ORNL-4257.txt

a 2t
. ORNL-4257 g k-
41 UC-4 — Chemistry L)
Ay

AN EMF STUDY OF LiF—BeF2 SOLUTIONS

B. F. Hitch
C. F. Baes, Jr.

OAK RIDGE NATIONAL LABORATORY
operated by

UNION CARBIDE CORPORATION
for the

- U.S5. ATOMIC ENERGY COMMISSION

¥

Printed in the United States of America. Availakle from Clearinghouse for Federal
Scientific and Technical Information, National Bureau of Standards,
U.S. Department of Commerce, Springfield, Virginia 22151
Price: Printed Copy $3.00; Microfiche $0.65

LEGAL NOTICE

This report was prepared as an account of Government sponsored work. Naither the United States,

nor the Commission, nor any person acting on behalf of the Commission:

A, Makes any warranty or representation, expressed or irnplied, with respect to the accuracy,
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8. Assumes any liabilities with respact to the use of, or for damages resulting from the use of
any information, apparatus, methed, or process disclosed in this report.

As used in the obove, ''person acting on behalf of the Commission’’ includes any employee or

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or controctor of the Commission, or employee of such centractor prepares, disseminates, or
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or his employment with such contractor.

- 4

o .,
DISCLAIMER

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ORNL~4257

Contract No. W-~74Q5-eng-26

REACTOR CHEMISTRY DIVISION

AN EMF STUDY OF LiF—BeF2 SOLUTIONS

B. F. Hitch and C. F. Baes, Jr.

LEGAL NOTICE

This report was prepared as an account of Government sponsored work, Neither the United
Siates, nor the Commission, nor any persen acting on behalf of the Commission:

A, Makes any warranty or representation, expreased or implied, with respect to the sccu-
racy, pl or H of the information contained in this report, or that the use
of any information, apparsatus, method, or process disclosed in this report may not infringe
privately owned rights; or

B. Assumes any liabilities with respect to the iise of, or for damages resulting from the
use of any information, apparetus, method, or process disclésed in this report,

As used in the above, “‘person acting on behalf of the Commisston’” includes any em-
ployes or contractor of the ¢ i or pl of such contractor, to the extent that
such employee or contractor of the € or pl of such contractor prepares,
disseminates, or provides acress 1o, any information pursuant to his employment or contract
with the C n, OF Dis t with such contractor.

JULY 1968

OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee
operated by
UNION CARBIDE CORPORATION
for the
U.S. ATOMIC ENERGY COMMISSION

iii

CONTENTS

Abstract ...ccececccacnnans cecsescanca v
Introduction e.sceoscosocesconososcnacsossce
Experimental ....ccivevecrecocencsacananas
Chemicals ..ccoceoscssososconsossacassesa
GASES .vvesssesssoasssscsacan seassas

Melt Components .....ceececensacs .o
Reagents OOOOOOO & & & 2 & O & b F O ¢ & 9 ¢ ° 0 B O ®

Apparatus ....... casesessecesscsne e .

Cell Design .o.ceveecannsoosancasns
HF-Ho Electrode ...cccveccneaconnss
Beryllium Electrode ........ ceeennn
Flow Control of Gases .......... coe
Hydrogen Fluoride ............ ‘oo
Hydrogen ...ccceececescsvscocssos
Helium ....... ceece et et antonnusnee
Titration Assembly ...ieeecccoascss
Electronic Equipment .......... .

Procedure ......c00c0000s00 csesenaoee

MeasurementsS ...csscoscnsssscsnssass

. Cell Potential .....ccee.. cesacas
Hy and HF Partial Pressures .....

Melt TemperatuUre ....cocooooeneas

Calculations ..ccceccos cesveseces

Systematic EXrors ...ceceecaccooces
Hydrogen Diffusion ......cc00ue.n
Thermal Diffusion ........ ceneens
Gas Cooling Effect on Electrodes
Melt ComposSition ....eceeoocacsaecs
Melt Impurities ....cccc.. cessana
Oxide Contamination of Melt..... .
SUMMATY s eecocesoscocansosasonnsascs

Random ErrOorSeeeseceosssssoesas e ensee

“ a % 2 3 Q¢ & 3 8T G

® 2 & % 0 2@ G > & 8 & O 6

--------------

b ® @ @ & * 6 6 & 5 8 6 B &
2 6 % o e & @ & 2 @ @ © ° B
. 8 @ @ o * & @ % & o @ ®©
9 & 2 2 0 @ @ B 8 B o

------ e o & ¢ 8 8 & 3
% @ 8 ° 8 ® 0 © 9 F B8 8 ?
4 @ & ¢ e » 9 & B @ & * 4 @
® 8 ¢ 6 3 8 % 3 ¥ 9 & O & O
e ® & & 5 ° ¥ B 2 0 ® s 6 ®

--------------

oooooooooooooo

Precision of Potential Measurements ....ccecoeoa

Melt Temperature ........ cesosnas
Melt Composition ....ccevveccacns

Titer Precision .eescecececocaseoa

» Temperature of Bubble-0-Meter ...
- Flow Rate Determination ....ecoee.

Endpoint Precision .....cs0000cee
. Hz and HF Flow Rates .........

e @ @ @ ¢ @ & @ O O B F

“« 2 6 0 ¢ & 8 @ % o & @ ¢ ©

* & 2 a ¢ & 0 & P & © & O ©

-~ Oy OO

4 ~J

10
11
11
11
11
11
13
13

13
13
14
14
15
15

18
18
19
19
19
20
21
22

22
22
22
22
23
23
23
23
23
iv

Statistical Error Analysis ....coen.

ReSUltS @ & 6 F & & @ b O O C @ b € % £ O ¢t D P BB E Y O G & D D B B O O

Ta—bulation ® & @ & B 6 & P T & O & S O 8 & O & G O S O B DO

EXperimEBtS > & O * & L] o -] e @ @ ® @ e & 9 * @ ® 3 - (-]
Corrected Cell Potentials ...ceeveas

DiSCuSSiOn ® & & & & © &8 b o b & © & 3 ¥ G B 6 & & O F b B 6 O B € 8 & & ¥ YL AT DS S & 6 O G 6 & O

Thermodynamics of LiF-BeFs .........

