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Kinetics and Mechanism of Oxidation of Hydrazinium Ion with Peroxomonophosphoric Acid in Acid Perchlorate Solutions and Role of Trace Iodide Ions T. PETER AMALA DHAS, D. K. MISHRA, R. K. MITTAL, and Y. K. GUPTA Department of Chemistry, University of Rajasthan, Jaipur, India

Abstract The reaction of peroxomonophosphoric acid and hydrazinium ion in acid perchlorate solutions occurs as per stoichiometry (i), and the rate law (ii) at large [NzH~'], (i) (ii)

2H3P05

+ N2H4

2H3PO4

+ NP + 2Hz0

-d[HaPOs]/dt = [H~PO~]T[H+] (ki + kz[NzHs+]/(K&+ [H']))

where K&is the first acid dissociation constant of H3P05 and k l and kz are rate constants found s-' and 5.0 x lo-' M-'s-', respectively, at 35".The reaction is greatly catato be 2.6 x lyzed by iodide ions. The mechanism involves a redox cycle 1-/12and the rate is independent of [N2H5'] in the presence of iodide ions. K&was found to be 0.55 M-' and independent of temperature.

Introduction The title reaction studied [l]in the past, reports limited reproducibility in the presence of trace amounts of iron(II1) and edta, but without ascribing any mechanistic role of the metal ion to reproducibility. In addition, the roles of iron(II1) and edta in any inorganic system are expected to be opposite. Thus, a re-investigation of the reaction seemed necessary. Another interest in the present work arose out of the reported [l]equilibrium between the normal and active forms of PMPA as in the case of hypophosphorus and phosphorus acids [2]. Only a few inorganic redox systems [3-61 involving peroxomonophosphoric acid (PMPA) have been studied in the past. Since trace amounts iodide catalyze the oxidation of hydrazine, the reaction was also studied in the presence of small amounts of iodide and iodine.

Experimental Materials

Solutions of PMPA were prepared each day whenever required by the hydrolysis of peroxodiphosphate in 0.5 M HC1O4 at 45" for about 1.5 h. It was standardized iodometrically. Solutions of lithium perchlorate were prepared by neutralizing 70%HC1O4(E. Merck) with BDH AnalaR lithium carbonate International Journal of Chemical Kinetics, Vol. 23, 203-213 (1991) 0 1991 John Wiley & Sons, Inc. CCC 0538-8066/91/030203-11$04.00

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DHAS ET AL.

to pH 6-8. Solutions of hydrazinium ion were prepared by dissolving appropriate quantities of BDH AnalaR hydrazinium sulphate in water and standardized by the bromate method [7] using indigo carmine as indicator. All other chemicals were either BDH AnalaR or E. Merck GR quality and were used as such. All solutions were prepared in double distilled water, and second distillation being from potassium tetraoxomanganate(VI1). All glass vessels were Corning make.

Kinetic Procedure Solutions of PMPA in aqueous HClO, in one flask and mixtures of hydrazinium sulphate, HClO, and other required chemicals in the other flask were separately temperature equilibrated in a thermostat a t the specified temperature. The reaction was initiated by adding the required quantity of PMPA to the other flask containing hydrazinium sulphate and other constituents, if any. Progress of the reaction was followed by decrease of [PMPA] by carrying out iodometric analysis in ice cold water. Aliquots of 5 ml were withdrawn after intervals of 10 or 15 min and added to ice cold 10% KI solution (10 ml). Although further hydrolysis of PMPA to H 2 0 2did not occur under the specified conditions, ice cold solutions were employed to avoid liberation of iodine from H202,if formed. Lithium perchlorate was employed to adjust the ionic strength. Since the reaction in presence of iodide ions was fast and PMPA had to be determined after each minute, the kinetics were followed in a different way. Several identical reaction mixtures (10 ml) in different small conical flasks were allowed to react for different desired times and then determined for PMPA iodometrically. Thus in this method no aliquots were withdrawn. Although trace amounts of iodide catalyze the oxidation of hydrazinium ion through 1-/12redox cycle, large amounts inhibit this reaction and thus iodine does not react with hydrazinium ion during iodometric analysis of the oxidant. The results in absence of iodide ions are quite reproducible provided reacting system is not contaminated with halide ions (iodide in particular). It appears that in the previous study [l]during iodometric determination of PMPA, traces of iodide could get into the reacting system causing irreproducibility. Preliminary experiments indicated that the kinetic results were unaffected by the presence of bisulphate ions, phosphate ions, and trace amounts of H202.The data were treated for the initial rates (vo)by the plane mirror method [S]. Rate constants were calculated from this and duplicate measurements were reproducible to 2 5% in absence of iodide ions and to 5 10% in presence of iodide ions.

