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Controlled potential and polarographic reduction of nitrosylruthenium-nitrato complexes
_-
_._- EiGl-ROANALGICAL
lzHEhasTRY
! Elsevier S$tioia S.A.; &ksamie - P&ted -_
in The Ngherlzgds -
-
.-
CO&O&ED REDUCTION
- -_ 1 -
%LECTROCHFMISTRii
INl-EWACti
-&ND
_
POTENTIAL-AND POLA&O&APHIC OF- NItidSYLRUTHENIUM-NIT%ATO
--
. _ -:
-*.
_
- -CO&&XXES
L. -&fORPURGO
_ ] !I&_
--_ _. --_ -
c
---I
-
di Roma
--
_
Laboratorio a%Cromatografi del C-N-R., Cftalyl (Received Mart+x 18th, 1969)
Istituto
di Chimica
Geizerale
ed Inorganica,
Universitd
INTRODUCTION
Nitric acid solutions of irradiated nuclear fuels contain several nitrosylruthenium-nitrato complexes which are difficult to separate from uranium and plutonium in purification processes. Attempts to destroy the RuN03+ group, using reducing agents such as hydrogen peroxide1 and hydrazine2, or by decomposition with u.v, light3, were only partially successful owing to slow generation of RuN03+ from Ru(II1) and nitric acid3. This paper reports on the reduction of nitrosylruthenium-nitrato complexes on a mercury pool cathode at controlled potential. It was hoped that formation of RuN03+ could be avoided in this method by maintaining the solution at low temperatures-
Materials
and methods
Stock solutions of 2 x lo- 2 M RuN03 + in 8 N HN03 were prepared as described -ti the literature4 from hydrated ruthenium trichloride (Fluka A-G.), and were aged for at least one week. Nitric oxide was obtained by reacting metallic copper with aqueous HN03 (diluted 1: 3)_ Traces of NOz were eliminated; for polarographic measurements, by passing_the gas through-a column filled with solid-NaOH. Poiarograp& measurements were carried-out using a dropping merCury electrode aqd a S_argeni model XXI polarograph. Controlled-potential electrolysis was carried out by pleans of & eel p&e-ntiostat- _Polariz&ion of the mercury pool cathode was avoided by magnetic stirring and by- bubbling _$&reampf nitrogen through therbulk of-the soltition. All potentials - -_ _ -._ are referred to-the saturated calomel electrode. . _ Spe&tr~:w~~e recorded on a_B&&&m -e&i2 spectrophotomete~, -fi+ - _~ : &ng -_-, .- .- -__- -_..= -_Corex~cellS_Z. -- -Chrotiaio&a@s we& OS&e& on celluio&-&in ia&-. MN-Pdlygrtim- &i < - 300 (Maclierek-Nagel and co), usi& p-but&o1 sa&ratGd_$&3 I$ HNO>.as_ehient_5. _- -??he Xeni$%ture f&r all pOlarOgraphi& and~cl@nat~@aphjc -wor&-was: O”c,- -
~~u+ss:&h&%v$e~~tafed~ =_- __;‘= _I 1 e-_ :
--.. _ ec __ _ --__ . _- -- __ -_
_;
-.- ‘__ _ -- ; __ : --.. /_‘T _ -._ -_ __ _- - .-_ -- -
I
--_
: 1 -1:
.
^ -_
-
i
=__
emI-_.__f-: __ I1 I_~--~_ ,j:--;-_-: L‘__-._5- =c--_ -.‘,--
_ -~~_-EZ~~t~~~~~~Ch~~~~~2~~1~69~~-~~~~~~~~~ - -‘ . -, m-_:.ai __ _ ’ :,. .z-.r=-
312
Ii.- M0RPuRGo
RESULTS
Polarography
_
A preliminary polarographic study of the system was made in view of the scarcity of information available on the subjec@-‘, especially for Iow temperatures. At 0°C the polarographic reduction of RuN03+-nitrato complexes takes pIace in two steps at concentrations of the depolarizer below about 3 x lob4 M, and in three steps at higher concentrations (Fig. 1). The half-wave potential of the first step is about + 0.01 V and is independent of pH ; its limiting current is proportional to the concentration of RuN03’ up to a value of about 3 x 10e4 M, and then remains constant for fisher increases (Fig_ 2)_ The second step is found at E+ equal above 3 x 10S4 M_ Its to about -0.15 V, starting from concentrations of RuN03+ -6
4-
f[uAl
Cb
al
/CJ.IAI -5
3-4
-3
2-
-2 l-1
0 a2
/: 0
-0.2”
I
t 0.1
0
-
0.2
-a4 v
Fig. 1. PoIarograms O-5 N; (b) ~uN03*J
of RuNO’* =4_7
I (a) IpuNO”]
x 10-O
lU,
[HNOJ
0
-0.6
CVolt1
=2.0 x 1Om4 M, [HN03] =0.15 IV, PaNO,] =0.5
=
6.0 x 10e2 IV, maNO,]
=
IV.
