Redox Chemistry of Hyponitrite: Part Ill-Oxidation by Hexacyanoferrateilll) in Aqueous Alkaline Solution

Indian Journal of Chemistry Vol. 28A, April 1989, pp. 280-283

Redox chemistry of hyponitrite: Part ill-oxidation by hexacyanoferrateilll) in aqueous alkaline solution

M R Goyal, Pankaj Bhatnagar, R K Mittal & Y K Gupta" Department of Chemistry, University of Rajasthan, Jaipur 302 004 Received 22 April 1988; revised 22 June 1988; accepted 8 July 1988

Oxidation of hyponitrite by hexacyanoferrate(ill) in alkaline solution occurs by two paths resulting in a stoichiometry of 1.21:1(oxidant.substrate), The finalproducts are N20,N2and NO; ,and the intermediates .'" are N02- and peroxonitrite. Rate has first order dependence in [Fe(CN)i - J and [N20~- J and no dependence on [OH - J. The second order rate constant isfound to be (0.87 ± 0.025) dm-mol" at 35° and 1=0.515 mol dm - 3. Rate significantlyincreases in the presence of alkali metal cations.

Studies on oxidation of hyponitrite by thalliumlllf)' strength as in the reaction mixture before measuring and ceriumll'Vj' in aqueous acidic solutions have in- absorbance. Initial rates were obtained by the plane directly indicated the formation of intermediates mirror method" and the results were reproducible H2N203 and HN02 or its isomer, though the final within ±3%. identified products are nitrate", N 0 and N . Oxida- 2 2 Results tion" by chloramine-T (CAT) provides no informa- Stoichiometry and products tion about the intermediates and the only product is The stoichiometry was determined using excess of found to be nitrate. These studies reveal that the inter- hexacyanoferratetlfl). Excess hypo nitrite could not mediates appear to be unstable in acidic solutions and be employed because of interference in its determina- hence an investigation in alkaline solution seemed to tion by hexacyanoferrate(II). For each mole ofhypon- be of interest. This prompted us to undertake the title itrite, 1.21 ± 0.01 mol of hexacyanoferrate(ill) were investigation. required when [Fe(CN)tl and [N20~-l were varied respectively from (1 to 6) x 10 - 3mol dm - 3and (0.5 to Materials and Methods 3) x 1O-3moldm-3inO.l moldm-3NaOH.Thepro- Hyponitrite was prepared and estimated as de- ducts were found to be N , N 0 and peroxonitrite. scribed earlier 'v-". Solution of potassium hexacyano- 2 2 Gaseous nitrogen and nitrous oxide were identified ferrate(ill) (BDH, AR) was prepared in doubly dis- and estimated as described earlier! .The relevant data tilled water whenever required and standardized co- are given in Table 1. Reaction mixtures were pale yel- lorimetrically' and iodometrically", All other rea- low at the end and gave a maximum at 302 nm corre- gents were either BDH (AR) or S Merck (GR) grade. sponding to peroxonitrite". These solutions yield All solutions were prepared in doubly distilled water, the second distillation being from tetraoxomangan- qualitative tests for nitrate. Formation of nitrogen, and such low consumption ate (MnOi). of one-electron oxidant in stoichiometric experi- ments can be explained only through the reduction of Kinetic procedure A solution containing potassium hexacyanoferr- hypo nitrite by some intermediate. No decomposition ateilll), sodium hydroxide and other ingredients was of hyponitrite would occur in the pH range employed 10.11 • On the basis of the previous reactions 1.2 and equilibrated at (35 ± 0.1 tc. A solution of known the intermediates reported therein, the stoichiometry quantity ofNa2N202 was also equilibrated at this tem- of the reaction studied presently can be accounted for perature for 5-7 min to avoid decomposition 7• The reaction was initiated by mixing the required amounts by Eqs (1) to (4). of the two solutions. Aliquots (5 ml each) were with- 2Fe(CN)~- + N20~- + H20-

drawn at intervals of 1or 2 min and absorbance was 2Fe(CN)~- + N20~- + 2H+ .00 (1) measured at 420 nm on an EC digital spectropho- N20~- +H+- tometer (E = 1070 ± 10 em - I). Solutions more con- centrated than 3.5 x 10-4 mol drn " were diluted ap- tN20+ tH20+ tN20~-(N02) 00. (2) propriately with cold NaOH solution of the same tN20~- +2H+ -N2 + tN20~-(N03)+ H20 ... (3) 280 GOYAL et al: OXIDATION OF HYPONITRITE BY HEXACYANOFERRATE(III)