Results and Discussion Stoichiomet ry M) the reaction is over in less than In presence of iodide ions (2 x 15 min and the stoichiometry could be determined without any difficulty. Excess [PMPA] was determined iodometrically and excess [N2H5+] was

COMMUNICATION: OXIDATION OF HYDRAZINIUM ION

205

determined by bromate method [7]. The results show that one mol of N2H5+ reacts with 1.97 or 2 mol of H3P05as per eq. (1). (1)

2H3P05

+ NzH4

__f

2H3P04

+ Nz + 2Hz0

The evolved gas did not respond to the tests of NzO, 02,and NO, and was insoluble in water and was thus concluded to be nitrogen. No ammonia could be detected and hence NzH5+ undergoes four electron change to Nz as is expected from a two or multi-electron oxidant such as chromium(V1) [9], thallium(II1) [lo], peroxydisulphate [ll], hydrogen peroxide [12], peroxodiphosphate [13], platimum(1V) [14] etc.

Reaction in Absence of Iodide Ions

PMPA Dependence The concentration of PMPA was varied in the range (1.03-9.3) x M at fixed concentrations of other reactants. A plot of initial rate vs. [PMPA] yields a straight line passing through the origin with a slope of 2.6 x s-' which can be regarded as first order rate constant at [HClO,] = 0.5 M, I = 2.0 M, [NzH5+]= 5.0 x lo-' M, and at 35".

Effect of FdIII) a n d Cu(1I) The concentration of these metal ions was varied in the range (1-100) x M, but there was no effect. The initial rate varied from (1.1 to 1.5) x M-' s-' at 25" with other conditions similar to those in the variation of [PMPA].

Variation of [NzHs+l The concentration of NzHs+ was varied in the range (1-100) x M at three temperatures, 25", 35", and 45",and at three [HC104]concentrations, 0.5 M, 1.0 M, and 2.0 M at constant I = 2.0 M. These results are shown in Figure 1. In the lower concentration range of hydrazine the experimental points fall on a curve but for [N2H5+]larger than 2.0 x M, the rate data points fall on a straight line which on extrapolation makes a n intercept on the rate axis. These results indicate a two term rate law such as (2)

d[H,POs]/dt = A

(Bk'C+"zH5+1 + k,[N,H,+]) C[NzHs+]

where A, B, and C are some constants. The first term reduces to a constant at large [NzH5+]and corresponds to the extrapolated intercept on the rate axis. This type of hydrazine dependence was completely missed in the previous study.

Hydrogen-lon Dependence The hydrogen-ion concentration was varied in the range (0.5-2.0 M at three temperatures and at constant I = 2.0 M adjusted with LiC104. The re-

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DHAS ET AL

103[N2H;1

/M

Figure 1. NzH5' dependence at different [H'] and temperatures; [H3PO5] = 2.2 x M; I = 2.0 M 0,[H'] = 1.0 M and 45"; 0 , [H'] = 2.0 M and 35"; A,[H'] = 1.0 M and 35";A, [H'] = 0.5 M and 35"; -,[H'] = 1.0 M and 25".

sults are given in Table I. The rate increases to a limiting value with the increase of [H+] conforming to an observed rate law (3).

