- Email: [email protected]
Properties of iodine and iodine compounds in molten potassium tetrachlorogallate (300°C)
J. Ekcrrcmnal
ChcmI,
I?l5micr Saquoia
PROPER-
POTASSIUM
139 (1982)
X3-328
323
Lausano e -
Printed
OF IODINE
AND
SA.
in The Nelhalands
IODtNE
IBRA GUE+E DtOUM, JACQUES VEDEL Lobnratoire
d’Elecrmchimie
Chimie de Paris. II.
(Reccivai
The
Analyrique
1982;
redor
and acid-base
BERNARD AsmciG
TREMILLON au CNRS.
Ecofe
Narionole
Suphieure
de
Curie. 75lW5. Paris (Frmce)
in revised form 24th
propertiia
of iodine
March 1982)
and its derivatives
have been determined
in molten
rctrachlorogalla~e (3OOT). as a function of chloroacidity.The following species have been
demonstrated:
for I(+
giving rise to IICI-.
mol kg-’
and
IN MOLTJW
(300°C)
et Appliquk
me Pierre er Marie
1501 timber
po~as.siun~
COMPOUNDS
TETRACHLOROGALLATE
I), rC1,
and ICI;
for I(-_),
The dis.sociationconsfanLs
and m(GaCI,I-)-m(CI-)/m(l-)=2X
I-
and GaCl,I-;
are. rcspativcly:
elcmenral
idinc
is a strong acid,
m(CI-)-m(ICl)/m(I,Cl-)=
1.6X IOPJ
IO-’ mol kge2.
INTRODUCTION
Iodine and iodide
ions are more easily oxidizable
us to study their electrochemical are compounds chloride
between
donor-acceptor
iodine
properties derivatives
pairs (chlorobases
can be varied in the solvent-that
than chloride ions, which allows
in molten chlorides.
and chloride
anions.
and chloroacids).
In addition, which
If the Cl-
is, if the solvent shows a partial
there
give rise to ion activity
autodissociation
ion exchange-then, the redox properties of iodine and its derivatives will depend on the acidity or, in other words, on the solvent cornpositionIn a previous study [l]. we have shown that molten potassium tetrachlorogallate -as molten sodium tetrachloroaluminate-has the required property. It dissociates, in terms of chloride
following
2 GaCl,
the equilibrium:
= Ga,Cl;
+ Cl-
of which the dissociation
(1) constant.
Kim,-,is:
Kim, = 10-4.5 mol’ kg-’ The acidity is defined by the equation pCI = -log m(Cl-) and either by addition of GaCl, or KCl. Its value is determined from the reduction voltammetric curve of the solvent_ In this study we describe the redox and acid-base properties of derivatives as obtained by interpretation of the i-V curves vs. acidity 00224728/82/tTHKUKW/SO2-75
Q 1W2 Envier Sequoia SA.
may be varied position of the iodine and its shift.
324 EXPERtMENTAL
The experimental conditions are the same as those of the previous study [l]_ Iodine and potassium iodine were Prolabo RP products, used without special purification. The working electrode was made of vitreous carbon as gold oxidizes at a lower potential than iodine (Fig. 1). Iodine may be adsorbed at the electrode surface, distorting some curves. In these cases it has to be eliminated treatment of the electrode. RESULTS
AND
by cathodic
DISCUSSION
Addition of iodine to molten salt gives a purple solution. Iodine slowly distills and condenses in the cold parts of the cells. Addition of iodide in the presence of oxygen is also accompanied by iodine evolution. Under a protective flush of nitrogen, a very slight purple colour appears when the GaCl, used for the titration is introduced. The resulting loss- of iodide was compensated for by addition of potassium iodide, in order to keep its concentration constant. Figure I shows the voItammetric curves obtained with a solution of iodine. An anodic and a cathodic wave are observed, the heights of which are approximately equal. Since the solution remains purple, there is no iodine dismutation, and the waves correspond respectively to the formation of iodine (+ I) and iodine (- I), following: I,--Ze-@21(+1)
(3)
and 12+2e+2
-60
1(-l)
(4)
0.5
0.7 potentiat
Fig. 1. Volrammcmic curve obhmd (2) residual cm-rent on vimous carbon;
0.9 / V wi& iodine solulioo. (1) Residual current (3) iodine solution an tiueous carban (ref.:
on
gold
~g/~g~i).
