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Polarographic activity of N2O
Electroanalytical Chemistry and Interfacial Electrochemistry, 46 (1973) 353-362
353
©Elsevier Seqooia S.A., Lausanne- Printed in The Netherlands
P O L A R O G R A P H I C ACTIVITY OF N20*
Z. P. ZAGORSKIand J. P. SUWALSKI Laboratory for Pulse Techniques in Radiation Chemistry, Institute of Nuclear Research Warsaw 91 (Poland)
(Received 15th January 1973)
INTRODUCTION Nitrogen suboxide is considered to be polarographically inactive, as stated by Walker t. However, some remarks by Barker et al. 2, given without detailed explanation, concern the reduction of this compound in connection with photopolarography. Investigation by polarography of solutions prepared for irradiation 3, has shown that the electrochemical behaviour of N 2 0 solutions is not simple. There is no distinct wave, but the final part of the polarization curve in the N2 O solution shows large current oscillations, differing substantially from those observed with the same D M E in the same solution swept with argon. Closer investigation of N 2 0 behaviour was needed for the purpose of understanding its secondary reactions in electrochemistry, as well as for the purpose of radiation chemistry, in which it serves extensively as a reagent for converting hydrated electrons into O - ion-radicals, with a very high rate constant (for e~ + N20--*O -, k = 5.6 x 1091 m o l - 1 s- 1)'k Little is known concerning the details of this reaction, and practically nothing about the form of N 2 0 in solution or the reasons for its high solubility in water (25°C:2.5 x 10 -2 mol 1-1)5. Nowadays the question of "dry electron" reactions with N 2 0 appears, complicating the interpretation of phenomena in radiation chemistry, but at the same time it brings the problem closer to electrochemistry where the theory of the reaction of the solvated electron alone 6 cannot explain all phenomena, and moreover leads us to express the opinion that reaction of the dry electron must prevail. The purpose of the present paper is to explain undoubted polarographic activity of N 2 0 ,in terms useful both for electrochemistry and radiation chemistry. EXPERIMENTAL Polarographs used were PO4 (Radiometer), PO (5122 CLA USSR, cf. ref. 7), with DME's of different m and t. Different vessels of low resistance, with 3 electrodes for oscillopolarography, were used. Nitrogen suboxide, oxygen content polarographically not detectable, was used. * Dedicated to ProfessorWiktor Kernula on the occasion of his 70th birthday.
354
Z, P. ZAGORSKI, J. P. SUWALSKI
All reagents were of A.R. grade. Tetramethyl and tetraethyl hydroxides were prepared through the reaction of Ag20 with tetraalkylammonium bromides and iodides (Feinchemie K-A Kalles or Merck "for polarographic purposes" and Fluka AG). Also 10% tetramethylammonium hydroxide prepared by Merck "for polarography" was used. A typical experiment consisted of the examination of Ar-swept solutions later mixed with known amounts of the same solution saturated with N20. Special precautions which were taken during some experiments (e.g. determination of temperature coefficient) are described in the relevant places in the Results section. RESULTS
Polarograms of N20 in various supporting electrolytes Figure 1 shows the conventional polarograms of N 2 0 in various simple supporting electrolytes, compared with those swept with argon. No differences in the anodic parts of curves were found. Figure 2 shows the i-t curves again in the presence and the absence of N20.
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b -1.0 -1.2 -1.4 - 1.6 -1.8 - 2 . 0
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-1.6 -1.8 -2__0-2.2
E/v~ SCE
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
V~
Fig. 1. Influence of N 2 0 on the final current rise in various electrolytes. (a) 0.0625 M K2504, or 0.125 M KC1, or 0.125 M NaOH, or 0.1 M KCI+0.005 M Na2CO 3 saturated with N 2 0 (1) and Ar (2); (b) slightly acidic solution; 0.03 M NazSO 4 + 10- 4 N H2SO4, acetate buffer pH 4.5 saturated with N 2 0 (1) and Ar (2). Fig. 2. i-t curves for the N 2 0 (a) or Ar (b) saturated solution, 0.125 M in N(CHa)4OH. Applied potential - 2.0 V (SCE).
The use of tetraalkylammonium hydroxides helps to widen the negative range of the polarographic spectrum. Figure 3 shows that in the negative region oscillations grow more and more, and although no limiting current is formed, the average intensity at a chosen potential (Fig. 4) is proportional to the concentration of N 2 0 and may be used for analytical purposes with the help of a calibration
POLAROGRAPHIC ACTIVITY OF N2 O
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20
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-2.3 -2.5 - 2.7 E//v l v$ SCE
I
2
3
4
5
6
7
8
9
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Fig. 3. Influence of N20 concentration on the final current rise and oscillations measured at - 2.5 V v s . SCE (at the right hand of the figure). Successive curves from (1) 0, (2) 1.2, (3) 2.27, (4) 3.25, (5) 4.17, (6) 5.0, (7) 6.5, (8) 7.14, (9) 8.34 mM N20. Fig. 4. Calibration curves for N20 current at various potentials. Average current measured from oscillations slightly damped (damping "1", Radiometer PO4 polarograph).
curve, with a precision of _ 3%. It may be remarked that at such high negative potentials, in the absence of N20, the surface tension of mercury is very low and the drop time is up to ten times shorter (cf. the end of Results section in connection with drop time). Because of the influence of N20 .on the natural drop time, an artificial drop timer was used. It was proved that the oscillations have larger amplitude even in the case of equal t (cf. Fig. 5).
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E/V vs SCE
Ffg. 5. Influence of N20 on the final current rise (a) in comparison to the same solution, 0.125 M N(CHa)4OH, saturated with Ar. The drop duration controlled in both cases at 0.3 s. Damping, "1" as in Fig. 4. Note the large amplitude of oscillations in N20 saturated solution.