Reference Electrodes . .veeoceooocescss
Beryllium Electrode .....coc0oceeeens

HF-H7 Electrode ...icccerececccnnennncoacanoas cene

REferenCeS ® & 4 6 8 & 4 & 6 F & 0 2 4 H 00 & T S 0 6B O B © O D OO

® 6 & & b ® & 5 O B

Page

23
25

25
25

26
29
29
39
39
41

42

.
AN EMF STUDY OF LiF~BeF2 SOLUTIONS

B. F. Hitch and C. F. Baes, Jr.
ABSTRACT
The potential of the cell

Be®|BeF,, LiF|HF,H

29
with the assumed cell reaction

23

Be(s) + 2HF(g) < Ber(d) + Hz(g)

was measured over a composition range of 0.30 to 0.90 mole

fraction BeF, and a temperature range of 500 to 900°C. Since
this cell potential is related to the activity of BeFs in the
solutions by

P a
o EO _.32 . H2 BeF2
2F PZ
HF

activity coefficients could be derived for BeFy (and by a
Gibbs-Duhem integration for LiF). Usefully accurate measure-
ments could not be made with pure BeFy in the cell, hence
values of E° were calculated using values for aBeFoy derived
from the phase diagram and previously reported heat of
fusion (1.13 kcal/mole) giving

EC = 2.4430 - 0.0007952T
This comparison of the emf data with the LiF-BeFy phase dia-
gram also indicated that the heat of fusion for BeFy is < 2.0
kcal/mole. A power series in xj4iF was assumed for log YBeF?
and the coefficients determined by a least squares fit to
the data. This gave

log v, = (3.8780 - 2333 2323.3y 2+ (=40.7375 + ggz%g_g) 3
2
+ (94.3997 - BBy (67,4178 + 222223y, 5

Formation free energies and heats for BeF, and Be( were also
calculated by combining the results of the present study with
available thermochemical data. The Be +|Be and HF,Hy|F~

electrodes performed acceptably for use as reference electrodes,
both being stable and reproducible.
I. INTRODUCTION

Molten fluoride mixtures of LiF and BeF2 are of considerable
interest at this Laboratory since they are the principal constituents
in the molten-salt reactor (MSRE) fuel and coolant salts.l Although
the molten LiFwBeF2 system has received considerable attention, only
a limited number of emf investigations have been attempted. The
development of reference electrode half-cells for molten fluorides in
general would facilitate the determination of electrode potentials for
various fluoride constituents and the detection of certain impurities

contained in these mixtures.

The purpose of this investigation was to study the cell reaction
Be®(s) + 2HF(g) 2 BeF,(d) + H,(g) (1)

+ —
in the molten LiF-BeF, system using Be2 lBeo and HF,H IF electrodes.

2 2

Emf data obtained in this study would hopefully extend and improve the
thermodynamics of this molten fluoride system. At the same time emf
measurements should demonstrate useful electrodes which may serve as
reference electrodes in future emf studies in this important molten
salt solvent system.

Thus far activity data for the LiF-BeF, systems has been obtained

2
. . 2,3 .

from mass spectroscopic studies of the vapor, from the phase dia-
4,5 6 . .

gram, from emf measurements, and from transpiration data where

gaseous HF-HZO mixtures were equilibrated with the molten fluoride mix-
7 : . . . .

ture. The values derived from the phase data, besides being non-iso-

thermal, have been limited in accuracy because the BeF, liquidus has

4,5,7,8,9

2

been difficult to determine. In addition, the reported wvalues

for the heat of fusion for BeF, are not in agreement. Mass spectro-

2

scopic and emf values of the activity were determined for only a limit-

ed number of compositions and temperatures. Probably the best activi-
, 7 . .

ty values are those determined by Mathews and Baes wusing a transpira-

tion method to equilibrate gaseous HF—HZO mixtures with molten LiF—Ber

mixtures. However, activities derived from these heterogeneous equi-
libria are somewhat limited in accuracy (+ 6%) and the equilibrium
quotients were measured in the presence of Be0 as a saturating solid
which might have influenced the activity values.

Direct determination of the activity of BeF, by emf measurements

2
should yield more accurate values over a greater composition range than

. . — 1
any of the methods previously mentioned. Dirian, Romberger and Baes 0

measured the potential of the following cell as a function of tempera-
ture.

- 0.67 LiF|HF,H,,Pd

Be®|0.33 BeF 5]

2

The cell reaction is that shown in eq. (1). The two electrodes -

2+

Be IBeO

and HF,HZIF_ - were judged by Dirian et al, to be reversible
from polarization measurements.

In the present investigation these two electrodes were used to
determine the cell potential over a composition range of 0.30 to (.90

mole fraction Ber and a temperature range of 500 to 900°C (Fig. 1).

An attempt was made to obtain measurements in pure BeF,, but results

25
of useful accuracy could not be obtained presumably because of its high
viscosity and/or high electrical resistivity. Even at 900°C, pure BeF,

, . . 11 .
is very viscous (about 180 poise 7). The reaction vessel was not heat-

ed gbove 900°C because of the tendency of nickel to soften at such

‘elevated temperatures. The melting points of mixtures below 0.33 BeF

2
increase rapidly as the concentration approaches pure LiF as shown in
the LiF—BeFZ phase diagram12 in Fig. 1. The lower BeF2 concentrations
(below 0.30 Ber) were not investigated therefore since the accessible
temperature range was so limited.

According to the assumed cell reaction (eq. 1), the cell potential
should be dependent on the activity of BeFZ, the activity of beryllium

metal, and the‘partial pressures of HF and H

.
Py aReF
E =g 2L, 2 7€ (2)
2F o2
HF 2Be®

Previous-measurementslo with thé HF—HZ electrode, as well as the pre-
sent ones, indicate the gases to be sufficiently ideal at the elevated
temperatures and low pressure levels involved to allow the use of
partial pressures in place of fugacities in this Nernst expression.

In this study mixtures of HF-H, were bubbled through molten

2
LiF-—BeF2 and the partial pressures of the gases determined by alkali-

metric titration and gas volume measurements. The measured cell po-

tential (E) was then corrected for the effect of the gas pressure

quotient.
Pu
E-E+%zn—2—% (3)
Pur

The corrected potential EC is related to the activity of BeF2 by

0 RT
EC = E - oF A0 aBeFZ (4)

L4 e a 0 -
assuming the activity of Be to be unity.
Notation

The following notation will be used:

X

e

///////////\

€ogi7) L
[(}]

mmmmmmmmmmmmmmm

0.00 BeF,, The number preceding "BeFZ” denotes mole
fraction.
PT’PHF’PH LV, T Total pressure(atm), partial pressures(atm),
2 ' volume (£) per unit time, and temperature(°K)
in the region where gas enters the melt. “
o 0 0 ,.0 .
PT’PH sV ,T Corresponding measurements at the Bubble-0-
2 Meter.
P ,PO Barometric pressure and vapor pressure of H,0
B’ H,0 : o 2
2 at T".
?SF The approximate partial pressure of HF at the
titrator. (see Calculations section below.)
AP Pressure drop required to maintain gas flow
through the melt.
E Cell potential measured experimentally for a
fixed BeF2 concentration and temperature.
Ec The observed cell potential corrected to a gas
pressure quotient of unity (eq. 3).
E° The standard cell potential with pure BeF, as i

2
the standard state.

IT. EXPERIMENTAL

Chemicals

Gases

Commercial HZ was purified by passage through a deoxo unit, a mag-

nesium perchlorate drying tube and, finally, a liquid N, trap. An-

2

hydrous HF (99.97%) was used without further purification. Commercial
He was purified by passage through an ascarite trap, a magnesium per-

chlorate trap and, finally, a liquid N, trap.

2

Melt Components g

Lithium fluoride (99.5%) was obtained from American Potash and

Chemical Corporation. Beryllium fluoride was from three sources: | .

Brush Beryllium Corporation, K and K Laboratories, Inc., and com-
mercial BeF2 distilled by the Reactor Chemistry Division at Oak Ridge

National Laboratory. Most of the commercial BeF2 contained impurities
which ""poisoned" the electrodes (see p. 20). With the exception of one

composition, the distilled BeF, was used throughout this investigation

2

since the purity was such that no electrode “poisoning' was encountered.
Reagents
Reagent grade 1IN NaOH from Fisher Chemical Company was standard-
ized with potassium acid phthalate.
Apparatus
Experiments were carried out in the apparatus shown in Fig. 2.

