ORNL-4257

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OWL-4 25 7

C o n t r a c t N o . U-7405-eng-26

REACTOR CHEMISTRY DIVISION AN EMF STUDY OF LiF-BeF2

SOLUTIONS

B. F. Hitch and C. F. B a e s , Jr.

*.? 1

c

JULY 1968

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

,


r


iii CONTENTS

............................................... Introduction ........................................... Experimental ........................................... Chemicals ......................................... Gases ........................................... Melt Components ................................. Reagents ........................................ Apparatus ......................................... Cell Design ..................................... HF-Hz Electrode ................................. Beryllium Electrode ............................. Flow Control of Gases ........................... Hydrogen Fluoride ............................. Hydrogen ..................................... Helium ........................................ Titration Assembly .............................. Electronic Equipment ............................ Procedure ........................................ Measurements .................................... Cell Potential ................................ H2 and HF Partial Pressures .................... Melt Temperature .............................. Calculations .................................. Systematic Errors ............................... Hydrogen D i f f u s i o n ............................ Thermal Diffusion ............................. Gas Cooling Effect on Electrodes .............. Melt Composition .............................. Melt Impurities ............................... Oxide Contamination of Melt .................... Summary ....................................... Random Errors .................................... Precision of Potential Measurements ........... Melt Temperature .............................. Melt Composition .............................. Titer Precision .............................. Temperature of Bubble-0-Meter ................. Flow Rate Determination ....................... Endpoint Precision ............................ H2 and HF Flow Rates .......................... Abstract

Page 1 2 6

7 7 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 i

ci ......................... Results ................................................. Tabulation ......................................... Experiments ...................................... S t a t i s t i c a l Error Analysis

.......................... Discussion .............................................. Thermodynamics of LiF-BeF2 ......................... Reference Electrodes ............................... Beryllium Electrode .............................. HF-H2 Electrode .................................. References .............................................. Corrected C e l l Potentials

i

23

25 m �

25 25

26 29

29 39 39 41

42


1

AN EMF STUDY OF LiF-BeF2 SOLUTIONS B. F. Hitch and C. F. Baes, Jr.

ABSTRACT The p o t e n t i a l of t h e c e l l Be

0

I BeF2,

LiF I HF ,H2 ,Pt

with t h e assumed c e l l r e a c t i o n

+

Be(s)

2HF(g)

t' BeF2(d) +

H2(g)

w a s measured over a composition range of 0.30 t o 0.90 mole f r a c t i o n BeF2 and a temperature range of 500 t o 900째C. Since t h i s c e l l p o t e n t i a l i s r e l a t e d t o t h e a c t i v i t y of BeF2 i n t h e s o l u t i o n s by D

P ?

c

a c t i v i t y c o e f f i c i e n t s could b e derived f o r BeF2 (and by a Gibbs-Duhem i n t e g r a t i o n f o r LiF). Usefully a c c u r a t e measurements could not b e made w i t h pure BeF2 i n t h e c e l l , hence values of EO w e r e c a l c u l a t e d using v a l u e s f o r aBeF2 derived from the phase diagram and previously reported h e a t of f u s i o n (1.13 kcal/mole) g i v i n g

Eo

=

2.4430

-

0.0007952T

This comparison of t h e emf d a t a w i t h t h e LiF-BeF2 phase diagram a l s o i n d i c a t e d t h a t t h e h e a t of f u s i o n f o r BeF2 i s < 2.0 kcal/mole. A power series i n xLiF w a s assumed f o r l o g YBeF2 and t h e c o e f f i c i e n t s determined by a least squares f i t t o t h e data. This gave log'BeF2

+

= (3.8780

(94.3997

-

2353*5)x2 T LiF

+

(-40.7375

+

84870*9)x4 T LiF

+

(-67.4178

+

36292.8

3 )xLiF

52923.5

5 lXLiF

Formation f r e e e n e r g i e s and h e a t s f o r BeF2 and Be0 were a l s o c a l c u l a t e d by combining t h e r e s u l t s of t h e g r e s e n t study w i t h a v a i l a b l e thermochemical data. The Be2+1 Be and HF,H2 Fe l e c t r o d e s performed acceptably f o r u s e as r e f e r e n c e e l e c t r o d e s , both being s t a b l e and reproducible.

I

/-

&> i


2

Molten fluoride interest

at this

ds

INTRODUCTION

I. mixtures

Laboratory

of LiF and BeF2 are of considerable since

they are the principal

*

constituents

r-

i

-4

in the molten-salt

reactor

(MSRE) fuel

and coolant

the molten LiF-BeF2 system has received a limited

number of emf investigations

considerable

electrode

general would facilitate

the determination

contained

fluoride

constituents

'

half-cells

Although

attention,

have been attempted.

development of reference

various

salts.

only The

for molten fluorides of electrode

and the detection

in

potentials

of certain

for

impurities

in these mixtures.

The purpose of this Be'(s)

investigation

was to study the cell

+ 2HF(g) 2 BeF2(d) + H2(g)

reaction

(1)

in the molten LiF-BeF2 system using Be2+ IBe0 and HF,H 2 IF- electrodes. Emf data obtained in this study would hopefully extend and improve the thermodynamics

of this

molten

fluoride

measurements should demonstrate reference salt

electrodes

solvent

in future

At the same time emf

electrodes

emf studies

in this

which may serve as important

molten

system.

Thus far activity from mass ,spectroscopic gram,

useful

system.

data for the LiF-BeF2 systems has been obtained of the vapor, 2,3 from the phase dia-

studies

495I from emf measurements,6

and from transpiration

data where

gaseous HF-H20 mixtures were equilibrated with the molten fluoride mixture. 7 The values'derived from the phase data, besides being non-isothermal,

have been limited

been difficult

to determine.

in accuracy because the BeF2 liquidus In addition,

the reported

has

values 4,5,7,8,9


3 f o r the h e a t of f u s i o n f o r BeF2 are not i n agreement. I

Mass s p e c t r o -

s c o p i c and emf values of t h e a c t i v i t y were determined f o r only a l i m i t ed number of compositions and temperatures.

Probably the b e s t a c t i v i -

t y values are t h o s e determined by Mathews and Baes

7

using a t r a n s p i r a -

t i o n method t o e q u i l i b r a t e gaseous HF-H20 mixtures w i t h molten LiF-BeF mixtures.

2

However, a c t i v i t i e s derived from t h e s e heterogeneous equi-

l i b r i a are somewhat l i m i t e d i n accuracy

(56 % )

and the e q u i l i b r i u m

q u o t i e n t s w e r e measured i n t h e presence of Be0 as a s a t u r a t i n g s o l i d which might have i n f l u e n c e d t h e a c t i v i t y values.

Direct determination of t h e a c t i v i t y of BeF2 by emf measurements should y i e l d more a c c u r a t e values over a g r e a t e r composition range than any of t h e methods p r e v i o u s l y mentioned.

D i r i a n , Romberger and Baes

10

measured t h e p o t e n t i a l of t h e following c e l l as a f u n c t i o n of temperature. BeO10.33 BeP2

- 0.67

LiFIJDl%,Pd

The c e l l r e a c t i o n i s t h a t shown i n eq. (1). Be2+IBeo and HF,H21F-

- were

The two e l e c t r o d e s

-

judged by Dirian e t a l , t o b e r e v e r s i b l e

from p o l a r i z a t i o n measurements. I n t h e p r e s e n t i n v e s t i g a t i o n these two e l e c t r o d e s w e r e used t o determine t h e c e l l p o t e n t i a l over a composition range of 0.30 t o 0.90 mole f r a c t i o n BeF2 and a temperature range of 500 t o 900째C (Fig. 1 ) . An attempt w a s made t o o b t a i n measurements i n pure BeF

2'

but results

of u s e f u l accuracy could n o t be o b t a i n e d presumably because of i t s high f

v i s c o s i t y and/or h i g h e l e c t r i c a l r e s i s t i v i t y .

is very viscous (about 180 p o i s e

11

1. The

Even a t 900째C, pure BeF

2

r e a c t i o n vessel w a s n o t heat-

ed above 900째C because of t h e tendency of n i c k e l t o s o f t e n a t such


4 The melting p o i n t s of mixtures below 0.33 BeF 2

e l e v a t e d temperatures.

W

i n c r e a s e r a p i d l y as t h e c o n c e n t r a t i o n approaches pure LiF as shown i n t h e LiF-BeF

2

12 phase diagram i n Fig. 1.

