

devices for the military and civilian agencies of the Government.

The work includes basic and applied research, development, engineering, instrumentation, testing, evaluation, calibration services,

and various scientific and technical advisory services. A major

portion of the NBS work is performed for other government agencies,

particularly the Department of Defense and the Atomic Energy Commission. The functions of the National Bureau of Standards are set

forth in the Act of Congress, March 3, 1901, as amended by Congress

in Public Law 619, 1950. The scope of activities is suggested in

the listing of divisions and sections on the inside of the front

cover.

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The results of the Bureau’s work take the form of either actual

equipment and devices or published papers and reports. Reports are

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Published papers appear either in the Bureau’s own series of publications or in the journals of professional and scientific societies.

The Bureau itself publishes three monthly periodicals, available

from the Government Printing Office : the Journal of Research, which

presents complete papers reporting technical investigations; the

Technical News Bulletin, which presents summary and preliminary re- ports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for

radio communications throughout the world. There are also five

series of i nonperiodical publications: the Applied Mathematics Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous Publications.

Information on the Bureau’s publications can be found in

NBS Circular 460, Publications of the National Bureau of Standards

($1.00). Information on calibration services and fees can be found

in NBS Circular 483, Testing by the National Bureau of Standards

(25 cents). Both are available from the Government Printing Of f ice.

Inquiries regarding the Bureau’s reports and publications should be

addressed to the Office of Scientific Publications, National Bureau

of Standards, Washington 25, D. C.