clecw&&
32s
The volatility of iodine hinders the study of these two redox systems during the -.neutralization~ of _&sic solutions with GaCl,. It is easier to use the i-E curves co&sponding to anodic ogdation-of the iodide species. Two of these curves are shown in ,Fig; 2, respectively obtained-in basic and in acidic medium. They present two waves, corr&pon~g to the previous redox reactions. As the solvent becomes more acidic; the curves are shifted to-wards positive potentials. In order to determine the nature of the.involved species, it was necessary to verify the reversibility of the electrode reaction and .to find a relation between acidity (pCI) and a parameter characterizing the position of the curves. The half wave potential was used (Fig. 3). On the E,,z vs. pC1 plot, the points are aligned upon four segments, the slopes of which being simple multiples of the coefficient 2.3RT/F (at 300°C. 2_3RT/F= 0.114V). An hypothesis was made on the nature of the species. This allowed the derivation of the equations of the i-E curves and the verification of the reversibility. The lack of points in the middle of the diagram is caused by the poor determination of the pCI in this range of acidity. The observed
variations
in, the electrochemical observed
values,
are explained
assuming
reactions show acid-base
the following
hypotheses,
that the iodine species involved
properties.
shown
To take into account
in Fig. 3 and Table
the
1, have been
made: (1) Iodine
(+
1) species have been described
bers, in molten NaAICI, ICI?-“-,
with n = 1.2, 3...
in the exchange
ICI,
Mamantov
and ICI =e
the conjugated
m(ICI)
of the type
. (5)
constant:
- m(CI-)/m(ICl,
2: Volt&metric
and Cham-
base and acid,
e ICI + cl-
-0 kg.
by Marassi.
assume the existence of compounds
Thus, ICl,
equilibrium:
with the dissociation K=
[2]. They
)
6.5 Curves olitaincd
mol kg-’
(‘9
is wilb
KI solutions
a~ (I)
pcl
=a4
anti (2)
pCI =
4.
I
cj
5 -
__. +
+
d
I
nn-4
=
+
Fig. 3. PotcntiaLpCI
diagram
for iodine
spkics.
(2) Elemental iodine exists in the form I&-, over the whole solvent acidity range. The species has been demonstrated by Kolthoff and Jordan in another acidic solvent, concentrated
chlorhydric
acid [3].
(3) Concerning the I( - 1) species, the observed slopes led us to suppose that Ianion is a weak base which is able to react upon the solvent, forming Cl- ions:
I- +GaCl;
=GaCI,I-
+CI-
(7)
with: K’ = m(GaC131-)
)/m(I-)
- m(Cl-
mol kg-’
(8)
The corresponding redo~ reactions, as well as the associated Nemst equations, are shown in Table 1. We assumed that the media were acidic or basic enough to buffer the acidity,.making pC1 independent of electrode reaction. This is’ another reason for not having -experimental points in the middle ‘of the acidity range. The relations
TAlkE
Anodic
2 oxidation
of potassium
iodide: &sewed
slopes of lhe
0.057 v Slopes
of N-t
Basic medium
Kl,‘V Acidic
medium
[email protected]
tnudOmlS.