The lower BeF2 c o n c e n t r a t i o n s

(below 0.30 BeF ) were not i n v e s t i g a t e d t h e r e f o r e s i n c e t h e a c c e s s i b l e 2 temperature range was so l i m i t e d . According t o the assumed c e l l r e a c t i o n (eq. l ) , t h e c e l l p o t e n t i a l should be dependent on t h e a c t i v i t y of BeF2, t h e a c t i v i t y of b e r y l l i u m

metal, and t h e p a r t i a l p r e s s u r e s of HF and H2. E=Eo--

RT En 2F

~

Previous measurements"

i

pH2

aBeF2

H 'F

a Beo

w i t h the HF-H2 e l e c t r o d e , as w e l l as the pre-

s e n t ones, i n d i c a t e t h e gases t o be s u f f i c i e n t l y i d e a l a t t h e e l e v a t e d temperatures and low p r e s s u r e l e v e l s involved t o allow t h e use of p a r t i a l p r e s s u r e s i n p l a c e of f u g a c i t i e s i n t h i s Nernst expression. I n t h i s study mixtures of HF-H

2

were bubbled through molten

c

LiF-BeF2 and t h e p a r t i a l p r e s s u r e s of t h e gases determined by a l k a l i m e t r i c t i t r a t i o n and gas volume measurements.

The measured c e l l po-

t e n t i a l (E) w a s then c o r r e c t e d f o r the e f f e c t of t h e gas p r e s s u r e quotient.

D

RT Ec=E+-Iln 2F

The c o r r e c t e d p o t e n t i a l E

j i

1 I I I

Ec = Eo

sH2 2

(3)

D

HF C

is r e l a t e d t o t h e a c t i v i t y of BeF by 2

-RT En a 2F BeF2

(4)

assuming t h e a c t i v i t y of Beo t o b e u n i t y . Not a t i o n

The following n o t a t i o n w i l l b e used:

L4


d

*

It

P

ORNL-DWG 66-7632R3

900

I

I

I

848

n

800

700

V

0

w

60C 555

[L

3

!az 2w 500 w

I

458

I-

I

I

BeF2 (HIGH QUARTZ TYPE) +LIQUID

4 00 360 Li F+ Li2BeF4

300

Li28eFq+ BeF2 (HIGH QUARTZ TYPE 1

I

I %CI

280I

1

Li2BeF4

+

1

Lm o,

LiBeF3

20c I

I

J

I

I

I

LiBeF3 + BeF, ( HIGH QUARTZ TYPE) LiBeF3+!BeF2 (LOWIQUARTZ TYPE), 220

I

1 1

c


6

14

The number preceding "BeF2" denotes mole f ra c t i o n .

0.00 BeF2

L

Total pressure(atm), p a r t i a l pressures(atm), volume(R) p e r u n i t t i m e , and temperature(OK) i n t h e r e g i o n where gas e n t e r s t h e m e l t . Corresponding measurements a t t h e Bubble-OMeter.

Po Po ,Vo,To T' H2

Barometric p r e s s u r e and vapor p r e s s u r e of H 0 2 a t To.

B "i20

The approximate p a r t i a l p r e s s u r e of HF a t t h e titrator. ( s e e C a l c u l a t i o n s s e c t i o n below.)

piF

P r e s s u r e drop r e q u i r e d t o maintain gas flow

AP

. through t h e m e l t . C e l l p o t e n t i a l measured experimentally f o r a fixed BeF2 concentration and temperature.

E

The observed c e l l p o t e n t i a l c o r r e c t e d t o a gas p r e s s u r e q u o t i e n t of u n i t y (eq. 3).

EC

The s t a n d a r d c e l l p o t e n t i a l w i t h pure BeF2 as t h e s t a n d a r d state.

EO

11.

EXPERIMENTAL Chemicals

Gases

Commercial H

2

w a s p u r i f i e d by passage through a deoxo u n i t , a mag-

nesium p e r c h l o r a t e drying tube and, f i n a l l y , a l i q u i d N2 t r a p . hydrous HF (99.9%) w a s used without f u r t h e r p u r i f i c a t i o n .

An-

Commercial

H e w a s p u r i f i e d by passage through an ascarite t r a p , a magnesium per-

c h l o r a t e t r a p and, f i n a l l y , a l i q u i d N2 t r a p .

M e 1t Components

*;-

. I

Lithium f l u o r i d e (99.5%) w a s obtained from American Potash and

Chemical Corporation.

Beryllium f l u o r i d e w a s from t h r e e s o u r c e s :

*

. ,

LJ


7

Brush Beryllium Corporation, K and K L a b o r a t o r i e s , I n c . , and comm e r c i a l BeF2 d i s t i l l e d by t h e Reactor Chemistry Division a t Oak Ridge National Laboratory.

Most of t h e commercial BeF2 contained i m p u r i t i e s

which "poisoned" t h e e l e c t r o d e s ( s e e p. 20).

With t h e exception of one

composition, t h e d i s t i l l e d BeF2 w a s used throughout t h i s i n v e s t i g a t i o n s i n c e t h e p u r i t y w a s such t h a t no e l e c t r o d e "poisoning" w a s encountered. Reapen t s Reagent grade 1 N NaOH from F i s h e r Chemical Company w a s standardi z e d w i t h potassium a c i d p h t h a l a t e . Apparatus Experiments were c a r r i e d out i n t h e apparatus shown i n Fig. 2. C e l l Design

A s k e t c h of t h e n i c k e l r e a c t i o n v e s s e l used t o c o n t a i n t h e LiF-BeF

mixtures is shown i n Fig. 3.

2

This v e s s e l w a s constructed of 2-1/2-in.

schedule 40 n i c k e l p i p e and w a s s e p a r a t e d i n t o two compartments by a 1/16-in.

n i c k e l s h e e t which extended t o w i t h i n 1/2-in.

bottom.

The nickel s h e e t w a s welded so t h a t t h e only contact between

of t h e v e s s e l

t h e two compartments w a s through t h e 1/2-in. opening a t t h e bottom. v e s s e l w a s 10-in. j

The

long.

Each compartment w a s equipped w i t h t h e following: a 3/4-in.

Swagelok f i t t i n g through which m e l t components could b e added o r an e l e c t r o d e i n s e r t e d , a 1/4-in.

-. -5 ' /

4

gas e x i t tube, and a thermocouple w e l l .

The Swageloks were equipped with Teflon seals when t h e e l e c t r o d e s were inserted.

vis provided an e l e c t r i c a l i n s u l a t o r as w e l l as a leak-

t i g h t f i t t i n g f o r the 1/8-in.

n i c k e l tubing.

Cooling c o i l s were wrap-

ped around each Swagelok t o provide cooling when t h e r e a c t i o n v e s s e l


8

ORNL-DWG 67-13719

ANHYD

I He

a-

-

AMPLl FIER -RECORDER

I 1

COMPART-

I

NaOH SOLN

COMPARTMENTED REACTION VESSEL

NaOH SOLN

METER

Fig. 2. Schematic Diagram of Apparatus Used to Measure Cell Potentials in Molten LiF-BeF2 Mixtures.


9

ORNL-DWG 67-13720

3

Fig. 3. Compartmented Cell Used to Contain Molten LiF-BeF2 Mixtures.


10 I

w a s a t e l e v a t e d temperatures. The r e a c t i o n v e s s e l w a s l o c a t e d i n s i d e an u p r i g h t tube furnace, t h e temperature w a s c o n t r o l l e d by an L & N Series 60 D.A.T.

B

Control c

The temperature of t h e r e a c t i o n v e s s e l w a s checked w i t h a c a l i -

Unit.

b r a t e d Chromel-Alumel thermocouple and an L & N K-3 potentiometer. A 4-in.

diameter vessel, f i t t e d w i t h 1/2-in.

diameter e l e c t r o d e

compartments, w a s used f o r preliminary measurements.

Electrodes used

i n t h e c y l i n d r i c a l compartments were i n s u l a t e d from t h e compartment

w a l l s w i t h boron n i t r i d e s p a c e r s , b u t even then a c c i d e n t a l electrical s h o r t s were a problem.

The l a r g e compartments of t h e r e a c t i o n vessel

used i n t h e p r e s e n t i n v e s t i g a t i o n e l i m i n a t e d ' t h e need f o r i n s u l a t i n g s p a c e r s except t h e Teflon s e a l a t t h e top of t h e compartment, and no problems from e l e c t r i c a l s h o r t i n g w e r e encountered. HF-H2

Electrode An H ,HF,Pd e l e c t r o d e of t h e t y p e used by D i r i a n and Romberger

10

2

w a s used i n some of t h e preliminary measurements. produced s t a b l e p o t e n t i a l s b u t w a s q u i t e n o i s y

This e l e c t r o d e

(2 1 mv).

Platinum

gauze w a s s u b s t i t u t e d f o r t h e palladium and w a s found t o be j u s t as responsive and capable of very low n o i s e l e v e l s (0.1 mv)

.

The p l a t i n -

um gauze t y p e (Fig. 4) w a s used f o r a l l measurements i n t h i s i n v e s t i gation.

E l e c t r o d e s were prepared by forming an egg-shaped bag w i t h th-

gauze and s l i p p i n g t h e open end over 1/8-in. i t s e c u r e l y w i t h s m a l l diameter n i c k e l w i r e .

w a s crimped t o g e t h e r s o t h a t t h e HF-H

2

n i c k e l tubing and t y i n g The o t h e r end of t h e bag

mixture, p a s s i n g down through

t h e n i c k e l tubing had t o pass through t h e gauze.

The 1/8-in.

nickel

tubing t r a n s m i t t e d t h e HF-H2 mixture and provided e l e c t r i c a l c o n t a c t .


