Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a process for preparing
ceric sulphate.
The use of cerium oxidants, for example ceric
sulphate, is well known in organic chemistry. Ceric sul-
phate can be used to prepare naphthoquinone from naphtha-
lene, p-tolualdehyde from p-xylene and benzaldehyde from
toluene.
In preparing a cerium oxidant for use in organic
snythesis it is important to prepare the oxidant in as
concentrated a form as possible. This is necessary to
increase reaction rates and reduce reactor size requirements
and manufacturing costs.
Kuhn in the Electrochemistry of Lead published by
the Academic Press in 1979, summarizes the prior art in the
oxidation of cerium (III) to cerium (IV). It is indicated
that prior workers such as Ramaswamy et al, Bull. Chem. Soc.
Jap. 35, 1751 (1962), and Ishino et al, Technol. Rep., Osaka
University. 10, 261 (1960), have observed that the current
efficiency for ceric sulphate production decreases with
increasing concentration of sulphuric acid, for example 0.26
to 2.6 molar, and with increasing current density, for
example 1 to 3.0 amps/dm2, i.e. 10 to 30 mamp/cm . The
current efficiency of ceric sulphate production was only 54%
at an anode current density of 1 amp/dm2 (10 mamp/cm2). The
"effective" anode current density was therefore only 5.4
mamp/cm2. Ishino et al. found the best electrolysis con-
ditions to be low anodic current density, for example 2
Amp/dm2 (i.e. 20 mamp/cm2), and low sulphuric acid con-
centration, for example 0.43M sulphuric acid.
r~
-1-
UO
The prior art fails to reveal how ceric sulphate
can be prepared in a concentrated form and at commercially
viable current densities, for example 100 mamp/cm2, and
commercially viable current efficiencies, for example 50~,
to give "effective" anode current densities of 50 mamp/cm2
or higher.
Kuhn, in the above publication, specifically
indicates that little information is available for the
reaction of oxidizing cerium (III) to cerium (IV).
However, the present application describes a
process able to achieve extremely high current efficiencies
for concentrated ceric sulphate preparation and very high
effective anode current densities using a wide variety of
anodes and cathodes and~acid strengths deemed detrimental by
others, specifically Ramaswamy et al and Ishino et al.
More specifically, the present invention is a
process for preparing ceric sulphate in solution that
comprises electrolyzing an at least saturated solution of
cerous sulphate at an anodic of or greater than 100 mamp/cm2
cathode current density of or greater than 1000 mamp/cm2 and
with vigorous agitation in the presence of 1 to 2 molar
sulphuric acid in the absence of a diaphragm.
The saturated cerous sulphate may be maintained as
such by electrolyzing a suspension of cerous sulphate, or by
carrying out the electrolysis of a saturated cerous sulphate
solution. A diaphragm is not used. The electrolysis of a
saturated cerous sulphate solution is carried out briefly
then the electrolyte is mixed with cerous sulphate crystals
to resaturate it with respect to cerous sulphate. Undissol-
ved cerous sulphate crystals are allowed to precipitate.
~l ,
llf~i`7~UU
The supernatant liquid is then re-electrolyzed.
The invention is illustrated in the following
examples:
Examples
Exeept where indicated otherwise in Table 1
electrolysis of a starting eleetrolyte comprising 25 grams
of eerous sulpha~e pentahydrate, 5.5 ml of eoneentrated
sulphurie aeid diluted to a volume of 100 ml with water to
give lM sulphuric acid was carried out with vigorous agi-
tation of the electrolyte during eleetrolysis. The results
and reaction conditions are set out in Table 1. A diaphragm
was not used in the eleetrolysis.
--3--
- I ~ Ll~ (30
., ,~ ~.
" ~ C o ~ C~ o ~ ~ o~
o ~ ~ ~ ~,~ ~oo ,, ~ ~ ~ ~ ~o
a ~ ~ ~ o~ O ,
:~
" C
o ~ ~ ~o ~CO ~O ~ ~ cr~
C~
u~ o ~ ~ ~ ~I ~
~ O ~ cn cn
U~
u~ o r~
U~ I I I I o I O S~
c~ o u) al ~o r~ .~:
E~ . . :1
0~ ~
. * T~
~1 X ~ X ~ * ~ X X X X
.a~ ~ ~ ~ o u~ X ~ ~ c~
0 d oo ~ co ~o ~ ~ o co ~ ,c
~ _I do o o o o o o o o o 3
u~ ~ o
~ 0
o ~ .
~ o ~ ~ 0
E~E~ ~ ~ .....
¢~ t~ ~t ~ ~ ~ ~ ~
sla~ o 0.. .. .... . . ...... .. .. P.