limiting current is not well defined and is difkult to measure accurately; however, it seems to increase linearly with RuN03+ concentration (Fig_ 3)- E+ of the third step depends on the hydrogen ion concentration of the solution and becomes more positive (by about 65 mV) for a ten-fold @crease (Table 1). The limiting c-krrent is proportional to RuN031‘ concentration -when this is below about 3 x 10S4 M, but also depends both bn Hi and NO, concktrations (Fig. 4). In HN03*
__ -
_ ?e __
--
REDUCTION
OF NITRO!WLR~
0
2
-NITRATO
4
6
10
8 @NO]
-
104~
Fig_ 2 Limiting height of the first wave as a function of RuN03+ IV NaN03; (A) 0.5 N HN03; (x) 0.5 IV HNO,+OS N NaNO,;
4
2
0
concn Supporting (+) 2 N HNO,.
8
6 [RuNo]
313
COMPLEXES
electrolyte:
(0)
0.5
electrolyte:
(0)
10
- i+hf
Fig. 3. Limiting height of the second wave as a function of RuN03+ 0.5 N NaNO,; (x) 0.5 N NaN03f0_5 N HN03_
concn.
Supporting
In Table 4 are reported the limiting currents of some polarograms for two different temperatures_ It can be seen that there is a greater percentage increase of ia of the third wave in 1.15 N HNO, than in 0.67 N HNO,. A similar behaviour is found also for the sum of the ia values of the first two waves ; ia of the first wave is insensitive to temperature variations_ Electrolysis
at controlled potential
At -02 V the_colour of the solution change gradually from orange-red to’. dark brown, Af$er a small initial increase the current st&ts to deere~eslovyly but_-_never goes beloy_30% of its Ztial value, irreqjective of the_time-of.elect.rofysi% A pola.rog&uu of &n el_ec$olyzed Solutiog (initialconcentration of R&NO3 ? F_ 5 x lo-? _; ._
_
-_*
_- _
_
-
I_--
_
_-
.-
_--_
--
:
-_ .l. E~y+tz&
&t&2
_.--
&i$3$318
,
_-
y.
_ _e- r
314
L.MORPURGO
TABLE1
[R~NO~+]- lo4iu
pH
E,/V
(caicd.) 6_87 x-45 202 2-58 3_29 4-00 O-29 l-45 2.02 2.58 -
TABLE
l-55 133 l-19 1.10 l-04 as9 0.32 0.28 0.27 0_26
-0.430 -0_415 -0_405 -0.4oo -0.395 -0.385 -0_350 -0.340 -0.345 -0-345
2 ~~~N0~+]=2_9~io--~ibf [HNO,]=o.s N
[R~N0~*]=5_7xlO-~M [HNOJ=z.o N h/Ci?l
id,;@
i,,/h - 10"
id,/@
i,,/h - 10'
53 63 73 83 93
093 L-09 136 1.59 1.68
1.70 l-74 I-79 1.93 1.82
1.03 l-15 l-32 1.56 1.76
1.9s l-83 l-81 1.89 1.89
TABLE3 [R~N0~+]=5.7xlO-~ [HNO~]=~.~ N h/cm
id,/@
53 63 73 83 93
5.70 5.52 5.70 5.64 594
_
M
[RuN03+]=2.9x [HNO~]=O.S N
lo-&M
id,/.@
id, x h-*
1.65 1.78 19s 2.11 225
0.22
0.23 0.23 023 0.23
TABLE4
O-67_ O-67- . I‘-- -.
_- o25
- _
18 4.8 - -_
_
--_
_
_
l-5 1.5 ,
_
1.6 23
3-7 -- 4.8
-
REDUCTION
OF NLTROSYLR uTHENIuM--NITRATocoMP~
-.