11 Beryllium E l e c t r o d e

These e l e c t r o d e s (Fig. 4) w e r e constructed by s l i p p i n g a b e r y l l i um metal c y l i n d e r (3/8-in.

over a 1/8-in.

1/8-in.

O.D.,

I.D.,

and 1/2-in.

length)

n i c k e l tube and crimping t h e n i c k e l tube s l i g h t l y on

each s i d e of t h e b e r y l l i u m c y l i n d e r t o hold i t s e c u r e l y .

w a s p o s i t i o n e d about 1/2-in.

The c y l i n d e r

from t h e t i p of the n i c k e l tube.

Lower-

i n g t h e b e r y l l i u m metal c l o s e r t o t h e t i p of t h e n i c k e l tube caused an increase i n the p o t e n t i a l noise.

This w a s probably due t o helium

bubbles temporarily i n s u l a t i n g t h e beryllium from t h e m e l t . i

The 1/8-

i n . n i c k e l tube w a s used t o bubble helium i n t o t h e compartment and t o provide e l e c t r i c a l c o n t a c t . Flow Control of Gases Hydrogen Fluoride.--

The HF manifold p r e s s u r e w a s c o n t r o l l e d by

* i

r e g u l a t i n g t h e temperature of t h e HF supply c y l i n d e r . c

w a s c o n t r o l l e d by a m a s s spectrometer l e a k valve.13

a monel tee where i t mixed w i t h H

2'

The flow of HF The gas flowed t o

The mixed gases were passed e i t h e r

d i r e c t l y t o t h e HF-H2 e l e c t r o d e o r a p o r t i o n w a s s p l i t o f f f o r i n fluent g a s analysis.

A p r e s s u r e r e l i e f v a l v e (Moore Products Company, d i f -

Hydrogen.--

f e r e n t i a l type flow c o n t r o l l e r , Model 63 BD, modified form) w a s used t o reduce t h e hydrogen manifold p r e s s u r e t o a c o n s t a n t v a l u e of 3.0 l b . gauge.

The flow w a s t h e n c o n t r o l l e d by a b r a s s needle v a l v e obtained

from Nuclear Products Company.

The H2 w a s then mixed w i t h the HF as

d e s c r i b e d above. Helium.--

Helium flow w a s c o n t r o l l e d w i t h t h e same t y p e n e e d l e

v a l v e used f o r t h e H2; no attempt w a s made t o c o n t r o l t h e manifold

I


12

ORNL-DWG

67

- 13 7 2 1 t

8

,

I

1

I

I

HF

- HZ

ELECTRODE

BERYLLIUM ELECTRODE

Fig. 4. HF-Hz and Beryllium Electrodes.

*

ib


13 p r e s s u r e s i n c e a constant flow through t h e beryllium e l e c t r o d e w a s unnecessary.

1

T i t r a t i o n Assembly

t

The NaOH t i t r a t i o n vessel w a s a 200-ml test tube.

A rubber stop-

p e r w a s i n s e r t e d i n t o the t e s t tube and w a s equipped w i t h t h e following: Teflon (1/4-in.

d i a . ) e n t r a n c e and e x i t tubes f o r g a s , a 5-ml Lab-

Crest microburet, and Beckman No. 39166 ( g l a s s , Ag-AgC1) e l e c t r o d e s .

A Beckman Zeromatic I1 pH meter w a s used t o determine t h e endpoint. The pH w a s maintained on t h e a l k a l i n e s i d e of t h e endpoint t o avoid

glass attack.

Duplicate t i t r a t i o n assemblies w e r e used t o measure in-

f l u e n t and e f f l u e n t HF c o n c e n t r a t i o n s . E l e c t r o n i c Equipment

. E

An L & N K-3 p o t e n t i o m e t e r , c a l i b r a t e d w i t h a s t a n d a r d c e l l from

Eppley Laboratory, I n c . , was used t o buck o u t most of the c e l l v o l t a g e . When the c e l l v o l t a g e exceeded t h e range of the p o t e n t i o m e t e r , a mercury b a t t e r y (Mallory Duracell No. RM 42R) w a s connected i n series t o extend t h e bucking v o l t a g e range.

The v o l t a g e of the b a t t e r y w a s

checked d a i l y w i t h the potentiometer, and i t proved t o be an extremely s t a b l e voltage source

0.02 mv/day).

The remainder of t h e c e l l v o l t -

age (< 100 mv) w a s coupled, through a P h i l b r i c k ModelMP s o l i d s t a t e o p e r a t i o n a l manifold equipped w i t h P65 AU a m p l i f i e r s , t o a Honeywell Brown E l e c t r o n i k r e c o r d e r . Procedure

Me as uremen ts s

W

The d a t a obtained f o r each c e l l measurement were c e l l p o t e n t i a l ,

c e l l temperature, and p a r t i a l p r e s s u r e s of HF and H2.


14 C e l l Potential.--

C e l l p o t e n t i a l measurements were always preced-

ed by s t a n d a r d i z a t i o n of the potentiometer a g a i n s t a s t a n d a r d c e l l and a check of t h e a m p l i f i e r zero on t h e recorder.

u I

During measurements of

t h e c e l l p o t e n t i a l , most of t h e v o l t a g e w a s bucked o u t by the potenrio-

W

meter ( o r potentiometer p l u s mercury b a t t e r y ) , and t h e remainder read o f f t h e recorder.

The n o i s e l e v e l - o f t h e p o t e n t i a l v a r i e d from 5 0.2

mv t o as high as 5 1.0 mv f o r t h e h i g h v i s c o s i t y m e l t s .

Although t h e

p o t e n t i a l f l u c t u a t e d as i n d i c a t e d , i t d i d n o t show any d r i f t toward a h i g h e r o r lower p o t e n t i a l . H2 and HF P a r t i a l Pressures.--

The d e t e r m i n a t i o n of p a r t i a l

p r e s s u r e s w a s made as follows: A measured volume of s t a n d a r d i z e d NaOH w a s added t o t h e

t i t r a t i o n vessel. The t i m e r e q u i r e d f o r t h e HF t o n e u t r a l i z e t h e b a s e w a s

determined. H

flow rates were determined by t h e use of a Bubble-0-

2 Meter.

I n f l u e n t p a r t i a l p r e s s u r e s were checked simultaneously i n experiments up t o .60 BeF2. P a r t i a l p r e s s u r e determinations were never begun u n t i l the

c e l l p o t e n t i a l had been s t e a d y f o r a t least 30 minutes.

Six t o t e n successive t i t r a t i o n s w e r e c a r r i e d out. The barometric p r e s s u r e , temperature of t h e t i t r a t i m assembly, and p r e s s u r e drop a c r o s s t h e system w e r e recorded. H e l i u m flowed through t h e beryllium e l e c t r o d e a t a rate of

30 t o 75 ml/min.

This flow rate provided adequate s p a r g i n g

f o r t h i s compartment and decreased t h e thermal g r a d i e n t s . H e l i u m a l s o p r o t e c t e d t h e e l e c t r o d e from any HF which might

bi


15

have e n t e r e d the compartment. M e l t Temperature.--

The temperature of the m e l t w a s determined by

a c a l i b r a t e d Chromel-Alumel thermocouple p o s i t i o n e d c a r e f u l l y t o the e x a c t depth of t h e e l e c t r o d e .

P o s i t i o n i n g w a s c a r r i e d o u t as accurate-

l y as p o s s i b l e t o reduce any e r r o r i n temperature readings caused by thermal g r a d i e n t s i n the m e l t . Calculations.-E

C

The r e q u i r e d c a l c u l a t i o n s t o e v a l u a t e P P and HF' H2

were c a r r i e d o u t as follows: (1)

The t i t r a t i o n t i m e s were averaged f o r a series of t i t r a t i o n s

of a f i x e d increment of s t a n d a r d base.

The gas volume passed w a s then

c a l c u l a t e d by (time of t i t r a t i o n ) / ( t i m e p e r 100 m l of gas) x 100 = m l of gas. (2)

The number of m i l l i m o l e s of HF removed from t h e H2 stream by

t h e t i t r a t i o n w a s c a l c u l a t e d by (ml NaOH)(conc. of NaOH) = millimoles HF.

(3)

It w a s found convenient f i r s t t o d e f i n e an approximate 0

p a r t i a l p r e s s u r e of HF (PHF) by i n t r o d u c i n g t h e s e measured q u a n t i t i e s i n t o t h e following s i m p l e gas l a w expression F ;'

-

-

.

( m o l e s HF) (0.08206) (abs t e m p . of Bubble-0-Meter) m l of H2 passed

The exact e x p r e s s i o n f o r t h e p a r t i a l p r e s s u r e of HF (PHF) a t the el e c t r o d e i n c l u d e s t h e e f f e c t of t h e p r e s s u r e drop (AP) caused by the p r e s s u r e r e q u i r e d t o maintain bubbling through t h e m e l t and subsequent t i t r a t o r , and the s a t u r a t i o n of t h e H2 stream w i t h w a t e r p r i o r t o measuring t h e flow rate.