Table I-Stoichiometry and Gaseous Products of Reaction between Fe(CN)~ - and N20~ - in Alkaline Solutions [NaOH] = 1.0 mol dm - 3; 35°; volume of reaction mixture = 100 ml 102[Fe(CN~-] W[N20~-] 1Q2[Fe(CN~-1nal MoleofFe(CN~- Vol of (mol dm=') (mol dm='] (mol dm - 3) reacting with one (N2 + N20Xml)* moleofNp~- 1.00 0.50 0.395 1.21 10.3(10.4) 15.9(16.0) 2.00 1.00 0.810 1.19 10.1(10.4) 15.8( 16.0) 2.10 1.00 0.890 1.21 10.4(10.4) 16.0(16.0) 2.30 1.50 0.485 1.21 15.7(15.6) 23.7(24.0) 3.10 1.50 1.26 1.23 14.8(15.6) 23.2(24.0) 4.00 2.00 1.60 1.20 21.0(20.8) 31.6(32.0) 6.00 3.00 2.34 1.22 11.8(12.5) 18.0(19.2) * Values in parentheses are the volumes on the basis of 1.21: 1 stoichiometry

Table 2-Initial Rates in Oxidation of NPi with Fe(CN)t in 2Fe(CN)~- + tN20~- + H20-2Fe(CN)~- Alkaline Solutions at 35· + tN 2026-(NO-)+3 2H+ ... (4) W[Fe(CN)~-) 103[NP2I W(vo) ~ [mol dm='] (moldm-3) (moldm-3s-l) (rnol "! drrr's" '] Reactions (1- 3) would conform to the stoichiometry ([Fe(CN)~-]/[N20~-])of 1:1 and reactions (1),(2)and Ionic strength=0.515 mol dm-3; [NaOH]=0.5 mol dm ? (4) indicate the stoichiometry of 4:1. Since the stoichi- 0.2 1.0 1.7 0.85 ometry is 1.21: 1, it can be surmised that 93% reaction 0.3 1.0 2.7 0.90 occurs by the first path and 7% by the second path. 0.5 1.0 4.6 0.92 Based on this, the expected volumes of N, and N20 OB 1~ 6B OB5 have been calculated and the values are given in Table 1.0 1.0 S.7 0.87 1. 1.2 1.0 9.6 0.80 Reaction mixtures containing excess N20~- if 1.5 1.0 13 0.S7 acidified, give back exactly original amount of I.S 1.0 17 0.85 Fe(CN)~ - .This shows that there is some oxidant (not 1.0 0.2 LS 0.90 nitrate or nitrite) in the system which oxidizes 1.0 0.5 4.1 0.82 Fe(CN)~ - in acid medium. This leaves peroxonitrite 1.0 1.0 8.6 0.86 as the active oxidant species. The absorption spectra 1.0 1.5 13 0.87 of several reaction mixtures containing excess of 1.0 2.0 18 0.90 N20~- in 1.0moldm -3 NaOHexhibited peaks at 302 1.0 2.5 23 0.91 nm, which is characteristic of peroxonitrite (E, 1670 1.0 3.0 26 0.S7 em - 1). Thus, the product appears to be peroxonitrite 1.0 4.0 36 0.90 which finally gives nitrate. Av=(0.S7 ±0.025) Rate dependence on reactants Ionic strength = 0.118 mQIdm "; [NaOH]=O.l mol dm? The results of variation of [Fe(CN)~ -], [N20~ -] and 0.35 1.0 0.84 0.24 [NaOH] are given in Table 2. The plot of rate versus 0.70 1.0 1.8 0.26 [Fe(CN)~-] or [N20~-] is linear passing through the 1.05 1.0 2.6 0.25 origin, indicating the reaction order to be unity in each 1.40 1.0 3.5 0.25 of these reactants. The rate is independent of [OH -]. 1.75 1.0 4.4 0.25 In all the cases the ionic strength was adjusted with O.SO 0.5 1.1 0.27 NaN03 except in stoichiometric runs. O.SO 1.0 2.2 0.27 0.80 1.5 2.8 0.23 O.SO 2.0 4.0 0.25 Effect of varying ionic strength 0.80 2.5 4.8 0.24 NaN03, NaCl, Na2S04, KCI and K2S04 were em- Av=(0.25 ±0.01) ployed to vary the ionic strength. The rate increases with increase in ionic strength but there is no correla- 3 3 Ionic strength = 2.01 mol dm- ;[NaOH]=0.1 to 2.0 mol dm- ti~m between the rate and ionic strength. Figure 1 1.0 1.0 29±0.1 (2.9±0.01) gives a plot of rate versus [Na +] or [K+].The results seem to be in accord with Eq. (5),

281 INDIAN J CHEM, SEC A, APRIL 1989

trites. However, other workers could not obtain the • NaCI(25 't) ~_fomiI4,15,though the products obtained could arise 36·0 • KCI( 25°) by the decomposition of ~-form. Therefore, it is likely G NaCI(35°) that in the oxidation of hypo nitrite by TI{ill)1,Ce{IV)Z • Na2S04 (35°) and Fe{CN)~ -, the immediate oxidation product A KCI (35·) (H2N203 or N20~ -) is a ~-form 'Which is very un- 28'0 X K2S04(35·) stable. , Increase in rate by the presence of alkali metal ..•.- • NaCI(45·) '6 101 KCI (3~) cations in systems containing anionic species, is very '0 common on account of the association of the metal 02 E ion with the anion. It is obvious from the slopes of line- ~ ar plot of Fig. 1that the association in the case ofK +is "'S; large. T'1e reported" association constants for K +

