Precipitation of nickel ferrites: Nature of intermediate reaction steps

Materials Chemistty and Physics, 19 (1988) 497-502

PRECIPITATION

S. ARDIZZONE Department

OF NICKEL

NATURE OF INTERMEDIATE

REACTION

STEPS

and L. FORMARO

of Physical

The University,

Received

FERRITES:

497

Chemistry

Via Golqi

and Electrochemistry,

19, 20133 Milan

1, 1987; accepted

December

(Italy)

January

14, 1988

ABSTRACT One of the reactions, oxidation examined ferrous

by nitrates using

the data referring

ferrites.

different occurs

by which

pH regimes

connected

of the Ni/Fetot

ferrites

are formed,

of the substituent

to the precipitation

The precipitation

for only fractional

function

substituted

of Fe '+in the presence

of all the oxides

of the oxidizing

is

of a family of nickel involves

by a very sharp pH transition.

additions

(controlled

cations)

two distinctly The transition

ions and is a continuons

ratio.

INTRODUCTION Metal methods

substituted

ferrous and quest cations simple and, provided metry and phase formula

composition

c71. Even a visual

is quite complicated are the result

control

observation

course,

however,

we report

of the reaction

oxygen,

present evidence

final properties

some observations

is rather the stoichio-

are very close to the projected

gives convincing

of several

(wet)

containing

procedure

of the products,

and that the favourable balance

by several

ions of solutions

are taken to exclude

of the final products

of a kinetic

In the following

by nitrate

is very often used. The reaction

some precautions

times in the reaction

better

(MexFei_x Fe 20 4 ) can be obtained

ferrites

Cl-61 and among them oxidation

intermediate

at different that the reaction

of the products reaction

which can be useful

steps.

to achieve

a

sequence.

EXPERIMENTAL All reagents tion. Solutions

were of reagent-grade were prepared

purity

and used without

with conductivity

water

passed

further purificathrough

a Milli-Q

apparatus.

0254-0584/88/$3.50

0 ElsevierSequoia/PrintedinTheNetherlands

498 The oxides were prepared Sidhu et al. the reaction of variable Results following

by a modification

of the procedure

developed

by

C61 to obtain magnetite

doped with foreign cations. Essentially 2+ oxidation of Fe by alkaline KNO3 in the presence

is a controlled 2+ of Me _ Details

amounts

in the following

pertain

of the procedure to preparations

are reported

performed

in c7f.

observing

the

conditions:

a) Oxidize

a constant

amount of the initial

amount of NO3 was dosed assuming

Fe 2+ (238.14 meq);

a 3 electron

exchange

the necessary

per NO; ion C61:

NO; + 4H+ + 3e- -----+ NO + 2H20

(1)

Vtml)

Fig. 1. Dependence alkaline-oxidizing of Ni 0.12Fe2.8804'

of the pH of the reacting mixture on the addition of the mixture. The curve specifically refers to the preparation

In the coordinates

of Fig.

The amounts

1 the addition

of KNO3 was completed

of Ni2+ and excess Fe2+ required

dosed by keeping

their molar

sum constant

b) Keep constant

the rate of addition

EKNO~I = 0.27 M 1 to the mixture c) Keep constant

by any sample stoichiometry

were

and equal to l/2 CPe 3+1 .

of the KOH + KNO3 solution

of the bivalent

the rate of stirring

at 294 ml.

cations.

of the reacting

solutions.

([KOHL = 3.333 M;

499

pH measurements the reacting after cooling

withdrawn

from the supernatant

times. Determinations

reaction

and an Amel

of

were performed

by means of a combined

at room temperature,

Ingold U402-~2

Ag/AgCl

336 pH meter.

AND DISCUSSION

The features Fig.

on samples

at different

the solution

Glass Electrode

RESULTS

were obtained

mixture

of the precipitation

1. The curve represents

addition

the variation

of the alkaline-oxidizing

trend observed II ratio.

of ferrites

of the reacting

on the basis of

solution

pH with the

the curve is representative

mixture;

in all the different

are examined

independently

preparations,

of the

of the initial

Ni/Fe

The ferrite initial

formation

stages pH shifts

appears

to a neutral

a great part of the reaction, Successively line

to involve

pH undergoes

value which

establishing

a rather

two distinct

pH domains;

remains

almost

a first neutral

sharp transition

in the

invariant

plateau

to a second,

for

region.

strongly

alka-

plateau.

At any stage of the reaction of a balance

between

the solution

the addition

which OH- ions are consumed

modifications

which

in passing

In the first part of the reaction, gel-like

with colours

at alkaline

starting

pH the phases

to definitely

(both Ni and Fe) which by calorimetric

the progressive

tests on samples

complete

in correspondence

region.

are voluminous

progressively

and flocculate

and

darker:

very quickly

withdrawn

solubility

of the precipitates formation

are further

with the neutral

plateau

t81, the formation

formation,

indications

of the

plateau

is, in fact,

at the relevant features

conand

of this occurence.

the only process

region.

found to be

values.

ions. The above quoted

is not, however,

of the reacting

giving rise both to the cation

the pH of the neutral

with that for the hydroxides

both of nickel and of ferrous

The hydroxide

to alkaline

ions

of the reaction;

ions was always

products

as the main process

and to the OH- consumption:

of the bivalent

from the supernatant

with the pH transition

can be considered

in good agreement

depletion

the precipitation,

from the beginning

of both nickel and ferrous

On the basis of room temperature

correspondence

and becoming aspect

undergoes

to the alkaline

which might occur during

takes place starting

the depletion

colours

the reaction,

pH, precipitates

lose their colloidal

concerns

mixture,

centrations

at neutral

from blue-green

the main processes,

the first observation

depletion

form during

from the neutral

black deposits.