0

The r e l a t i o n s h i p (eq. 11) between PHF and


16

Lid

PHF which i s r e q u i r e d w a s developed as follows: (3.1)

The system may b e v i s u a l i z e d as c o n s i s t i n g of t h r e e

-

t h e c e l l v e s s e l ( a t P , T I , t h e t i t r a t i o n assembly, and T t h e Bubble-0-Meter (at Pi, To). regions

(3.2)

Assuming t h a t t h e number of moles of HF p a s s i n g

through t h e c e l l assembly and i n t o t h e t i t r a t o r is t h e same f o r

a given t i m e i n t e r v a l , then

%F

-

PiF

vo

RTo

and

i II

'HF

I

I

E

%F=

/

RT

(Note t h a t t h e f i r s t expression i s merely a rearranged form of t h e previous equation which d e f i n e s P

0

HF

).

Combining t h e s e two e q u a t i o n s ,

'HF

- 0 - 'HF

(3.3)

-

Vo T TO

(5)

Moles of H2 passing through t h e c e l l and through t h e

Bubble-0-Meter

may be t r e a t e d i n a l i k e manner

-Vo= T -

V To

Po

HZ (3.4)

Combining equations (5) and (6)

(7)

i


17 The t o t a l p r e s s u r e a t t h e HF-H2 e l e c t r o d e i s

(3.5) ! j

z

PT = PB

+ AP

= 'HF

+

p ~ 2

1 !

'

3

..

H2

=pB+Ap-

'HF

and the t o t a l p r e s s u r e of the gas a t the Bubble-0-Meter

is

0

P; = PB = PO H2 + 'H20

.- .

P H2 O =

(3.6)

.

PB

0 - 'H20.

S u b s t i t u t i n g (8) and (9) i n t o equation (7)

.

and s o l v i n g e q u a t i o n (10) f o r P gives HF

c

I

8

PHF can b e e v a l u a t e d s i n c e a l l t h e o t h e r q u a n t i t i e s are known.

values f o r P

HF

The

then can be s u b s t i t u t e d i n t o equation (8) and t h e P H2

determined.

(4) Using t h e measured c e l l p o t e n t i a l (E) and t h e p a r t i a l pres) the corrected cell p o t e n t i a l was then

s u r e s of HF (PHF) and H2 (P

H2 c a l c u l a t e d by

D

H2

RT

EC

=E+-Rn2F

D2 A

The c e l l p o t e n t i a l

.

HF

E w a s recorded a f t e r gas e q u i l i b r a t i o n w i t h t h e

m e l t w a s accomplished, and t h e m e l t temperature w a s c o n s t a n t .


18

Gi

Systematic Errors The preceding method of c a l c u l a t i n g p a r t i a l p r e s s u r e s does n o t i n clude c o r r e c t i o n s f o r t h e d i f f u s i o n of H

2

t

through t h e w a l l s of t h e

n i c k e l r e a c t i o n v e s s e l , nor does i t consider t h e e f f e c t of thermal d i f fusion.

M e l t p u r i t y and composition should a l s o be considered as

sources of s y s t e m a t i c e r r o r s . Hydrogen Diffusion.--

According t o published d i f f u s i o n coef-

f i c i e n t s , 1 4 t h e d i f f u s i o n of H2 out of t h e n i c k e l r e a c t i o n vessel could b e a few m i l l i l i t e r s p e r minute a t e l e v a t e d temperatures.

The

r a t e of H d i f f u s i o n w a s measured experimentally by Mathews and Baes 2

i n a v e s s e l s i m i l a r t o t h e one used i n t h e p r e s e n t experiment. obtained t h e following rates: 6OO0C, 0.015 ml/sec.

Extxapolatioh of t h e s e measurements t o 8OOOC and

Typically emf experiments were conducted with a H

rate of 2.5 ml/sec.

They

7OO0C, 0.035 ml/sec; 65OoC, 0.025 ml/sec;

900째C y i e l d s d i f f u s i o n rates of 0.055 ml/sec and 0.075 m l / s e c , spectively.

7

re2 flow

This means t h a t t h e measured volume of H would 2

be i n e r r o r by 0.6% a t 6OO0C, 1.40% a t 7OO0C, 2.20% a t 800째C, and 3.00%

a t 900OC.

These e r r o r s i n p a r t i a l p r e s s u r e s would cause t h e c a l c u l a t e d

p o t e n t i a l t o be about 1.0 mV lower a t 7OO0C, 1.5 mV lower a t 800째C, and 2.0 mV a t 900째C.

As mentioned p r e v i o u s l y , t h e HF-H2 mixtures w e r e

analyzed b e f o r e and a f t e r e n t e r i n g t h e HF-H

2

e l e c t r o d e compartment, and

no discrepancy between t h e two could be detected.

These analyses were

performed p e r i o d i c a l l y on a l l compositions up t o .60 BeF and tempera2 t u r e s up t o 700OC.

?

Melts of h i g h e r BeF2 concentrations were n o t check-

ed i n t h i s manner because d i f f i c u l t y w a s encountered i n keeping t h e flow rate constant through t h e s e high v i s c o s i t y m e l t s when t h e s p l i t

1c

bi


19 flow technique w a s a t tempted. Since the H

2

d i f f u s i o n e f f e c t w a s not observed a t temperatures up

t o 7OO0C, the e r r o r s c a l c u l a t e d above seem t o b e somewhat high. Thermal Diffusion.--

Recognizing t h e p o s s i b i l i t y t h a t H2 and HF

might tend t o s e p a r a t e along t h e thermal g r a d i e n t i n t h e r e a c t i o n v e s s e l , w e c a r r i e d o u t an experiment t o see i f t h i s e f f e c t w a s s i g n i f i cant. The t o t a l flow of HF-H2 p a s s i n g through t h e HF-H

v a r i e d between 23 ml/min and 160 ml/min.

2

electrode was

The c e l l p o t e n t i a l i n c r e a s e d

by about one m i l l i v o l t over t h i s range; t h e i n c r e a s e i n c e l l p o t e n t i a l over t h e range of flow rates where p o t e n t i a l measurements were a c t u a l l y made (100-160 ml/min) w a s less than 0.3 m i l l i v o l t .

This i s thought t o

b e s u f f i c i e n t evidence t h a t t h e thermal d i f f u s i o n e f f e c t w a s not s i g *

=

n i f icant. I '

8 '

Gas Cooling E f f e c t on Electrodes.--

The experiment mentioned above

a l s o i n d i c a t e s t h a t cooling of t h e e l e c t r o d e s by gas flow could n o t have caused more t h a n a one m i l l i v o l t e r r o r i n t h e p o t e n t i a l measurements.

It w a s f e l t t h a t although t h e p o s s i b i l i t y of a s m a l l e r r o r in

p o t e n t i a l might r e s u l t from flow rates of 100-160 ml/min, vigorous agit a t i o n of t h e m e l t w a s necessary t o reduce the e f f e c t of thermal gradients i n the m e l t .

Melt Composition.--

A composition e r r o r which w a s discovered a t

t h e end of run No. 4 w a s a p p a r e n t l y caused by d i s t i l l a t i o n of BeF2 t o I

>

_

*

c o o l e r regions of t h e apparatus. 3

W ,

I n t h i s p a r t i c u l a r run, many a t t e m p t s

were made t o measure t h e c e l l p o t e n t i a l of pure BeF over a temperature 2 range of 800째C t o 900째C.

A t t h e s e e l e v a t e d temperatures t h e vapor


20

p r e s s u r e of BeF2 is a p p r e ~ i a b 1 e . l ~Evidently l o s s of BeF2 occurred during t h e p e r i o d s a t h i g h temperatures.

Subsequent a d d i t i o n s of LiF

w e r e made t o b r i n g t h e BeF composition down t o 0.90 BeF2, 0.80 BeF2, 2 0.70 BeF2, and f i n a l l y t o 0 . 3 3 BeF2.

The e r r o r w a s discovered a t 0 . 3 3

BeF s i n c e t h e p o t e n t i a l s d i d not correspond t o previous measurements. 2 A t t h i s p o i n t enough BeF. w a s added t o b r i n g t h e composition up t o a

2.

book value of 0.40 BeF temperature. LiF-BeF

2

2

p e r m i t t i n g thermal a n a l y s i s of t h e l i q u i d u s

Thermal -a n a l y s i s w a s c a r r i e d o u t and compared w i t h t h e

l i q u i d u s d a t a . l6 R e s u l t s i n d i c a t e d t h a t t h e t r u e composition

w a s 0 . 3 8 8 BeF2.