356
Z.P. ZAGORSKI, J. P. SUWALSKI
In slightly acidic solutions (5 x 10 -5 M H 2 S O 4 ) where the plateau due to Ha O+ reduction is visible, the phenomena in the negative region are basically similar, except that they appear on the background of the hydrogen wave (Fig. lb). The temperature coefficient of the N 2 0 current was measured between 5 and 50°C. To keep the N 2 0 concentration constant the solution was saturated with N20 at 50°C, cut off from the gas (the vessel was without the gas phase) and cooled down gradually. Cooling was interrupted from time to time and polarograms were recorded at a stepwise stabilized temperature (Fig. 6). Under these conditions the slope of the i/T straight line was 1.4 #A per °C. Taking the current at 25°C as 100%, the temperature coefficient is 2.2% per °C. The results of an investigation of the dependence of N20 current on the height of the mercury reservoir at various potentials are shown in Fig. 7. The current plotted is an average of the extensive oscillations not usually encountered in diffusion-controlled waves. ///~A 110 100 9C
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Fig. 6. Influence of temperature on the N20 current, measured at -2.5 V. Supporting electrolyte, 0.125 M N(CH3)4OH. Fi~. 7. Dependence of N20 current measured at various poteniials, on the height of the mercury reservoir.
Similar results are obtained with NO3, benzoate, and adenosine. All three compounds have been chosen as belonging to very different categories, and all are considered to be polarographically inactive in simple supporting electrolytes. These compounds have high rate constants for reaction with electrons: 1.1 x 101°, 3.6 x 109 and 1.0 × 101° respectively. The polarograms in Fig. 8 show that all these compounds exhibit the same phenomenon as N20 i.e. earlier rise of current with increased oscillations. Sodium nitrite gives basically the same effect, e.xcept that it appears on the background of the Na ÷ wave (it proved impossible to prepare tetraethylammonium nitrite without Na ÷ ions).
357
POLAROGRAPHIC ACTIVITY OF N20 /~A
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(a)
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-2.0
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E/V vs SCE
Fig. 8. Influence of nitrates (a), benzoate (b) and adenosine (c). (a) Successive curves from above: 10.0, 8.75, 7.35, 5.84, 4.12, 2.19, 0.0 mM N O ~ ; (b) successive curves from above: 11.8, 6.75, 3.18, 1.2, 0.0 mM benzoate; (c) successive curves from above: 7.0, 5.26, 4.12, 1.56, 0.0 mM adenosine. Supporting electrolyte -0.125 M N(CH3)4OH.
Influence o f N20 on Cd 2 ÷ wave As the next approach to the problem of N 2 0 electrochemistry, its influence on the behaviour of polarographically active species was investigated. First of all the influence of N 2 0 and, for the sake of comparison, of N O 2 and NO~-, on the Cd 2+ wave was shown on polarograms. This experiment was performed by the multisweep technique on the oscillopolarograph. A slowly dropping ( t = 15 s) D M E was polarized with a potential from E o to E 1 (respectively the beginning and end of a sweep) many times during its life. The switch from E 1 to E 0 was of a millisecond duration. E o was typically - 0 . 5 V (SCE) and E1 was varied between - 1 . 4 and - 2 . 1 V (SCE). The sweep duration was typically 2 V c m - i . Figure 9 shows that, in the presence of N 2 0 and with a final potential more negative than - 1.8 V (SCE), two peaks appear for cadmium. The new wave is diminished again in the presence of O - scavengers like C2HsOH or SO 2-. Both scavengers have a high rate constant with O - but a low one with electrons. ~/.tA
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358
Z.P. ZAGORSKI, J. P. SUWALSKI
The N 2 0 effect m a y be s i m u l a t e d again, this time b y H 2 0 2 , which p r o d u c e s O - at l o w e r negative potentials. If the limit o f the p o t e n t i a l sweep is m o r e negative t h a n - 1.1 V ( S C E ) a similar s e c o n d w a v e o f c a d m i u m a p p e a r s a n d m a y be supp r e s s e d b y C E H s O H o r S O 2 - . T h e wave is d i m i n i s h e d also b y N O 2 which acts here as a p u r e scavenger, b e c a u s e the p o t e n t i a l is n o t negative e n o u g h to r e a c h the r e d u c t i o n of this ion itself. T a b l e 1 s u m m a r i s e s the p h e n o m e n a . A similar s e c o n d p e a k m a y be o b s e r v e d with M n 2+ a n d Z n 2+ b u t n o t w i t h Tl +. TABLE 1 CONDITIONS OF THE 2nd CADMIUM PEAK FORMATION
No.