In considering

hydroxides

the net result

KNO 3 and the rate by

in the reaction.

The aspect of the precipitates, significant

pH is, necessarily,

rate of the alkaline

which occurs

in

The amount of KOH, added at

500

I

‘\O \ \

ixlo-

\

\

I

\

0 \

=

I

5 m-

\

0

>

\

\

0

\

\

\

O

0

'0 .

.

.

-o - - o-

soI

1

0.2

0.3

I

0

0.1

-

Fig. 2. Amount of alkaline-oxidizing mixture added at the inflection point of the pH vs Volume curves as a function of cationic composition of the starting mixture.

inflection

point

of the curve,

stoichiometrically Further

reguired

in Fig. 2 it can be seen that the pH transition

which become

lower the higher

very close similarity also observing reactions,

that the initial

is, apparently,

with H' consuming

tion of the hydroxides. of the first-formed transformations is stable

for KOH amounts,

in the starting mixture. On the 2+ and Ni2+ hydroxides, and

of Fe

(Ni + Fe) is a constant

in the different

in Fig. 2 cannot be interpreted

that when still in the neutral

effects

The progressive

precipitates,

take place

change,

is in keeping

of the hydroxides. towards oxidation,

to Fe 30 4 through

denomination oxidation.

of

occurs

considering

of the hydroxides.

The only conclusion

oxidized

amount

content products

the shift of the transition

other processes

hydroxide

the nickel

of the solubility

only the precipitation

mical

is in fact, in any case, lower than the amount 2+ the hydroxides of Fe and Ni2+.

to precipitate

the formation

While,

proposed

especially

apparent

in the present

nickel

[9-111 to be

which have the general

stoichiometries

cl21 for the intermediates

in the colour of some che-

conditions,

is reported

of intermediates,

pH region,

with the precipita-

with the occurence

iron hydroxide

of green rusts, and have different The formulae

together

and degrees

of

are:

(A)

501 [ FeIIF.eIIIOx(OH)

](s-2x-y)

(B)

Y It is worth noting hydroxide of the

acidity

the higher

of alkalinity

It is proposed passes

partially

the positive

produced

(A and B

depending

charge

) from the

on the stoichiometry

of the complex,

will be; the opposite

that in the present

through

substituted

The isomorphism ferrites,

followed

the greater

effect holds good if the

during

for by nickel

allow

be the process hydroxide

which,

related

introduce increase

of the precipitation

occurring

to some effect,

variations

rusts).

that a common

and inverted

reaction

the alkalinization

overlap

therefore,

with

of the reacting

green rusts should bear a positive

caused

path is

with increasing

the mixture: charqe.

Ni content

by the extent of the Ni for Fe substitution can be indicated:

the formation

in their stoichiometry

in a first instance

of the intermediates

(increasing

positive

and might

charge)

or an

in the rate of their formation.

Secondly

the variation

of nickel

are further oxidized

intermediates

were positive,

acidifying

reaction

the nickel

At the present

the oxidation

the rate by which

the

If the charge of the

would occur as an

in Fig. 2 would originate reaction

while

from

increasing

of the intermediates. state of the work,

the oxidation

to ferrites

point the amount

+ 3Ni(OH)2

results do not allow a discrimination

Independently

Note. For example the formation following reactions:

Noi + 6Fe(OH)2

to ferrites

of the rate of the oxidation

these possibilities.

the inflection

may affect

shift of the pH transition

lowering

content

content

to the final products.

(see note).

Thus the backward a progressive

mediates

green

megnetite

into green rusts might well,

Two possibilities

of Ni might concern

1s

of these compounds.

the shift of the pH transition

intermediates

between

between

with a total or a partial

determines

the intermediate

in the intermediates. the influence

(nickel substituted

similarities,

of the hydroxides

precipitation,

From the above, appears

ions

of nickel-substituted 2+

to A and B, in which Fe

it in fact to be assumed

the course

in this hypothesis

case the formation

intermediates,related

and the general

The transformation

NO;

of intermediates

or alkalinity

is negative.

ferrites

spine1

either

(green rust):

the amount charge

that the formation

mav produce

cannot be completed

of added nitrate of NiFe204

+ f3n+l)H+

+ 3CNiFe IIFexl*Ox(OH)y]m+

of the nickel

+ k(H20)

contents

of the inter-

in the first plateau

is in any case in defect

can occur

through

II

--J

NO + 3CNiFe

3

NO + 3NiFe204

the sequence

Fe lllOx(OH)y~n+

+ (3~lfHS

(at

in of the

+ wH20

502

respect

of the stoichiometry

rapidly

quenched

ions is excess In order geneous matrix

of reaction

at the end of the neutral

over the final ferrite

to complete

charge

reaction,

inherently

processes

the products

it is necessarily

between

takes place.

within

plateau,

nitrates

contain

is

iron

II

required that a heteroII ions in the solid

and Fe

This latter reaction

the bulk of the solids.

slow and is completed

if the reaction

composition).

the oxidation

transfer

of the intermediate,

or transport

Cl1 and, further,

in the alkaline

involves

It therefore

diffusion

becomes

plateau.

CONCLUSIONS The formation which

includes

of ferrites

formation

Notwithstanding

products.

ions, the increase modifications

appears

the chemical

of the Ni/Fe

either

to occur

and subsequent

through

similarities

in the starting

in the composition

a complex

transformation

reaction

of different

between

mixture

sequence

intermediate

nickel

and ferrous

determines

significant

or in the reactivity

of the intermediate

products.

ACKNOWLEDGEMENTS Financial

support

by ENEA

(Rome) is gratefully

acknowledged.

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