Assuming t h a t a l l or most of t h e BeF2 w a s l o s t b e f o r e

t h e L i F a d d i t i o n s w e r e begun, t h e compositions a t 0.90, 0.80,

and 0.70

BeF w e r e reduced t o 0.896, 0.791, and 0.686 BeF2, r e s p e c t i v e l y . 2 in E

C

i n t h e 0.60 BeF

2

Changes

t o 0.80 BeF2 regions are s m a l l ; t h u s , t h e compo-

s i t i o n e r r o r a t t h e s e concentrations should n o t b e s i g n i f i c a n t .

ever, t h e v a l u e s obtained f o r 0 . 3 3 BeF

2

How-

i n t h i s run w e r e more s e r i o u s l y

a f f e c t e d by t h e u n c e r t a i n t y i n composition and were n o t used. M e l t Impurities.--

Early i n t h e experimental work i t w a s found

t h a t commercial BeF2 which contained about 1000-2000 ppm s u l f u r and 1000 ppm t o t a l of Fe, C r , Cu, and N i caused "poisoning" of both electrodes.

The c e l l p o t e n t i a l would decay as much as 0.5 v o l t s .

Spectro-

graphic examination of t h e Be e l e c t r o d e showed t h a t Fe, C r , and N i had been reduced on t h e s u r f a c e of t h e e l e c t r o d e ; a l s o metallic i m p u r i t i e s o r i g i n a l l y i n t h e beryllium m e t a l w e r e much h i g h e r i n c o n c e n t r a t i o n on t

t h e e l e c t r o d e s u r f a c e a f t e r exposure t o t h e m e l t . Mn, and T i , t h e most abundant one being aluminum.

These included A l , 1


21 The platinum gauze on t h e HF-H2 e l e c t r o d e w a s darkened by exposure t o t h e s e contaminated m e l t s .

This e f f e c t w a s n o t pursued s i n c e i t w a s

a l r e a d y e v i d e n t t h a t BeF2 with t h e i m p u r i t i e s mentioned above w a s not 3

s u i t a b l e f o r use w i t h t h e beryllium e l e c t r o d e . D i s t i l l e d BeF2, containing only trace amounts of i m p u r i t i e s , w a s t e s t e d ; and none of t h e problems mentioned above were encountered.

This

material w a s used f o r a l l measurements i n t h i s i n v e s t i g a t i o n . Oxide Contamination of M e l t . - -

The oxide chemistry of LiF-BeF2

mixtures must b e considered s i n c e t h e r a w materials c o n t a i n s m a l l amounts of moisture, and s i n c e beryllium oxide is an impurity i n beryllium metal.

Moisture contamination of t h e m e l t w a s kept t o a minimum

by f r e e z i n g t h e mixtures p r i o r t o a d d i t i o n s of r a w materials.

Beryl-

l i u m f l u o r i d e w a s s t o r e d i n a dry box p r i o r t o use.

A s t a n d a r d p u r i f i c a t i o n procedure f o r removing oxides17 from f luo-

r i d e mixture is HF-H2 sparging.

This procedure w a s used throughout t h e

p r e s e n t i n v e s t i g a t i o n t o remove oxide i m p u r i t i e s from t h e m e l t s . tinuous use of HF-H

2

Con-

as one of t h e e l e c t r o d e materials undoubtedly k e p t

t h e oxide c o n c e n t r a t i o n s m a l l .

T y p i c a l l y , b e r y l l i u m m e t a l contains a-

bout 1000 t o 4000 ppm oxygen.’*

Using t h e l a r g e r v a l u e f o r oxide con-

tamination, ‘the m a x i m u m c o n c e n t r a t i o n of oxide c o n t r i b u t e d by t h e b e r y l I

lium e l e c t r o d e would have been 1 x

moles/kg, which is about one

o r d e r of magnitude below t h e s o l u b i l i t y of Be0.l’

Therefore, no s i g -

n i f i c a n t e r r o r i s expected due t o oxide contamination of t h e m e l t by a d d i t i o n of r a w materials o r from t h e m e t a l e l e c t r o d e .


22 Summary.-

The only known s y s t e m a t i c e r r o r s t h a t might b e s i g n i f i -

c a n t are t h o s e a t t r i b u t e d t o H

2

d i f f u s i o n and gas cooling of the

L4 *

These are o p p o s i t e i n e f f e c t , and both are expected t o b e

electrodes.

f.

w i t h i n t h e experimental scatter of t h e data. Random E r r o r s I n o r d e r t o o b t a i n a s a t i s f a c t o r y estimate of t h e expected prec i s i o n of t h e experimental measurements, t h e v a r i o u s random e r r o r s and t h e i r probable magnitude w e r e considered. P r e c i s i o n of P o t e n t i a l Measurements.-t e n t i a l f l u c t u a t i o n s were about

5

0.2 mV f o r mixtures up t o 0.60 BeF

These f l u c t u a t i o n s i n c r e a s e d t o about However, even a t t h e h i g h e r BeF

2

Typically s h o r t term po2'

2 1.0 mV f o r 0.90 BeF2 mixtures.

c o n c e n t r a t i o n s , t h e average v a l u e of

t h e p o t e n t i a l d i d n o t change more than

2

1.0 mV over a p e r i o d of one t o

two hours. Melt Temperature.--

+ 0.2OC.

The temperature of t h e m e l t w a s c o n t r o l l e d t o

The temperature g r a d i e n t i n t h e m e l t v a r i e d from about 2째C i n

m e l t s up t o 0.60 BeF2, t o a maximum of 9째C i n 0.90 BeF2.

However, care

w a s taken during each p o t e n t i a l measurement t o p o s i t i o n t h e thermocouple i n t h e m e l t so t h a t i t coincided w i t h i n rode depths. no more t h a n

If 1/8-in. of t h e elect-

This should have reduced t h e temperature u n c e r t a i n t y t o

2 0.5OC,

which corresponds t o an u n c e r t a i n t y of

5 0.4

mV

i n the potential. M e l t Composition.--

The LiF-BeF

2

mixtures w e r e prepared by adding

weighed amounts of t h e components t o t h e r e a c t i o n v e s s e l .

c i s i o n of weighing and t r a n s f e r r i n g materials t o t h e v e s s e l w a s about

+ 0.2%.

*

The preI

di


23

Commercial lN NaOH i n one q u a r t b o t t l e s w a s

T i t e r Precision.--

s t a n d a r d i z e d w i t h potassium a c i d p h t h a l a t e .

Agreement of t h e standard-

i z a t i o n values w a s about f.0.1% of t h e average value. i n b u r e t t e readings during HF t i t r a t i o n s w a s about Temperature of Bubble-0-Meter.--

The u n c e r t a i n t y

2 0.5%.

The Bubble-0-Meter

temperature,

which w a s t h e temperature of t h e gas as i t s volume w a s being measured, v a r i e d no more than & 0.1OC during a given experiment.

The same tem-

p e r a t u r e v a r i a t i o n a p p l i e s t o t h e t i t r a t i o n vessel where t h e H2 w a s s a t u r a t e d w i t h water.

~

Flow Rate Determination.--

rates w a s 0.1 s e c p e r 100 m l .

The p r e c i s i o n i n t h e timing of H2 flow For a t y p i c a l v a l u e of 50 seconds p e r

100 m l , t h i s would amount t o 0.2%. The p r e c i s i o n i n timing of t h e endpoints w a s

Endpoint Precision.--

about f. 0.3% of t h e measured value. H2 and HF Flow Rates.--

t h e measured value.

of HF.

2

flow r a t e w a s c o n s t a n t a t

2 0.1% of

Successive HF t i t r a t i o n s v a r i e d as much as & 1.0%

from t h e average value. I

The H

This was apparently due t o i r r e g u l a r flow rates

The v a r i a t i o n w a s random and no b i a s w a s noted.

The e r r o r in-

volved i n HF flow rate is much greater than any of t h e o t h e r e r r o r s ass o c i a t e d w i t h t h e c a l c u l a t i o n of PHF; t h e r e f o r e t h e o t h e r s w i l l be considered negligible. S t a t i s t i c a l E r r o r Analysis The probable e r r o r t o be expected i n t h e c a l c u l a t e d c e l l p o t e n t i a l , E f

C

w a s determined from t h e estimated magnitude of random e r r o r s i n

v a r i o u s q u a n t i t i e s which appear i n eq. 3.

I f t h e c o n t r i b u t i o n of each

observable i s r e l a t e d t o a c a l c u l a t e d f u n c t i o n Q and expressed as


24

CJ Q = f (a,b,c----),

t h e c o n t r i b u t i o n of such e r r o r s can b e c a l c u l a t e d .

l a w a normal d i s t r i b u t i o n , i t can be shown2'

I f the errors fol-

t

t h a t t h e following equaf

t i o n allows f o r p a r t i a l c a n c e l l a t i o n of e r r o r s of opposite s i g n :

x2

aQ2 = (aa) a a 2

+

x 2ab2 + (ac) 3 2 2 U c + ....

(ab)

Applying t h i s r e l a t i o n t o

P EC

RT =E+-!LnnF

H2 2 'HF

gives -

(13)

HF

D i f f e r e n t i a t i o n of t h e above q u a n t i t i e s may be c a r r i e d out t o o b t a i n D

-a = EC

aE

1

-a=E c

alr

-a +E

~R

R =n H2 y

,

aT 'HF

and

-

= PB + AP PHF. (It may b e r e c a l l e d t h a t P H2 AP may b e neglected, t h u s , P = PB PHF) H2

-

For t h e p r e s e n t a n a l y s i s

.

