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LOS AMMOS SCIENTIFIC LABORATORY

This document consists of 9 pages

THERMODYNAMIC Properties OF mrrom HEWTJJORIDE

A Freliminaq Report

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A, E. Florin

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Data

have been

-S found

ABSTRACT

obtained in a study of the equilibrium

PuF4 + F2 *PuF6 ,

used to determine AF, AH, and AS for the reactiona It

that AH for the reaction is positive, i.e.,a higher

temperature favors the formation of PuF6.

The data reported are preliminary and are contingent upon

the establishment of a proof that the method employed to measure

the equilibrium does in fact measure the equilibrium.

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A preliminary study of the equilibrium, PuF4 + F2 @ puF6,at several

temperatures has melded data from which preliminary values for AH, AF and.AS

of reaction have been calculated. Although more work is required for a complete understanding of the equilibrium process and to improve the accuracy and

extend the temperature range of the data, the unusual finding that PuF6 has a

positive heat of reaction (i.e.,it is more stable at high temperature than at

low), at least over a moderate temperature range,ties it desirable to report

the preliminary results at this time.

The finding of a positive heat of reaction for p@6 is not original in the

present work, but the result was not on a very sound basis previous to the

present work. First Brown and Hill?, later Davidson, Katz and 0rlemann2 and

again later Newton, Fisher, Vaslow and Tevebaugh3 invoked the possibility of

a positive heat of formation for PuF6 to explain perplexing results of attempts

to measure the vapor pressure of PuF6. Indeed, the latter group of authors

concluded this was the case, but since at the time of their investigations PuF6

had not yet been isolated arxiidentified, high reliability could not be attached

to their data.

In the present work

temperatures in both the

phase of the e~ilibrium

data, AH, AF, and AS for

the equilibrium PuF4 + F2 ~ p@?6 was studied at several

forward axxlreverse directions by analyzing the gas

mixture fOr lmth PUF6 and F2. From these equilibrium

the reaction have been calculated.

Apparatus and EXPerinwntal Procedure.

The reaction chamber for establishing the equilibrium is

It consisted of a welded nickel can (2 in. o.d. x2 in. 10W)

shown in Fig&e

with a special,

--r

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ED

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I

high-temperature, nickel valve at one end. The reaction chamber was connected

through the valve by nickel and copper tubing to a manifold to which p@’6 or

F2 could be supplied and from which samples could be withdrawn for analysis.

To study the decomposition of MS a sample of PuF6 was distilled into the

reaction chamber and the valve closed. The reaction chamber was heated to the

temperature at which an experimental point was desired and kept at that

temperature for a sufficient time to permit the reacting materials to approach

equilibrium. At the end of that time the nickel valve was opened and the

volatile contents of the reactor were pumped out through the manifold into the

analysis apparatus. It was assumed that these gases represented the equilibrium

concentrations of p@6 and F2. In the analysis apparatus PuF6 was frozen out in

a liquid nitrogen cold trap and later washed out of the cold trap with nitric acid

and.determined by alpha counting. The

tube filled with NaBr at 150° where F2

out in a liquid nitrogen cold trap and

F2 passed through the cold trap into a

wae converted to Br2. The Br2 was frozen

later allowed to react with a solution

of KI to produce a solution of 12 in KI. This final solution was titrated

with 0.1 N thiosulfate to determine 12. The molar amount of F2 emerging’frcm

the reaction chamber was then equal to

titration.

At a given temperature, the ratio

the molar amount of 12 found in the

PF2]

thus determined was taken as the

equilibrium constant K for the reaction, PuF4 + F2 ~ W6. After several deter-

!dnatiOn of equilibrium M)nSt~tS using ~6 as the starting material, enough

PuF4 had been deposited in the reaction chamber so that it was only necessaryto

add F2 to permit approach to equilibrium in the forward direction. After

addition of F2 the procedure in these runs was the same as described above.