~ 0 ~ l ~ ~ ,1 C~ ~
~1 ~ ~
E~ ~ ~ , o ~
:4a~ ~o~
o3 ~ o o o o o o o ~ o o a~
E~ ~ ~ o $ o $ 1~ 1~ 1~ o o ~ æ
¢~ ~ ~ ~ ~ o
P~ ,i
0 al w
a a) O
d ~ ~ o~
d d~ ~ d ~ d d d~ a
a~ ~ ~0 u a
u Vu ~n u d d u u u
o ~ q~ O
v 0 d ~q d 0u0 d ~ d ~
_ ~ o
c~ o c~i .
~a
o o o o o o o o o o
o o o o o O o O o o ~ ~
v ~ J ~~~ u
a~ ~ ~I tJ
U3 . ~J ~
3 Q _ a ~
~ ~ a
u~ u~ u~
,~ , ~ ~0 ~ 0 ~
~ NN ~ X ~ X ~1 X
a) ~ ~ a v ~U Q U
U ~ O~ o U U ~ U~ ~ U
o 0 0 a~ a 0 ~ 0 0~ 0
~ ~ ~ 0~ ~ ~I ~ ~ ~~ ~ ~ ~ _I ~
r~
. ~4~
o
In addition to the above experiments illustrating
the present invention experiments were carried out to
attempt to reproduce the results of Ramaswamy et al, re-
ferred to above, by using an anodized lead anode and a lead
cathode at current densities of 20 mamp/cm2 and 300 mamp/cm2
respectively using the electrolyte, electrolysis cell and
electrolyte agitation defined in Table 1 above. Table 2
below summarizes the current efficiencies obtained during
this experiment as a function of ceric ion concentration of
the electrolyte.
V
Ir) N
Nt`
~O .
oo a~
O
~. . ~
N1`~,
~~ I` O
N .
O~ ~
~r ~co o
O Ne~
l_ . .
Ll~ .
CO O
o\ o\ N
N ~1~1
a~ . .
N O~ ,~
E~ ~ Il') c5~ o
d~ d~a~
CO OCO
N . N
Na~ a~ O
o\oo~o
~ ~ N
~ . .
or~
Na~ 0
~ ~ o
a
u~
~-~
o ~
s~o
o ~
a)-" o a) o
Et
0
The results show that applicant was unable to
generate ceric sulphate above 0.37M concentration by opera-
ting at a low anodic current density, that is 20mamp/cm
and a low cathode current density of 300mamp/cm using
Ramaswamy et al's suggested electrolysis conditions.
Further, once the ceric sulphate concentration approaches
0.3 molar the anodic current e~ficiency began to drop
rapidly. Inspection of the lead cathode used in this
electrolysis revealed that it was covered with a thick
deposit of lead. This depositicn has not been observed
during the high current density electrolysis described in
Table 1 and has the following significance:
1. The fact that lead is plated on ~he
cathode indicates that the lead
dioxide film on the anodized lead
anode is not stable during low current
density electrolysis once the ceric
ion concentration of the electrolyte
builds up much above 0.3 molar concen-
tration.
2. If the anode is unstable, current is
being wasted in the following possible
ways:
(a) Ceric ion in the electrolyte de-
composes by reacting with lead
atoms to form lead (11) ions which
migrate to the cathode and plate
out.
i.e. 2Ce4 + Pb 2Ce3+ + pb2 (anode)
Pb + 2e- Pb ~cathode)
o
The overall reaction is:
2Ce4+ + 2e~- 2Ce3+
(b) The lead dioxide film produced
by anodizing the lead electrode
is not sufficiently polarized
at low current densities to
prevent its being decomposed by
sulphuric acid to form lead sul-
phate.
2 2 4 2e
~ PbS04 + 2H20
If the lead dioxide (PbO2) film
is lost in whole or part, the
anode is incapable of generating
ceric sulphate and the underlying
lead is susceptible to attack by
ceric sulphate generated previously.
3. If lead electrodeposits on the cathode,
the cathode current density is reduced
and ceric sulphate decomposition is
enhanced according to the following
reaction:
Ce4+ + le ~ Ce3+
All three factors alone or in combina-
tion can have a disastrous effect on
current efficiency for ceric ion pro-
duction as is evident from Table 2.
~i~66(J 0
The above problems can be avoided if a platinum
anode is used instead of the lead dioxide anode used in
Table 2. However, the use of platinum at low current
densities of 20 mamp/cm2 is too expensive.
Thus the present invention has illustrated that
high current efficiencies obtained at high "effective"
current densities and high ceric sulphate concentration when
electrolysis is carried out at high anodic and cathodic
current densities. It is important to maintain the maximum
dissolved cerous ion concentration in the electrolyte for
the entire electrolysis.