_-
_
-
-
315
=OS M) .s&vs OI& two reduction steps. Their ia is similar to thatbf the first two steps of the initial solution, but E, is shifted towards negative values by about 50 mV_ The third wave is missing. Thin layer chromatograms of the solution before and after the electrolysis are shown in Fig. 5. The ele&.rolyzed8olution was not &ar and a faint dark precipitate formed on standing. It was probably this precipitate that gave the spot at the application point of the second chromatogram. Once clear. the spectrum of the electrolyzed solution does not vary appreciably with time if kept at 0°C but does so at room temperature (Fig_ 6).
M, [@NO,]
ET ;----, ;-__-i --____-’
0
2
6
4 [RUNO]
- 104 M
Fig. 5. (a) Thin layer chromatogram electrolytic reduction at -0.1 V-
of a soln. of 5 x
b
a
Fig. 4. Limiting height of the third wave as a function of RuNOsc COnCn. Supporting IV NaNO, ; (b) 05 IV HNO, ; (c) 0.5 N HN03 f0.5 N NaN03 ; (d) 2 N HN03.
10m3 M RuN03+
electrolyte:
in 0.5 ZVKNOJ
(a) OS
; (b) a~ (a) after
DISCUSSION
-The behaviour of id of the first wave at varying RuN03+ concentrations, its linear relationship to the height of the mercury reservoir and the ver$ small temperature coefficient show clearly its adsorption character__The second wave, which only appears at higher concentrations pf tie depolarizer,_when -the likiting height -of the adsorption-wave-is becoming-constant, is the_“normal” wave_‘?_ It is’originated by _ the same electrochemi&l process; but- the- reduction product, -no-h&g& adsorb&l on -- - : , _ _ -: _ . _ _ _- the .electrode surface~~remains~insolution_ r- - _’ _--- - -.. i _In dilute acid solutionS the lim&g height cf the -third wave’& approxim&ly _ &p.xk to -the_ su&of -~the_otlier_&A$ wave heights $&d is- diEusion-controlle&_rhis ind&at~ a subsequent reduction process involving the sari+&inber~of elect&$s &,G1 ‘-&G.jrst__‘~-:~ I__ - -‘=_;_;-=- -1; __I _ ‘-; _= _ - _ -= -_ _ :;_---;__;:_-;.:“_-__$1 _:;‘,_c(y-~~_-~ c_‘
z._ _ 1. . -- _’ _ _- - z-_ :- .~ _-_>_ _ .- - -z ~-
. -- - - _ _ __:_- __ .-- ;‘_ __ (--:- -2; --c_ _ : __.r_ q+&&;lhr[&&~-‘& _ c- __-_ _-_
__ .f_ _ --__ _- - -=,__ --: (<&j~j.&~;~&= _ _A
_----
L. MORPURGO
316
Fig. 6. Spectra of a soIn. of RuNO 3+ in 8 N HN03, dihted to 0.5 N HN03 : (I) before electrolytic reduction; (2) after electrolytic reduction and kept for 2 daqs at O’C ; (3) as (2) after 45 d2ys; (4) electrolyzed soh. kept at room temp_ for 30 days; (5) as (4) after 15 days.
Adsorption waves have already been observed for RuC13 and RuNO(NO,), in sulphuric acid medium at room temperature7.They were thought to be due to the reduction of polynuclear species and were found to involve the adsorption of some ruthenium derivative. “Xormal” waves were not observed_ Our soIutions, however, are known to contain a mixture of labile nitrosylruthenium-nitrato com-
plexes of general forrnula5*9 I_RuNO(NO,),(H,O), _JC3--“)+. Polynuclear species are only present in traces and cannot alone be responsible for the adsorption wave. In our opinion this and the “normal” wave are due to all species in solution that dissociate prior to reduction. The electrode reaction may then be: RuN03++e
-
RuZf+NO
with Ru*+ adsorbed on the first step_ The NO reduced along the third step:
NO+e+H+
-
produced
by this reaction
may be
HNO
Moreover Ruz f is very likely to be reoxidized by HN03 to give Ru3+ or possibly Ru4+ derivatives- This reaction scheme is also substantiated by the following considerations : The polarographic reduction of Ru(III) to Ru(I1) has already been observed at -0-02 V in p-toluenesulphonic acidlo, at - 0.28 V in 1 N HClgb and at -0-34 in perchloric acid solutionsll, Le. in the potential range of the first two waves.