Systeminvestigated
pH
1st peak Cd -0.65 V
2nd peak Cd -0.73 V
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Cd 2+ +Ar Cd2+ +Y20 Cd 2+ +N20 Cd2++N20+H ÷ Cd2+ +N20 +CHaCH2OH Cd2+ +N20+SO~ Cd2+ +H202 Cd 2+ + H202 Cd2+ +H202+H + Cd2+ + H202+CH3CH2OH Cd2+ +H202+SO3zCd2+ +H202+NO~ , Cd2+ +H2Oz+NO~ Cd2 + + H202 + N20 +CH3CH2OH Cd 2+ +NO3 Cd 2+ +NO~Cd 2÷ +NO3 +CHaCH2OH
6.9-7.1 6.9-7.1 6.9-7.1 4.2-5 6.9-7.1 ~7.5 6.9-7.1 6.9-7.1 4.2-5.0 6.9-7.1 ~7.5 6.9-7.1 6.9-7.1 6.9-7.1 6.9-7.1 6.9-7.1 6.9-7.1
+ + + + + + + + + + + + + + + + +
+ + + +
15
16 17
+ -
Final potential of sweep/V -2.1 1.7 -2.1 -2.1 -2.1 -2.1 1.0 -
-
l
more negative than - 1.1 V (SCE) 1.8 -2.1 -2.1 1.7 -2.1 -2.1 -
-
Acidity changes around the drop It has been f o u n d t h a t the flow o f c u r r e n t in n e u t r a l s o l u t i o n s c o n t a i n i n g N 2 0 is a c c o m p a n i e d b y a n increase in a l k a l i n i t y in the vicinity o f the d r o p . This w a s d e t e c t e d b y the m e t h o d o f K e m u l a a n d G r a b o w s k i 8 w h i c h consists o f the a d d i t i o n of a n i n d i c a t o r to the solution. I n o u r case it was p h e n o l p h t h a l e i n . If the s o l u t i o n is s a t u r a t e d with N 2 0 , a l r e a d y a t a p o t e n t i a l o f - 1 . 7 V ( S C E ) the p i n k c o l o u r a p p e a r s a n d b e c o m e s m o r e a n d m o r e intense as the negative p o t e n t i a l increases. A l k a l i n i t y at the e l e c t r o d e - s o l u t i o n interface a p p e a r s a t m u c h higher p o t e n t i a l s o n l y ( T a b l e 2) in the case o f a r g o n - b u b b l e d s o l u t i o n a n d is a n o b v i o u s p h e n o m e n o n . T h e r e m a r k a b l e fact o f the increase o f surface t e n s i o n o f H g in the s o l u t i o n o f N 2 0 at high negative p o t e n t i a l s is s h o w n in Fig. 10. T h e effect is very m a r k e d a n d m a y be d e t e c t e d b y a low-sensitivity m e t h o d , s i m p l y the m e a s u r e m e n t o f the d r o p time o f a c o n v e n t i o n a l D M E . DISCUSSION T h e p r e s e n t e d results show t h a t N 2 0 in a q u e o u s s o l u t i o n c a n n o t be con-
POLAROGRAPHIC
ACTIVITY
359
OF N20
TABLE 2 ACIDITY
CHANGES
No.
Potential/V (SCE)
AROUND
THE DROP
Colour around DME N20
Ar
1
- 1.35 "'b
-
-
2
- 1.50 a'b
-
-
3 4
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5
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6
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1
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7
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8 9
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3 3
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10
-- 2,30 b
4
--
11
-- 2.40 b
4
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12
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5
2
" F o r 0.12 M ( C H 3 ) 4 N B r . b F o r 0 . 1 2 5 M K 2 S O 4.
/s 3(
25 2O 15
a
10
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18 ~ ,
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I i I ,
E/V vs SCE F i g . 10. T h e n a t u r a l t i m e o f t h e m e r c u r y d r o p - l i f e i n N 2 0 0.125 M in N ( C H a ) 4 O H .
s a t u r a t e d (a) a n d A r (b) s a t u r a t e d solution,
sidered as polarographically inactive. It is reducible at the D M E from - 1 . 4 V (SCE) in practically all supporting electrolytes, but without formation of a limiting current even in N E t 4 O H solution. The detection of the start of the reduction depends on the sensitivity of measurement and the ease of distinguishing the capacity current. The N 2 0 current is highly reproducible and may serve for analytical purposes if the potential of the reference electrode is well stabilized. , The lack of current plateau is overcome by these means. Because of the rather high current intensities it is advisable to use reference electrodes of checked polarizability 9. The lack of a plateau indicates that neither diffusion nor another form of depolarizer transport controls the process of reduction, it being rather the concentration in the bulk of the solut~o,p which matters. The phenomenon does not
360
z.P. ZAG()RSKI,J. P. SUWALSKI
seem limited to N 2 0 only: we have observed it for other species which exhibit a high rate constant for the reaction with solvated electron (NO3, benzoate, adenosine). The most conclusive fact seems to be the easily observed, unusually wide oscillations of current. These also appear when an artificial drop timer is used (the drop is removed with every movement of the capillary I o) and are of much larger amplitude than in the case of a diffusion controlled electrode reaction. This conclusion is supported by the shape of the i - t curve of N 2 0 (or NO3) reduction (no tendency toward the steady-state current formation). From this behaviour we may conclude that the obvious first reaction N20+e- ~N2+O-
(1)
must be followed by some others. First of all the reaction O - + e- ~ 0 2 - (water)
(2)
may occur, which uses an additional electron supplied by the electrode. O - may, however, react with N 2 0 according to the equation O - + N 2 0 -~ N2 + O f
(3)