0
SUPPLEMENTARY DISCLOSURE
Further work done on the invention described in
the principal disclosure has indicated that the process
there described is of a broader aspect. According to the
further work the present invention is a process for prepa-
ring ceric sulphate in solution that comprises electrolysing
an at least saturated solution of ceric sulphate at an
anodic current density in the range 100 to 400 mamp/cm2, a
cathode current density in the range 1,000 to 4,500 mamp/cm2
and with vigorous agitation in the presence of dilute
sulphuric acid.
As in the principal disclosure the saturated cerous
sulphate may be maintained as such by electrolyzing a
suspension of cerous sulphate, or by carrying out the
electrolysis of a saturated cerous sulphate solution. A
diaphragm is not used. The electrolysis of a saturated
cerous sulphate solution is carried out briefly then the
electrolyte is mixed with cerous sulphate crystals to resaturate
it with respect to cerous sulphate. Undissolved cerous
sulphate crystals are allowed to precipitate. The super-
natant liquid is then re-electrolyzed.
The following examples illustrate this development:
Examples
Except where indicated otherwise in Table 3 elec-
trolysis of a starting electrolyte comprising 25 grams of
cerous sulphate pentahydrate, 6.6 ml. of concentrated sulphuric
acid diluted to a volume of 100 ml with water to give lM
sulphuric acid was carried out with vigorous agitation of
the electrolyte during electrolysis. The results and reaction
conditions are set out in Table 1. A diaphragm was not used
in the electrolysis.
_ ~ _
11~
o~ ~ u~
a~ 0 O~ ~ Ir) o~
' ~ a
~ ~ O ~ ~ ~ ~ O ~1 1` 0
C~ ~ ~ ~ 0 ~ ~ u~ ~ ~ u~
~ C~ ~ ~ ~ ~ ~ ~ D ~ U~ ~ `D,
~o ~ I I o ~ o
. ~ ~ ~ U~ U~ U~
E~ .
_~ V S ~ o~ U~ o ~o ~ r~ ~
~o ~ ~ ~ ~ u~ O O _l O ~a
~n ~ 0 0 0 0 O O O O S~ 0 0 0 0 ~ ~0
~ . . ~
X o J-
o ~ ~o ~ .~
3~w~ ggo og~o
~¢ ~ ~ ~ ~ a~
~1 ~ c~ ~ d
~1 ~ ~ ~ . ~ ~-
O ~ El O o o o o o o o o o o o o ~d ~ O
oooo ooooo oooo ,~
oZ ~ :~ u~ O O O ~0 0 0 ~ ~ ~ ~r ~ ~ o. ~ a
~ .c C ~ ~ ~
~ ' t~ ~ . . ::1 V
~' ~ ~C
C ~ C C ~ C ~ OC~ C ~ ~ ~; 3
1 V JJ ~J J- ~ U JJ J ~ V J- ~ m
o 0 ~ o co 0 ~ o~ g
~ 3 ~ X ~ ~ z c~ ~ ~ 0 ~ ~
-
,, . . ~ ~ ~
. .K o~ UO
~ ~ goog ggggo gggg ~
C~
. ,.' . ~ ~ S
V
1~
'OU J~ ~o t~ 3r:
/t
V
Thus the invention of this supplementary disclosure,
like the invention of the principal dlsclosure, has illus-
trated that high current efficiencies are obtained at high
"effective" current densities and high ceric sulphate con-
centration when electrolysis is carried out at high anodic
and cathodic current densities. Again it is important to
maintain the maximum dissolved cerous ion concentration in
the electrolyte for the entire electrolysis. With regard to
the present process the generally higher molarities of the
final ceric sulphate should be noted.
Further information applicable to the present
application is:
Cathode current densities much in excess of
4500 mamp/cm2 (e.g. 6000-8000 mamp/cm2) may
result in polymerization of ceric sulphate on the
cathode due to an excessive hydrogen production
rate and increase in pH at the cathode surface.
Formation of the polymer can be eliminated by
operating in an electrolyte of slightly higher
acidity or lower temperature or a combination of
both. This polymer can be redissolved from the
cathode by exposing it to a mixture of dilute
nitric acid and hydrogen peroxide. The polymer
can also be dissolved with a mixture of dilute
sulphuric acid and hydrogen peroxide.
The significance of operating at high cathode
current densities is two fold:
(a) Ceric sulphate exists in the form
H2Ce~SO4)3 in solution - ("sulfatoceric
acid") which partially dissociates to form
HCe(SO4)3- (anion). This negatively charged
anion may be repelled from the negatively
_ ~ _
V
charged cathode by increasing cathode current
density thereby preventing its decomposition.
(b) The higher the cathode current density, the lower
is the cathode surface area and the less likely
is any form of ceric ion e.g. H2Ce(S04)3 or
HCe(S04)3, etc. to make contact with the cathode,
thereby reducing ceric ion decomposition.
