188 V. K. JINDAL, M. C. AGRAWAL, AND S. P. MUSHRAN

Oxidation of Hydrazine by Alkaline Ferricyanide in Water-methanol Mixtures V. K. Jin d a l , M. C. A graw al , and S. P. M ush ra n Department of Chemistry, University of Allahabad, Allahabad, India

(Z. Naturforsdi. 25 b, 188—190 [1970] ; eingegangen am 2. September 1969) Kinetics of the oxidation of hydrazine by ferricyanide was investigated in water-methanol mix­ tures using several buffer solutions. The reaction showed first order dependence in both hydrazine and ferricyanide. The order with respect to hydroxide ion concentration was zero. Increase in con­ centration of methanol had a retarding influence on the rate while the addition of neutral salts showed a specific ion effect. The energy and entropy of activation were calculated as 12.3 kcals. mole- 1 and — 20.8 cals. deg-1 mole-1 respectively. A suitable mechanism has been proposed which suggests the primary rate determining reaction between N2H4 and Fe(CN)63e. Nitrogen was found to be the product of the reaction. Hydrazine and its derivatives have been exten­ Several buffer solutions were prepared by mixing sively used as reducing titrants in both acidic and suitable amounts of sodium carbonate and bicarbonate (0.2 m ) and final pH was adjusted in 50% methanol. alkaline media.1 The principal product of the oxi­ The following table gives the pH of the buffers employed dation is nitrogen. A little work has been done on in aqueous and 50% methanolic solutions. the kinetics of the oxidation of hydrazine in alkaline solutions 2. In an acidic medium detailed kinetics of CO,20 a + HC03q a pH pH in 50% the oxidation of hydrazine by iron (III) has been [ml] [ml] MeOH H ig g in so n WRIGHT3. studied by and Velocity of 4.0 46.0 9.2 9.90 the oxidation of hydrazine by ferricyanide was stud­ 13.0 37.0 9.5 10.80 ied by Gilbert 4 at a pH of about 6. 19.5 30.5 9.7 11.20 25.0 25.0 9.9 11.30 Oxidation of hydrazine by ferricyanide is very 30.0 20.0 10.1 11.38 fast in an alkaline medium. In a medium of 2.5 to 35.5 14.5 10.3 11.45 5% KOH hydrazine can be directly titrated against Table I. Influence of Methanol on pH. a Total volume was ferricyanide at 70° 5 where the stoichiometry was made to 200 mis and 10% of the buffer was used in the found to be 1: 4.0. In presence of methanol, the rate investigations. of the reaction was considerably slowed down. In pH measurements were made on a Leeds and the present investigation, the results of the kinetics Northrup type direct reading pH-meter using glass- of oxidation of hydrazine by alkaline ferricyanide electrode. Bidistilled water was used to prepare all the in 50% methanol have been recorded and sub­ solutions and diluting where necessary. sequently used for the formulation of a suitable The kinetics of the reaction wTere followed by esti­ mating the amount of unreacted ferricyanide colori- mechanism. metrically using Klett-Summerson Photoelectric Colori­ meter with blue filter No. 42 (transmission 400 — 450 Experimental m/v). The absorption cell was chilled before adding the Aqueous solutions of hydrazine sulphate are stable reaction mixture and readings were taken within 10 even for a period of twro months and therefore a stock seconds. solution was prepared from a recrystallised sample of hydrazine sulphate (A. R., B. D. H.). Potassium ferri­ Results cyanide solution was prepared by dissolving weighed The kinetics of the oxidation of hydrazine bv amount of the AnalaR sample of the reagent. All other solutions wTere prepared from the reagents of analyti­ ferricyanide was investigated at several concentra­ cal grade and their concentrations were determined by tions of the oxidising and reducing agent. It was appropriate methods. observed that the rate of disappearance of ferri-

