The use of A ZrZrO2 electrode as an indicator electrode in the potentiometric acid-base titrations in fused KNO3

Materials

THE

Chemistry

USE

OF

and Physics,

A Zr/Zr02

POTENTIOVlETRIC

A.

BARAKA,

Cairo

ELECTRODE

ACID-BASE

-

!A.I.

Chemistry

9 (1983) 583-595

AS

AN

INDICATOR

TITRATIONS

AEIDEL-RUHMAN

Department,

583

and

Faculty

IN

E.A.

FUSED

ELECTRODE

IN

TI-IE

KNOX

EL-TAHER

of Science,

Cairo University,

(Egypt)

Received 24 May 1983; accepted 27 June 1983

ABSTRACT The

Zr/ZrO*

oxidation used

as

tions.

of

molten

indicator

oxide-ion

KN03

with

on

in

mV relative

and

Ag/Ag(I)

responds

molten

KNOX.

of [02-]

electrolytic

electrode

acid-base

potential

at [02-] = 1, is

be

titra-

theoretically

and the slope

reference

can

Its potential

The standard

the potential

to the

This

the

in potentiometric

the logarithm

i.e.

by

zirconium.

to 67 mV at 35OOC.

electrode,

prepared

reversibily

concentration

linearly

was

electrode

behaves

It

amounts

electrode

to the

varies

of the line of

computed

Zr/Zr02,02as

-1698

electrode.

INTRODUCTION

Metal/metal-oxide

for the determination acid-base

titrations

In continuation titrations

in molten

0254-0584/83/$3.00

electrodes

have

of oxide-ion in molten

KN03

using

activity

salts

of our work

been

used and

as indicator potentiometric

[l-9].

on potentiometric

the

acid-base

metal/metal-oxide

0

Elsevier Sequoia/Printed

electrodes

in The Netherlands

584

as indicator

electrodes

preparation

of a Zr/Zr02

electrode

for oxide-ion

acid-base

titrations

the present

~0,111,

electrode.

reports

The feasibility

determination

in fused

paper

KM03

on the

of this

and for potentiometric

is established.

EXPERIMENTAL

The technique ten salts was used

of potentiometric

as developed

by Shams

in the present

out in tall unlipped in length).

heated

perature

of the furnace

former.

The temperature

tight

crucible-type

fitting

Pyrex-glass

ness.

X 12 cm elec-

of the tem-

a variable

was measured

trans-

by means

indicator

was the Zr/Zr02 zirconium

sheet,

was of dimensions

20 cm long

carried

of a

(+ 3'C).

by means

of a

tube.

was cut from pure

afm of about

through

from the main melt

electrode

The electrode

in mol-

in a small

Regulation

and a temperature

was separated

The indicator electrode

furnace.

of the melt

were

(4.9 cm diameter

was maintained

was affected

thermocouple

The thermocouple

Titrations

tubes

vessel

titrations

and his co-workers[9,12-191

investigation.

Ihe titration

Pt-Pt-lO%Rh

El-Din

Pyrex-glass

trically

acid-base

This

electrode. having

0.1 cm 'chick-

IX1 cm and with a side

and 0.2 cm width,

used

for the electrical

connection. The electrode cally

pickled.

surface

The mechanical

and 3/O emery papers a soft

cloth

obtained. onds

polishing

then rubbing

of cotton

After

was mechanically

wool,

bath.

after

This

was affected

the electrode

this the electrode

in a pickling

polished

which

consisted

49% HF, 4 ml 70% HN03 and 5 ml distilled

2/O

against

surface

was

for IO to 20 sec-

of a mixture water;

using

surface

a bright

was dipped

and chemi-

of 1 ml

this composition

585 is the same electrode cold

as that used

was then rapidly

distilled

trode

was immersed

a thick

grey

these

in molten

layer

electrode

nitrate-potassium

The electrode

was separated

at high

temperature

KNO3 pure

acted

sample

pelled

ccator

potassium hours.

pure

g of KNO3,

dichronate

electrical

and the last

crushed

traces

for a period

in a mortar

were

of 1 hour.

and kept

together

with

were

the weighed

exThe

in a desi-

were

melted

then slowly

over

lowered

of the steady-state

quantity

weighed

quantities

of the base.

Changes

depended

the neutralization

was mechanically

out by adding

in potential

The measured

than 2 IO mV.

During

of 2

o-f the in-

were ob-

on the type of, and on the progress

process.

to better

a period

potential

was then carried

These

of the

into oositions.

the titration

potentials the whole

were

of,

repro-

time the melt

stirred.