V
The third wave is missing in the polarogram of solutions electrolyzed at -02 V_ The only explanation we have for this is that the wave is due to NO produced on the two preceding steps_ During the electrolysis NO is displaced from the solutions by thestream of nitrogen as soon as it is formed_ The potential of the other two waves is shifted after electrolysis towards_- negative valuesindicating that they are then due J-_-Ehxmadl.
- _
Chime,
22 (1969)
321-318
-_
REDUCTION
OF E
COMPLEXES
317
to the reduction of differentchemical species. These are possibly cationic, as indicated by their chromatographic behavior?, and are probably polymericRu(II1) derivatives_ The third wave acquires some catalytic character when the concentration of HNOs increases, i.e. when the reoxidation of Ru(I1) becomes easier. More NO is possibly produced by this reaction. A somewhat similar behaviour was observed during electrolysis at controlled potential of yellow solutions of Ru(II1) in per&lo& acidlo, i-e- the colour changed to brown, large quantities of chloride ions were produced, and the intensity of the current never dropped to zero_ A reduction step at -OS V (NCE) is reported by some authors12*13 for NO in acid solutions- It has some kinetic character, a pH-independent E, and is assigned 2- . We have verified that a wave at about the to the reaction 2 NO P(NO)~~N~O~ same potential also occurs in the reduction of NO in nitric acid solutions. For RuN03 + the properties of the third wave, which we have assigned to the reduction of NO, are different, but so are the conditions under which the wave is produced, owing to the presence on the electrode of the metal ions. A different reduction pattern has also been found by other authors under different conditions14. Since the ruthenium compounds produced by electrolysis of RuN03+ apparently contain no NO and, at low temperature, are stable cationic species with different chromatographic properties from the starting compounds, it is possible that the electrolytic method could be used to separate ruthenium from irradiated nuclear fuels_ ACKNOWLEDGEMENT
This work was carried out under the contract N. 365/RB with the International Atomic Energy Agency (Vienna). SUMMARY
The polarographic behaviour of nitrosylruthenium-nitrato complexes has been examined at low temperatures, in nitric acid and sodium nitrate solutions_ Two waves are observed at concentrations of RuN03+ below about 3 x 10B4 M, and three at higher concentrations. A reduction scheme is proposed and a method for group by controlled potent&l &ectrolysis is described. decomposmg the RuN03+ REFERENCES U_K_At.E_Authority Rept., AERE-R, 4537 (1964). 1 C. E_ LYON AND D. SCAFtGILJi, 2 A. G. WAIN, R. J. W_ S-nsrrr ON, E. N. JENKINS,J. M. FLETCHER AND P. G. M. BROWN, U_K_At.E.Authority Rept.. AERE-R, 3509 (1960). 3 J. M. FLFTCHER AND J. L. WOODHEAD, J_ Inorg_ NucI_ Chem., 27 (1965) 1517. 4 3. M. FLETCHER, I_ L. JENKIN s, F_ M_ LEVER, F. S_ MARTIN, A. R. POWELL AND R. TODD, J_ Inorg. NucZ. Chem., 1 (1955) 378. 5 L. MORPURGO, Ric. Sci., 36 (1966) 553. 6 E_ N_ JENKINS, U_K-At.E.Authority Rept., AERE-R, 3491 (1960). 7 J_ BLAZEK AND D. M_ WAGNEROVA, Collection Czech..Chem. Commun., 29 (1964) 915. 8 L. MEITES, Polarographic Techniques, Inter-science, 2nd ed., 1965, a, p_ 187; b, p- 629. 9 D_ SCARGILL, C, E. LYON, N. R. LARGE AND J. M. FLETCHER, J_ I..rg_ &zl_ Chem., 27 (1965) 161. J_ Electroad.
Chem, 22 (1969) 311-318
318
L.
10 Em E. MJZRCER AND R. R. BUCKLEY, Innorg. Chem., 4 (1965) 1962. 1I L. W_ NIEDRACH AND A. D. ~EVEBXUGH, J_ Am. Chem. Sot., 73 (1951) 2835. 12 L. RICCOBOM, P. LAXZA AND P. FAVERO, Atti iVa=. Lincei, Mem. Ckzsse Sci. Es. 3 (1950) 8. 13 J_ !vL=s+, Z_ Anal_ Chem., 224 (1967) 99. 14 D- L. EI+MN AND D_ T. SAWWZR, J. ECecrrounuL Chem., 16 (1968) 541_ J.