The expected u n c e r t a i n t y of Ec can now b e c a l c u l a t e d by

t


25 I

i

Evaluating t h e above equation numerically f o r t h e u n c e r t a i n t y i n t h e c a l c u l a t e d p o t e n t i a l , using t y p i c a l p a r t i a l p r e s s u r e s of HF and H2 a t a temperature of 1000째K and using t h e estimated u n c e r t a i n t i e s of

+ 0 . 5 O C , & 0.001 o2

=

2 0.5

mV,

a t m r e s p e c t i v e l y i n E, T , and PHF, one o b t a i n s

(5 x 10-4)2

+

-4 2 -1 2 (8.95 x 10 ) (5 x 10 )

+

-3 2 (2.103)2(1 x 10 )

EC

a2

= (2.50

+

+

(2.00

4.42

103

EC

a2

=

4.87 x

=

+ .0022

EC (J i

i

E

C

volts.

The c a l c u l a t e d s t a n d a r d d e v i a t i o n of

2

2.2 mV i s s l i g h t l y l a r g e r t h a n

t h e s t a n d a r d d e v i a t i o n obtained from least squaring t h e a c t u a l data. The range of t h e s t a n d a r d d e v i a t i o n f o r t h e a c t u a l d a t a w a s about 1.0 mV t o 2.0 mV.

The major u n c e r t a i n t y i s i n t h e PHF term and is due t o t h e

l i m i t e d p r e c i s i o n of HF flow. 111.

RESULTS

Tabulation Table 1 contains t h e d a t a obtained from each experiment, arranged according t o m e l t compos i t ion. Experiments Four s e p a r a t e series of experiments were conducted during t h i s investigation.

The c e l l p o t e n t i a l of each m e l t composition w a s determined


26

f o r a range of temperatures.

I n the f i r s t series the r e a c t i o n vessel

w a s i n i t i a l l y loaded w i t h 0.33 BeF2.

Subsequent a d d i t i o n s of BeF w e r e 2

0.50 BeF2, and t o made t o b r i n g t h e c o n c e n t r a t i o n s up t o 0.40 BeF 2’ 0.60 BeF2. I n t h e second series t h e r e a c t i o n v e s s e l w a s i n i t i a l l y loaded with pure BeF

2’

enough LiF w a s then added t o g i v e 0.90 BeF2, 0.80 BeF2, 0.70

BeF2, 0.60 BeF2, 0.50 BeF2, 0.40 BeF2, and f i n a l l y 0.33 BeF2. o r two determinations w e r e made f o r 0.60 BeF

2

Only one

and lower BeF compo2

s i t i o n s s i n c e t h e s e were checks of previous measurements. Pure BeF w a s a l s o t h e i n i t i a l composition f o r t h e t h i r d series of 2 measurements.

S u b s e q u e n t a d d i t i o n s of L i F w e r e m a d e t o give 0 . 9 0 , 0 . 8 0 ,

0 . 7 0 , 0.60, and 0.33 BeF2. BeF

2

A s p r e v i o u s l y mentioned t h e d a t a f o r 0.33

w a s considered t o be i n e r r o r and w a s n o t used. I n t h e l a s t series of measurements t h e r e a c t i o n v e s s e l w a s loaded

i n i t i a l l y w i t h 0.33 BeF 2’

Enough L i F w a s added t o g i v e 0.30 BeF2; t h e n

BeF w a s added t o b r i n g t h e composition back t o 0.33 BeF2. 2 Corrected C e l l P o t e n t i a l s The c o r r e c t e d c e l l p o t e n t i a l s (E ) f o r v a r i o u s compositions are C

p l o t t e d as a f u n c t i o n of temperature and shown i n Fig. 5.

E

C

i s assumed

t o be a l i n e a r f u n c t i o n of temperature, Ec = A+BT, over the temperature

range i n v e s t i g a t e d .

The d a t a p o i n t s f o r each composition were least

squared, and t h e c a l c u l a t e d l i n e s are a l s o shown i n Fig. 5.

The para-

meters from t h e least squaring treatment are l i s t e d i n Table 2.


27

(YINL-DWG 67-137231

I

I

0.10 BeF, Q80. 0.90 &F2

600

650

700

600

650

Is0

650

100

100 ,150

800 TEMPERATURE CCI

750 800 850

800 850

900

850

900

950

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


28

Table 1. Pressure Corrected C e l l P o t e n t i a l s (Ec) Obtained from Measurements i n Molten LiF-BeF, 'BeF

Temp. 2

("a

E (volts) C

0.30 0.30 0.30 0.30 0.30 0.30

585.1 646.0 696.2 732.0 609.2 681.0

1.8864 1.8447 1.8126 1.7882 1.8680 1.8213

0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.33

562.9 514.0 601.0 649.5 698.0 624.1 565 .O 720.8 529.0 617.2 617.2 550.0 624.5 671.0 812.0

1.8812 1.9152 1.8561 1.8230 1.7891 1.8397 1.8757 1.7653 1.9035 1.8436 1.8396 1.8857 1.8356 1.8016 1.7081

0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40

597.5 609.5 609.5 661.2 686.6 685.8 528.2 528.2 536.5 535.0 706.0 706.0

1.8069 1.8000 1.7991 1.7623 1.7430 1.7478 1.8578 1.8578 1.8540 1.8522 1.7337 1.7316

0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50

632.8 561.5 728.0 707.0 658.0 617.3 568.3 546.6 503-3 511.8

1.7516 1.8022 1.6765 1.6972 1.7340 1.7629 1.7991 1.8127 1.8467 1.8387

Temp. ("C)

E (volts)

0.50 0.50 0.50 0.50

608.5 613.1 685.2 681.0

1.7671 1.7648 1.7116 1.7163

0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60

637.0 566.5 735.0 637.0 591.1 521.0 591.0 591.0 521.2 710.0 709.0 662.0

1.7263 1.7856 1.6555 1.7283 1.7656 1.8160 1.7672 1.7667 1.8170 1.6760 1.6771 1.7128

0.70 0.70 0.70 0.70 0.70 0.70

760.0 706.5 609.6 562.0 663.0 794.9

1.6310 1.6736 1.7477 1.7844 1.7085 1.6079

0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80

800.4 751.0 656.5 632.5 704.9 754.0 681.0 633.5 611.5 639.7 887.5 810.0 735.4 663.0 616.0 814.1

1.6027 1.6335 1.7132 1.7252 1.6725 1.6321 1.6907 1.7205 1.7484 1.7206 1.5304 1.5967 1.6482 1.7051 1.7387 i.5909

XBeF

2

C

h3

1

XBeF

2 0.90 0.90 0.90 0.90 0.90 0.90 0.90 0.90

Temp.

Ec ( v o l t s )

804.1 866.0 754.8 706.2 778.0 876.5 802.5 702.0

1.5819 1.5307 1.6208 1.6613 1.5996 1.5277 1.5905 1.6620

("a

t

?


, 29

Table 2.

Parameters from Correlation of E as a Function of C

Temperature a t Specified Compositions (Ec = a+bT) 'BeF2

Intercept (a)

5u

5 u)x

(Slope

UEcx10 3 ( v o l t s )

(b 1

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 a c t i v i t y of BeF2 may be c a l c u l a t e d from eq. (4) using the emf d a t a (Ec) obtained i n t h i s study i f t h e a p p r o p r i a t e values f o r t h e s t a n d a r d c e l l p o t e n t i a l (Eo) are known. 0

a1 attempts w e r e m a d e t o determine E

As mentioned previously, sever-

experimentally, b u t values of use-

f u l accuracy could not b e obtained because of high m e l t v i s c o s i t y and p o s s i b l y because of high electrical r e s i s t i v i t y . The values of Ec obtained f o r v a r i o u s BeF2 compositions do not lend

themselves t o d i r e c t e x t r a p o l a t i o n t o E

0

.

However, t h e standard c e l l

p o t e n t i a l may b e c a l c u l a t e d by r e l a t i n g d a t a i n t h i s study w i t h the BeF2 l i q u i d u s d a t a i n t h e following manner. 0

I f t h e assumption i s made t h a t

t h e standard c e l l p o t e n t i a l (E ) v a r i e s l i n e a r l y w i t h temperature


30

(Eo = A+BT)*

E

LiJ

then eq. (4) may b e w r i t t e n

C

= (A+BT)

. . fin aBeF2

-E fin

0

aBeF2

2F[(A+BT) =

- Ec]

i

RT

It is t h e n p o s s i b l e t o equate eq. (15) t o

fin aBeF2 (BeF2 s a t ' n ) =

AHf 1 1 - -(R T - -1 Tf

(16)

(where AHf is t h e h e a t of f u s i o n of BeF2 and Tf i s the melting p o i n t of pure BeF ) t o o b t a i n a r e l a t i o n s h i p between A, B, AHf and E c a l c u l a t e d 2 C

a t t h e BeF l i q u i d u s temperatures and compositions. 2

Combining eqs. (15)

and (16).