AND DISCUSSION

The variation the amounts

different

Pyrex-

conductance

of uater

electrode,

with

w/w).

The analytically

dicator

RESULTS

dipping

(2.04;: AgN03

in the titration,

(as an acid)

the establishment

ducible

wire

required.

The electrodes

served.

the elec-

the main melt by a solid

dry oxygen

salt was then

50.00

with

at 3.%IoC, whereby

silver

melt

sufficient

as a diluent

was ore-melted

until

times

[9-191.

by bubbling

solidified

nfter

lhis possessed

tubing.

the

on its surface.

nitrate

from

J;

treatments

was a 99.9+$

into a silver

glass

surface

[21

several

for few hours

KNO,

of Zr02 formod

The reference

and rinsed

removed

After

water.

[20 ] and Young

by Tegart

amounts

of the i!r,/Zr02electrode

of K2CO3

and Na202

of K2Cr207

added

followed

potential

to KN03

typical

melts

at 350°C containing

potentiometric

ti-

586 tration ves drops

curves

of

these in

Figs.

the

The

results

errors

when

titrated the

K2Cr207

to

show

potential

tage

hence

similar

of

those

that of

at

the

Figs.

given

1

the

and

suitablity

under

these

K2C03

or

of

the

2

Na202

that

known

did

and

not

2.

points

the

exceed

Ihe

recorded.

average

2

of 1

cur-

distinct

are

quantities

potentiometric

conditions

1

electrode

show

of

Figs.

equivalence

indicator

in the determination with

in

oercenthe

acid

percent,

determination

of

is justified,

K2CO3 Al Fig.

1. Potentiometric titration of KzCr207 with K2C03 in KNOB melt 11 1.000 g, 21 1.502 g, 31 2.000 g, 41 3.000 g K2CrzO7.

at 35O'C

587

-4oc -500 -600 -7oc

>

E -800 -900 -lOCO -1100 -1200 -1300 (L

-14oc

0” -1500 Y L

rG

-1600

I

I

I

I

I

I

I

I

I

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Na202

a9

Fig. 2. Potentiometric titration of K2Cr207 with NazOz in KNO, melt at 350"C 11 0.500 g, 21 1.015 g, 31 1.633 g, 41 2.005, 51 2.917 g K2Cr*0,. In Fig. the inflexion are plotted K2Cr207

3 the weights points

of the titration

as a function

in the nitrate

of figures

is again

of the two titrants

of the corresponding

melt.

quite

curves,

The agreement

clear.

consumed

Figs.

1 and 2,

weights

between

up to

of

the two sets

588

l.! s % i

1.c

5 z 0 ._

.C OC

1.0 Weight

0.0

Fig.

2.0 of K2Cr207

(g)

3. Relation

between the quantities of bases and the concentration of the dichromate.

The neutralization melt under

Cr*+

3.0

2-

+

Specifically

reactions

consideration 2II

+

2co3

2Cr207

+

2O2

=

=

Lux's

place

acid-base

2CrOZ-

=

mentioned,

2Cr207

follow

taking

in the nitrate definition,

(1)

the reactions

are:

2cro;-

+

co2

(2)

2cro$-

+

&02

(3)

v&.,

589

For an oxygen cro;-

electrode

ions

the potential

EO'

_

in equilibrium

of this

electrode

with

Cr20$-

and

is:

[cro;-]2 =

E

(4) CCr20;-1

1Jhen the ZrjZr02 assumed

Zr02

that the ZrO2

=

zr+4

+

electrode

is used

dissociates

into:

as indicator,

20*-

then the potential

it is

(5)

of the zirconium

electrode

would

be given

as: E

zr

=

G/Zr

4+

+

rz

E0 +4 Zr/zr

+

=

E

where

EO "

-

g

[Zr+4]

(6)

%

In ,,,!;,

(7)

constant

of reaction

02- potential,

i.e.

(5) and EC" is

the potential

(4) in the case of the Zr/Zr02

electrode

at

[b2-]=1.

becomes:

[Cr0;-]2

EO " -

%

In

In this work

of this

(9)

[CrO;-j

a trial

lity of the Zr/Zr02 potential

(8)

In [C'-]

Zr/Zr02,

Equation

=

In

K' is the ionization

the standard

E

g

was made

electrode

electrode

The experimental quantities

of Na202

potentials

to establish.

to establish

and to determine

in molten

procedure

to 50.000

KN03

implied

g KN03

the rewersibi-

the standard

at 35O'C.

the addition

and uaiting

of small

for constant

590

In Table 1 the results of these measurements are grouped. Column I gives the molarities of oxide-ion in the nitrate melt, while in column II the corresponding equilibrium potentials are given.

