selective fluoride electrode mechanism studies

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ANALYTICAL LETTERS, 6 (3), 211-216 (1973)

SELECTIVE FLUORIDE ELECTRODE MECHANISM STUDIES

KEY WORDS: fluoride, electrode, mechanism, lanthanum fluoride

H. M. Stahr and Darrell O. ClardyChemistry Lab

Veterinary Diagnostic LaboratoryIowa State University

Ames, Iowa

ABSTRACT

We wanted to determine the mechanism of charge transport

through the LaF crystal in Orion's 94-09 and 94-09A fluoride3 I

electrodes. Voltage was applied to the electrode while it

was in 15% Sodium Acetate Solution (pH 6.3) with or without

(FIB ). We concluded that the mechanism of conduction through

LaF 3 was one of ionic diffusion. FIB accumulated on the

electrode surface very rapidly. Diffusion into the body of

the LaF 3 (deeper than 300 ~) was also very rapid.

It is well known that the alkali halides contain

Schottky defects. LaF3

is composed of 0.1% neutral Schottky

defects at 27 oC, which cannot act as charge carriers. l The

conduction of charge could be due to ionic diffusion, elect-

ronic conduction, or a mixture of these mechanisms. Since

the Debye temperature is B7 0 C for LaF3

, the fluoride anion

is very mobile. Orion has taken advantage of this fact by

doping their crystal with Europium and, thus, greatly improv-.. d.. 21ng 1ts con uct1v1ty.

211

Copyright © 1973 by Marcel Dekker, Inc:. All Rights Reserved. Neither this work nor any part may be reproducedor transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, andrecording, or by any information storage and retrieval system, without permission in writing from .he publisher.

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.. , ""STAHR AND CLARDY

We wanted to determine the mechanism of charge transport

through the LaF3

crystal in Orion's 94-09 and 94-09A fluoride

electrodes. We applied voltage to the silver wire in the

fluoride electrode and to the reference electrode for 15

seconds. This was done before each measurement. We wanted

to see what the effects would be on the crystal's conductivity.

r.tiJ, I (6'''~1'' H :y""J• Figure/,~ shows that as we made the crystal positive with

K)/J f,.rP1V~~~_j5p~r(etrespect to the reference electrode, the sol~bility product

'.

decreases. This could be due to the formation of a film

which possess capacitive effects. This film could consist

of LaF3 without europium, of the LaF3_xAcx complex, or of

fluoride anions saturating the crystal's surface and piling

up in the solution around the crystal.

To check this concept, we applied voltage to the crystal

and measured the current that flowed. Figure 2 shows that

with a negative applied voltage (with respect to the reference

electrode), we observed an increase in current flow with an

KSp FOR LAF3 IN THE ORION ELECTRODE

~ KSP~~Aj ~-J3ORION'S SPECIFIC ION ELECTRODENEWSLETTER, JAN./FEB. 1971.(94-09) ~.3xIO-30

J. Ross IN R. A. DURST'S "IoNSELECTIVE ELECTRODES," NBS SPECIALPUBLICATION #31~

P. A. EVh ~ £T Ah., LAB. PRACT.,20:644 (1971). (9Q-09A)

THIS PAPER: 94-09A94-09 (PLASMA FLAME)IN 15% NAAc pH 6.2-6.4

94-09A94-09

FIGURE 1

212

3.5xlO-28

3,5x10-28

7.1xlO-307.5xlO-27

1.2xlO-254.2xlO-22

Xsf (~I(I..t Il(fr,p

/YD"" f ~f i~llech'~")

fe ,'/If".

-lr.D ELECTRODE MECHANISM STUDIES

/4--1+--- - NEG.

ORION 94 - 09

8 L-- --'- .......2_}J_Q. -J

3L.----.-...

FIGURE 2

increase in fluoride concentration. However, with a positive

applied voltage we observed a decrease in current flow with

an increase in fluoride concentration. This is consistent

with the formation of a capacitive film being formed on the

surface of the crystal.

To determine if this film prevented the flow of the

fluoride anion through the crystal, we generated rIB from

LiC0 3 in the Ames Labs Research Reactor. We put the F18

in a 0.025 M NaF solution in 1.83 M Na Ac, pH 6.3. Table 1

shows the experiments carried out. The electrode was first

exposed to the rIB without any applied voltage for 10 minutestFig.3).

We then applied a -7V to the crystal for five minutes and18

measured the decays per minute. Next we exposed the crystal to F for

five minutes with an applied voltage of +7V. Theoretical

213

STAHR AND CLARDY

Fl8- EXPERIMENT

SAMPLE

1. r l8 - as irradiatedin solution 15t NaAc

2. F18- after experimentin solution 15% NaAc

3. Electrode un~tasedexposed to F - solutionfor 10 minutes

a) fluoride electrodeb) calomel electrode

4. Fluoride electrodebiased negatively 7 voltsrelative to calomel


ACTIVITYC.P.M.

126,325

86,100

7,688337

6,106276

CALCULATED DECAY~

MEASURED DECAY **

*CD-32\)HlMD-27\

Net loss890 C.P.M.

5. Fluoride electrodebiased positively 7 volts


6. Fluoride electrode abraided3x with 500 grit emery paper

7. Fluoride electrode coveredexcept for crystal surface;with tape and exposed againto solution without bias for5 minutes

11,088C.P.M.3l2C.P.M.

7,102C.P.M.

Net gain5428 C.P.M.

Net loss3570 C.P.M.

**MD-6l89 C.P.M.~CD-626" C.P.M.

values were calculated using In COle = 0.0063+, where t is

in minutes. It can be seen that there is a marked increase

over the theoretical dpm when the voltage was applied. We

then abraided the surface of the crystal three times with

500 grit emery cloth to remove a few 100 ~ and remeasured

the dpm. There is a 33% decrease from the theoretical value

indicating that most of the Fl8 is not concentrated as a

film on the surface of the crystal. We checked to see if

214

.r

I

enoI

'etenZoCEo

~Ol x[.:!]

215

ELECTRODE MECHANISM STUDIES

o2+

>:eo

ooNI.

STAHR AND CLARDY

any rIB was absorbed by the epoxy housing. We found no takeup

of r I8by the housing.

The mechanism, therefore, must be one of ionic diffusion.

When we applied a positive voltage to the crystal, the active

sites must be saturated with fluoride anions; thus, creating

the decrease in current flow with an increase in fluoride

concentration.

ACKNOWLEDGMENTS

We wish to acknowledge the Veterinary Diagnostic Laboratory,

I. S. U.. for time to conduct the study; Ames Labs Research Reactor Chief,

Dr. Voigt, and Reactor persormel, Ken Malaby and Dr. O'Toole, under

whose authority the experiment was conducted.

REFERENCES

1. A. Sher, R. Solomon, K. Lee, and M. W. Muller, PhysicalReview 144 (2). 593 (1966).

2. Josef Vesely, Chern. Listy. .§2. (1), 86 (1971).

Received February 9. 1973

Accepted February 16, 1973

216

selective fluoride electrode mechanism studies

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