The peroxide radical reacts further at the electrode at the highly negative potentials applied thr.oughout the investigation. Our considerations do not distinguish at the moment between the dry and hydrated electron. From radiation chemistry it is known that the rate constant of reaction (2) with ea~ is 2.2 x 101° mo1-1 s -a. Our view is supported by the shape of the calibration curves (current vs. N 2 0 conc., Fig. 4). The higher the N 2 0 concentration, the smaller is the electrochemical contribution, because more of the O - has the opportunity to react with N 2 0 and O - . Both O - and O~ partly escape the electrode reaction. In other words, at all N 2 0 concentrations reaction (1) contributes to the current 100%, but at low concentrations the current is increased by reaction (2) and the reduction of 0 2 formed in reaction (3). With the increase of the N 2 0 concentration the charge per N2 O unit of concentration decreases, because the product of the reaction ( O - ) steals some N 2 0 molecules. At the same time some O - radicals escape the reduction, because with growing concentration the chance of them entering into reaction (3) increases. The second generation of products (O~) has an even loWer chance of taking electrons from the electrode. The possibilities of proving this mechanism of N 2 0 reduction by pure electrochemical methods are poor. In view of the fact that the shapes of the depolarization curves obtained with N20, NO~- or other electron scavengers differ greatly in comparison with the conventional polarographic curves, application of the usual tests for the electrode reaction mechanism is not correct. Thus, knowledge of the temperature coefficient (Fig. 6) does not permit any conclusions to be drawn and also the lack of dependence of current on the pressure of mercury (Fig. 7) is not conclusive because of unusual oscillations and the lack of steady state on the mercury drop. There is a similar uncertainty concerning the measurement of other electrochemical features--these were therefore necessarily supported by different investigations. We have concentrated on the consequences of the reduction of N 2 0 and the fate of O -, a very active ion-radical, well known from radiation chemistry. One
POLAROGRAPHIC ACTIVITY OF N20
361
has to stress that the spectrum of this species is very inconvenient for investigation. Its maximum lies in the far end of the common u.v. spectrum (2 = 240 nm) and the molar extinction coefficient is low 11 240 1 mol-1 cm-1. Prospects for the detection of O - near the electrode are poor or negligible in comparison with the relative ease of spectral detection of the hydrated electron, distinguished by a very high and convenient position of the maximum. The investigation of the consequences of N 2 0 reduction on the electrode was concentrated on the study of pH changes during N 2 0 reduction and on the influence of N 2 0 reduction products on the cadmium wave. It is evident from Fig. 9 that the first voltage sweep in the solution containing Cd 2÷ and N 2 0 results in one peak of Cd 2+ reduction. All subsequent sweeps during the same mercury drop show the presence of an additional peak. The latter can be caused only by the reduction of species formed at more negative potentials and adsorbed on the electrode. Close investigation shows that (a) The N2 O' reduction potential must be reached in order to develop the new peak (i.e. O- is essential). (b) Even higher negative potentials in the presence of Ar do not give rise to the peak (i.e. e-, Na, NH4, H may be excluded). (c) All O - or O H scavengers remove the peak (i.e. O- and not O H - is essential; the positive effect of non-ionic scavengers e.9. C2HsOH indicates that it is not alkalinity which causes the phenomenon). (d) The phenomenon occurs both in neutral and acidic solution (i.e. the acidic form of O - , namely OH, reacts similarly). (e) A similar phenomenon is observed if NzO is replaced by NO3, and in some conditions (neutral solution only) by H202 (i.e. the first product of reduction is also O - or the OH radical). ( f ) A similar phenomenon is observed with other divalent cations like Mn 2÷ but not with monovalent T1 ÷ (i.e. the phenomenon is limited to cations with at least 2 valency states). The indirect experimental evidence tends to make us express the view, that O - or O H (both hydrated) react with Cd 2÷, forming most probably Cd(OH)~ which is adsorbed at the electrode and is reduced at more negative (by 100 mV) potentials, beyond the Cd z ÷ peak. A quantitative interpretation of the results is not possible because of several reactions proceeding in parallel in the system. The investigation is continued along the line of combination of electrochemistry with radiation chemistry. SUMMARY
During an investigation of basic radiation chemical reactions of N 2 0 in aqueous solution, the electrochemistry of this compound was investigated. In the literature, the polarographic activity of N 2 0 has been discussed usually in connection with photo-ejected electrons, but this time reactions in the dark were also investigated. At sufficiently negative potentials N 2 0 reacts with an electron yielding O - which may further react with another electron, with N20, with itself or with specific additives present in the solution. The latter case was illustrated by the reaction with Cd 2÷ which shows two oscillopolarographic waves in the presence
362
Z.P. ZAGORSKI, J. P. SUWALSKI
of N20. similar currents as in the N20 case were shown by other solutes which do not give diffusion controlled waves, but have a high rate constant for the reaction with electrons, determined by radiation chemical methods. Nitrates, benzoates and adenosine were investigated by us. REFERENCES 1 2 3 4
5 6 7 8 9 10 11
D. C. Walker, Can. J. Chem., 44 (1966) 2226. G. C. Barker, A. W. Gardner and D. C. Sammon, J. Electrochem. Soc., 113 (1966) 1182. Z. P. Zag6rski, K. Sehested and S. O. Nielsen, J. Phys. Chem., 7-5 (1971) 3510. M. Anbar and P. Neta, A Compilation of Rate Constants for Reactions of e~q, H and OH with Inorganic and Organic Compounds in Aqueous Solution, Israel Atomic Energy Commission, Soreq Nuclear Research Center, 1966. Handbook of Chemistry and Physics. The Chemical Rubber Co., Cleveland, Ohio, 51st edn., 1970-71. D. C. Walker and G. A. Kenney, Electroanalytical Chemistry, A series of Advances. Vol. 5, Hydrated Electrons and Electrochemistry, Marcel Dekker, New York, 1971, p. 2. Z. P. Zag6rski and A. Blum, J. Electroanal. Chem., 41 (1973) 447. W. Kemula and Z. R. Grabowski, Rocz. Chem., 25 (1951) 350. Z. P. Zag6rski, Proc. 2nd Polaroor. Conor., Vol. 3, Oxford, 1960, p. 1132. Z. P. Zag6rski, Przem. Chem., 11/34 (1955) 693. A. K. Pikaev, Solvatirovanyi Elektron v Radyatsyonnoy Khimii, Nauka, Moscow, 1969.