Table

1.

Variation of Zr/Z'rfl2electrode potent ial w ith the

concentration of Na202 in KM03 melt at 35O'C

Plolarity of Na202

Zr/ZrD2 erectrode potential vs. Ag/Ag(I), mV. -

1.00

x

1o-3

- 1495

1.58 X 1O-3

- 1506

2.00 X lO-3

- 7515

3.2

x 10'3

- 1528

5.00 X 10-3

- 1540

1.00 X 1o-2

- 1560

1.60 x 1o-2

- 1577

2.00 x 10 -2

- 1562

3.20 X IO-2

- 1588

5.00 x WY2

- 1610

1.00 x 10'1

- 1630

In Fig. 4 the potential of the Zr/Zr02 electrode is plotted as a function of the logarithm of the molarity of the oxide ion in the melt.

The experimental points fall on a perfect straight

line whose slope amounts to 67 mV.

This value is in agreement

with the magnitude of the term 2.303 RT/ZF at the corresponding temperature (61.6 mV at 35ClOC). This means that the Zr/Zr02 electrode behaves reversibily in these melts and can be used

591

0

a -1SOC

[02-I

Log Fig.

4. Calibration KN03

as indicator the E

0”

electrode

value

the Ag/Ag(I)

at [02-J reference

The variation function

of time,

(bases):

NHbV03,

NsHAs04,

K2C03

is shown

in Fig.

for 0

Z-

-ion

in molten

determination.

= 1 , is computed

of the Zr/Zr02

NaP03,

NaH2P04,

and Na202 5.

is given

From

Fig. 4,

02- electrode,

i.e.

as - 1698 mV relative

to

electrode. electrode

in 10 -2filKNOX melts

the addition

potential

electrode

(in eqn. 8) of the Zr/ZrO*,

the potential

after

for the Zr/ZrO,

curve

at 35O'C.

relative

In this

K2HP04,

Ag/Ag(I)

to the nitrate

As is seen,

reference

melt,

constant

as a acids

K2Cr04,

Fig. the time is given

of the salt in mV.

of the following

K2Cr207, to

potential

Na4P207,

electrode

in miuntes while

potentials

the are

l -1

‘

I, , I

593 for

the

Zr/ZrC2

in

arily,

zero

35O'C)

dc.wiation

Ki,:fl 3 is

acidity

taken

as

calculations

given

a

neg-tiiic 0

tivity.

lases

clcncting

an

acid

city)

trode be

The

of

2.

ID -%1

Acid

acidity

are

given

in

denoting

,

basic

the

results by

as

(base)

in

Zr/Zr02 -vs.

2.303

2.

ar::

2

that

in

KN03

electrode lAg/Ag(I),

the a

is

at

the

ZrjZr02

siyn,

acidity

an

(basi-

seruence ,oxygEn

electrode

in

acidity

ac-

The stronger

sa:ne

using

of

are

positive

[221,

these

(basicity)

elkscan

melts.

number

350°C

potential mV

at

pure

results

ilcidity (basicity) number

NH4V03

-

450

-

15.42

NaPO

-

500

-

14.6

NaH2P04

-

540

-

13.96

K2Cr207

-

580

-

13.31

K2HP04

-

600

-

9.74

K2CrO4

-

925

-

7.7

Na4P207

-

980

-

6.62

Na2HAsO4

-

1200

-

3.25

KN03

-

1400

K2C03

-

1500

+

1.62

Na202

-

1560

+

2.60

3

of

oxide-ion

its

electrode

mV

oxyanions

shoiti the

and

(61.6

The

civcn

arbitr-

that

activity.

indicator

molten

from

!\ciC

decrease

El-Sin

potenticls

RT/2F

(base)

Table

Table

means

assigned,

(basicity).

a

of

This

is

(numerically)

Shams

Steady-state

acid

oxide-ion

higher

use d

oxyanions

the

KiiO3 each

qxyanions

in

indicator.

satisfeztorily

of

of

siqn and

and

unit

obiained

as

Table

a

(base),

nrewiously

pure

potential

incrcasc

number.

melts,

(basicity) in

these

the

these

0.00

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