-d[H,POs]/dt = D[H+]/(Ki + [H']) (3) where D and KA are constants. Effect of Ionic Strength Ionic strength was varied in the range (0.5 to 2.0) M by employing LiC104 and the results show only a 10%increase in the rate. TABLEI. Hydrogen-ion dependence in H3P05-NzH5' reaction at 35" and 45".*

[HCIOdM

35"

45"

0.05 0.15 0.20 0.30 0.50 0.70 0.90 1.20 1.50

0.37 1.8 2.4 3.0 3.6 4.1 4.6 4.9

1.2 4.4 7.2 10.0 11.5 13.5 14.5 15.5


207

Mechanism of PMPA-N2H5+ Reaction Hydrazine would predominantly be present as NzH5+ in acid perchlorate solutions since its acid dissociation constant is very small [15]. It is also the reactive species since a n assumption for NzH4 would involve a n inverse H’ dependence. Peroxomonophosphoric acid is a tribasic acid and its dissociaand 1.6 x at 25” tion constants are reported to be 0.089, 3.16 x and I = 0.15 M by Battaglia and Edwards [16]. There is some uncertainty about the first acid dissociation constant. It is reported to be 0.3 in a review and 0.24 at 35” in article [17], 0.34 at 35” by Mahapatro and coworkers [MI, the earlier investigation [l]. In any case PMPA would be present as H3P05 and H2P05-in the acid solutions employed and since the rate tends to a limiting value with the increase of [H’], H3P05 should be the reactive species. As already mentioned Figure 1 indicates that NZH5+ dependence involves two terms, one of which is independent for its large concentrations. Another significant observation is the absence of any effect of trace metal ions like Cu(I1) and Fe(II1) commonly present in the reagents and distilled water. Thus, involvement of hydrazine in complex formation with any of the trace metal ions is out of the question. These observations can possibly be explained by assuming that the redox step is preceded by a n equilibrium step involving PMPA in two forms, one of which is highly reactive and is present in small concentration. The other form is less reactive but is present perdominantly. Such active forms have been assumed in several studies [2] on hypo-phosphorus and phosphorus acids. In general 3,5 and 7-coordinated phosphorus compounds are highly reactive [19] and four coordinated tetrahedral ones are quite stable. In case of hypophosphorus H2PO(OH) and phosphorus HPO(0H)z acids which have quadruply connected phosphorus atom, there is slow conversion of these acids into reactive triply connected phosphorus atom-acids. The two reactive forms are HP(OH)2and P(OH)3respectively. However, in the present investigation peroxymonophosphoric acid is likely to be converted into pentacoordinated structure as given below and this may have trigonal bipyramidalhquare pyramidal structure [20] with one oxygen occupying the equatorial/apical site. This is more facile for the attack by hydrazinium OP(OH)z(O,H) normal, four coordinated

= OPO(OH)3 active, five coordinated

ion as compared to the attack on the peroxide linkage of the normal form of H3P05. The linear part of Figure 1 represents another path for the reaction between the predominent form of H3P05 and NzH5+. Thus, following mechanism may be suggested for the reaction. Writing H3P05(n)and H3P05(a) for the less reactive and highly reactive forms respectively, we obtain the following.

208

DHAS ET AL

Path 2 (6)

H3P05(n) + NzHs+

kz

&Po4

+ N2H2 + H30+

These are followed by the fast step (7)

&Po& + N2H, fast\ Hap04 + N2 + H 2 0 Writing [H3P05]for [H3P05(n)]since [H3POs(a)]is very small and also including acid dissociation of H3P05 (eq. @)), (7)

Kd

H3P05e H2PO5- + H +

(8)

rate law (9) can be obtained.