353
©Elsevier Seqooia S.A., Lausanne- Printed in The Netherlands
P O L A R O G R A P H I C ACTIVITY OF N20*
Z. P. ZAGORSKIand J. P. SUWALSKI Laboratory for Pulse Techniques in Radiation Chemistry, Institute of Nuclear Research Warsaw 91 (Poland)
(Received 15th January 1973)
INTRODUCTION Nitrogen suboxide is considered to be polarographically inactive, as stated by Walker t. However, some remarks by Barker et al. 2, given without detailed explanation, concern the reduction of this compound in connection with photopolarography. Investigation by polarography of solutions prepared for irradiation 3, has shown that the electrochemical behaviour of N 2 0 solutions is not simple. There is no distinct wave, but the final part of the polarization curve in the N2 O solution shows large current oscillations, differing substantially from those observed with the same D M E in the same solution swept with argon. Closer investigation of N 2 0 behaviour was needed for the purpose of understanding its secondary reactions in electrochemistry, as well as for the purpose of radiation chemistry, in which it serves extensively as a reagent for converting hydrated electrons into O - ion-radicals, with a very high rate constant (for e~ + N20--*O -, k = 5.6 x 1091 m o l - 1 s- 1)'k Little is known concerning the details of this reaction, and practically nothing about the form of N 2 0 in solution or the reasons for its high solubility in water (25°C:2.5 x 10 -2 mol 1-1)5. Nowadays the question of "dry electron" reactions with N 2 0 appears, complicating the interpretation of phenomena in radiation chemistry, but at the same time it brings the problem closer to electrochemistry where the theory of the reaction of the solvated electron alone 6 cannot explain all phenomena, and moreover leads us to express the opinion that reaction of the dry electron must prevail. The purpose of the present paper is to explain undoubted polarographic activity of N 2 0 ,in terms useful both for electrochemistry and radiation chemistry. EXPERIMENTAL Polarographs used were PO4 (Radiometer), PO (5122 CLA USSR, cf. ref. 7), with DME's of different m and t. Different vessels of low resistance, with 3 electrodes for oscillopolarography, were used. Nitrogen suboxide, oxygen content polarographically not detectable, was used. * Dedicated to ProfessorWiktor Kernula on the occasion of his 70th birthday.
354
Z, P. ZAGORSKI, J. P. SUWALSKI
All reagents were of A.R. grade. Tetramethyl and tetraethyl hydroxides were prepared through the reaction of Ag20 with tetraalkylammonium bromides and iodides (Feinchemie K-A Kalles or Merck "for polarographic purposes" and Fluka AG). Also 10% tetramethylammonium hydroxide prepared by Merck "for polarography" was used. A typical experiment consisted of the examination of Ar-swept solutions later mixed with known amounts of the same solution saturated with N20. Special precautions which were taken during some experiments (e.g. determination of temperature coefficient) are described in the relevant places in the Results section. RESULTS
Polarograms of N20 in various supporting electrolytes Figure 1 shows the conventional polarograms of N 2 0 in various simple supporting electrolytes, compared with those swept with argon. No differences in the anodic parts of curves were found. Figure 2 shows the i-t curves again in the presence and the absence of N20.
~./#A
(a)
3.0
15 a
2£
2.0
10
1.0
5
b -1.0 -1.2 -1.4 - 1.6 -1.8 - 2 . 0
E/Vvs SCE
-1.6 -1.8 -2__0-2.2
E/v~ SCE
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
V~
Fig. 1. Influence of N 2 0 on the final current rise in various electrolytes. (a) 0.0625 M K2504, or 0.125 M KC1, or 0.125 M NaOH, or 0.1 M KCI+0.005 M Na2CO 3 saturated with N 2 0 (1) and Ar (2); (b) slightly acidic solution; 0.03 M NazSO 4 + 10- 4 N H2SO4, acetate buffer pH 4.5 saturated with N 2 0 (1) and Ar (2). Fig. 2. i-t curves for the N 2 0 (a) or Ar (b) saturated solution, 0.125 M in N(CHa)4OH. Applied potential - 2.0 V (SCE).
The use of tetraalkylammonium hydroxides helps to widen the negative range of the polarographic spectrum. Figure 3 shows that in the negative region oscillations grow more and more, and although no limiting current is formed, the average intensity at a chosen potential (Fig. 4) is proportional to the concentration of N 2 0 and may be used for analytical purposes with the help of a calibration
POLAROGRAPHIC ACTIVITY OF N2 O
I/ ppA
9 8 7
20
355
y.A
6
~
5
-2.5V
15 4
- 2.4V
3
1O
5
2
3
1
12
iii;
10
-21 V
: ov
,1 I
-1.9
-2.1
1
-2.3 -2.5 - 2.7 E//v l v$ SCE
I
2
3
4
5
6
7
8
9
I
10
Fig. 3. Influence of N20 concentration on the final current rise and oscillations measured at - 2.5 V v s . SCE (at the right hand of the figure). Successive curves from (1) 0, (2) 1.2, (3) 2.27, (4) 3.25, (5) 4.17, (6) 5.0, (7) 6.5, (8) 7.14, (9) 8.34 mM N20. Fig. 4. Calibration curves for N20 current at various potentials. Average current measured from oscillations slightly damped (damping "1", Radiometer PO4 polarograph).
curve, with a precision of _ 3%. It may be remarked that at such high negative potentials, in the absence of N20, the surface tension of mercury is very low and the drop time is up to ten times shorter (cf. the end of Results section in connection with drop time). Because of the influence of N20 .on the natural drop time, an artificial drop timer was used. It was proved that the oscillations have larger amplitude even in the case of equal t (cf. Fig. 5).
2C
a
15
1C
5
b
=
.17
-1
'
i
,
i
-1,9 -2.1
i
i
,
-2.3
i
,
i
-2.5 -2.7
E/V vs SCE
Ffg. 5. Influence of N20 on the final current rise (a) in comparison to the same solution, 0.125 M N(CHa)4OH, saturated with Ar. The drop duration controlled in both cases at 0.3 s. Damping, "1" as in Fig. 4. Note the large amplitude of oscillations in N20 saturated solution.
356
Z.P. ZAGORSKI, J. P. SUWALSKI
In slightly acidic solutions (5 x 10 -5 M H 2 S O 4 ) where the plateau due to Ha O+ reduction is visible, the phenomena in the negative region are basically similar, except that they appear on the background of the hydrogen wave (Fig. lb). The temperature coefficient of the N 2 0 current was measured between 5 and 50°C. To keep the N 2 0 concentration constant the solution was saturated with N20 at 50°C, cut off from the gas (the vessel was without the gas phase) and cooled down gradually. Cooling was interrupted from time to time and polarograms were recorded at a stepwise stabilized temperature (Fig. 6). Under these conditions the slope of the i/T straight line was 1.4 #A per °C. Taking the current at 25°C as 100%, the temperature coefficient is 2.2% per °C. The results of an investigation of the dependence of N20 current on the height of the mercury reservoir at various potentials are shown in Fig. 7. The current plotted is an average of the extensive oscillations not usually encountered in diffusion-controlled waves. ///~A 110 100 9C
.?;A
80 7C 60 50 4£
°
-
.