TheOrCtid
v&x
328
and pCI are &o indicated in between formal (E,O), half-wave (E,,, ) potentials -Table 1. They contain the I- ion concentration_ If this concentration remains approximately constant, the E,,? vs. pC1 varititions allows us to define the nature of reacting species The reversibility of electrode reaction was checked by plotting the variation of E vs. lo&i/( i, - i)‘) and E vs. log(i’/(i, - i)) for the two waves of each I-E curve. These relations are deduced from eqns. 9-12 in Table I. The observed slopes are reported in Table 2, in good agreement with the theoretical value 0.057 V. The electrode reactions are then Nernstian. allowing the use of E,,? as characteristic potential. The values of K and K’ may be determined owing to some simplifying hypothesis. First. combining eqns. (6). (9) and (10) and (8) (11) and (12). we obtain Ef=f+0_114logK
(13)
E;=E,O+O.l14logK
04)
Then, assuming that D(I-)
= D(I,Cl-)
= D(GaCI,I-)
05)
and for constant m(I-) values, one can show that the intersection of lines I and 2 in Fig. 3 occurs for pC1 = -log K’. and that the intersection of lines 3 and 4 occurs for pCl = -log K. Thus, the following values are derived: K’/mol
kg-’
= 2 - lo-’
and K/mol
kg-’
= 1.6 - 10-j
In conclusion, the following points are consistent with the electrochemical detertinations: (1) Elemental iodine is a strong acidic species, reacting upon the solvent according to the equation: I2 + 2 GaClJ
- I,CI-
+GazCIT
The conjugated base, I$does not dissociate and is a very weak base. (2) Iodine (+ 1) e_xists under a basic (ICI, ) and an acidic (ICL) form. According to the suggestion of Mamantov et al. [2], the species is stabilized by Cl- ions, but there is no evidence of the existence of ICI:-. (3) Iodine (- 1) is a weak base, moderately dissocisted, capable of reacting with Ga&l; ions, more active in acidic media, according to the reaction (iden5cal to reaction (7)): Ga?CIT
+ I- --, GaCI,I-
+GaCl,
REFERENCES I 1-G. Dioum. J. Vedel and B. Tn?miUon. J. Elecuoanal. Chem.. 137 (1982) 219. 2 R hfarnssi. G. Mamamov and J.Q. Chamlxrs Inorg. Nucl. Chem., Letters. I1 (1975) 245. 3 I.M. Kolrhoff and J. Jordan. J. Am. Chcm Sot.. 75 (1953) 1571.
ChcmI,
I?l5micr Saquoia
PROPER-
POTASSIUM
139 (1982)
X3-328
323
Lausano e -
Printed
OF IODINE
AND
SA.
in The Nelhalands
IODtNE
IBRA GUE+E DtOUM, JACQUES VEDEL Lobnratoire
d’Elecrmchimie
Chimie de Paris. II.
(Reccivai
The
Analyrique
1982;
redor
and acid-base
BERNARD AsmciG
TREMILLON au CNRS.
Ecofe
Narionole
Suphieure
de
Curie. 75lW5. Paris (Frmce)
in revised form 24th
propertiia
of iodine
March 1982)
and its derivatives
have been determined
in molten
rctrachlorogalla~e (3OOT). as a function of chloroacidity.The following species have been
demonstrated:
for I(+
giving rise to IICI-.
mol kg-’
and
IN MOLTJW
(300°C)
et Appliquk
me Pierre er Marie
1501 timber
po~as.siun~
COMPOUNDS
TETRACHLOROGALLATE
I), rC1,
and ICI;
for I(-_),
The dis.sociationconsfanLs
and m(GaCI,I-)-m(CI-)/m(l-)=2X
I-
and GaCl,I-;
are. rcspativcly:
elcmenral
idinc
is a strong acid,
m(CI-)-m(ICl)/m(I,Cl-)=
1.6X IOPJ
IO-’ mol kge2.
INTRODUCTION
Iodine and iodide
ions are more easily oxidizable
us to study their electrochemical are compounds chloride
between
donor-acceptor
iodine
properties derivatives
pairs (chlorobases
can be varied in the solvent-that
than chloride ions, which allows
in molten chlorides.
and chloride
anions.
and chloroacids).