Cell Design

A sketch of the nickel reaction vessel used to contain the LiF-BeF2
mixtures is shown in Fig. 3. This vessel was constructed of 2-1/2-in.
schedule 40 nickel pipe and was separated into two compartments by a
1/16=in. nickel sheet which extended to within 1/2-in. of the vessel
bottom. The nickel sheet was welded so that the only contact between
the two compartments was through the 1/2-in. opening at the bottom. The
vessel was 10-in. long.

Each compartment was equipped with the following: a 3/4-in.
Swagelok fitting through which melt components could be added or an
electrode inserted, a 1/4-in. gas exit tube, and a thermocouple well.
The Swageloks were equipped with Teflon seals when the electrodes were
inserted. This provided an electrical insulator as well as a leak-

tight fitting for the 1/8-in. nickel tubing. Cooling coils were wrap-

ped around each Swagelok to provide cooling when the reaction vessel

ORNL-DWG 67-13719

ANHYD
HF

He

POTENTIOMETER

AMPLIFIER RECORDER

POTENTIAL MEASUREMENT

I
[._l

Be
ELECTRODE | EXHAUST
| COMPART-
I MENT

HF-H, ELECTRODE :
COMPARTMENT | i=2||~

]

T, 7
-

, BUBBLE -0-
ggeu COMPARTMENTED SO0N METER
REACTION VESSE L

Fig. 2. Schematic Diagram of Apparatus Used to Measure Cell Potentials
in Molten LiF-BeF, Mixtures.

'3
ORNL-DWG 67-13720

L S

o

L

.

VA Vd L Z Z =

-

SIS ISP I

UL e

SIS IIAIAIAS,

SIS,

Fig. 3. Compartmented Cell Used to Contain Molten LiF-BeF, Mixtures.

10

was at elevated temperatures.

The reaction vessel was located inside an upright tube furnace,
the temperature was controlled by an L & N Series 60 D.A.T. Control
Unit. The temperature of the reaction vessel was checked with a cali-
brated Chromel-Alumel thermocouple and an L & N K~3 potentiometer.

A 4—in. diameter vessel, fitted with 1/2-in. diameter electrode
compartments, was used for preliminary measurements. Electrodes used
in the cylindrical compartments were insulated from the compartment
walls with boron nitride spacers, but even then accidental electrical
shorts were a problem. The large compartments of the reaction vessel
used in the present investigation eliminated the need for insulating
spacers except the Teflon seal at the top of the compartment, and no
problems from electrical shorting were encountered.

HF—H2 Electrode

An HZ,HF,Pd electrode of the type used by Dirian and Rombergerlo
was used in some of the preliminary measurements. This electrode
produced stable potentials but was quite noisy (+ 1 mv). Platinum
gauze was substituted for the palladium and was found to be just as
responsive and capable of very low noise levels (0.1 mv). The platin-
um gauze type (Fig. 4) was used for all measurements in this investi-
gation. Electrodes were prepared by forming an egg-shaped bag with th-
gauze and slipping the open end over 1/8-in. nickel tubing and tying

it securely with small diameter nickel wire. The other end of the bag

was crimped together so that the HF-H, mixture, passing down through

2

the nickel tubing had to pass through the gauze. The 1/8-in. nickel

tubing transmitted the HF—H2 mixture and provided electrical contact.

11

Beryllium Electrode

These electrodes (Fig. 4) were constructed by slipping a berylli-
um metal cylinder (3/8-in. 0.D., 1/8-in. I.D., and 1/2-in. length)
over a 1/8-in. nickel tube and crimping the nickel tube slightly on
each side of the beryllium cylinder to hold it securely. The cylinder
was positioned about 1/2-in. from the tip of the nickel tube. Lower-
ing the beryllium metal closer to the tip of the nickel tube caused an
increase in the potential noise. This was probably due to helium
bubbles temporarily insulating the beryllium from the melt. The 1/8-
in. nickel tube was used to bubble helium into the compartment and to
provide electrical contact.

Flow Control of Gases

Hydrogen Fluoride.-- The HF manifold pressure was controlled by

regulating the temperature of the HF supply cylinder. The flow of HF
was controlled by a mass spectrometer leak valve.13 The gas flowed to
a monel tee where it mixed with H2. The mixed gases were passed either

directly to the HF-H, electrode or a portion was split off for in-

2
fluent gas analysis.

Hydrogen.-- A pressure relief valve (Moore Products Company, dif-
ferential type flow controller, Model 63 BD, modified form) was used
to reduce the hydrogen manifold pressure to a constant value of 3.0 1b.
gauge. The flow was then controlled by a brass needle valve obtained
from Nuclear Products Company. The H2 was then mixed with the HF as
described above.

Helium.—-— Helium flow was controlled with the same type .needle

valve used for the Hz; no attempt was made to control the manifold
12

ORNL-DWG 67-13721

BERYLLIUM ELECTRODE

HF = H, ELECTRODE

Fig. 4. HF-H, and Beryllium Electrodes.

13

pressure since a constant flow through the beryllium electrode was un-
necessary.

Titration Assembly i

The NaOH titration vessel was a 200-ml test tube. A rubber stop-
per was inserted into the test tube and was equipped with the following:
Teflon (1/4-in. dia.) entrance and exit tubes for gas, a 5-ml Lab-
Crest microburet, and Beckman No. 39166 (glass, Ag-AgCl) electrodes.

A Beckman Zeromatic II pH meter was used to determine the endpoint.
The pH was maintained on the alkaline side of the endpoint to avoid
glass attack. Duplicate titration assemblies were used to measure in-
fluent and effluent HF concentrations.

Electronic Equipment

An L & N K-3 potentiometer, calibrated with a standard cell from
Eppley Laboratory, Inc., was used to buck out most of the cell voltage.
When the cell voltage exceeded the range of the potentiometer, a
mercury battery (Mallory Duracell No. RM 42R) was connected in series
to extend the bucking voltage range. The voltage of the battery was
checked daily with the potentiometer, and it proved to be an extremely
stable voltage source (+ 0.02 mv/day). The remainder of the cell volt-
age (< 100 mv) was coupled, through a Philbrick Model MP solid state
operational manifold equipped with P65 AU amplifiers, to a Honeywell
Brown Electronik recorder.

Procedure

Measurements

The data obtained for each cell measurement were cell potential,

cell temperature, and partial pressures of HF and H2.

14

Cell Potential.—— Cell potential measurements were always preced-

ed by standardization of the potentiometer against a standard cell and
a check of the amplifier zero on the recofder. During measurements of
the cell potential, most of the voltage was bucked out by the potentio-
meter (or potentiometer plus mercury battery), and the remainder read
off the recorder. The noise level of the potential varied from + 0.2
mv to as high as + 1.0 mv for the high viscosity melts. Although the
potential fluctuated as indicated, it did not show any drift toward a
higher or lower potential,

H2 and HF Partial Pressures.—— The determination of partial

pressures was made as follows:

(1) A measured volume of standardized NaOH was added to the

titration vessel.

(2) The time required for the HF to neutralize the base was

determined.

(3) H2 flow rates were determined by the use of a Bubble-0-

Meter.
(4) 1Influent partial pressures were checked simultaneously in

experiments up to .60 Ber.