:~
: . . . . : ; : . .
-2.5V
.
.
-
"
- . . . . - : : : .
-2.4V
•
:
•
. - . . .
2O "
15
30
•
-2.2V
2C -
;
- ; . z . : 7 . : : : ; ;
B
-2.0V
lC
I;
31o
;o d.o 7.'o n~/eml/2
9'.0 6.o
Fig. 6. Influence of temperature on the N20 current, measured at -2.5 V. Supporting electrolyte, 0.125 M N(CH3)4OH. Fi~. 7. Dependence of N20 current measured at various poteniials, on the height of the mercury reservoir.
Similar results are obtained with NO3, benzoate, and adenosine. All three compounds have been chosen as belonging to very different categories, and all are considered to be polarographically inactive in simple supporting electrolytes. These compounds have high rate constants for reaction with electrons: 1.1 x 101°, 3.6 x 109 and 1.0 × 101° respectively. The polarograms in Fig. 8 show that all these compounds exhibit the same phenomenon as N20 i.e. earlier rise of current with increased oscillations. Sodium nitrite gives basically the same effect, e.xcept that it appears on the background of the Na ÷ wave (it proved impossible to prepare tetraethylammonium nitrite without Na ÷ ions).
357
POLAROGRAPHIC ACTIVITY OF N20 /~A
~A
(a)
(b)
~A
(c)
6(3 5C
15
4C IC
3O 2C
,J<,
1C -2.0 -22 -2.4 E/V vs SCE
-2.0
-2.0 -2.2 -2.4
-2.2
-2.4
E/VvsSCE
E/V vs SCE
Fig. 8. Influence of nitrates (a), benzoate (b) and adenosine (c). (a) Successive curves from above: 10.0, 8.75, 7.35, 5.84, 4.12, 2.19, 0.0 mM N O ~ ; (b) successive curves from above: 11.8, 6.75, 3.18, 1.2, 0.0 mM benzoate; (c) successive curves from above: 7.0, 5.26, 4.12, 1.56, 0.0 mM adenosine. Supporting electrolyte -0.125 M N(CH3)4OH.
Influence o f N20 on Cd 2 ÷ wave As the next approach to the problem of N 2 0 electrochemistry, its influence on the behaviour of polarographically active species was investigated. First of all the influence of N 2 0 and, for the sake of comparison, of N O 2 and NO~-, on the Cd 2+ wave was shown on polarograms. This experiment was performed by the multisweep technique on the oscillopolarograph. A slowly dropping ( t = 15 s) D M E was polarized with a potential from E o to E 1 (respectively the beginning and end of a sweep) many times during its life. The switch from E 1 to E 0 was of a millisecond duration. E o was typically - 0 . 5 V (SCE) and E1 was varied between - 1 . 4 and - 2 . 1 V (SCE). The sweep duration was typically 2 V c m - i . Figure 9 shows that, in the presence of N 2 0 and with a final potential more negative than - 1.8 V (SCE), two peaks appear for cadmium. The new wave is diminished again in the presence of O - scavengers like C2HsOH or SO 2-. Both scavengers have a high rate constant with O - but a low one with electrons. ~/.tA
///#A
(a)
(b)
~,~A
2O
2C
20
15
15
'15
-0.6
-1.0
-1.4
E/Vvs SCE
-0.6 -1.0 -1.4 ElM vs SCE
-0.6
(c)
-1.0
-1.4
-1.8
E/V vs SCE
Fig. 9. Influence of N 2 0 presence on the Cd 2+ peak in the multisweep d.c. oscillopolarography. (a) Saturated with Ar; (b) saturated with N20. The second peak of cadmium appears (c) only if the sweep of voltage reaches the zone of N 2 0 reduction.
358
Z.P. ZAGORSKI, J. P. SUWALSKI
The N 2 0 effect m a y be s i m u l a t e d again, this time b y H 2 0 2 , which p r o d u c e s O - at l o w e r negative potentials. If the limit o f the p o t e n t i a l sweep is m o r e negative t h a n - 1.1 V ( S C E ) a similar s e c o n d w a v e o f c a d m i u m a p p e a r s a n d m a y be supp r e s s e d b y C E H s O H o r S O 2 - . T h e wave is d i m i n i s h e d also b y N O 2 which acts here as a p u r e scavenger, b e c a u s e the p o t e n t i a l is n o t negative e n o u g h to r e a c h the r e d u c t i o n of this ion itself. T a b l e 1 s u m m a r i s e s the p h e n o m e n a . A similar s e c o n d p e a k m a y be o b s e r v e d with M n 2+ a n d Z n 2+ b u t n o t w i t h Tl +. TABLE 1 CONDITIONS OF THE 2nd CADMIUM PEAK FORMATION
No.
