Nov 23, 1998 - 38A, March 1999, pp. 280-285. Oxidation of Fe2. + and formation of hydrogen peroxide during electric discharges between a liquid electrolyte.
Indi an Journal of Chemistry Vol. 38A, March 1999, pp. 280-285
Oxidation of Fe 2+ and formation of hydrogen peroxide during electric discharges between a liquid electrolyte surface and an electrode above it 1M Piskarev Institute of Nuclear Physics, Moscow State Uni versity, Russia, 119899, Moscow, Worob 'ewy go ry Received 17 March 1998; revised 23 Novembe r 1998
Oxidation of Fe 2+ and formation of H20 2 have been in vesti gated in course of spark electric di scharges (I = 0.5 mA ) between a liqui d electrol yte surface and an electrode above it. Yields have been calculated by ass umin g th at acti ve radi als (OH, H), created in the di scharge, interact with solution components in surface laye r hav ing thi ckness 0.1 mm (with posi ti ve di scharge electrode) and thi ckness 0.05 mm (with negative di scharge electrode) . Additional active particles are created in the li quid by energised positi ve ions of water accelerated in cat hode drop of the di sc harge. Experimental data are reaso nab ly reproduced by calcul ation model. Some new features for chem ica l effe cts of electric discharge at smal l current compared to well known GDE at higher cu rrent have been noted.
field in discharge gap would be useful for dete rmining the mechanism of chemical effects of spark di scharges.
Experimental Spark discharge electrolyses were carried out in a glass cell plugged with a Teflon stopper (Fig. I ) with solution volume of 20 ml and gas volume at atmospheric pressure of 16 cm3 . Thickness of liqu id layer was 20 mm. The cell had ports for electrodes and inle t/outlet tubes for blowing-through a gas (velocity - 0.5 cm3/min). Both the electrodes were of platinum (0.3 mm dia .). The contact electrode was of 10 mm le ngth imme rsed to 10 mm depth , and enclosed in a sac of a filter-pape r box . The e lectrode was placed 2-3 mm above the liquid sur-
9
10
(j) 12
11
Elec tro lys is with an elec tric disc harge betwee n a liq uid e lec tro lyte surface and one e lectrode above it in the gas space at low pressure known as g low di sc harge e lectrol ys is (GDE) was studied in detaW" . Another kind of GD E where both the e lectrodes are imme rsed into the liquid is co ntact g low di sc harge e lectrol ys is (CG DE)" 11-1 1
GDE was studi ed by di sc harge c m rent of te ns to hun dred s mA . However, c he mical e ffec ts of co rona and spark di sc harges with c urrent even less th an I m A we re not reported. Th e sc reening of e lec trostati c fi e ld in electrode-l iquid gap for co rona and spark discharge mod es (current 0.1 - 0.5 mA ) is esse nti all y less th an that for arc and glow di sc ha rge modes (50 - 100 m A) and for corona and spark disch arge the stre ngth of e lec tric fie ld is essentiall y more than that for arc and g low modes. In fact, new characte ri sti cs of chem ica l effec ts we re noted for suc h processes e.g. H 1 0 } formation and cyanide deco mpo siti on22.23. Oxidation of Fe}+- and fonnati on of H 2 0 2 are well known probes and th e ir investi gati on at small curre nt (0 . 1 - 0 .5 m A) and hi gh e lectri c
--3---
1
---13
Fig. 1- Schematic representati on of experimeJ1l: I - Reacti on vessel; 2 - Te non stopper: 3 - Electrolytc liqu id; 4 - Gas inl ct tu be; 5 - Gas outl et tu be; 6 - Cuntact electrode; 7 - Di scharge electrode; 8 - Ballast resistor i 0 MQ ; 9 - Power supp ly (dc. V = 6 - 10 kV ); 10, II - Res istor chain divider, 10 - R=3 kQ, II - R=IOO kQ ; 12 - Osci llograph , 13 - Paper- filt er box.