(5) Partial pressure determinations were never begun until the

cell potential had been steady for at least 30 minutes.
(6) Six to ten successive titrations were carried out.

(7) The barometric pressure, temperature of the titratica as-

sembly, and pressure drop across the system were recorded.

(8) Helium flowed through the beryllium electrode at a rate of
30 to 75 ml/min. This flow rate provided adequate sparging
for this compartment and decreased the thermal gradients.

Helium also protected the electrode from any HF which might

15

have entered the compartment.

Melt Temperature.-- The temperature of the melt was determined by

a calibrated Chromel-Alumel thermocouple positioned carefully to the
exact depth of the electrode. Positioning was carried out as accurate-
ly as possible to reduce any error in temperature readings caused by
thermal gradients in the melt.

Calculations.—— The required calculations to evaluate PHF’PH and
2

EC were carried out as follows:

(1) The titration times were averaged for a series of titrations
of a fixed increment of standard base. The gas volume passed was then
calculated by (time of titration)/(time per 100 ml of gas) x 100 = ml
of gas.

(2) The number of millimoles of HF removed from the H2 stream by

the titration was calculated by

(m1l NaOH) (conc. of NaOH) = millimoles HF.
(3) It was found convenient first to define an approximate
partial pressure of HF (PEF) by introducing these measured quantities
into the following simple gas law expression

o _ f(mmoles HF)(0,08206) (abs.temp. of Bubble-0O-Meter)

PHF ml of H2 passed

The exact expression for the partial pressure of HF (PHF) at the e-
lectrode includes the effect of the pressure drop (AP) caused by the
pressure required to maintain bubbling through the melt and subsequent

titrator, and the saturation of the H, stream with water prior to

2

measuring the flow rate. The relationship (eq. 11) between PEF and
16

PHF which is required was developed as follows:

(3.1) The system may be visualized as consisting of three
regions ~ the cell vessel (at PT,T), the titration assembly, and
the Bubble-O-Meter (at P2, T°).

(3.2) Assuming that the number of moles of HF passing

through the cell assembly and into the titrator is the same for

a given time interval, then

nHF - o

and

"F T RT
(Note that the first expression is merely a rearranged form of the

o) y.

previous equation which defines PHF

Combining these two equatioms,

PHF v _ PHF v
RTo RT
o
o) v T
P_ =P . (5)
HF EF v To

(3.3) Moles of H2 passing through the cell and through the

Bubble-0-Meter may be treated in a like manner

O O
P \Y% P V
o, - B o B
2 RT RTO
P
vo T i,
S = o - (6)
Vv T PH
2
(3.4) Combining equations (5) and (6)
o P.
Pur = Pup __Q_z_ (72
th

17

(3.5) The total pressure at the HF—H2 electrode is

- PH2=PB+AP—PHF (8)

and the total pressure of the gas at the Bubble~O-Meter is

0 o o
P,.=P_ =P + P
T B H2 HZO
o o)
P =P - P . (9
Hz B H20

(3.6) Substituting (8) and (9) into equation (7)

, _ 50 PB 4+ AP - PHF (109
HF HF PB - PH 0
2
and solving equation (10) for PHF gives
PB + AP
P =P 5 (11)
HF HF PB PHZO + PHF

PHF can be evaluated since all the other quantities are known. The

values for P then can be substituted into equation (8) and the P

HF H

2
determined.

(4) Using the measured cell potential (E) and the partial pres-

sures of HF (PHF) and H (PH ) the corrected cell potential was then

2 2
calculated by
PH
RT 2
EC = B + 5F n P2 . (12)
HF

The cell potential E was recorded after gas equilibration with the

melt was accomplished, and the melt temperature was constant.
18

Systematic Errors

The preceding method of calculating partial pressures does not in-
clude corrections for the diffusion of HZ through the walls of the
nickel reaction vessel, nor does it consider the effect of thermal dif-
fusion. Melt purity and composition should also be considered as

sources of systematic errors.

Hydrogen Diffusion.-— According to published diffusion coef-

ficients,l4 the diffusion of H2 out of the nickel reaction vessel

could be a few milliliters per minute at elevated temperatures. The

rate of HZ diffusion was measured experimentally by Mathews and Baes7

in a vessel similar to the one used in the present experiment. They
obtained the following rates: 700°C, 0.035 ml/sec; 650°C, 0.025 ml/sec;
600°C, 0.015 ml/sec. Extrapolatioh of these measurements to 800°C and
900°C yields diffusion rates of 0.055 ml/sec and 0.075 ml/sec, re-

spectively. Typically emf experiments were conducted with a H, flow -
y yP y P 9

rate of 2.5 ml/sec. This means that the measured volume of H2 would

be in error by 0.67% at 600°C, 1.407%7 at 700°C, 2.20% at 800°C, and 3.00%
at 900°C. These errors in partial pressures would cause the calculated
potential to be about 1.0 mV lower at 700°C, 1.5 mV lower at 800°C,

and 2.0 mV at 900°C. As mentioned previously, the HF-H, mixtures were

2

analyzed before and after entering the HF-H, electrode compartment, and

2

no discrepancy between the two could be detected. These analyses were

performed periodically on all compositions up to .60 BeF, and tempera—

2

tures up to 700°C. Melts of higher BeF2 concentrations were not check-

ed in this manner because difficulty was encountered in keeping the =

flow rate constant through these high viscosity melts when the split .

19

flow technique was attempted.

Since the H2 diffusion effect was not observed at temperatures up

to 700°C, the errors calculated above seem to be somewhat high,

Thermal Diffusion.-- Recognizing the possibility that H2 and HF

might tend to separate along the thermal gradient in the reaction
vessel, we carried out an experiment to see if this effect was signifi-
cant.

The total flow of HF-—-H2 passing through the HF—-H2 electrode was
varied between 23 ml/min and 160 ml/min. The cell potential increased
by about one millivolt over this range; the increase in cell potential
over the range of flow rates where potential measurements were actually
made (100-160 ml/min) was less than 0.3 millivolt. This is thought to
be sufficient evidence that the thermal diffusion effect was not sig-

nificant.

Gas Cooling Effect on Electrodes.-- The experiment mentioned above

also indicates that cooling of the electrodes by gas flow could not
have caused more than a one millivolt error in the potential measure-
ments. It was felt that although the possibility of a small error in
potential might result from flow rates of 100-160 ml/min, vigorous agi-
tation of the melt was necessary to reduce the effect of thermal
gradients in the melt.

Melt Composition.-- A composition error which was discovered at

the end of run No. 4 was apparently caused by distillation of BeF2 to

cooler regions of the apparatus. TIn this particular run, many attempts

were made to measure the cell potential of pure BeF, over a temperature

2

range of 800°C to 900°C. At these elevated temperatures the vapor

20

15

pressure of BeF, is appreciable. Evidently loss of BeF, occurred

2 2

during the periods at high temperatures. Subsequent additions of LiF

were made to bring the BeF, composition down to 0.9C BeF,, 0.80 BeF

2 2? 2°

0.70 Ber, and finally to 0.33 BeF The error was discovered at (.33

2

Ber since the potentials did not correspond to previous measurements.