In addition, which
If the Cl-
is, if the solvent shows a partial
there
give rise to ion activity
autodissociation
ion exchange-then, the redox properties of iodine and its derivatives will depend on the acidity or, in other words, on the solvent cornpositionIn a previous study [l]. we have shown that molten potassium tetrachlorogallate -as molten sodium tetrachloroaluminate-has the required property. It dissociates, in terms of chloride
following
2 GaCl,
the equilibrium:
= Ga,Cl;
+ Cl-
of which the dissociation
(1) constant.
Kim,-,is:
Kim, = 10-4.5 mol’ kg-’ The acidity is defined by the equation pCI = -log m(Cl-) and either by addition of GaCl, or KCl. Its value is determined from the reduction voltammetric curve of the solvent_ In this study we describe the redox and acid-base properties of derivatives as obtained by interpretation of the i-V curves vs. acidity 00224728/82/tTHKUKW/SO2-75
Q 1W2 Envier Sequoia SA.
may be varied position of the iodine and its shift.
324 EXPERtMENTAL
The experimental conditions are the same as those of the previous study [l]_ Iodine and potassium iodine were Prolabo RP products, used without special purification. The working electrode was made of vitreous carbon as gold oxidizes at a lower potential than iodine (Fig. 1). Iodine may be adsorbed at the electrode surface, distorting some curves. In these cases it has to be eliminated treatment of the electrode. RESULTS
AND
by cathodic
DISCUSSION
Addition of iodine to molten salt gives a purple solution. Iodine slowly distills and condenses in the cold parts of the cells. Addition of iodide in the presence of oxygen is also accompanied by iodine evolution. Under a protective flush of nitrogen, a very slight purple colour appears when the GaCl, used for the titration is introduced. The resulting loss- of iodide was compensated for by addition of potassium iodide, in order to keep its concentration constant. Figure I shows the voItammetric curves obtained with a solution of iodine. An anodic and a cathodic wave are observed, the heights of which are approximately equal. Since the solution remains purple, there is no iodine dismutation, and the waves correspond respectively to the formation of iodine (+ I) and iodine (- I), following: I,--Ze-@21(+1)
(3)
and 12+2e+2
-60
1(-l)
(4)
0.5
0.7 potentiat
Fig. 1. Volrammcmic curve obhmd (2) residual cm-rent on vimous carbon;
0.9 / V wi& iodine solulioo. (1) Residual current (3) iodine solution an tiueous carban (ref.:
on
gold
~g/~g~i).
clecw&&
32s
The volatility of iodine hinders the study of these two redox systems during the -.neutralization~ of _&sic solutions with GaCl,. It is easier to use the i-E curves co&sponding to anodic ogdation-of the iodide species. Two of these curves are shown in ,Fig; 2, respectively obtained-in basic and in acidic medium. They present two waves, corr&pon~g to the previous redox reactions. As the solvent becomes more acidic; the curves are shifted to-wards positive potentials. In order to determine the nature of the.involved species, it was necessary to verify the reversibility of the electrode reaction and .to find a relation between acidity (pCI) and a parameter characterizing the position of the curves. The half wave potential was used (Fig. 3). On the E,,z vs. pC1 plot, the points are aligned upon four segments, the slopes of which being simple multiples of the coefficient 2.3RT/F (at 300°C. 2_3RT/F= 0.114V). An hypothesis was made on the nature of the species. This allowed the derivation of the equations of the i-E curves and the verification of the reversibility. The lack of points in the middle of the diagram is caused by the poor determination of the pCI in this range of acidity. The observed
variations
in, the electrochemical observed
values,
are explained
assuming
reactions show acid-base
the following
hypotheses,
that the iodine species involved
properties.
shown
To take into account
in Fig. 3 and Table
the
1, have been
made: (1) Iodine
(+
1) species have been described
bers, in molten NaAICI, ICI?-“-,
with n = 1.2, 3...
in the exchange
ICI,
Mamantov
and ICI =e
the conjugated
m(ICI)
of the type
. (5)
constant:
- m(CI-)/m(ICl,
2: Volt&metric
and Cham-
base and acid,
e ICI + cl-
-0 kg.
by Marassi.
assume the existence of compounds
Thus, ICl,
equilibrium:
with the dissociation K=
[2]. They
)
6.5 Curves olitaincd
mol kg-’
(‘9
is wilb
KI solutions
a~ (I)
pcl
=a4
anti (2)
pCI =
4.