1 A. B e r k a , J. V u l t e r i n , and J. Z y k a , Chemist-Analyst 52, 3 W . C. E. H i g g i n s o n and P. W r i g h t . J. chem. Soc. 1955, 56 [1963]. 1551. 2 W. C. E. H i g g i n s o n , The Chem. Soc. [London], Spl. publi­ 4 E. C. G i l b e r t , Z. physik. Chem. A, 142. 139 [1929]. cation No. 10, pp. 95. 5 J. V u l t e r i n and J. Z y k a , Chem. Listy 48. 1762 [1954]. OXIDATION OF HYDRAZINE BY ALKALINE FERRICYANIDE 189

Expt. [Fe(CN)639] [Hydrazine] kt x103 b k2 c = k1/ No. a M x 104 Mx 103 [min-4] [Hydrazine] [/•mole-1 min-1] 1 3.2 4.0 63.6 15.9 2 3.6 4.0 66.8 16.7 3 4.0 4.0 66.0 16.5 4 4.4 4.0 65.0 16.2 5 4.0 1.0 38.7 38.7 6 4.0 2.0 74.4 37.2 7 4.0 3.0 116 38.6 8 4.0 4.0 149 37.4 9 4.0 7.0 235 33.6 10 4.0 9.0 299 33.3

Table II. Effect of Reactants Concentration. a Methanol = 50%, pH = 11.3, Temp. = 25° and [NaClOJ = 0.1 m, only in expts. Nos. 5 to 10. b First order constants in ferricyanide. c Second order rate constants. cyanide follows first order dependence at all concen­ pH /cj x 103 /l*2 trations of hydrazine (Fig. 1). However, the con­ [min-1] [/•mole-1 min-1] centration of hydrazine increased the first order con­ 9.90 51.6 12.9 stants in ferricyanide almost linearly showing that 10.80 57.6 14.4 11.20 58.5 14.6 the reaction is also of first order in hydrazine (Table 11.30 65.9 16.5 II). The reaction is, therefore, of second order 11.38 66.8 16.7 whose constants k.2 have been calculated by dividing 11.45 75.3 18.8 the pseudo-first order constants in ferricyanide by Table III. Effect of pH. [N2H4] = 4 x 1 0 - 3 m, [Fe(CN)63e] the concentration of hydrazine. = 4 x 10-4 m, Methanol = 50% and Temp. 25°. It is observed from the pH study that a large variation in the alkalinity of the reaction mixture causes a slight increase in the rate constant. It is therefore concluded that the reaction has an insignifi­ cant pH effect. Effect of Neutral Salts Influence of several neutral salts on the rate of the reaction was studied and it was observed that though the addition of different salts usually enhanced the reaction rate (Table IV), the effect was singularly specific. Salt [^1 [M] [/•mole-1 min-J] None added 0.0 16.5 Fig. 1. Plot of loga/a—x vs time, [N2H4] as 1.0, 2.0, 3.0, 0.02 KC1 0.02 28.1 4.0, 7.0 and 9.0 x l0 ~ 3 m in I, II, III, IV, V and VI respec­ 0.04 KC1 0.04 46.5 tively. 0.01 Na,S04 0.03 19.1 0.02 Na,S04 0.06 22.0 0.03 Na.,S04 0.09 25.6 The reactions were studied in presence of buffer 0.02 NaC104 0.02 18.9 and 0 .1 m NaC104 (Experiments 5 — 10 of Table 0.04 NaC104 0.04 22.7 II), in order to avoid any variations in the reaction 0.06 NaC104 0.06 27.1 rate due to ionic strength and pH. 0.08 NaC104 0.08 30.0 Table IV. Effect of Neutral Salts [N,H4] = 4 x 10-3 m, Effect of pH [Fe(CN)630] = 4 x 10-4 m, pH = 11.3, Methanol = 50% and Temp. 25°. The effect of pH was studied between the range 9.9 to 11.45 using several sodium carbonate-bicar- It is evident from the above table that the accel­ bonate buffers (Table III). erating effect of neutral salts is not due to a change 190 OXIDATION OF HYDRAZINE BY ALKALINE FERRICYANIDE in ionic strength but appears to be a specific salt Cahn and Powell 6, which is further oxidised to effect. K® ions have more pronounced effect than nitrogen in several subsequent fast steps as follows: f Q cf Na® ions while SO420 ions have a retarding in­ N2H3 + Fe(CN)63® — N2H2 + Fe(CN)640. (2) fluence. 