At this point enough BeF2 was added to bring the composition up to a

book value of 0.40 BeF2 permitting thermal analysis of the liquidus
temperature. Thermal analysis was carried out and compared with the

. . . 16 . 14 A
L1F~BeF2 liquidus data. Results indicated that the true composition

was 0.388 Ber. Assuming that all or most of the Ber was lost before

the LiF additions were begun, the compositions at 0.90, 0.80, and 0.70

BeF, were reduced to 0.896, 0.791, and 0.686 BeF

2 respectively. Changes

23
in Ec in the (.60 Ber to 0.80 BeF2 regions are small; thus, the compo-
sition error at these concentrations should not be significant. How-

ever, the values obtained for 0.33 BeF2 in this run were more seriously

affected by the uncertainty in composition and were not used.

Melt Impurities.-- Early in the experimental work it was found

that commercial Ber which contained about 1000-2000 ppm sulfur and
1000 ppm total of Fe, Cr, Cu, and Ni caused "poisoning"™ of both elect-
rodes. The cell potential would decay as much as 0.5 volts. Spectro-
graphic examination of the Be electrode showed that Fe, Cr, and Ni had
been reduced on the surface of the electrode; also metallic impurities
originally in the beryllium metal were much higher in concentration on

the electrode surface after exposure to the melt. These included Al,

Mn, and Ti, the most abundant one being aluminum.

21

The platinum gauze on the HF-H, electrode was darkened by exposure

2

to these contaminated melts. This effect was not pursued since it was

already evident that BeF, with the impurities mentioned above was not

2

suitable for use with the beryllium electrode.

Distilled BeF containing only trace amounts of impurities, was

29
tested; and none of the problems mentioned above were encountered. This

material was used for all measurements in this investigation.

Oxide Contamination of Melt.-- The oxide chemistry of LiF-BeF

2

mixtures must be considered since the raw materials contain small a-
mounts of moisture, and since beryllium oxide is an impurity in beryl-
lium metal. Moisture contamination of the melt was kept to a minimum
by freezing the mixtures prior to additions of raw materials. Beryl-
lium fluoride was stored in a dry box prior to use.
. o , . . 17

A standard purification procedure for removing oxides from fluo-—
ride mixture is HF—~H2 sparging. This procedure was used throughout the
present investigation to remove oxide impurities from the melts. Con-
tinuous use of HF—H2 as one of the electrode materials undoubtedly kept
the oxide concentration small. Typically, beryllium metal contains a-

18 ) .
bout 1000 to 4000 ppm oxygen. Using the larger value for oxide con-
tamination, the maximum concentration of oxide contributed by the beryl-
L] —3 , L
lium electrode would have been 1 x 10 ~ moles/kg, which is about one
. . o 19 .

order of magnitude below the solubility of BeO. Therefore, no sig-

nificant error is expected due to oxide contamination of the melt by

addition of raw materials or from the metal electrode.

22

Summary.-— The only known systematic errors that might be signifi-
cant are those attributed to H2 diffusion and gas cooling of the
electrodes. These are opposite in effect, and both are expected to be

within the experimental scatter of the data.

Random Errors

In order to obtain a satisfactory estimate of the expected pre-
cision of the experimental measurements, the various random errors and
their probable magnitude were considered.

Precision of Potential Measurements.-- Typically short term po-

tential fluctuations were about + 0.2 mV for mixtures up to 0.60 Ber.

These fluctuations increased to about + 1.0 mV for 0.90 BeFZ mixtures.
However, even at the higher Ber concentrations, the average value of
the potential did not change more than + 1.0 mV over a period of one to

two hours.

Melt Temperature.-— The temperature of the melt was controlled to

+ 0.2°C. The temperature gradient in the melt varied from about 2°C in

melts up to 0.60 BeF,, to a maximum of 9°C in 0.90 BeF,. However, care

2’ 2

was taken during each potential measurement to position the thermo-
couple in the melt so that it coincided within<i_l/8~in. of the elect-
rode depths. This should have reduced the temperature uncertainty to
no more than + 0.5°C, which corresponds to an uncertainty of + 0.4 mV
in the potential.

Melt Composition.—- The LiF-BeF, mixtures were prepared by adding

2

weighed amounts of the components to the reaction vessel. The pre-
cision of weighing and transferring materials to the vessel was about

+ 0.2%.

23

Titer Precision.-- Commercial 1IN NaOH in one quart bottles was

standardized with potassium acid phthalate. Agreement of the standard-
ization values was about + 0.17%Z of the average value. The uncertainty
in burette readings during HF titrations was about + 0.57%.

Temperature of Bubble-0-Meter.—- The Bubble-0O-Meter temperature,

which was the temperature of the gas as its volume was being measured,
varied no more than + 0.1°C during a given experiment. The same tem-
perature variation applies to the titration vessel where the H2 was

saturated with water.

Flow Rate Determination.—- The precision in the timing of HZ flow

rates was 0.1 sec per 100 ml. For a typical value of 50 seconds per
100 ml, this would amount to 0.27%.

Endpoint Precision.-~ The precision in timing of the endpoints was

about + 0.3% of the measured value.

H, and HF Flow Rates.-- The H2 flow rate was constant at + 0.17% of

the measured value. Successive HF titrations varied as much as + 1.0%
from the average value. This was apparently due to irregular flow rates
of HF. The variation was random and no bias was noted. The error in-
volved in HF flow rate is much greater than any of the other errors as-

sociated with the calculation of P therefore the others will be con~-

HF’
sidered negligible.

Statistical Error Analysis

The probable error to be expected in the calculated cell potential,
EC was determined from the estimated magnitude of random errors in
various quantities which appear in eq. 3. If the contribution of each

observable is related to a calculated function Q and expressed as

24

Q=1 (a,b,C“—“—'),

the contribution of such errors can be calculated.

- » L4 4 20
low a normal distribution, it can be shown

If the errors fol- -

that the following equa-

tion allows for partial cancellation of errors of opposite sign:

0Q? (——Q) oa +<—9> ob +<—+Q> gt 4 .

‘Applying this relation to

P
H
E =E + Bz-fin 2
c nF P2
HF
gives
02 BEC 2 O2 'aEc 2 02
EC = SEP E + 5—“~ T

BEC 2
+ oP
BPEF HF

(13)

Differentiation of the above quantities may be carried out to obtain

EE.C.__,]_ .?E_c_= 9E , R

oE ’ oT aT 2F
and

oFE ] P P )

_c . Rry . f, OB CHF}|_

SPHF 2F P2

HF
e BPHF —
(It may be recalled that Py =Pp+ AP ~ P
2

AP may be neglected, thus, Py = PB PHF)'

The expected uncertainty of EC

HF®

Hz

2 3

PHF
E ZPB —' PHF

F|2P (PB PHF)

For the present analysis

can now be calculated by

25

~ n 2 - 12
. H 2P - P
2 2 JE R 2 2 RT ( B HF ) 2
o = g +l==+ = n— | o_ + |- — s . (14)
E E T 2 2 -
c b ¥ P b PN 2P (PpPypd/ | Pyg

Evaluating the above equation numerically for the uncertainty in the

calculated potential, using typical partial pressures of HF and H2 at a

temperature of 1000°K and using the estimated uncertainties of + 0.5 mV,

+ 0.5°C, + 0.001 atm respectively in E, T, and P one obtains

x HF’
2 4.2 _ - _
of = (5% 107H% + (8.95 x 10 206 x 107092 + (2,102 x 10792
(o4
2 - - -
0f = (2.50 x 10 'y % (2,00 x 1077y + 4.42 x 1079
C
02 = 4,87 x 10_6
E
C
8]
- E. = + .0022 volts.