If [NzH5+]% (kl'/k), rate law (9) reduces to (10) uo =

[H3P051[H,+1 [H+l + K d

(kl

+ k2[N2H5+])

The two terms, kl[H3P05][H+]/(Ki + [H']) and kz[H3P05][H+][N2H5+]/ (K&+ [H']) at different [H+]can be known from the intercept and slopes of the straight line portion in Figure 1.Further plots of (Intercept)-' vs. [H+]-', and (Slope)-' vs. [H+]-' at 35" also yield straight lines with nonzero intercepts. From the intercepts and slopes of these two plots, k l and k2 were found to be 2.6 x low4s-' and 5.0 x M-' s-', respectively, at 35" and I = 2.0 M. The values of Kh were found to be 0.55 and 0.54 from these two plots. If a plot (u0)-' vs. [H+]-' at constant [N2Hs+]is made directly from eq. (lo),one obtains the value of Ki as 0.55 which is the same as found from the derivative plots. Similar plots of eq. (11) at 45" yield K&value of 0.6. These values are almost independent of temperature, but considering the values reported by others, they seem to depend on ionic strength. However, these are all kinetically determined indirect values. The only actually determined value is 0.089 at 25" and I = 0.15 M from spectrophotometric measurements by Battaglia and Edwards [16]. These workers mention that there is uncertainity about this determination even under controlled conditions and that the spectrophotometric method is not suitable for the evaluation of the dissociation constants of H3P05 as a function of ionic strength. Equation (9) can be verified in the following way. At constant [H3P05] and [H+], it can be re-written as (11). uo =

Ak'"zH5+1 + [N2Hsf] + Ak2[N2Hs+]

ki/k

where A is a constant. Or

Reciprocal plots of both the sides of eq. (12), yield the values of k;/k equal 3.5 x and 2.8 x at 25", 35" and 45", respectively. If to 6.2 x the values of kl,k2, and K&and k ;/k are substituted in eq. (9),one obtains a calculated value of uo. The agreement (within ~ 1 0 % in ) the calculated and

209


observed values of the rates proves the validity of eq. (9). A comparison of the present reaction with that of hydroxylamine [6] indicates that the latter M-' S-' bimolecular reaction is much slower (rate constant = 1.15 x at 45").

Reaction i n Presence of Iodide Ions This reaction was studied to prove the catalysis by iodide ions in the reaction and the intermediacy of reactive iodine. This has already been studied by Secco and Venturini [4], but not under the conditions of our investigation M. The variato 3 x i.e., [H'] range of 0.05-2.0 M and [I-] = 1 x tion of [H3P05]was done in the range (8.52-60.2) x and the order in H3P05was one. The first order rate constant at fixed [NzH5'] = 2.0 x s-'. A M, [I-] = 7.0 x M, at I = 0.75 M and 35" was 15.4 x variation of [NzH5+]from 1.0 x M to 2.0 x lo-' M showed that the rate is independent of [NzH5+]. Hydrogen-ion concentration was varied from 0.05 M to 2.0 M at constant I = 2.0 M adjusted with LiC104. The results at three temperatures given in Table I1 show that the rate increases and tends to be limiting with the increase of [H']. Thus, it is the same type as found in the reaction in absence of iodide ions.

Iodide Dependence The concentration of iodide ion was varied in the range 1 x lo-' M to 3 x M and then also extended to 1 x lo-' M. Figure 2 along with two insets shows these results. There is no change in the rate from 1 x M to 1 x M [I-] (inset 1)and it is only when [I-] is 7 x M that the rate increases considerably and a plot of rate vs. [I-] (from 7 x M to 3 x M) is linear, which does not pass through the origin. The slopes of these lines divided by [H3P05]yield the value of 9 x lo3 M-' s-'. This may be regarded as a second order rate constant of the PMPA-I- reaction. Thus, our results confirm the orders of one in H3P05and I - previously found. The variation of iodide in the range 1 x to 1 x lo-' M has been shown as log-log plot of rate vs. [I-] in the inset 2. The rate decreases at higher [I-] as explained in the next section. TABLE 11. Hydrogen-iondependencein the iodide catalyzed H3P05-NzHs' reaction. [H~POB] = M; [I-] = 7.0 x M; and I = 2.0 M(LiC104). 2.15 x M; [NzH5'] = 2.0 x 105(vo)/Ms-' at