Systeminvestigated
pH
1st peak Cd -0.65 V
2nd peak Cd -0.73 V
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Cd 2+ +Ar Cd2+ +Y20 Cd 2+ +N20 Cd2++N20+H ÷ Cd2+ +N20 +CHaCH2OH Cd2+ +N20+SO~ Cd2+ +H202 Cd 2+ + H202 Cd2+ +H202+H + Cd2+ + H202+CH3CH2OH Cd2+ +H202+SO3zCd2+ +H202+NO~ , Cd2+ +H2Oz+NO~ Cd2 + + H202 + N20 +CH3CH2OH Cd 2+ +NO3 Cd 2+ +NO~Cd 2÷ +NO3 +CHaCH2OH
6.9-7.1 6.9-7.1 6.9-7.1 4.2-5 6.9-7.1 ~7.5 6.9-7.1 6.9-7.1 4.2-5.0 6.9-7.1 ~7.5 6.9-7.1 6.9-7.1 6.9-7.1 6.9-7.1 6.9-7.1 6.9-7.1
+ + + + + + + + + + + + + + + + +
+ + + +
15
16 17
+ -
Final potential of sweep/V -2.1 1.7 -2.1 -2.1 -2.1 -2.1 1.0 -
-
l
more negative than - 1.1 V (SCE) 1.8 -2.1 -2.1 1.7 -2.1 -2.1 -
-
Acidity changes around the drop It has been f o u n d t h a t the flow o f c u r r e n t in n e u t r a l s o l u t i o n s c o n t a i n i n g N 2 0 is a c c o m p a n i e d b y a n increase in a l k a l i n i t y in the vicinity o f the d r o p . This w a s d e t e c t e d b y the m e t h o d o f K e m u l a a n d G r a b o w s k i 8 w h i c h consists o f the a d d i t i o n of a n i n d i c a t o r to the solution. I n o u r case it was p h e n o l p h t h a l e i n . If the s o l u t i o n is s a t u r a t e d with N 2 0 , a l r e a d y a t a p o t e n t i a l o f - 1 . 7 V ( S C E ) the p i n k c o l o u r a p p e a r s a n d b e c o m e s m o r e a n d m o r e intense as the negative p o t e n t i a l increases. A l k a l i n i t y at the e l e c t r o d e - s o l u t i o n interface a p p e a r s a t m u c h higher p o t e n t i a l s o n l y ( T a b l e 2) in the case o f a r g o n - b u b b l e d s o l u t i o n a n d is a n o b v i o u s p h e n o m e n o n . T h e r e m a r k a b l e fact o f the increase o f surface t e n s i o n o f H g in the s o l u t i o n o f N 2 0 at high negative p o t e n t i a l s is s h o w n in Fig. 10. T h e effect is very m a r k e d a n d m a y be d e t e c t e d b y a low-sensitivity m e t h o d , s i m p l y the m e a s u r e m e n t o f the d r o p time o f a c o n v e n t i o n a l D M E . DISCUSSION T h e p r e s e n t e d results show t h a t N 2 0 in a q u e o u s s o l u t i o n c a n n o t be con-
POLAROGRAPHIC
ACTIVITY
359
OF N20
TABLE 2 ACIDITY
CHANGES
No.
Potential/V (SCE)
AROUND
THE DROP
Colour around DME N20
Ar
1
- 1.35 "'b
-
-
2
- 1.50 a'b
-
-
3 4
- 1.60 a'b - 1.70 "'b
-
-
5
- 1.8() "'b
1
-
6
- 1.90 "'b
1
-
7
- 2.00"'b
2
--
8 9
- - 2 . 1 0 "'b -- 2,20 b
3 3
---
10
-- 2,30 b
4
--
11
-- 2.40 b
4
1
12
-- 2,50 b
5
2
" F o r 0.12 M ( C H 3 ) 4 N B r . b F o r 0 . 1 2 5 M K 2 S O 4.
/s 3(
25 2O 15
a
10
d
÷ .6 ~0.2 -0.2 ' ' ' -0.6 ~ ' -1.0 ' ' -1.4 ~ '-1
18 ~ ,
. . . . .
2-2.6-3.0
I i I ,
E/V vs SCE F i g . 10. T h e n a t u r a l t i m e o f t h e m e r c u r y d r o p - l i f e i n N 2 0 0.125 M in N ( C H a ) 4 O H .
s a t u r a t e d (a) a n d A r (b) s a t u r a t e d solution,
sidered as polarographically inactive. It is reducible at the D M E from - 1 . 4 V (SCE) in practically all supporting electrolytes, but without formation of a limiting current even in N E t 4 O H solution. The detection of the start of the reduction depends on the sensitivity of measurement and the ease of distinguishing the capacity current. The N 2 0 current is highly reproducible and may serve for analytical purposes if the potential of the reference electrode is well stabilized. , The lack of current plateau is overcome by these means. Because of the rather high current intensities it is advisable to use reference electrodes of checked polarizability 9. The lack of a plateau indicates that neither diffusion nor another form of depolarizer transport controls the process of reduction, it being rather the concentration in the bulk of the solut~o,p which matters. The phenomenon does not
360
z.P. ZAG()RSKI,J. P. SUWALSKI
seem limited to N 2 0 only: we have observed it for other species which exhibit a high rate constant for the reaction with solvated electron (NO3, benzoate, adenosine). The most conclusive fact seems to be the easily observed, unusually wide oscillations of current. These also appear when an artificial drop timer is used (the drop is removed with every movement of the capillary I o) and are of much larger amplitude than in the case of a diffusion controlled electrode reaction. This conclusion is supported by the shape of the i - t curve of N 2 0 (or NO3) reduction (no tendency toward the steady-state current formation). From this behaviour we may conclude that the obvious first reaction N20+e- ~N2+O-
(1)
must be followed by some others. First of all the reaction O - + e- ~ 0 2 - (water)
(2)
may occur, which uses an additional electron supplied by the electrode. O - may, however, react with N 2 0 according to the equation O - + N 2 0 -~ N2 + O f
(3)
The peroxide radical reacts further at the electrode at the highly negative potentials applied thr.oughout the investigation. Our considerations do not distinguish at the moment between the dry and hydrated electron. From radiation chemistry it is known that the rate constant of reaction (2) with ea~ is 2.2 x 101° mo1-1 s -a. Our view is supported by the shape of the calibration curves (current vs. N 2 0 conc., Fig. 4). The higher the N 2 0 concentration, the smaller is the electrochemical contribution, because more of the O - has the opportunity to react with N 2 0 and O - . Both O - and O~ partly escape the electrode reaction. In other words, at all N 2 0 concentrations reaction (1) contributes to the current 100%, but at low concentrations the current is increased by reaction (2) and the reduction of 0 2 formed in reaction (3). With the increase of the N 2 0 concentration the charge per N2 O unit of concentration decreases, because the product of the reaction ( O - ) steals some N 2 0 molecules. At the