2 N2H3 — N2H2 + N2H4. (3) Effect of Dielectric Constant of the Medium N2H2 N2 . (4) Increase in concentration of methanol or a de­ The stoichiometry of the reaction agrees well with crease in dielectric constant of the medium shows a the above mechanism. marked inhibition on the rate of oxidation of hydra­ Now as step (1) is slow and rate-determining the zine by ferricyanide (Table V). reaction would have first order dependence in both hydrazine and ferricyanide and due to its reversible Methanol D o5o k2 nature it would have significant retarding influence [%] [Z • mole-1 min-1] of ferrocyanide. Our experimental observations are 40 60.18 27.1 50 55.59 16.5 in accordance with these conclusions (Tables II 55 53.29 23.0 and VI). 60 50.99 11.1 The rate determining step (1) involves the inter­ 70 46.40 9.0 action between an uncharged molecule and a nega­ Table V. Effect of Dielectric Constant (Conditions same as in Table IV). tively charged ion and therefore should correspond to a positive dielectric effect and a negative entropy Other Effects change 7. This has been found to be true from the Addition of ferrocyanide retards the rate of the kinetic data recorded earlier. reaction (Table VI). Rise in temperature increases The rate of oxidation of hydrazine by ferricyanide the rate and the temperature coefficient has been in an alkaline medium is greatly influenced by the found to be 1.87. The reaction was studied at several addition of inert ions but the effect has been found temperatures (Table VI), wherefrom the energy to be exclusively specific. It was not, therefore, pos­ and entropy of activation were calculated as 12.3 sible to separate out the influence of ionic strength kcals. mole“1 and — 20.8 cals. deg-1 mole-1 re­ on the rate of the reaction, which should have been spectively. insignificant according to proposed mechanism. How­ ever, it has been ascertained that there are specific Mechanism ions like Na® and K® which enhance the rate whilst The independence of the reaction rate on hydrox­ ions like S042t retard the rate of reaction. ide ion concentration predicts that N2H4 , rather than It is, therefore, concluded that during the oxida­ N2H5®, is the main reacting species. As the reaction tion of hydrazine by alkaline ferricyanide in 50?o shows first order dependence in both hydrazine and methanol, neutral hydrazine molecule (N2H4) is ferricyanide, the primary rate determining step may attacked by ferricyanide, due to which the reaction be written as follows: unlike other oxidations by ferricyanide8’9, shows N2H4 + Fe (CN) 63® ^ N2H3 + Fe (CN) 64®. (1) zero order dependence in hydroxide ion. The authors wish to thank Council of Scientific and The formation of N2H3 intermediate, during the Industrial Research, New Delhi and State C.S.I.R.. oxidation of hydrazine has also been suggested by Lucknow, for fellowships to MCA and VKJ.

[Fe(CN))64°] k. Temperature k2 MxlO4 [/-mole-1 min-1] [°C] [/•mole-1 min-!] 0.0 16.5 15 8.9 1.6 14.6 20 12.7 3.2 13.3 25 Table VI. Effect of Ferrocyanide and 6.4 12.5 30 öl '. Temperature (Conditions same as in 8.0 11.3 35 Table IV). 6 J. W. Ca h n and R. E. P ow ell. J. Amer. chem. Soc. 76, 8 U. S. M e h r o t r a , M . C. A g r a w a l , and S. P. M u s h r a n , J. 2568 [1954]. physic. Chem. 73. 1996 [1969]. 7 M. C. Agrawal and S. P. Mushran, J. physic. Chem. 72, 9 M . C. A g r a w a l , V. K. J in d a l , and S. P. M u s h r a n , J. in­ 1497 [1968], org. nuclear Chem., in press.