Efecmoonaf_ Chem.,
22 (1969) 311-318
..
hfORPuRG0
Mat. Nat- Se=. II”,
_._- EiGl-ROANALGICAL
lzHEhasTRY
! Elsevier S$tioia S.A.; &ksamie - P&ted -_
in The Ngherlzgds -
-
.-
CO&O&ED REDUCTION
- -_ 1 -
%LECTROCHFMISTRii
INl-EWACti
-&ND
_
POTENTIAL-AND POLA&O&APHIC OF- NItidSYLRUTHENIUM-NIT%ATO
--
. _ -:
-*.
_
- -CO&&XXES
L. -&fORPURGO
_ ] !I&_
--_ _. --_ -
c
---I
-
di Roma
--
_
Laboratorio a%Cromatografi del C-N-R., Cftalyl (Received Mart+x 18th, 1969)
Istituto
di Chimica
Geizerale
ed Inorganica,
Universitd
INTRODUCTION
Nitric acid solutions of irradiated nuclear fuels contain several nitrosylruthenium-nitrato complexes which are difficult to separate from uranium and plutonium in purification processes. Attempts to destroy the RuN03+ group, using reducing agents such as hydrogen peroxide1 and hydrazine2, or by decomposition with u.v, light3, were only partially successful owing to slow generation of RuN03+ from Ru(II1) and nitric acid3. This paper reports on the reduction of nitrosylruthenium-nitrato complexes on a mercury pool cathode at controlled potential. It was hoped that formation of RuN03+ could be avoided in this method by maintaining the solution at low temperatures-
Materials
and methods
Stock solutions of 2 x lo- 2 M RuN03 + in 8 N HN03 were prepared as described -ti the literature4 from hydrated ruthenium trichloride (Fluka A-G.), and were aged for at least one week. Nitric oxide was obtained by reacting metallic copper with aqueous HN03 (diluted 1: 3)_ Traces of NOz were eliminated; for polarographic measurements, by passing_the gas through-a column filled with solid-NaOH. Poiarograp& measurements were carried-out using a dropping merCury electrode aqd a S_argeni model XXI polarograph. Controlled-potential electrolysis was carried out by pleans of & eel p&e-ntiostat- _Polariz&ion of the mercury pool cathode was avoided by magnetic stirring and by- bubbling _$&reampf nitrogen through therbulk of-the soltition. All potentials - -_ _ -._ are referred to-the saturated calomel electrode. . _ Spe&tr~:w~~e recorded on a_B&&&m -e&i2 spectrophotomete~, -fi+ - _~ : &ng -_-, .- .- -__- -_..= -_Corex~cellS_Z. -- -Chrotiaio&a@s we& OS&e& on celluio&-&in ia&-. MN-Pdlygrtim- &i < - 300 (Maclierek-Nagel and co), usi& p-but&o1 sa&ratGd_$&3 I$ HNO>.as_ehient_5. _- -??he Xeni$%ture f&r all pOlarOgraphi& and~cl@nat~@aphjc -wor&-was: O”c,- -
~~u+ss:&h&%v$e~~tafed~ =_- __;‘= _I 1 e-_ :
--.. _ ec __ _ --__ . _- -- __ -_
_;
-.- ‘__ _ -- ; __ : --.. /_‘T _ -._ -_ __ _- - .-_ -- -
I
--_
: 1 -1:
.
^ -_
-
i
=__
emI-_.__f-: __ I1 I_~--~_ ,j:--;-_-: L‘__-._5- =c--_ -.‘,--
_ -~~_-EZ~~t~~~~~~Ch~~~~~2~~1~69~~-~~~~~~~~~ - -‘ . -, m-_:.ai __ _ ’ :,. .z-.r=-
312
Ii.- M0RPuRGo
RESULTS
Polarography
_
A preliminary polarographic study of the system was made in view of the scarcity of information available on the subjec@-‘, especially for Iow temperatures. At 0°C the polarographic reduction of RuN03+-nitrato complexes takes pIace in two steps at concentrations of the depolarizer below about 3 x lob4 M, and in three steps at higher concentrations (Fig. 1). The half-wave potential of the first step is about + 0.01 V and is independent of pH ; its limiting current is proportional to the concentration of RuN03’ up to a value of about 3 x 10e4 M, and then remains constant for fisher increases (Fig_ 2)_ The second step is found at E+ equal above 3 x 10S4 M_ Its to about -0.15 V, starting from concentrations of RuN03+ -6
4-
f[uAl
Cb
al
/CJ.IAI -5
3-4
-3
2-
-2 l-1
0 a2
/: 0
-0.2”
I
t 0.1
0
-
0.2
-a4 v
Fig. 1. PoIarograms O-5 N; (b) ~uN03*J
of RuNO’* =4_7
I (a) IpuNO”]
x 10-O
lU,
[HNOJ
0
-0.6
CVolt1
=2.0 x 1Om4 M, [HN03] =0.15 IV, PaNO,] =0.5
=
6.0 x 10e2 IV, maNO,]
=
IV.