NOTES
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Concentration of Fe2+. molll
Fig. 2·De pendcnce o f total yield of Fe2 + as fun ctio n qu antity o f elec· tri city, passed th rough the circuit, fo r positi ve polari ty of vo lt age on di scharge elec trode. C harge of 0.3 Coul o mb was coll ectcd durin g 10 min by the ave rage current value 0.5 mA o [Fe2+ 1 = 2. 5 x I 0.2 mol d m"( I ), 6.2 x I 0'] mo l d m''(2), I .Sx 10'] mol dm'\3) and 0.5 x 10'] mol dm·](4).
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face. The ambi ent te mperature ma inta ined was 20 Hi gh stre ngth of e lectri c fi e ld in liquid-e lectrode gap and sm all radiu s of di sc harge e lectrode (0.15 mm) cause corona and spark di scharges se lf-supporting. The di scharge curre nt was preset by ba ll ast res istor 5- 10 MQ connec ted in series with the di sc harge gap and by varying voltage ove r 6-10 kY. The d ischarge curre nt was recorded on milliammete r. The yi e ld at the di sc harge e lectrode was reported as its G- va lue w ith G defined as the rati o of the yie ld observed (m ol)( mol e lectron )·l and the faradaic yie ld in mo l per mo l e lectron. Fe 2+ ox idati on was studied usin g 0.0004 - 0.025 M FeS0 4 in 0.4 M H 2S04 and a ir as gas ph ase . F e 3 + fo rm a ti o n w as m o nitor e d s p ec tr o ph otometrica ll y at A = 300 nm and by titration for hi g h concentrati on of Fe)+ (re f .. 4). Yi e ld of HP 2 for spark di scharge co nditi o n was de te rmin ed with pure o xygen in gas space as a fun c ti o n o f p H ( 1.5 - 12.0) a ft er deozoni zati o n of the so luti on ( b~ bl o win g a ir fo r 10 min ) a nd by titrati on w ith 0.002 M KMn04 so luti o n in acid medium 22 .
Conditions of electric discharges (i).ln corona di scharge mode, average curre nt was 0.05-0. 1 mAo The c harge carri e rs were separate electro ns and ion ava la nc hes. Hi g h vo ltage ac ross the gap was - 9 kV ( refs 23, 24). (ii ) In .spark di sc harge mode, average curre nt was 0.5 mA o The c harge carri ers were the same, but w ith numero us ava lanc hes fo rmin g in the who le spark c han-
Fig. 3- De pendence of initi al yield (in un its of fa radaic yield) of reacti o n Fe 2+ ~ Fe·1+ o n [FeSO~ 1 (mol dm']). Experimental va lue is shown as crosses ( positi ve e lectrode) and squares (negati ve e lectrode). Bold line (I) re presents the results calcul ated fo r relati ve thi ckn ess of acti ve layer A = 5 x 10'] (0. 1 mm). Dotted lines are th e result s calcul ated for A = 0 .08 mm (2), 0.05 mm (3) and 0.02 mm (4).
ne l. When the spark c hanne l was fo rmed , pote ntia l di fference across the gap w as nearl y ze ro (as ball ast res isto r is of 10 MQ in thi s reg ime) and di sch arge di sco ntinued . Afte r di scharge was ove r, the pote ntial on e lec trode increased again up to the breakdown value and spark d isch arge re peated. Hi g h voltage across the gap was - 5 kV (refs 23, 24). (iii ) In arc di sc harge m ode, a ve rage curre nt was 2 mA or more. Under the arc di sc harge the spark cha nne l was hi g hl y ioni zed and in fact it was a curre nt-heated conductor. Di sc harge current was hi g h e nough to provide continuous ioni zati on in gas. The m ain c harge ca rrie rs in arc were positive ion s and e lectrons . The ex te rna l e lectrostati c fi e ld was just entirc ly sc reened. Hi g h vo ltage across th e gap was 1.2 kV (refs 23, 24) . O sc ill ogra ms o f di scharge curre nt we re imm ediate ly observed by means of res istor cha in vo ltage di vider. C ircuit for curre nt osc ill ograms observation was shown in Fi g. I (res isto rs 10, II and osc ill ograph 12) . For corona di sc harge the curre nt was pul sed w ith amplitude of 0.0 I A and durati on 0.05-0.1 /-ls. Pul ses of voltage on the surface o f liquid had amplitude 100 - 200 Y. F or spark di sc harge pul ses of current was about 0.1 - 0.2 A, d urati o n 0.3 - 0. 5 /-ls. Pul ses of vo ltage on th e surface of li quid had a mplitude - 5 kY. Fo r a rc di sc harge the c urre nt was continu ous w ith out pUl ses.