2F[ (A+BT)

-

Ec] =

RT

-

AHf 1 -(R

T

1 - -Tf 1

which s i m p l i f i e s t o 2F Ec

--

T

-

(2FB

-

+

-)

(2FA

+ AHf)r1 .

Tf

This r e l a t i o n s h i p permits c o r r e l a t i o n of t h e d a t a (Ec) obtained i n t h e p r e s e n t i n v e s t i g a t i o n w i t h t h e phase diagram data.12

A p l o t of t h e a-

bove expression as 2FE /T vs 1 / T should b e l i n e a r s i n c e t h e i n t e r c e p t C

and s l o p e c o n t a i n only constants.

Using t h e least s q u a r e parameters f o r

each composition (Table 2 ) , v a l u e s f o r E

C

were c a l c u l a t e d f o r t h e

l i q u i d u s temperatures a t 0.515, 0.60, 0.70, 0.801and 0.90 BeF2 (see f o o t n o t e ) and t h e n p l o t t e d according t o eq. (17) (Fig. 6 ) .

The r e s u l t -

i n g p o i n t s follow t h e p r e d i c t e d l i n e a r r e l a t i o n s h i p w i t h i n t h e i r pre-

* Using

t h e h e a t c a p a c i t y d a t a i n t h e JANAF Tables (Ref. 21), t h e ACp e f f e c t over a temperature range of 450" 900째C w a s evaluated and found t o be n e g l i g i b l e .

-

z


31

dicted uncertainties. Intercept =

Parameters f o r t h

-

Slope = 113.84 The p o i n t a t 0.90 BeF

2

0.03805

least squared l i n e are:

5 0.0034,(Kcal/'K)

5 2.48 ,(Kcal)

= 2FA

= 2FB

+ AHf

-AH

(18)

Tf (19)

w a s n o t used because the u n c e r t a i n t y i n E

C

is

relatively large. Values f o r A and B were c a l c u l a t e d using v a r i o u s l i t e r a t u r e values f o r t h e h e a t of f u s i o n of BeF2 and a melting p o i n t of 555째C f o r pure BeF2.

A t a b u l a t i o n of t h e s e v a l u e s is shown i n Table 3.

The v a l u e s of

A and B which are c o n s i s t e n t w i t h v a l u e s obtained i n t h e p r e s e n t in-

v e s t i g a t i o n are those f o r which t h e h e a t of f u s i o n is assumed t o b e 2.0 kcal/mole o r less as shown i n Fig. 7.

A heat of f u s i o n f o r BeF2 >2.0

kcal/mole y i e l d s v a l u e s of Eo which are g r e a t e r than corresponding v a l u e s of Ec a t t h e h i g h e r BeF2 concentration.

This c r e a t e s an impossible

s i t u a t i o n where t h e a c t i v i t y of BeF i n t h e mixture is g r e a t e r t h a n t h e 2 a c t i v i t y of pure l i q u i d BeF2.

Thus measurements i n t h e p r e s e n t study

c l e a r l y support t h e lower v a l u e s f o r t h e h e a t of f u s i o n f o r BeF

8 99 2'

The e m f data (Ec) obtained i n t h e p r e s e n t study w e r e f i t t e d by

least s q u a r e s t o t h e expressions l i s t e d i n Table 4 , using t h e lowest l i t e r a t u r e v a l u e (1.13 kcal/mole) f o r t h e h e a t of f u s i o n of BeF2. p l o t of t h e smoothed l i n e s f o r E shown i n Fig. 8.

C

A

a t v a r i o u s BeF c o n c e n t r a t i o n s i s 2

The p r e s s u r e c o r r e c t e d c e l l p o t e n t i a l s (E ) d i f f e r e d C

by 1.4097 s t a n d a r d d e v i a t i o n s from t h e smoothed v a l u e s given by eq. 4-1 (Table 4 ) , and t h e average d e v i a t i o n of Ec from smoothed v a l u e s w a s approximately

2

2.5 mV.

The average d e v i a t i o n is c o n s i s t e n t w i t h the

v a l u e p r e d i c t e d by t h e e r r o r a n a l y s i s f o r E

C

.


32

ORNL- DWG 67-13722

I

a14t

a5t5 Be5

0.13t

/

7

0.t2c

elu"

h

v

LL (u

0.110

0.100

0.090

I

1.20

1.25

1.30

t .40

1.35

1.45

t.50

t.55

looo/TPK)

Fig. 6. Correlation of E C

with BeF2 Liquidus Data.

,-


33

ORNL-DWG 68-230

1.825

5

1.800

1.775

v)

4-

Y

1.750

lu" L3

2 1.725 0

lu

A fff =2.00kcaI/mole' A Hf =5.80 kcal/mole

'

1.700

1.675

1.650 550

575

600

TI

625

650

675

700

T ("C)

Fig. 7. Effect of the Heat of Fusion of BeF2 in Determining Eo The solid lines represent Ec for various mole fraction (dashed lines). of BeF2.

4

U


34

Table 3.

Calculated Parameters f o r the Standard C e l l P o t e n t i a l of

Pure BeF2 (Eo = A

+ BT)

Assuming Various Values f o r the

Heat of Fusion of BeF2 Heat of Fusion of BeF2

(kcal/mole)

,(a>

0.70

2.4525

1.00

2.4460

1.13

2.4432

1.60

2.4330

2.00

2.4241

2.50

2.4135

5.80

2.3420

(a)

From eq. (19).

(b)

From eq. (18).

(b) Bx 10

-

Ref,

No.

.8065 .7986 .7952

9

.7829

8

.7725 .7594 .6730

7


35

Expressions f o r C e l l P o t e n t i a l s and A c t i v i t y

Table 4.

C o e f f i c i e n t s i n the LiF-BeF2 System

Eq. No.

i

c

-

2.3RT a 2F l o g xBeF2 -- 2F

4- 1

EC

4-2

Eo = 2.4430

4- 3

log'BeF2

4-4

L

(a)

= Eo

-

0.000795211

= (3.8780

(-40.7375

+

(94.3997

+

(-67.4178

l o g yLiF = 0.9384

-

(a>

2353.5)x2 T LIF

-

+

log'BeF2

+

-

36292. 81x3 LiF

84870.9

+

4 IXLiF

52923.5

5 IXLiF

232.08 T

+

(-36.9734

+

+

(126.0947

-

+

(-158.4173

+

(67.4178

14652.7)x2 BeF2 74588.5

+

T

3 IXBeF2

113592.3

4 IXBeF2

- 52923.5)x5 T BeF2

C a l c u l a t e d u s i n g a heat of f u s i o n f o r BeF2 = 1.13 kcal/mole.


36

ORNL-DWG 67- I3718

2.0(

1.9c

1.80

-n

c

> 0

lu" 1.70

\ \

1.60

\ t.50

500

600

700

800

i2

900

T ("C)

Fig. 8. Correlation of Pressure Corrected Cell Potentials Ec as a Function of Temperature Using Smoothed Parameters.


37

A Gibbs-Duhem i n t e g r a t i o n of t h e expression f o r y i

BeF2

Table 4 ) w a s

1..

c a r r i e d o u t t o g i v e t h e corresponding e x p r e s s i o n f o r yLiF (eq. 4-4, Table 4 ) .

The i n t e g r a t i o n c o n s t a n t f o r eq. 4-4 w a s determined by com-

p a r i s o n with yLiF v a l u e s derived from t h e l i q u i d u s d a t a l 2 and a h e a t of f u s i o n of 6.47 kcal/mole f o r LiF.

A more a c c u r a t e e v a l u a t i o n of t h e

i n t e g r a t i o n c o n s t a n t should b e p o s s i b l e when t h e h e a t of mixing measurements of Holm and Kleppa22 become a v a i l a b l e f o r t h e LiF-BeF2 system. Smoothed v a l u e s of yBeF2 are shown as a f u n c t i o n of composition a t s e v e r a l temperatures i n Fig. 9.

These r e s u l t s are c o n s i s t e n t w i t h t h o s e

obtained by Mathews and Baes7 over a composition range 0.30 t o 0.60 BeF 2' However, a t x

BeF2

> 0.60 t h e r e s u l t s are not i n agreement.

The v a l u e s

o b t a i n e d i n t h e p r e s e n t s t u d y are thought t o be t h e more r e l i a b l e s i n c e they are c o n s i s t e n t w i t h b o t h t h e phase d a t a and w i t h a low h e a t of c

f u s i o n f o r BeF 2' I

The previous measurements a t compositions > 0.60 BeF2

might be i n e r r o r because of d i f f i c u l t i e s i n mixing LiF-BeF

2

at high

BeF2 c o n c e n t r a t i o n s and because of t h e e f f e c t s of Be0 s a t u r a t i o n .

In

t h e p r e s e n t s t u d y i t was found t h a t a t 0.90 BeF2, a well-mixed m e l t w a s n o t o b t a i n e d u n t i l t h e temperature w a s raised above 8 5 0 째 C .

This pro-

cedure w a s followed f o r a l l h i g h BeF2 c o n c e n t r a t i o n s t o ensure proper mixing of the LiF-BeF2.

