A KINETIC STUDY OF THE OXIDATION OF NITRITE ION BY PEROXODISULPHATE

Dr. Eeljali Elobeid Elhassan 1 , Prof. Salah edin Mohamed Elamin 2 and Dr. Ibrahim Mukhtar Ahmed 3 . 1. Assistant Professor, Chemistry Department, Faculty of Education, Khartoum university, P.O.Box406 Omdurman-Sudan. 2. Professor, Head, Teachers’ Research Resource Unit (RRSU), Ahfad University for Women. 3. Associate Professor, Chemistry Department, Faculty of Education, Khartoum university, P.O.Box406 Omdurman-Sudan. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

The oxidation of nitrite ion NO 2 -by Peroxodisulphate ion S 2 O 8 2-was studied in high ionic strength and in phosphate and acetate media at around pH=7 and 60 0 C .Under controlled conditions the reaction is of overall second order being first order with respect to reactants.Cations (Na + , K + , Ca +2 , Mg +2 and Cu +2 catalyze the reaction and the extent of catalysis depends on the nature and concentration of the cations.The temperature effect on the redox reactions was studied and the activation energy is 60.01 kJmol -1 .The mechanism has been proposed.Based on experimental data and kinetic simulation, we have modified previous models for this reaction mechanism and show that the new model proposed here is consistent with experimental data.The stoichiometry was determined to be:-S 2 O 8

…………………………………………………………………………………………………….... Introduction:-
Peroxodisulphate ion S 2 O 8 2-is one of the strongest oxidizing agents known in aqueous solution (1) .Peroxodisulphuric acid and its salts were suggested to be among the most useful oxidizing agents in aqueous solution (2,3) .Many studies on the thermal decomposition of Peroxodisulphate ions were made by many workers such as Cotton, Kolthoff, Miller, Vasudeva, Taha and Wasif (4) and there is general agreement that it follows first order kinetics.Reactions involving peroxodisulphate ion are generally slow at ordinary temperatures (5) but become more rapid in the presence of some metal ions like Ag(I), Fe(II) and Cu(II).Therefore in this work we used Na + and K + in the redox reaction between Peroxodisulphate ion and nitrite ion.The kinetic study of the reaction of nitrite ion has been of increasing interest due to its biological and chemical implications (6,7) .Nitrite ion is a common contaminant in water, and because its complexing ligand (with Fe(II) )its presence can be dangerous.The kinetics of the redox reaction of S 2 O 8 2-and NO 2 -was studied by a number of workers.Elamin (8,9) , who found that the oxidation of nitrite ion by peroxodisulphate was second order, being first order with each reactant.

Materials:-
All chemicals used are of high purity (98%).The redox reaction of peroxodisulphate with nitrite ion proceeds at measurable rate at about 60 0 C and it is for this reason, that the temperature range 60 0 -75 0 C was chosen.For each run two solutions A and B were prepared.A was freshly prepared -K 2 S 2 O 8 -while B was a mixture of NaNO 2 , added salts (NaNO 3 and KNO 3 ) which maintained the constant ionic strength and phosphate buffer (Na 2 HKPO 4 -KH 2 PO 4 ).Also solutions A and B were prepared in the same manner and acetate (C 2 H 3 O 2 Na -C 2 H 4 O 2 ) buffer is used instead of phosphate buffer.Solutions A and B were made in such way that the concentration of each species was double the value required in the final reaction mixture.Water carefully redistilled from alkaline KMnO 4 was used.

Kinetic measurements:-
The progress of the reaction was followed by examining the concentration of peroxodisulphate ion at various time interval.The concentration of S 2 O 8 2-was determined iodometrically during the kinetic run.Purified nitrogen gas was passed through the quenched iodine flask contain 10cm 3 sample so as to drive away any dissolved oxygen.Then 4gms of KI ware added.It was then placed in the dark for about 10minutes till all the iodine was liberated.The liberated iodine was then titrated against standard sodium thiosulphate using starch as an indicator.The concentration of nitrite ion was determined before the kinetic runs by two methods; (a) Titration method: By using standard potassium permanganate solution.(b) Spectrophotometric method: By reacting nitrite ion with a series of organic materials to form a purple dye, so that the measured absorption A was plotted vs. nitrite concentration.A straight line passing through the original point was obtained and thus providing that the solution obeys Beer's law.

Kinetic order with respect to Peroxodisulphate ion
The order of the reaction-in its initial stages-with respect to S 2 O 8 2-was investigated under the constant conditions given in the captions of table (1) which shows the variation of the initial rate of disappearance of peroxodisulphate ion,R 0 , with its initial concentrations.A plot of R 0 vs. [S 2 O 8 2-] 0 (fig. 1) gives a straight line indicating that the reaction is first order with respect to S 2 O 8 2-, thus: Where k obs is initial-pseudo first order rate constant.From fig( 1) the slope = k obs = 3.7398X10 -5 s -1 , at 60 0 C.
R 1 represents the thermal decomposition of peroxodisulphate in the absence of nitrite ion while the second term represents the redox (S 2 O 8 2--NO 2 -)path.Since the reaction is first order in S 2 O 8 2-equation ( 2) can be rewritten as: Where k 1 is the thermal decomposition of peroxodisulphate and k 2 is second order rate constant.