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I

The equilibrium constant, K, for the reaction PuF4 + F2 a ~G$ at each

tempe~ature, T, can then be written K =

f-+

PuF

F2 “

By plotting In K agaitibtlJT

the heat of reaction, AH, haa been calculated (see

the line obtained in the plot. The free energy

by applying the relation AF - -RT in K, where R

of reaction, AS, was obtained from the relation

of

is

AF

Figure 2) from.the b~ope of

reaction, AF, was obtained

the gas constant. The entropy

- AH - TAS. The equilibrium

data and calculated quantities

Thermodynamic Values

Temperature

“c.

167 .Ooo1

2(X) .00025

.0021

:: .0055

.011

& .017

167 to 308

308 to600

All the quoted values are

are summarized in Table I.

Table

for the

subject

method used to meaxure the equilibrium

Since plutonium fluorides intermediate

and of comparatively low stability may

r

A

Reaction PuF4 + F2 * PuF6

AF AS

Kcal &_ eou.

*8.05

+7.8

+7.0

+6.97

+6.99

+7.03

+~. 5 +7*I3

+ 6.77 4.3

to revision pending proof that the

does in fact measure the equilibrium.

in composition between PuF4 and ~G

exist in the equilibrium mixture, some

doubt maybe justly raised concerning the accuracy and applicabilityof the

measured equilibrium values. It is thought, hcwever, that such intermediate

fluorides, even if shown to exist, would have little or no effect on the

measured equiMbrium values.

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Thus far, the only reasonably reliable equilibrium data were obtained at

elevated temperature with ~G present only in the gas phase. The AH values

derived from these data showan inorease from +6.7 to +11.5 kcal/mole with

decreasing temperature. The data are preliminary; howsver, the change appears

to take place all at one temperature (more data muld be necessaryto prove this)

rather than gradually over a range of temperatures. This would suggest that

the change results from a phase ohange in the reacting material. One can

speculate that at temperatures lower than those for which reasonably reliable

equilibrium values have been obtainedO additional phase changes may occur and

possibly cause the AH value to become negative with the consequence of inmeasing

stability for ~G at still lowsr temperatures. It would be expecteds that

liquid phase p@G (heat of vaporization = 7.4 kcal) and especially solid phase

p@G (heat of sublimation = 12.1 kcal) would provide additional stability for

hFGo at low temperat~es. It ie doubtful that such effects even if of

appxwciable magnitude at very low temperatures could prevent the @f-alpha

radiodecompositionof ~G unless a catalyst could be found that would allow

recombination by a path not requiring as high an activation energy as would

be necessary for the direct synthesis. A three month observation of the decomposition of solid p@G in equilibrium with its own vapor at room temperature

showed no trend away from the decomposition rate of roughly 1$ per day which

can wsl.1.be attributed to self-alpha

Bibliography:

1. Rsport CN-696

2. Report Ck-987, Oct. 1943

3- Report CN-1783, Aug. 1944

radiodecompostion4$5.

4. A. Ee Florin, Report LAMS-11.18,Oct. 1950

5* A. E. Florin, Report LA-).168,NOV. 1950

-Milm .-,..-A

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—-- - -1

.—

——— .—. ...—

. -.— ~~~THRusT BEARING

/

BELLOWS ( SILVER SOLDERED )

FIG. I

EQUILIBRIUM c E OF NICKEL

I

1

I

MANIFOLD

GUIDE

AL VALVE

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.. .

-1.5

-2.0

-2.5

x

o

_3,5

-4.C

AH = +/i77 Kcal I

Y

LOGIO K =

-1.48 X 103 _ 0’0644

T

.

POSSIBLY INDICATES

PHASE CHANGE OF

ONE OF REACTING

MATERIALS

AH =

o

A

PuF4 +

POINTS

POINTS

,

K = 1,7133 -

2.5136 X 103

T

F2 #E PuF6

~ [PuF6]

=

[F21

APPROACH EQU

APPROACH EQU

L

L

BRIUM

BRIUM

5 1.0 2.0

(l/T) X ;;3

FIG.2

EQUILIBRIUM DATA FOR PUF6

=!!!--

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