I
cj
5 -
__. +
+
d
I
nn-4
=
+
Fig. 3. PotcntiaLpCI
diagram
for iodine
spkics.
(2) Elemental iodine exists in the form I&-, over the whole solvent acidity range. The species has been demonstrated by Kolthoff and Jordan in another acidic solvent, concentrated
chlorhydric
acid [3].
(3) Concerning the I( - 1) species, the observed slopes led us to suppose that Ianion is a weak base which is able to react upon the solvent, forming Cl- ions:
I- +GaCl;
=GaCI,I-
+CI-
(7)
with: K’ = m(GaC131-)
)/m(I-)
- m(Cl-
mol kg-’
(8)
The corresponding redo~ reactions, as well as the associated Nemst equations, are shown in Table 1. We assumed that the media were acidic or basic enough to buffer the acidity,.making pC1 independent of electrode reaction. This is’ another reason for not having -experimental points in the middle ‘of the acidity range. The relations
TAlkE
Anodic
2 oxidation
of potassium
iodide: &sewed
slopes of lhe
0.057 v Slopes
of N-t
Basic medium
Kl,‘V Acidic
medium
[email protected]
tnudOmlS.
TheOrCtid
v&x
328
and pCI are &o indicated in between formal (E,O), half-wave (E,,, ) potentials -Table 1. They contain the I- ion concentration_ If this concentration remains approximately constant, the E,,? vs. pC1 varititions allows us to define the nature of reacting species The reversibility of electrode reaction was checked by plotting the variation of E vs. lo&i/( i, - i)‘) and E vs. log(i’/(i, - i)) for the two waves of each I-E curve. These relations are deduced from eqns. 9-12 in Table I. The observed slopes are reported in Table 2, in good agreement with the theoretical value 0.057 V. The electrode reactions are then Nernstian. allowing the use of E,,? as characteristic potential. The values of K and K’ may be determined owing to some simplifying hypothesis. First. combining eqns. (6). (9) and (10) and (8) (11) and (12). we obtain Ef=f+0_114logK
(13)
E;=E,O+O.l14logK
04)
Then, assuming that D(I-)
= D(I,Cl-)
= D(GaCI,I-)
05)
and for constant m(I-) values, one can show that the intersection of lines I and 2 in Fig. 3 occurs for pC1 = -log K’. and that the intersection of lines 3 and 4 occurs for pCl = -log K. Thus, the following values are derived: K’/mol
kg-’
= 2 - lo-’
and K/mol
kg-’
= 1.6 - 10-j
In conclusion, the following points are consistent with the electrochemical detertinations: (1) Elemental iodine is a strong acidic species, reacting upon the solvent according to the equation: I2 + 2 GaClJ
- I,CI-
+GazCIT
The conjugated base, I$does not dissociate and is a very weak base. (2) Iodine (+ 1) e_xists under a basic (ICI, ) and an acidic (ICL) form. According to the suggestion of Mamantov et al. [2], the species is stabilized by Cl- ions, but there is no evidence of the existence of ICI:-. (3) Iodine (- 1) is a weak base, moderately dissocisted, capable of reacting with Ga&l; ions, more active in acidic media, according to the reaction (iden5cal to reaction (7)): Ga?CIT
+ I- --, GaCI,I-
+GaCl,
REFERENCES I 1-G. Dioum. J. Vedel and B. Tn?miUon. J. Elecuoanal. Chem.. 137 (1982) 219. 2 R hfarnssi. G. Mamamov and J.Q. Chamlxrs Inorg. Nucl. Chem., Letters. I1 (1975) 245. 3 I.M. Kolrhoff and J. Jordan. J. Am. Chcm Sot.. 75 (1953) 1571.