The calculated standard deviation of + 2.2 mV is slightly larger than
the standard deviation obtained from least squaring the actual data.
The range of the standard deviation for the actual data was about 1.0 mV
to 2.0 mV. The major uncertainty is in the PHF term and is due to the
limited precision of HF flow.
ITI. RESULTS
Tabulation
Table 1 contains the data obtained from each experiment, arranged

according to melt compositiomn.

Experiments

. Four separate series of experiments were conducted during this in-

. vestigation. The cell potential of each melt composition was determined

26

for a range of temperatures. In the first series the reaction vessel

was initially loaded with 0.33 BEFZ" Subsequent additions of BeFZ were
made to bring the concentrations up to 0.40 Ber, 0.50 Ber, and to
0.60 Ber.

In the second series the reaction vessel was initially loaded with

. ,» 0.80 BeF,, 0.70

Ber, 0.60 BeFZ, 0.50 Ber, 0.40 Ber, and finally 0.33 BeF

pure BeF_, enough LiF was then added to give 0.90 BeF

9 Only omne

or two determinations were made for 0.60 BeF2 and lower BeF2 compo-

sitions since these were checks of previous measurements.

Pure Ber was also the initial composition for the third series of

measurements. Subsequent additions of LiF¥ were made to give 0.90, 0.80,

0.70, 0.60, and (.33 Ber. As previously mentioned the data for 0.33

BeF2 was considered to be in error and was not used.

In the last series of measurements the reaction vessel was loaded

initially with 0.33 BeF Enough LiF was added to give 0.30 BeF then

2" 23

BeF2 was added to bring the composition back to 0.33 BeF

o
Corrected Cell Potentials
The corrected cell potentials (EC) for various compositions are
plotted as a function of temperature and shown in Fig. 5. Ec is assumed
to be a linear function of temperature, Ec = A+BT, over the temperature
range investigated. The data points for each composition were least

squared, and the calculated lines are also shown in Fig. 5. The para-

meters from the least squaring treatment are listed in Table 2.

ORNL-DWG 67-137234

£, {valis)

B
1.60 \ - ™.

/
vald’4
/

/
/S /S S LSS

55 N
0.90
\\Q\;:
. ) \
0.30, 0.33, 040, 050, 0.60 BeF, 550 600 650 700 750 800 850
0.70 BeF, 600 650 700 750 800 850 900
0.80, 0.90 BeF, 850 700 ., 750 800 850 200 950

TEMPERATURE {*C)

Fig. 5. Correlation of Pressure Corrected Cell Potentials for Various
Compositions as a Function of Temperature.

238

OO OO0 C OO OO OO 0O Q-

Table 1. Pressure Corrected Cell Potentials (Ec) Obtained from
Measurements in Molten LiF—Ber
X Temp. E (volts) X Temp. E (volts) X Temp. E (volts)
BeF (°C) c BeF (°C) c BeF2 °C) c
0.30 585.1 1.8864 0.50 608.5 1.7671 0.90 804.1 1.5819
0.30 646.0 1.8447 0.50 613.1 1.7648 0.90 866.0 1.5307
0.30 696.2 1.8126 0.50 685, 2 1.7116 0.90 754.8 1.6208
0.30 732.0 1.7882 0.50 681.0 1.7163 0.90 706.2 1.6613
0.30 609.2 1.8680 0.90 778.0 1.5996
0.30 681.0 1.8213 0.60 637.0 1.7263 0.90 876.5 1.5277
o o 0.60 566.5 1.7856 0.90 802.5 1.5905
33 562.9 1.8812 0.60 735.0 1.6555 0.90 702.0 1.6620
33 514.0 1.9152 0.60 637.0 1.7283
33 601.0 1.8561 0.60 591.1 1.7656
33 649.5 1.8230 0.60 521.0 1.8160
33 698.0 1.7891 0.60 591.0 1.7672
33 624.1 1.8397 0.60 591.0 1.7667
33 565.0 1.8757 0.60 521.2 1.8170
.33 720.8 1.7653 0.60 710.0 1.6760
.33 529.0 1.9035 .60 709.0 1.6771
.33 617.2 1.8436 0.60 662.0 1.7128
.33 617.2 1.8396
.33 550.0 1.8857 0.70 760.0 1.6310
.33 624.5 1.8356 0.70 706.5 1.6736
33 671.0 1.8016 0.70 609.6 1.7477
.33 812.0 1.7081 0.70 562.0 1.7844
0.70 663.0 1.7085
0.40 597.5 1.8069 0.70 794.9 1.6079
0.40 609.5 1.8000
0.40 609.5 1.7991 0.80 800.4 1.6027
0.40 661.2 1.7623 0.80 751.0 1.6335
0.40 686.6 1.7430 0.80 656.5 1.7132
0.40 685.8 1.7478 0.80 632.5 1.7252
G.40 528.2 1.8578 0.80 704.9 1.6725
0.40 528.2 1.8578 0.80 754.0 1.6321
0.40 536.5 1.8540 0.80 681.0 1.6907
0.40 535.0 1.8522 0.80 633.5 1.7205
0.40 706.0 1.7337 0.80 611.5 1.7484
0.40 706.0 1.7316 0.80 639.7 1.7206
0.80 887.5 1.5304
0.50 632.8 1.7516 0.80 810.0 1.5967
0.50 561.5 1.8022 0.80  735.4 1.6482
0.50 728.0 1.6765 0.80 ° 663.0 1.7051
0.50 707.0 1.6972 0.80 616.0 1.7387
0.50 658.0  1.7340 0.80 814.1  1.5909
0.5 617.3 1.7629
0.50 568.3 1.7991
(.50 546.6 1.8127
0.50 503.3 1.8467
0.50 511.8 1.8387

29

Table 2. Parameters from Correlation of EC as a Function of

Temperature at Specified Compositions (Ec = atbT)

XBeF Intercept + o (Slope + o)x 1()“3 0Ecx103(volts)
2 (a) (b)
0.30 2.45212 0.0053 - 0.6607 0.0080 0.98
0.33 2.46021 0.0048 - 0.6944 0.0077 2.28
0.40 2.4258  0.0040 - 0.7091 0.0065 1.56
0.50 2.4179  0.0041 - 0.7369 0.0065 1.68
0.60 2.4162  0.0054 - 0.7537 0.0061 1.98
0.70 2.4226  0.0049 - 0.7643 0.0071 1.42
0.80 2.4149  0.0072 - 0.7595 0.0101 3.28
0.90 2.4296  0.0162 - 0.7861 0.0205 3.54

IV. DISCUSSION

Thermodynamics of LiF—BeF2

The activity of BeF, may be calculated from eq. (4) using the emf
data (Ec) obtained in this study if the appropriate values for the
standard cell potential (E°) are known. As mentioned previously, sever-—
al attempts were made to determine E° experimentally, but values of use-
ful accuracy could not be obtained because of high melt viscosity and
possibly because of high electrical resistivity.