INDIAN J CHEM , SEC. A, MARCH 1999
282 10.00
In the present work, the dependence of H 20 2 yield on solution pH at discharge current of 0.5 mA, with positive and negative polarities of di scharge electrode and a gas space of pure oxygen was studied. Pure oxygen was used to avoid production of nitrogen compounds at a flow of 0 .5 cm} min-I. The results are presented in Fig. 4 .
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Fig. 4 -The dependence of HP2 yie ld (in unit s of faradaic yield) in water on acidity of solution. Solid line represents calcul ated va lues and croS$es represent ex pe rim ental data for positive potential on di scharge electrode whil e dotted line represents calcul ated values and squares experimental data fo r negati ve potential on di scharge electrode.
Results Oxidation Fe 2 + ~ Fe 3 + The dependence of initial yie ld G o(Fe3+) on [Fe 2+ was in vesti gated for spark di sc harge mode at 0.5 rnA with different polariti es of di scharge e lectrode. For the e lectrode-liquid distance of -2.5 mm, the voltage across the gap was 5 kY. The plot of the integ ral yield vs quantity of e lec tricity up to 0 .3 C was s ho wn in Fig . 2. The Go(Fe 3+) vs [Fe 2+ plots (Fig.3) showed that Go(Fe 3+) tended to a maximum with increase in [Fe 2 +] However, the maximum Go (ref. I 7) appeared independent of the polarity of the electrode. Howeve r, with pos itive polarity of di scharge electrode the max imum yield was obtai ned at lower [Fe 2+ ] (Fig. 3). To in vesti gate the role of volume gas space, if any, where active spec ies would form, yields of Fe3+ were measured for 0 .005 , 0.0005 ,0.00005 M Fe 2 + with negati ve polarity on the discharge electrode at 0.5 rnA, when (a) gas was connected with atmosphere via two holes (di a. 6 mm each) in stopper and (b) ho les were closed . In the first case where the loss of active particles from the gap space could take place, the yie lds decreased . Go(Fe1+) values were IA3; 0 .53 and 0.064 (with holes open) as against Go= 7 .3 ; 3A and 0.25 (with holes closed) for [Fe 2+] at 0 .005, 0 .0005 and 0.00005 /VI respectivel y.
Hydrogen peroxide formation Hydrogen peroxide formation under the same experimental conditions as Fe2+ oxidation was studied earlier22.
Comparison with GDE data Oxidation of Fe2+ from FeS04 in OA M H 2S04 by positive polarity of di scharge e lectrode for glow di sc harge mode (at 100 rnA) was studied by Hickling et a/ U .5. An appreciable yield of Fe 3+ was detected when the discharge e lectrode was positive but a very small yield was obtained when the polarity was negative. Further, yield of Fe 3+ by GDE was found to be independent of current of di scharge, gas pressure and gas phase composition. The dependence of Go(Fe 3+) on [Fe 2+ ] is simil ar to th at for the present experiment ; the max imum yield was less and equal to 7.1 (from 0 .025 M Fe2+) compared to 17 ± I found in the present study. The yield of HP2 at GDE 1,2 was about Go - 1.6 - 1.9 for neutral solution, at CGDEI 8, 19 was Go - 3.8 . Under spark discharge mode the reported value for neutral solution was about Go - 2.2 (ref.22) . Mechanism of reactions It appears that charged partic les in e lectric di sc harges through collision with mol ecules in the gas lead to several rad icals and active spec ies like OH, H, 0 , 3 in the gas phase. Further, with positive polarity on di sc harge e lectrode (H 20 gas) the main charge carriers acce lerated though - 400 V, the cathode fall near the liquid would form Hand OH radi cals through co llisi on w ith liquid H 20 mol ecul es. Yi eld of exci ted water mol ecul es breaking up to H and OH radi cals is equal to 2 molecul es/ I00 e V (ref.25). Thus for cathode drop of potential -400 V about 8 radica ls each of OH and H would form. These radicals or active spec ies will add to active particles, created in the who le volume of gas under di scharge action in the case of positive voltage polarity on di sc harge electrode. The motion of charged particle in co rona and spark pulse di scharges (s mall current de nsity and absence of continuou s spark channe l, when di scharge is pulsed) is si milar to th at with an initial energy value equal to difference of potentials to th e gas-liquid gap (-10k V). Mean free path of e lectron with energy - 10 ke V on a ir at atmospheric pressure is about I mm (compared to 2 - 3 mm for discharge gap in the experiment). The energetic electron will lose its energy in colli sions with gas
°
283
NOTES
Table 1- Reaction between neutral acti ve parti cles and Fe2+ ions. Reacti ons between charged particles see in ref. (27). The compl ete set of reactions, taken into acco unt, is 64.