limiting current is not well defined and is difkult to measure accurately; however, it seems to increase linearly with RuN03+ concentration (Fig_ 3)- E+ of the third step depends on the hydrogen ion concentration of the solution and becomes more positive (by about 65 mV) for a ten-fold @crease (Table 1). The limiting c-krrent is proportional to RuN031‘ concentration -when this is below about 3 x 10S4 M, but also depends both bn Hi and NO, concktrations (Fig. 4). In HN03
__ -
_ ?e __
--
REDUCTION
OF NITRO!WLR~
0
2
-NITRATO
4
6
10
8 @NO]
-
104~
Fig_ 2 Limiting height of the first wave as a function of RuN03+ IV NaN03; (A) 0.5 N HN03; (x) 0.5 IV HNO,+OS N NaNO,;
4
2
0
concn Supporting (+) 2 N HNO,.
8
6 [RuNo]
313
COMPLEXES
electrolyte:
(0)
0.5
electrolyte:
(0)
10
- i+hf
Fig. 3. Limiting height of the second wave as a function of RuN03+ 0.5 N NaNO,; (x) 0.5 N NaN03f0_5 N HN03_
concn.
Supporting
In Table 4 are reported the limiting currents of some polarograms for two different temperatures_ It can be seen that there is a greater percentage increase of ia of the third wave in 1.15 N HNO, than in 0.67 N HNO,. A similar behaviour is found also for the sum of the ia values of the first two waves ; ia of the first wave is insensitive to temperature variations_ Electrolysis
at controlled potential
At -02 V the_colour of the solution change gradually from orange-red to’. dark brown, Af$er a small initial increase the current st&ts to deere~eslovyly but_-_never goes beloy_30% of its Ztial value, irreqjective of the_time-of.elect.rofysi% A pola.rog&uu of &n el_ec$olyzed Solutiog (initialconcentration of R&NO3 ? F_ 5 x lo-? _; ._
_
-_*
_- _
_
-
I_--
_
_-
.-
_--_
--
:
-_ .l. E~y+tz&
&t&2
_.--
&i$3$318
,
_-
y.
_ _e- r
314
L.MORPURGO
TABLE1
[R~NO~+]- lo4iu
pH
E,/V
(caicd.) 6_87 x-45 202 2-58 3_29 4-00 O-29 l-45 2.02 2.58 -
TABLE
l-55 133 l-19 1.10 l-04 as9 0.32 0.28 0.27 0_26
-0.430 -0_415 -0_405 -0.4oo -0.395 -0.385 -0_350 -0.340 -0.345 -0-345
2 ~~~N0~+]=2_9~io--~ibf [HNO,]=o.s N
[R~N0~*]=5_7xlO-~M [HNOJ=z.o N h/Ci?l
id,;@
i,,/h - 10"
id,/@
i,,/h - 10'
53 63 73 83 93
093 L-09 136 1.59 1.68
1.70 l-74 I-79 1.93 1.82
1.03 l-15 l-32 1.56 1.76
1.9s l-83 l-81 1.89 1.89
TABLE3 [R~N0~+]=5.7xlO-~ [HNO~]=~.~ N h/cm
id,/@
53 63 73 83 93
5.70 5.52 5.70 5.64 594
_
M
[RuN03+]=2.9x [HNO~]=O.S N
lo-&M
id,/.@
id, x h-*
1.65 1.78 19s 2.11 225
0.22
0.23 0.23 023 0.23
TABLE4
O-67_ O-67- . I‘-- -.
_- o25
- _
18 4.8 - -_
_
--_
_
_
l-5 1.5 ,
_
1.6 23
3-7 -- 4.8
-
REDUCTION
OF NLTROSYLR uTHENIuM--NITRATocoMP~
-.