The values of EC obtained for various BeF, compositions do not lend

2

themselves to direct extrapolation to E, However, the standard cell
potential may be calculated by relating data in this study with the BeF2

liquidus data in the following manner. 1If the assumption is made that

the standard cell potential (EO) varies linearly with temperature

30

(E° = A+BT)#* then eq. (4) may be written

RT
EC = (A+BT) - T n aBeFZ

2F[ (A+BT) - EC]

L tmag e = RT (15)

It is then possible to equate eq. (15) to

in a (BeF

BeTF 5 sat'n) = - —(= - =) (16)

2 £

(where AHf is the heat of fusion of BeF,, and Tf is the melting point of

pure Ber) to obtain a relationship between A, B, AH_ and EC calculated

f

at the BeF, liquidus temperatures and compositions. Combining eqs. (15)

2
and (16).

2F[(A+BT) - E_] - AHf(_l‘ 1

RT R T T
f

which simplifies to

2F Ec AHf 1

T = (2FB - "Egfi + (2FA + AHf'E . (17)

This relationship permits correlation of the data (EC) obtained in the
present investigation with the phase diagram data.12 A plot of the a-
bove expression as ZFEC/T vs 1/T should be linear since the intercept
and slope contain only constants. Using the least square parameters for
each composition (Table 2), values for Ec were calculated for the

liquidus temperatures at 0.515, 0.60, 0.70, 0.80 and 0.90 BeF., (see

2
footnote ) and then plotted according to eq. (17) (Fig. 6). The result-

ing points follow the predicted linear relationship within their pre-—

* Using the heat capacity data in the JANAF Tables (Ref. 21), the ACp
effect over a temperature range of 450° - 900°C was evaluated and
found to be negligible.

31

dicted uncertainties. Parameters for the least squared line are:

AH

Intercept = - 0.03805 + 0.0034,(Kcal/°K) = 2FB -7 (18)
f

Slope = 113.84 + 2.48 ,(Kcal) = 2FA + AHf (19)

The point at 0.90 BeF, was not used because the uncertainty in EC is

2

relatively large. |

Values for A and B were calculated using various literature values

for the heat of fusion of BeF, and a melting point of 555°C for pure

BeFZ. A tabulation of these wvalues is shown in Table 3. The values of

A and B which are consistent with values obtained in the present in-
vestigation are those for which the heat of fusion is assumed to be 2.0

kcal/mole or less as shown in Fig. 7. A heat of fusion for Ber >2.0

kcal/mole yields values of E® which are greater than corresponding values

of Ec at the higher BeF2 concentration. This creates an impossible

situation where the activity of BeF, in the mixture is greater than the

2
activity of pure liquid BeF Thus measurements in the present study

8,9
-

9°
clearly support the lower values for the heat of fusion for BeF
The emf data (EC) obtained in the present study were fitted by
least squares to the expressions listed in Table 4, using the lowest
literature value (1.13 kcal/mole) for the heat of fusion of Ber. A

plot of the smoothed lines for EC at various BeF, concentrations is

2
shown in Fig. 8. The pressure corrected cell potentials (Ec) differed
by 1.4097 standard deviations from the smoothed values given by eq. 4-1

(Table 4), and the average deviation of Ec from smoothed values was

approxXimately + 2.5 mV. The average deviation is consistent with the

value predicted by the error analysis for Ec'

32
ORNL-DWG 67-13722
0.515 Bef,
0440 —
0430 ////////J S
0120 080 Bef, ] ]
-
>
u
[19
od
0410 070 Bek, ]
0.80 Bef | /
Q%fiféfi/4
0400 /f —
Trm FOR Bef
H
0.090 |
1.20 1.25 130 1.35 1.40 1.45 1.50 1.55

100 g

Fig. 6. Correlation of Ec with BeF, Liquidus Data.

33

ORNL-DWG 68-230

1.825
1.800
1.775
®
£ 1750
Luo
fa
z A H¢=143 keal /mole
< 1.725 —
i OLu A H;=2.00 kcal/mole
A H¢=5.80 kcal/mole
1.700
1.675
1.650
550 575 600 625 650 675 700
7 (°C)

Fig. 7. Effect of the Heat of Fusion of BeF, in Determining E°
(dashed lines). The solid lines represent Ec for various mole fraction

of BeF;.

34

Table 3. Calculated Parameters for the Standard Cell Potential of
Pure BeF (EO = A + BT) Assuming Various Values for the

i Heat of Fusion of BeF2
Heat of Fusion
(kzliiie) A3 8P} 3 No.
x 10 °
0.70 2.4525 - .8065
1.00 2.4460 - .7986
1.13 2.4432 - .7952 9
1.60 2.4330 - .7829 8
2.00 2.4241 - 71725
2.50 2.4135 - .7594
5.80 2.3420 - .6730 7

(a) From eq. (19).

(b) From eq. (18).

35

Table 4. Expressions for Cell Potentials and Activity

Coefficients in the LiF—BeF2 System

Eq. No.

o  2.3RT 2. 3RT
4-1 E.=E - =95 log XBeF2 - T oF 198 YBeF2

(a)

T
]
t
it

2.4430 - 0.0007952T

2353.5)X2
T LiF

4-3 log YpeF (3.8780 -

2

+(-40.7375 + 202283
+ (94,3997 - SRRy
+ (=67.4178 + égg%é;égxiiF
44 log v, = 0.9384 - 23208

14652.7, 2

T )XBer

+ (-36.9734 +

74588.5, 3

T XpeF
2

113592.3. 4

X
T BeF2

+ (126.0947 -

+ (-158.4173 +

52923.5,.5

T )XBeFZ

+ (67.4178 -

(a) Calculated using a heat of fusion for BeF2 = 1.13 kcal/mole.

36

ORNL-DWG 67-13718

2.00

1.80

£, (volts)

NN |

.70 AN : x
160 \\
0.80
0.80
PURE BefF,
2 N
1.50 | %\
5G0 600 700 800 S00
7 (°C)
Fig. 8. Correlation of Pressure Corrected Cell Potentials Ec as a
Function of Temperature Using Smoothed Parameters.

37

A Gibbs-Duhem integration of the expression for Y peF Table 4) was
. 2
carried out to give the corresponding expression for Y14F (eq. 4-4,

Table 4). The integration constant for eq. 4-4 was determined by com-—
parison with Y14iF values derived from the liquidus data12 and a heat of
fusion of 6.47 kcal/mole for LiF. A more accurate evaluation of the
integration constant should be possible when the heat of mixing measure-

22 '
ments of Holm and Kleppa =~ become available for the LiFmBer system.
Smoothed values of Ypep are shown as a function of composition at
2
several temperatures in Fig. 9. These results are consistent with those

obtained by Mathews and Baes7 over a composition range 0.30 to 0.60 Ber.
However, at X > 0.60 the results are not in agreement. The values
2

obtained in the present study are thought to be the more reliable since
they are consistent with both the phase data and with a low heat of

fusion for BeF,. The previous measurements at compositions > 0.60 BeF

2 2

might be in error because of difficulties in mixing LiF-BeF, at high

2

BeF2 concentrations and because of the effects of BeO saturation. In

the present study it was found that at 0.90 BeFZ, a well-mixed melt was

not obtained until the temperature was raised above 850°C. This pro-

cedure was followed for all high BeF, concentrations toc ensure proper

2

mixing of the LiF-BeF Another possible error in the previous measure-

20
ments could have been the presence of Be0 as a saturating solid. The

solubility of Be0O might tend to influence the BeF, activity more at high

2

BeF2 concentrations.