S.No.
;,
I. 2. 3. 4. 5. 6. 7. 8. 9. 10. II . 12. 13. 14. 15. 16. 17. IS. 19. 20. 2 1. 22. 23.
Constant , k by 293 OK dm 3 mol-' sol 3 x lO'o 3 x l07 4. 5x 107 6.0 x I07 1.5 x l O'" 3 x lO' 5 x IO') 5.3x 10') 2.0x 10'0 l Ax 10" 1.8 x IO" 1,2·x IO " l ·x IO 'o i x lO" 1.5x 10" 56 2.S x lO'o 7.0 x lO" 4 x l07 10 x lO" S.5 x l O' 2 x I0'0 (l iqui d) 1.6x 107 (gas) l Ax 10"
Reacti on OH + ° ----7 ° 2 + H OH + 0 3 ----7 H0 2 + 0 2 OH + HP 2 ----7 H0 2 + Hp H + HP 2----7 OH + Hp 0.1 + H ----7 0H +02 0 3 + H0 2----7 OH + 202 ° + H0 2 ----7 OH + 0 2 OH + OH ----7 HP 2 H + H0 2----7 HP 2 OH +OH ----7 Hp +O H + H0 2 ----7 H2 + 0 2 OH- + ° ----7 H0 2 + e.,,! 0 - + I-I p + ----7 Hp + OH Fe2• + OH ----7 Fe.!+ + OHFe 2+ + HO? ----7 Fe 3+ + HO?2Fe2+ + H20 2 ----72 Fe3+ + -20HOH + H ----7 Hp OH + 1-1 0 1 ----7 0 2 + Hp OH + H2 ----7 H + Hp H + H ----7 H2 HOI + H0 2 ----7 HP 2 + 0 2 H + 0 2 ----7 H0 2 1-102- + H,o+ ----7 HP 2 + Hp
Reference [29 ] [29 ] [27 , 28] [27 , 28] [29 ] [29 ] [29] [28] [28J [29] [29] [2S] [2S] [28] [28] [28] [27] [27] [27] [27] [27] [27] [27]
... 0 J- and Fe2+ by Bugaenko el alY , conside rin g add iti onal reac tion s inTable I and th e fo llowing equilibri a
molecules . The probability of c reati on of acti ve particles de pends s lightly upon e nergy of elec tron for e nergy va lue apprec iably abo ve the thres hold of its appearin g. The refore th e radi o lys is data of air and wate r vapour26 may be used fo r calcul ation of acti ve particles y ie ld. The compos iti on of gas phase is oxygen, nitrogen and water vapour ( IS To rr at 20 °C). M ain products o f wate r vapour radi olys is in air a re OH radi ca ls, and H ato ms and molecul es o f ozone, OJ' The absorbti on of e nergy by molecul es o f a substance is proportional to its partial pressure. Thus, for the vo ltage applied (S kV) the e nergy absorbed by water vapour was 116 e V (atmos ph eri c pressure was 770 Torr). From th e data of the y ie ld s of pure wate r va pour radi olys is [I .OS; 6.22 and 7 .38 ( 1/ 100 c V ) for 0, OH a nd H res pecti vely 16 a nd th e va lue of abso rbed e nergy ( 116 e V) y ie ld s of acti ve parti c les for the passage o f one e lectron w ith e nergy S ke V were ca lcul ated . Th e total y ield s a re c lose ly s imilar: G,(O) = 1.2S; G,(OH ) = 7. 3 and G,(H ) = S.44.