0.05 0.15 0.30 0.50 0.75 1.00 2.00

35"

25"

15"

0.43 0.87 1.50 2.00 2.50 2.75 2.90

0.33 0.57 0.95 1.25 1.55 1.80 -

0.175 0.42 0.70 0.95 1.15 1.50 -

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DHAS ET AL

1O6[I-I /M Figure 2. Plot of uo vs. [I-] at [HClOI] = 0.5 M; I = 2.0 M. 0,[H3P05] = 2.11 x M; [NZH5+] = 2.0 x M; 0 , [ H ~ P O E=] 1.05 X M; [NzHs'] = 2.0 x M; M; [NzHS'] = 1.0 x M. a, [H3P05] = 2.11 x

Reaction in Presence of Iodine

M and The concentration of iodine was varied in the range (1-9.2) x the results are given in Table 111. A plot of rate vs. [I2]is linear passing through the origin and the second order rate constant is 8.2 x lo3 M-' s-' which is quite comparable to the second order rate constant of the iodine catalyzed reaction. The fact that no catalysis is observed for [I-] < 7 x M and the linear plot of Figure 2 does not pass through the origin, but that with iodine the linear plot passes through the origin, proves the intermediacy of iodine in the iodide-catalyzed reaction. PMPA oxidizes iodide into iodine which in turn oxidizes hydrazine and iodide is obtained back. Thus PMPA-I- reaction becomes rate controlling in presence of traces of iodide and the oxidation of hydrazine with iodine is fast. The situation is reversed in the presence of large [I -1 when oxidant is determined iodometrically. Under this situation hydrazine-iodine reaction becomes very, very slow.

211


TABLE111. Iodide catalyzed H3P05-NZHs' 0.2 M; and [LiClOd] = 1.0 M, 35". 106[121/M ~ O ~ ( ~ , s-l )/M

reaction. [H3PO5] = 2.12 x

2.76 2.35

0.92 0.71

4.60 3.75

6.44 5.2

M; [HC104] = 9.20 7.3

This is obvious from the rate law (13)obtained in a latest article [21] on this reaction and equilibrium(l4) [22] showing formation of unreactive Is-. = A[NzH4] [I21

-d[L-]/dt (13) (14)

= B"zH5'1

+ I- e KT

I2

x3-

(KT

[L-I/[H'l [I-]

= 830 M-')

It has been reported that at low concentrations of iodide, the latter is oxidized [23] to hypoiodite and traces of iodite and iodate. Hypoiodite is present as HOI since it is a weak acid [24]. Perhaps this does not react with N2H5+ or N2H4and there is insufficient I- for the reaction of HOI and Ito liberate iodine. This appears to happen if [I-] < 7.0 x loe7M. At iodide concentration larger than this, iodine becomes available from the reaction of HOI and I- and there is tremendous catalytic activity. If the small contribution to the rate for [I-] < 7.0 x M is ignored, following mechanism for the iodide catalyzed reaction can be suggested and the rate law (19) can be obtained. (15)

+ 01+ 1- 2 01- + H' e HOI HOI + I+ OH212 + N2H.I fast\ Nz + 4H' + 41-

&Po5

rapid

(16) (17) (18)

(19)

fast

12

-d[HSPOs]/dt = k3[H3P05] [I-] =

K~[H~PO~]T[H'][I-]/(KA + [H'I)