The free energy and heat of the cell reaction (eq. 1) were calcu-

lated assuming a heat of fusion for BeF2 = 1.13 kcal/mole. These wvalues

combined with the available thermochemical wvalues for HF21 were used

ORNL-DWG 68- 231

O O 0.2 0.3 04 0.5 0.6 0.7 0.8 0.3 1.0

X,
BeF2

Fig. 9. Activity Coefficients in Molten LiF-BeF, Mixtures.

39

to derive the free energy and heat of formation for liquid Ber at 900°K
(Table 5). Mathews and Baes7 measured the equilibrium quotient for the

reaction

HZO(g) + Ber(d) Z 2HF(g) + Bel(s) (20)

If eq. (20) is combined with the reaction in eq. (1) the resulting re-
action is

HZO(g) + Be(s) Z BeO(s) + Hz(g) (21)

The free energy and heat of this last reaction thus could be obtained by
combining the two sets of measurements, and, since the thermochemical
data for H20 are accurately known, improved free energy and heats of

formation for Be0 could be calculated. These are shown in Table 5. The

preceding calculations for BeF, and Be0 were made for a temperature of

2
900°K. Values for other temperatures were generated using the heat

capacity data in the JANAF Tables.21

Reference Electrodes
Both electrode half-cells used in the present investigation per-
formed acceptably for use as reference electrodes, both being stable and
reproducible.

Beryllium Electrode

The BelBeZ+ electrode should work well in any melt containing beryl-
lium ions and no reducible cations. Potential fluctuations due to this
electrode were masked in the present study by the fluctuations due to
the HF-H, electrode, but should be less than + 0.1 mV. Beryllium e~

lectrodes were fabricated from three different batches of beryllium

metal and no discrepancies in potentials were noted when the electrodes

Table 5. Formation Heats and Free Energies of BeF2 and Be(
Compound Temp. State AHf AGf
(°K) (kcal/mole) (kcal/mole)
22 22
Ber 298 Cryst. - 246.01 (=242.30 + 2) - 234.39 (~230.98 + 2)
Ber 800 Cryst. - 244,75 - 215.50
Ber 900 Liquid - 243,12 (+ 1.1) - 211.90 (+ 1.1)
Ber 1000 Liquid - 242,54 - 208.47
o~
o
22 22
BeO 298 Cryst. - 145,85 (-143.10 + 0.1) - 138.36 (=136.12 + 0.1)
Bel 800 Cryst. - 145.68 - 125.70
BeQ 900 Cryst. - 145,57 (+ 1.5) - 123.20 (+ 1.5)
BeO 1000 Cryst. - 145,46 - 120.73

41

were interchanged. Therefore, the electrode response does not appear to
be a function of a particular batch of beryllium metal.

The beryllium electrode does not appear to be suitable for small
cell compartments since mass transfer causes the electrode to become en-
larged due to spongy deposition of the Be metal, and eventually electri-
cal shorts develop between the electrode and the cell compartment wall.

HF~H2 Electrode

The Pt, HF,Hle_ electrode should be a suitable reference electrode
in any fluoride—containing melt where there is no possibility of oxida-

tion by HF or reduction by H The solubility of HF in LiF-BeF, is

2° 2

low23 (about 0.0003 mole fraction for the partial pressures of HF used
in this study), and no significant solubility of HZ is expected in this
system. Potential fluctuations due to this electrode appear to be a
function of the melt viscosity. Fluctuations are about + .1 mV in melts
with a viscosity of one poise or less.

The precision of this electrode was limited somewhat in the present
study by the method of HF delivery, as previously mentioned. In future

experiments the H,-HF mixture will be obtained by passing H, through a

2 2

thermostated NaHF2 bed. It is hoped that this will be a more precise

method of producing mixtures of HF and H, of constant composition.

2
10.

11.

12.

13.

14.

15.

16.

42

REFERENCES

W. R. Grimes, MSRP Semiann. Prog. Rept. July 31, 1964, ORNL-3708,

p. 230.

J. Berkowitz and W. A. Chupka, Ann. N. Y. Acad. Sci., 79, 1073 (1960)

A. Buchler and J. L. Stauffer, "Vaporization in the Lithium Fluoride-

Beryllium Fluoride System,” SM-66/26 in Thermodynamics, vol. 1,

IAEA, Vienna, 1966.

T. Férland, "Thermodynamics of Fused Salt Systems," p. 156 in Fused
Salts, ed. by B. R. Sundheim, McGraw-Hill, New York, 1964,

J. Lumsden, Thermodynamics of Molten Salt Mixtures, p. 227, Academic

Press, London, 1966.

A. Buchler, Study of High Temperature Thermodynamics of Light Metal

Compounds, Army Research Office (Durham, N. C.) Progr. Rept. No.

9 (Contract DA-19-020-ORD-5584) Sept. 30, 1963.

A. L. Mathews and C. F. Baes, Jr., J. Inor. Chem., 7, 373 (1968).

J. A. Blauver et al., J. Phys. Chem., 69, 1069 (1965)

A. R. Taylor and T. E. Gardner, Some Thermal Properties of Beryllium

Fluoride from 8° to 1,200°K, U.S. Bureau of Mines, Rept. No.

RI-6644 (1965).

G. Dirian, K. A. Romberger, and C. F. Baes, Jr., Reactor Chem. Div.

Ann. Progr. Rept. Jan. 31, 1965, ORNL-3789, pp. 76-79.

C. T. Moynihan and S. Cantor, Reactor Chem. Div. Ann, Progr. Rept.

Dec. 31, 1966, ORNL-4076, p. 25.

R. E. Thoma et al., submitted for publication in J. Nucl. Mat.

Diaphragm Type Adjustable Leak Valve (Ref. No. C-I 24492A) obtained

from ORGDP, Oak Ridge, Tenn.

S. Dushman, Scientific Foundations of Vacuum Technique, pp. 607-618,

Wiley and Sons, N. Y., 1949.

S. Cantor et al., Reactor Chem. Div. Ann. Progr. Rept. Dec. 31, 1965,

ORNL-3913, p. 27.

Temperature-Composition values used here for the BeF
supplied by S. Cantor of this Laboratory.

2

liquidus were

17.

18.

19.

20.

21.

22.

23.

43

J. H. Shaffer, MSRP Semiann. Prog. Rept. July 31, 1964, ORNL-3708,
p. 288.

G. E. Darwin and J. H. Buddery, Beryllium, p. 85, Butterworths
Scientific Publications, London, 1960.

B. F. Hitch and C. F. Baes, Jr., Reactor Chem. Div. Ann. Progr.
Rept. Dec. 31, 1966, ORNL-4076, p. 19.

H. D. Young, Statistical Treatment of Experimental Data, p. 96,
McGraw-Hill, New York, 1962.

JANAF Thermochemical Tables, Clearing House for Federal Scientific
and Technical Information, U.S. Dept. of Commerce, Aug., 1965.

0. J. Kleppa, private communication.

P. E. Field and J. H. Shaffer, J. Phys. Chem., 71, 3320 (1967).

45

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