[H02-] + [OH-] + [0 -1 + [e",-] + [0 2- J+ [0 3- ]- [H P+ ]= constant
Ca/cu/atioll of Fe 3 + yield Fo ll owin g the proposed reacti on sc he me between OH, H, 0 , H 2 , 0 " H0 1, H 2 0 }, H0 2-, OH-, 0 -, HJO+, eal)' 0 1-'
It was proposed furth e r th at th e reacti ons between primary ac ti ve particles occur in th e gas ph ase, but the ir inte raction s with Fe 2+ occur onl y in the thin surface layer. As Fe 2+ consumed in the surface laye r is suppleme nted from the bulk , [Fe 2+] in th e w hole volume re mai ns
°
HP 2 + Hp Hp +Hp
~
Hp+ + H0 2 -
~ Hp + +OH
e"q' + H 20 ~ H0 2-H HP 2 + OH- ~ H0 2- + Hp 0 - + Hp ~ OH- + OH (comprisin g 64 reactions) the yie ld o f Fe J+ for th e.reaction time o f ISO sec and for [Fe 2 +] = Sx· 10-1, I. S·x I OJ , Sx · 1O-J and 2.Sx· 10-2 mol dnY\ was calcul ated. T he system o f 16 equ ati o ns , desc ribin g formati on and co nsumpti on of the acti ve pa rticles was so lved by Run geKutta method to dete rmin e [FeJ+] Tn the syste m of equ ation s the c harge conservati on was accounted as :
INDI AN J CHEM, SEC. A, MARCH 1999
2~4
unifonn, i.e. in the course of reaction only part of ions in treated solution layer can participate, which is equal to A[Fe 2+ ] , where A is ratio the thickn ess of surface layer to full thickness of the liquid in the cell. Production of Fe3+ ion s may thus be written as:
J IF c 3 + 1 = A . IF e 2 + J
I' 10
H J . k 14
+ ,\ . I r e 2 + J . I H 02 1 . k 15
I
+1 /2·i\ ·IFe2+)·IH2021·k I 6
... ( I)
An analogous equation can be written for the formation and consumption of H?O? and other particles in the gas and liquid phase ( fro~ 16 equations) . To calculate hydrogen peroxide production in liquid phase it was proposed that transfer of hydrogen perox ide from gas to liquid phase took place in the layer with thickness ,1X. Value of ,1X was taken equal to thickness of active layer, i.e. ,1X=A part of liquid volume (20 ml) which corre·· sponds to part A,= 1.25 A of gas volume ( 16 cm 3). The express ion for hydrogen peroxide accumulation in water may be written as: 0 d I H2 2 \I' I = I H2 0 2 I . r\ 1 - A . I H 20 2 \V I . I dI - A ·IH202w)·IH1 · k4
°
ness of liquid 20 mm). The dotted bold line in Fig.3 is the result of calculation for negative polarity and thickness of active layer was 0.05 mm . It is seen that th e computation fairly satisfactorily reproduces the characteristic of the process. The maximum calculated yield of Fe 3+, Go - 15, and it was obtained for [Fe 2 + = 0.25x·IO- ' mol dm-3 by sol id line. Experimental yield is 2 Go = 17 + - 1 for the same concentration of Fe +. For lower 2 3 concentrations of [Fe + the yield of Fe + decreased as active radicals would interact among themselves before they could interact with Fe 2+. In the case of GDE for positive polarity on discharge electrode, the potential drop is virtually near the cathode fall, where active particles would generate. In the gas phase active particles cannot generate as di scharge channel is a current heated conductor. For the case of GDE with negative polarity on discharge electrode the active particles cannot generate at all. According to the proposed model of calculation, yield of active particles for positive polarity (because of cathode drop) has been equal to 8 OH radical s and 8 H atoms. It wi ll give yie ld of Fe 3+ formation , G, about 7.5 , and no yield for negative polarity. These results are in accordance with GDE data.