12·0 and Na + are 31 mol r ldm' and 7 mol r ldm'' respectively at 25° and I= 0.1 mol dm - 3.A point to be noted is that no limiting rates are obtained even at 2 mol dm - 3concentration of these metal ions. Similarly in oxidation of hypo phosphite 17 and iodide" by hexacyanoferrate{III), no limiting rates are obtained for large o 0·50 ',00 1·50 2·00 concentrations of alkali metal ions. From this, it ap- [NQ+]or[K1/moi dm-3 pears that the role played by alkali metal ions is not confined to mere association with the anions; perhaps · I-Effect of alkali metal cations on initial rate (IR) F19. 10-3 I they are part of the transition state 19,20in an independ- ([ 2 P2]= 1.0 x 1O-3moldm-3;[K Fe(CN)6] = 1.0 x mo Na N 3 ent way. Na + and K + have the same role in the ex- dm-3;and[OH-]= 0.1 moldm-3) change reaction?' ofFe{ CN)~- and Fe{CN)~ -, but the authors do not consider any alkali metal ion catalysis. The mechanism of this reaction" is somewhat differ- ... (5) ent being outer sphere type and also seems to be mon- itored by trace metal ions since the results are irre- where M + is Na + or K +.The intercepts are the same producible in the absence of EDTA. for Na + and K + at any temperature. The values of ~ There is one more way to explain how limiting rates are found to be 0.080, 0.20 and 0.46 mol +drn's -I at are not obtained at large Na + or K +.The association 25°,35° and 45° respectively. ~ values for Na + and constants given above are for Fe(CN)63- whereas an K+ are 0.41, 0.81; 0.89,2.2; and 1.4,2.8 at 25°,35° association with N20~- is also possible and it is this and 45° respectively. From these values E, for kl was which might be taking place at higher concentrations. found to be 71 kJmol- 1.Being composite in nature, Thus all the following three reactions are likely to oc- E, of the ~ path was not calculated. cur. Fe{CN)~- +N20~-; MFe{CN)~- +N20~-; and

Fe{CN)3- +MN20i· Discussion The rate of reaction is independent of [OH -j.The hyponitrite is present as N20~- since the acid disso- 7 11 References elation" ,12constants are -1O- and -10- . 1 Goyal M R, Mittal R K & Gupta Y K, Indian J Chern, 27 A The present reaction does not provide any evid- (1988) 487. ence for the intermediate trioxodinitrate, Nz05-, 2 Goyal M R, Bhatnagar Pankaj, Mittal R K & Gupta Y K, Indi- though the earlier studies 1,2gave ample evidence. The an] Chern, (1988). 3 Hughes M N & Stedman G, J chem SociA), (1963) 4230. presence of peroxonitrite (ONOO-) h~ ~e:n r: 4 GoyalM R, MinaI R K & Gupta Y K, Indian J Chern, 27A tinned spectroptiotometricelly. Peroxonitritets llkely to be obtained from isostructural peroxohyponitrite (1988)626. 5 KolthoffI M, MeehanE J, Tsao M S & Choi Q M,J phys Chern, (NOO-), an isomer of NOi. Our earlier reports'v 66 (1962) 1233. have also shown that trioxodinitrate is not formed as 6 KolthoffI M & Belcher R, Volumetric analysis, (Interscience, an intermediate in the oxidation of hyponitrite; but New York) 1957, p 344. the product does have the composition Na2N203. Al- 7 Hughes M N, Quart Rev, 22 (1968) 1. though there is no evidence, it could well have the 8 Latshaw M, J Arn chem Soc; 47 (1925) 793. structure (ON.NOO F- in alkaline solution. Addison 19 HughesMN &NicklinHG'/chemSoc(AJ, (1968)450. and coworkers 13have reported a- and ~-oxyhyponi- o Buchholz] R & Powell R E,J Arn chem Soc 87 (1965) 2350' 85(1963)509. " 282 GOYAL et al.:OXIDATION OF HYPONITRITE BY HEXACYANOFERRATE(III)

11 Hughes M N & Stedman G,] chern SocIA), (1963) 1239. 17 Mohan D, Kumar V & Gupta Y K,] chern Sac Dalton Trans, 12 Polydoropoulos C N & Pipinis M, Zeit phys Chern, 40 ( 1964) (1975) 1737. 322. 18 Abdul Majid Y & HowlettK E,] chern Soc(A), (1968) 679. 13 Addison C C, Gamlen G A & Thompson R, ] chern Sac (A), 19 Taube H, Meyers H & Rich R L,] Am chern Sac, 75 (1953) (1952) 346. 4119. 14 Hughes M N & Nicklin H G, Chern Commun, (1969) 80. 20 Taube H, ] Am chern Sac, 77 (1955) 4481. 15 Vosper AJ,] chern Sac (A), (1968)2403. 21 CampionRJ,DeckCF,King(Jr)P& WahIAC, lnorg Chern, 6 16 ChlebekR W & Lister M W, Can] Chern, 44 (1966) 437. (1967) 672.

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