same time some O - radicals escape the reduction, because with growing concentration the chance of them entering into reaction (3) increases. The second generation of products (O~) has an even loWer chance of taking electrons from the electrode. The possibilities of proving this mechanism of N 2 0 reduction by pure electrochemical methods are poor. In view of the fact that the shapes of the depolarization curves obtained with N20, NO~- or other electron scavengers differ greatly in comparison with the conventional polarographic curves, application of the usual tests for the electrode reaction mechanism is not correct. Thus, knowledge of the temperature coefficient (Fig. 6) does not permit any conclusions to be drawn and also the lack of dependence of current on the pressure of mercury (Fig. 7) is not conclusive because of unusual oscillations and the lack of steady state on the mercury drop. There is a similar uncertainty concerning the measurement of other electrochemical features--these were therefore necessarily supported by different investigations. We have concentrated on the consequences of the reduction of N 2 0 and the fate of O -, a very active ion-radical, well known from radiation chemistry. One
POLAROGRAPHIC ACTIVITY OF N20
361
has to stress that the spectrum of this species is very inconvenient for investigation. Its maximum lies in the far end of the common u.v. spectrum (2 = 240 nm) and the molar extinction coefficient is low 11 240 1 mol-1 cm-1. Prospects for the detection of O - near the electrode are poor or negligible in comparison with the relative ease of spectral detection of the hydrated electron, distinguished by a very high and convenient position of the maximum. The investigation of the consequences of N 2 0 reduction on the electrode was concentrated on the study of pH changes during N 2 0 reduction and on the influence of N 2 0 reduction products on the cadmium wave. It is evident from Fig. 9 that the first voltage sweep in the solution containing Cd 2÷ and N 2 0 results in one peak of Cd 2+ reduction. All subsequent sweeps during the same mercury drop show the presence of an additional peak. The latter can be caused only by the reduction of species formed at more negative potentials and adsorbed on the electrode. Close investigation shows that (a) The N2 O' reduction potential must be reached in order to develop the new peak (i.e. O- is essential). (b) Even higher negative potentials in the presence of Ar do not give rise to the peak (i.e. e-, Na, NH4, H may be excluded). (c) All O - or O H scavengers remove the peak (i.e. O- and not O H - is essential; the positive effect of non-ionic scavengers e.9. C2HsOH indicates that it is not alkalinity which causes the phenomenon). (d) The phenomenon occurs both in neutral and acidic solution (i.e. the acidic form of O - , namely OH, reacts similarly). (e) A similar phenomenon is observed if NzO is replaced by NO3, and in some conditions (neutral solution only) by H202 (i.e. the first product of reduction is also O - or the OH radical). ( f ) A similar phenomenon is observed with other divalent cations like Mn 2÷ but not with monovalent T1 ÷ (i.e. the phenomenon is limited to cations with at least 2 valency states). The indirect experimental evidence tends to make us express the view, that O - or O H (both hydrated) react with Cd 2÷, forming most probably Cd(OH)~ which is adsorbed at the electrode and is reduced at more negative (by 100 mV) potentials, beyond the Cd z ÷ peak. A quantitative interpretation of the results is not possible because of several reactions proceeding in parallel in the system. The investigation is continued along the line of combination of electrochemistry with radiation chemistry. SUMMARY
During an investigation of basic radiation chemical reactions of N 2 0 in aqueous solution, the electrochemistry of this compound was investigated. In the literature, the polarographic activity of N 2 0 has been discussed usually in connection with photo-ejected electrons, but this time reactions in the dark were also investigated. At sufficiently negative potentials N 2 0 reacts with an electron yielding O - which may further react with another electron, with N20, with itself or with specific additives present in the solution. The latter case was illustrated by the reaction with Cd 2÷ which shows two oscillopolarographic waves in the presence
362
Z.P. ZAGORSKI, J. P. SUWALSKI
of N20. similar currents as in the N20 case were shown by other solutes which do not give diffusion controlled waves, but have a high rate constant for the reaction with electrons, determined by radiation chemical methods. Nitrates, benzoates and adenosine were investigated by us. REFERENCES 1 2 3 4
5 6 7 8 9 10 11
D. C. Walker, Can. J. Chem., 44 (1966) 2226. G. C. Barker, A. W. Gardner and D. C. Sammon, J. Electrochem. Soc., 113 (1966) 1182. Z. P. Zag6rski, K. Sehested and S. O. Nielsen, J. Phys. Chem., 7-5 (1971) 3510. M. Anbar and P. Neta, A Compilation of Rate Constants for Reactions of e~q, H and OH with Inorganic and Organic Compounds in Aqueous Solution, Israel Atomic Energy Commission, Soreq Nuclear Research Center, 1966. Handbook of Chemistry and Physics. The Chemical Rubber Co., Cleveland, Ohio, 51st edn., 1970-71. D. C. Walker and G. A. Kenney, Electroanalytical Chemistry, A series of Advances. Vol. 5, Hydrated Electrons and Electrochemistry, Marcel Dekker, New York, 1971, p. 2. Z. P. Zag6rski and A. Blum, J. Electroanal. Chem., 41 (1973) 447. W. Kemula and Z. R. Grabowski, Rocz. Chem., 25 (1951) 350. Z. P. Zag6rski, Proc. 2nd Polaroor. Conor., Vol. 3, Oxford, 1960, p. 1132. Z. P. Zag6rski, Przem. Chem., 11/34 (1955) 693. A. K. Pikaev, Solvatirovanyi Elektron v Radyatsyonnoy Khimii, Nauka, Moscow, 1969.