_-
_
-
-
315
=OS M) .s&vs OI& two reduction steps. Their ia is similar to thatbf the first two steps of the initial solution, but E, is shifted towards negative values by about 50 mV_ The third wave is missing. Thin layer chromatograms of the solution before and after the electrolysis are shown in Fig. 5. The ele&.rolyzed8olution was not &ar and a faint dark precipitate formed on standing. It was probably this precipitate that gave the spot at the application point of the second chromatogram. Once clear. the spectrum of the electrolyzed solution does not vary appreciably with time if kept at 0°C but does so at room temperature (Fig_ 6).
M, [@NO,]
ET ;----, ;-__-i --____-’
0
2
6
4 [RUNO]
- 104 M
Fig. 5. (a) Thin layer chromatogram electrolytic reduction at -0.1 V-
of a soln. of 5 x
b
a
Fig. 4. Limiting height of the third wave as a function of RuNOsc COnCn. Supporting IV NaNO, ; (b) 05 IV HNO, ; (c) 0.5 N HN03 f0.5 N NaN03 ; (d) 2 N HN03.
10m3 M RuN03+
electrolyte:
in 0.5 ZVKNOJ
(a) OS
; (b) a~ (a) after
DISCUSSION
-The behaviour of id of the first wave at varying RuN03+ concentrations, its linear relationship to the height of the mercury reservoir and the ver$ small temperature coefficient show clearly its adsorption character__The second wave, which only appears at higher concentrations pf tie depolarizer,_when -the likiting height -of the adsorption-wave-is becoming-constant, is the_“normal” wave_‘?_ It is’originated by _ the same electrochemi&l process; but- the- reduction product, -no-h&g& adsorb&l on -- - : , _ _ -: _ . _ _ _- the .electrode surface~~remains~insolution_ r- - _’ _--- - -.. i _In dilute acid solutionS the lim&g height cf the -third wave’& approxim&ly _ &p.xk to -the_ su&of -~the_otlier_&A$ wave heights $&d is- diEusion-controlle&_rhis ind&at~ a subsequent reduction process involving the sari+&inber~of elect&$s &,G1 ‘-&G.jrst__‘~-:~ I__ - -‘=_;_;-=- -1; __I _ ‘-; _= _ - _ -= -_ _ :;_---;__;:_-;.:“_-__$1 _:;‘,_c(y-~~_-~ c_‘
z._ _ 1. . -- _’ _ _- - z-_ :- .~ _-_>_ _ .- - -z ~-
. -- - - _ _ __:_- __ .-- ;‘_ __ (--:- -2; --c_ _ : __.r_ q+&&;lhr[&&~-‘& _ c- __-_ _-_
__ .f_ _ --__ _- - -=,__ --: (<&j~j.&~;~&= _ _A
_----
L. MORPURGO
316
Fig. 6. Spectra of a soIn. of RuNO 3+ in 8 N HN03, dihted to 0.5 N HN03 : (I) before electrolytic reduction; (2) after electrolytic reduction and kept for 2 daqs at O’C ; (3) as (2) after 45 d2ys; (4) electrolyzed soh. kept at room temp_ for 30 days; (5) as (4) after 15 days.
Adsorption waves have already been observed for RuC13 and RuNO(NO,), in sulphuric acid medium at room temperature7.They were thought to be due to the reduction of polynuclear species and were found to involve the adsorption of some ruthenium derivative. “Xormal” waves were not observed_ Our soIutions, however, are known to contain a mixture of labile nitrosylruthenium-nitrato com-
plexes of general forrnula5*9 I_RuNO(NO,),(H,O), _JC3--“)+. Polynuclear species are only present in traces and cannot alone be responsible for the adsorption wave. In our opinion this and the “normal” wave are due to all species in solution that dissociate prior to reduction. The electrode reaction may then be: RuN03++e
-
RuZf+NO
with Ru*+ adsorbed on the first step_ The NO reduced along the third step:
NO+e+H+
-
produced
by this reaction
may be
HNO
Moreover Ruz f is very likely to be reoxidized by HN03 to give Ru3+ or possibly Ru4+ derivatives- This reaction scheme is also substantiated by the following considerations : The polarographic reduction of Ru(III) to Ru(I1) has already been observed at -0-02 V in p-toluenesulphonic acidlo, at - 0.28 V in 1 N HClgb and at -0-34 in perchloric acid solutionsll, Le. in the potential range of the first two waves.