Effect of hydrogen ion:-
The pH of the redox reaction was changed in the range (4.65-8.41)by changing the ratio [HPO 4 2-]/[H 2 PO 4 -] under otherwise constant experimental conditions table (3).The pH was measured directly using a pH-meter.From table (3.3) there is no clear effect on R 0 .Therefore it is safe to conclude that the rate of the reaction is independent of pH.

Effect of added salts:-
The effect of added the salts NaNO 3 ,KNO 3 , Mg(NO 3 ) 2 , Ca(NO 3 ) 2 and Cu(NO 3 ) 2 on the redox reaction was shown in figs.(3and 4) by varying the concentration of the cation under otherwise constant experimental conditions given in tables (5 and 6).To investigate the effect of divalent cations (Mg +2 , Ca +2 and Cu +2 ) the medium was changed from phosphate buffer to acetate buffer which does not form precipitates with the cations.Effect of Na + and K + were repeated in acetate medium.It is clear from figs.( 3and 4) that the rate of the reaction is linearly to cations concentrations ([M +n ], n=1or 2), thus the rate of the reaction may be expressed by eq.4.R 0 = C + k obs [M +n ] (4) In which C is y-intercept at [M +n ] = 0 and k obs is slope of the straight lines, as defined by the equations ( 5)and ( 6) where k M +n is the catalytic rate constant for the cation M +n under study, k Mi +n is that of the residual cation M +n and k 2 0 is the second order rate constant at zero cation concentration.Substituting the relevant values of C and k obs from each plot and the appropriate values of the other terms applicable to it is leads to the results given in tables( 5 and 6).Thus it is evident that the cations catalyze the redox reaction and that they do so in the ascending order Na + < K + < Mg 2+ < Ca 2+ < Cu 2+ .
Effect of temperature:-The effect of temperature on the k 2 path was studied over the temperature range 60-75C 0 at otherwise constant experimental conditions of: .20 mol dm -3 I=1.24 pH=6.47The values of the first order rate constant for the thermal decomposition of S 2 O 8 2-,k 1 , calculated by Elamin, were used in calculating k 2 at each temperature from eqation (5).Results are summarised in table (7).The effect of temperature on k 2 components (i.e.k 2 0 and k M +n ) was also studied in the range 60-75 0 C.For each temperature k 2 was calculated from equation(3), and then k M +n was calculated from y-intercept of figures resulting from plot of [M +n ] vs.R 0 by using the equation R From plot of logk Mn + against 1/T the relevant Arrhenius parameters are calculated in table (7) for each path. 2-] 0 (11) Which corresponds to the right hand side (r.h.s.) of equation (2).Some SO 4 . radicals seem to end up forming oxygen according to well established mechanism discussed by House (10) , while most SO 4 . radicals seem to oxidize NO 2 -ion in some faster steps represented by the overall equ.(8) given by Elamin (7) Fig.2.Plot of [NO 2 -] vs. R

A: Calculated amount of K 2 S 2 O 8 (A.R) was dissolved in double distilled water to make 500cm 3 of solution A. (ii) Solution B: Calculated
amount of NaNO 2 , NaNO 3 , KNO 3 and Phosphate buffer (Na 2 HPO 4 -KH 2 PO 4 ) or acetate buffer (C 2 H 3 O 2 Na -C 2 H 4 O 2 ) are dissolved in double distilled water to make 500cm 3 of solution B. The additional components under study (i.e.allylacetate was added in each kinetic run to solution B. The redox reaction was carried out in the conditions: [S 2 O 8 2-] 0 =[NO 2 -] 0 , high ionic strength around one (I=0.80-1.34),Phosphate buffer [Na 2 HPO 4 ]=[KH 2 PO 4 ]=0.20 mol dm 3 or acetate buffer (Na 2 HPO 4 -KH 2 PO 4 ).

order with respect to Nitrite ion:-
The effect of varying the initial concentration of nitrite ion -whilekeeping [S 2 O 8 2-] 0 and other factors constant, was investigated by plotting of initial rate of the disappearance of peroxodisulphate against the respective [NO 2 -] 0 values, (fig 2) and (Table2), a straight line intercepting the y-axis was obtained suggesting a rate equation of the form: R 0

Table ( 3) Variation of R 0 with pH:-
The effect of adding allylacetate in multi concentrations was studied by carrying out the runs in experimental conditions given in the caption of table(4).From this table no clear effect is observed.This probably shows that free radicals are not involved in the rate-determining step although the result does not exclude the presence of free radicals in other fast steps.

Table ( 7) Thermodynamics parameters of activation
Therefore the rate equation of this path may represent by the equation(7) Rate = k 1 [S 2 O 8