HI' k 3
... (2)
It was supposed that intermixing of liqu id is the result of applying an electrostatic field to liquid (by means of the second electrode immersed into liquid) and passing pulsed electric >current through liquid. With continuous current of 0.1 - 0.5 mA, intermixing during the reaction time (up to ] 0 min) is impossible, the electrostatic field acting in liquid being small : 0.1 - 1 Vfcm . During spark di scharge, current pulsed almost to full potential of 5 kV was applied to liquid sUlface and the large electric field (- 2.5 kVlcm) caused intermixing of the liquid. From calculation it was seen that the main contribution to Fe z+ oxidation was determined by OH and H radicals, and for H?OZ production the main contribution was determined by OH, H, HOz and OH- and HO?-. The relative thickness of active layer A was varied from Ix 10-3 to 5xlO-3 (i .e. thickness of active layer varied from 0.02 mm to 0.1 mm respectively ; thickness of total liquid layer was 20 mm) to obtain the agreement between results of experiment and those of calculation . In Fig. 3 the results of calculation were plotted by solid line for positive polarity (cathode drop was taken into account) and value of A = 5x· 10- 3 (this value corresponds to thickness of active layer 0.1 mm and full thick-
Ca lculation of H2 0 2 yield The calculated yield of H 20 zw (hydrogen peroxide in water solution) for a time period of I hour as function of pH (Fig. 4) is in accordance with experi mental data for pH 2 - 7 with positive discharge electrode. The model also predicts decrease in yield for negative polarity22. The calculated yield of H?O? at pH = 12 or higher is negligible in accordance wi-th -the experimental data due to interactions of active particles with OH-. The calculated increase in H?O? yield at pH = 8 - lOi s much hi gher than that fOUild -by experiments, indicating that not all particles (e.g. excited 0 2 molecules) were taken into account. It was observed that yield of HOO? for corona discharge is less compared to that for spa-rk -discharge . The field for corona discharge, being more than that for spark, would cause the energy of active particles in discharge increase. For high energy active particles, for example, in conditions of upper atmosphere 29 , instead of reactions (8, 9) reactions (10, II) take place. In the present case the replacement of reactions (8,9) by reactions ( 10, II ) would occur on varying discharge mode spark ----) corona. By changing the discharge mode, spark ----) corona, the energy of active particles is increased, owing to increase in electric field and by analogy with processes happening in upper atmospherez9 ; instead of reactions
NOTES
(8,9) leading to hydrogen peroxide formation, reactions ( 10, II) would take place which lead to no HP2production . It was calculated in the case of H2 0 2 accumulation, the variation of discharge mode, spark ~ corona, and substitution of reactions (8 , 9) by (l0, II ) would cause abrupt decrease in HP2yield (approximately 105 times) . Experimentally22 the abrupt decrease in HP2 yield by vari ation of discharge mode, spark ~ corona, was observed (from G = 2.2 to G < 0.04) indicating a significant participation of reactions (10, II) in corona discharges. To conclude an electric discharge between liquid surface and an electrode above it for corona and spark modes have essentially the followin g characteri stics. (i) The gas phase composition has significant influence on the chemical products formed in solution.(ii) Polarity of voltage across di scharge gap has influence on yield, but appreciable yield was observed for both polarities of voltage. (iii) The initiation of chemical reacti on in the liquid seems to be connected with the existence of electrostatic fie ld in gap between electrode and liquid surface. Active radicals were originated in gas phase. This radicals may then undergo competitive reactions with themselves (in gas phase) and with any substance in the thin sUli'ace layer of liquid hav ing thickness of 0.05 - 0. 1 mm . Additional active particle production can occur in the cathode drop of potential near liquid surface (i n the case of positi ve potential on di scharge electrode). (iv) Intermixin g of liquid was realized by means of diffusion under action unidirected pu lsed electric field. The mechani sm proposed bears a close resemblance to mechani sm advanced for interpret ing non-faradai c chemical effects of anodic CGDE 'x. Electric di scharges in variolls modes can initiate reactions on gas/liqu id boundary, where electrodes are absent. For initiating such reactions vari ous sources of ioni zation may also be used and such processes was named as "e lec trode less -::Iectrochemica l reacti ons":1O
285
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24 25 26 27 28 29 30
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