V
The third wave is missing in the polarogram of solutions electrolyzed at -02 V_ The only explanation we have for this is that the wave is due to NO produced on the two preceding steps_ During the electrolysis NO is displaced from the solutions by thestream of nitrogen as soon as it is formed_ The potential of the other two waves is shifted after electrolysis towards_- negative valuesindicating that they are then due J-_-Ehxmadl.
- _
Chime,
22 (1969)
321-318
-_
REDUCTION
OF E
COMPLEXES
317
to the reduction of differentchemical species. These are possibly cationic, as indicated by their chromatographic behavior?, and are probably polymericRu(II1) derivatives_ The third wave acquires some catalytic character when the concentration of HNOs increases, i.e. when the reoxidation of Ru(I1) becomes easier. More NO is possibly produced by this reaction. A somewhat similar behaviour was observed during electrolysis at controlled potential of yellow solutions of Ru(II1) in per&lo& acidlo, i-e- the colour changed to brown, large quantities of chloride ions were produced, and the intensity of the current never dropped to zero_ A reduction step at -OS V (NCE) is reported by some authors12*13 for NO in acid solutions- It has some kinetic character, a pH-independent E, and is assigned 2- . We have verified that a wave at about the to the reaction 2 NO P(NO)~~N~O~ same potential also occurs in the reduction of NO in nitric acid solutions. For RuN03 + the properties of the third wave, which we have assigned to the reduction of NO, are different, but so are the conditions under which the wave is produced, owing to the presence on the electrode of the metal ions. A different reduction pattern has also been found by other authors under different conditions14. Since the ruthenium compounds produced by electrolysis of RuN03+ apparently contain no NO and, at low temperature, are stable cationic species with different chromatographic properties from the starting compounds, it is possible that the electrolytic method could be used to separate ruthenium from irradiated nuclear fuels_ ACKNOWLEDGEMENT
This work was carried out under the contract N. 365/RB with the International Atomic Energy Agency (Vienna). SUMMARY
The polarographic behaviour of nitrosylruthenium-nitrato complexes has been examined at low temperatures, in nitric acid and sodium nitrate solutions_ Two waves are observed at concentrations of RuN03+ below about 3 x 10B4 M, and three at higher concentrations. A reduction scheme is proposed and a method for group by controlled potent&l &ectrolysis is described. decomposmg the RuN03+ REFERENCES U_K_At.E_Authority Rept., AERE-R, 4537 (1964). 1 C. E_ LYON AND D. SCAFtGILJi, 2 A. G. WAIN, R. J. W_ S-nsrrr ON, E. N. JENKINS,J. M. FLETCHER AND P. G. M. BROWN, U_K_At.E.Authority Rept.. AERE-R, 3509 (1960). 3 J. M. FLFTCHER AND J. L. WOODHEAD, J_ Inorg_ NucI_ Chem., 27 (1965) 1517. 4 3. M. FLETCHER, I_ L. JENKIN s, F_ M_ LEVER, F. S_ MARTIN, A. R. POWELL AND R. TODD, J_ Inorg. NucZ. Chem., 1 (1955) 378. 5 L. MORPURGO, Ric. Sci., 36 (1966) 553. 6 E_ N_ JENKINS, U_K-At.E.Authority Rept., AERE-R, 3491 (1960). 7 J_ BLAZEK AND D. M_ WAGNEROVA, Collection Czech..Chem. Commun., 29 (1964) 915. 8 L. MEITES, Polarographic Techniques, Inter-science, 2nd ed., 1965, a, p_ 187; b, p- 629. 9 D_ SCARGILL, C, E. LYON, N. R. LARGE AND J. M. FLETCHER, J_ I..rg_ &zl_ Chem., 27 (1965) 161. J_ Electroad.
Chem, 22 (1969) 311-318
318
L.
10 Em E. MJZRCER AND R. R. BUCKLEY, Innorg. Chem., 4 (1965) 1962. 1I L. W_ NIEDRACH AND A. D. ~EVEBXUGH, J_ Am. Chem. Sot., 73 (1951) 2835. 12 L. RICCOBOM, P. LAXZA AND P. FAVERO, Atti iVa=. Lincei, Mem. Ckzsse Sci. Es. 3 (1950) 8. 13 J_ !vL=s+, Z_ Anal_ Chem., 224 (1967) 99. 14 D- L. EI+MN AND D_ T. SAWWZR, J. ECecrrounuL Chem., 16 (1968) 541_ J.
Efecmoonaf_ Chem.,
22 (1969) 311-318
..
hfORPuRG0
Mat. Nat- Se=. II”,