Another p o s s i b l e e r r o r i n t h e previous measure-

ments could have been t h e presence of Be0 as a s a t u r a t i n g s o l i d .

The

s o l u b i l i t y of Be0 might tend t o i n f l u e n c e t h e BeF2 a c t i v i t y more a t h i g h BeF

2

concentrations. The f r e e energy and h e a t of the c e l l r e a c t i o n (eq. 1 ) w e r e calcu-

3

ki

l a t e d assuming a h e a t of kusion f o r BeF2 = 1.13 kcal/mole.

These v a l u e s

combined w i t h t h e a v a i l a b l e thermochemical v a l u e s f o r HF21 w e r e used


38

ORNL-DWG 68- 231

h 1.0

0.8

t

h’ 0.6

n

z

4

,p0.4 u

m

h

0.2

0.I 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

XBeF,

Fig. 9. Activity Coefficients in Molten LiF-BeF2 Mixtures.

1.O


39 t o d e r i v e t h e f r e e energy and h e a t of formation f o r l i q u i d BeF2 a t 900°K (Table 5).

Mathews and Baes7 measured t h e e q u i l i b r i u m q u o t i e n t f o r t h e

react i o n

I f eq. (20) i s combined with t h e r e a c t i o n i n eq. (1) t h e r e s u l t i n g rea c t i o n is

The f r e e energy and h e a t bf . t h i s l a s t r e a c t i o n thus could b e obtained by combining t h e two sets of measurements, and, s i n c e t h e thermochemical d a t a f o r H 0 are a c c u r a t e l y known, improved f r e e energy and h e a t s of 2 formation f o r Be0 could be c a l c u l a t e d .

These are shown i n Table 5.

The

preceding c a l c u l a t i o n s f o r BeF2 and Be0 w e r e made f o r a temperature of II

900°K.

Values f o r o t h e r temperatures were generated using t h e h e a t

c a p a c i t y d a t a i n t h e JANAF Tables.

21

Reference Electrodes Both e l e c t r o d e h a l f - c e l l s used i n t h e p r e s e n t i n v e s t i g a t i o n performed acceptably f o r use as reference e l e c t r o d e s , both being s t a b l e and

reproducible.

I

B e r y l l i u m Electrode

The BelBe”

e l e c t r o d e should work w e l l i n any m e l t containing beryl-

I

lium i o n s and no r e d u c i b l e c a t i o n s .

P o t e n t i a l f l u c t u a t i o n s due t o t h i s

e l e c t r o d e were masked i n t h e p r e s e n t study by t h e f l u c t u a t i o n s due t o t h e HF-H2 e l e c t r o d e , b u t should b e less t h a n

5 0 . 1 mV.

Beryllium e-

I B

W

l e c t r o d e s w e r e f a b r i c a t e d from t h r e e d i f f e r e n t batches of beryllium

metal and no d i s c r e p a n c i e s i n p o t e n t i a l s were noted when t h e e l e c t r o d e s


Formation Heats and F r e e Energies of BeF, and Be0

Table 5. Compound

Temp.

State

AHf (kcal/mole)

(OK)

AGf (kcal /mole) ~

BeF2

29 8

BeF2

800

. crys t .

BeF2

900

Liquid

BeF2

1000

Liquid

crys t

- 246.01

(-242.30

~~

2 2)22

- 244.75 -

243.12

(2 1.1)

242.54

- 234.39 - 215.50 - 211.90 - 208.47

(-230.98

2

2) 22

P

.

Be0

29 8

crys t

Be0

800

Cryst.

Be0

900

c r y st

Be0

1000

Cryst.

.

0

-

145.85

(-143.10

145.68 145.57 145.46

(2 1.5)

2 0.1)22

- 138.36 - 125.70 -

123.20 120.73

(-136.12

1.5)

2 0.1)

22


I

41

i

were interchanged.

Therefore, t h e e l e c t r o d e response does not appear t o

be a f u n c t i o n of a p a r t i c u l a r b a t c h of b e r y l l i u m metal. The b e r y l l i u m e l e c t r o d e does n o t appear t o be s u i t a b l e f o r s m a l l

c e l l compartments s i n c e m a s s t r a n s f e r causes t h e e l e c t r o d e t o become enl a r g e d due t o spongy d e p o s i t i o n of t h e B e metal, and e v e n t u a l l y e l e c t r i c a l s h o r t s develop between the e l e c t r o d e and t h e c e l l compartment w a l l .

HF-H

2

Electrode

The P t , HF,H2 IF- e l e c t r o d e should be a s u i t a b l e r e f e r e n c e e l e c t r o d e

i n any f l u o r i d e - c o n t a i n i n g m e l t where t h e r e i s no p o s s i b i l i t y of oxidat i o n by HF o r r e d u c t i o n by H2.

The s o l u b i l i t y of HF i n LiF-BeF2 i s

low23 (about 0.0003 mole f r a c t i o n f o r t h e p a r t i a l p r e s s u r e s of HF used i n t h i s s t u d y ) , and no s i g n i f i c a n t s o l u b i l i t y of H2 is expected i n t h i s system.

P o t e n t i a l f l u c t u a t i o n s due t o t h i s e l e c t r o d e appear t o be a

f u n c t i o n of t h e m e l t v i s c o s i t y .

F l u c t u a t i o n s a r e about

2

.1 mV i n m e l t s

w i t h a v i s c o s i t y of one p o i s e o r less. The p r e c i s i o n of t h i s e l e c t r o d e w a s l i m i t e d somewhat i n t h e p r e s e n t s t u d y by t h e method of HF d e l i v e r y , as previously mentioned.

In future

experiments t h e H -HF mixture w i l l b e obtained by p a s s i n g H through a 2 2 thermostated NaHF2 bed.

It is hoped t h a t t h i s w i l l be a more p r e c i s e

method of producing mixtures of HF and H2 of c o n s t a n t composition.


42

REFERENCES 1. W. R. G r i m e s , MSRP Semiann. Prog. R e p t . J u l v 31. 1964, ORNL-3708, p. 230.

12, -- 1073

2.

J. Berkowitz and W. A. Chupka, Ann. N. Y. Acad. Sci.,

3.

A. Buchler and J. L. S t a u f f e r , "Vaporization i n t h e Lithium FluorideBeryllium Fluoride System," SM-66/26 i n Thermodynamics, v o l . &, IAEA, Vienna, 1966.

4.

T. Fdrland, "Thermodynamics of Fused S a l t Systems," p. 156 i n Fused S a l t s , ed. by B. R. Sundheim, McGraw-Hill, New York, 1964.

5.

J. Lumsden, Thermodynamics of Molten S a l t Mixtures, p. 227, Academic P r e s s , London, 1966.

6.

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)

(1960)

Sept. 30, 1963.

-z,

7.

A. L. Mathews and C. F. Baes, Jr., J. Inor. Chem.,

8.

J. A. Blauer e t a l . , J. Phys. Chem.,

9.

A. R. Taylor and T. E. Gardner, Some Thermal P r o p e r t i e s of B e r y l l i u m Fluoride from 8" t o 1,200째K, U.S. Bureau of Mines, Rept. No. RI-6644 (1965).

$2, --

373 (1968).

1069 (1965).

10.

G. Dirian, K. A. Romberger, and C. F. Baes, Jr., Reactor Chem. Div. Ann. Progr. Rept. Jan. 31, 1965, Om-3789, pp. 76-79.

11.

C. T. Moynihan and S. Cantor, Reactor Chem. Div. Ann. Progr. Rept. D ~ c . 31, 1966, ORNL-4076, p. 25.

12.

R. E. Thoma e t a l . , submitted f o r p u b l i c a t i o n i n J. Nucl. Mat.

13.

Diaphragm Type Adjustable Leak Valve (Ref. No. C-I 244928) obtained from ORGDP, O a k Ridge, Tenn.

14.

S. Dushman, S c i e n t i f i c Foundations of Vacuum Technique, pp. 607-618, Wiley and Sons, N. Y., 1949.

15.

S.. Cantor e t a l . , Reactor Chem. D i v . Ann. Progr. R e p t . Dec. 31, 1965, ORNL-3913, p. 27.

16.

Temperature-Composition values used h e r e f o r t h e BeF l i q u i d u s were 2 supplied by S. Cantor of t h i s Laboratory.

bd


43

17.

J. H. S h a f f e r , MSRP Semiann. Prog. Rept. J u l y 31, 1964, ORNL-3708, p. 288.

18.

G. E. Darwin and J. H. Buddery, Beryllium, p. 85, Butterworths

S c i e n t i f i c P u b l i c a t i o n s , London, 1960.

I

c

19.

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

20.

H. D. Young, S t a t i s t i c a l Treatment of Experimental Data, p. 96, M c G r a w - H i l l , New York, 1962.

21.

JANAF Thermochemical Tables, Clearing House f o r Federal S c i e n t i f i c and Technical Information, U.S. Dept. of Commerce, Aug., 1965.

22.

0. J. Kleppa, p r i v a t e communication.

23.

P. E. F i e l d and J. H. S h a f f e r , J. Phys. Chem.,

11, -- 3320

(1967).



45

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