A plot of (rate)-' vs. [H+]-' is linear with nonzero intercept yielding the values of K3(M-' s-') and KA(M-') as 1.5 x lo4 and 0.63 at 15", 2.0 x lo4 and 0.60 at 25", and 3.0 x lo4 and 0.60 at 35", respectively. Once again KA seems to be independent of temperature. E, for k3 was calculated to be (26.8 ? 1.7) K J mol-'. Step (15) has already been suggested [4] before and rate constant K 3 corresponds to k l of the previous study [4] of H3PO5-1- reaction. This was found to be 1.57 x at 25" and I = 0.2M which is smaller by a factor of 10 than that of our study. This may be due to the high ionic strength of 2.0 M employed in our investigation. Incidentally, the same mechanism has been reported [3] earlier in the oxidation of halide ions with PMPA. To further substantiate this we also employed bromide and chloride ions as catalysts instead of iodide ion. The results given in Table IV show that the rate is catalyzed and here also exists a n optimum concentration of halide ions, above which the catalysis is observed. This shows that Cl2 and not HOC1, and Br2 and not HOBr are the active intermediates operating through cycle of C1-/CIz and Br-/Br2, re-

212

DHAS ET AL.

TABLEIV PMPA-N2H5+ reaction in presence of Br- and C1-. [H3P05] = 2.0 x M; [HC104] = 0.5 M; and I = 2.0 M, 35". [N2H5'] = 2.0 x

M;

Effect of Br105[Br-]/M 10s(vo)/M s - ~

0.10 0.50

0.20 0.50

0.50 0.50

1.0 2.0 0.55 0.82 Effect of C1-

5.0 1.65

10.0 3.0

15.0 4.4

103[C1-]/M 106(Vo)/Ms - ~

0.10 0.50

0.20 0.50

0.50 0.50

1.0 0.50

5.0 1.2

7.5 1.65

10.0 2.15

2.0 0.60

spectively. It may also be mentioned that the decrease in rate as found for large concentrations of iodide, is not observed in case of bromide and chloride, since the formation of tribromide and trichloride ions is not significant [25,26] and free Brz and Clz is available for oxidation. KT (equilibrium 14) for Br3- and C13- are about 17 and 0.18, respectively. A comparison of the present reaction with that of the oxidation of hydroxylamine [6] shows that both the reactions are catalyzed by iodide ions, though the latter reaction is quite complicated. Part of the catalysis is explained through intermediacy of iodine and part by a termolecular transition state consisting of H3P05, NH30H, and I-. One important difference is that the oxidation of hydroxylamine is catalyzed by Fe(II1) also whereas the present reaction is not. This is not understandable in view of the fact that hydrazine [27] and hydroxylamine [28], both are oxidized with Fe(II1) in acid medium with almost the same rate.

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[19] J. R. Van Wazer, Phosphorus and its Compounds, Interscience, New York, 1958, Vol. I, p. 71. [20] R. R. Holmes, Pentacoordinated Phosphorus, ACS Monograph 176, Washington DC, 1980, Vol. 11, 9.87. [21] T.Sh Bah and Mock Chup New, J. Singapore Natl. Acad. Sci.,6,24 (1977). [22] R.W Ramett and R.W. Sanford, Jr., J. Am. Chem. Soc., 87, 5001 (1965); J. H. Espenson, Inorg. Chem., 4, 1834 (1965). [23] M. Kuhn and A.C. Wahl, J. Chem. Phys., 21, 1185 (1953); H.M. Eiland and M. Kuhn, J. Phys. Chern., 65, 1317 (1961). [24] E A. Cotton and G. Wilkinson, Advanced Znorg. Chem., John Wiley & Sons, 5th Ed., 1988, p. 565. [25] D. B. Scaife and H. J. Tyrrell, J. Chem. Soc., 386, (1958); G. Jones and S. Baeckstrom, J. Am. Chem. Soc., 56, 1517 (1934). [26] M. Eigen and K. Kustin, J. Am. Chem. Soc., 89,1355 (1962). [27] S. S.Gupta and Y. K. Gupta, J. Chem. Soc., Dalton Trans., 547 (1983). [28] K. Arora, A. P.Bhargava, and Y. K. Gupta, J. Chem. Soc., Dalton Trans., Communicated.

Received May 11, 1990 Accepted August 31, 1990