The Inhibitory and Catalyzing Effects of Different Materials on the Thermal Decomposition of Potassium Peroxydisulphate in Aqueous Solution at 70

The present study is an investigation of the inhibitory effect of halides ions and the mineral salt potassium sulphate; and the catalyzing influences of electrolytes; non – electrolytes and the mineral salt mercuric chloride on the thermal decomposition of potassium peroxydisulphate at 70 C. The study showed that the specific inhibitory effect of halides ions on the thermal decomposition is in the order: F< Cl < Br < I; while the catalyzing influences of electrolytes and non – electrolytes on the decomposition in the order: HCl > KCl > H2SO4 > K2SO4 > Urea. It was also found that potassium sulphate acts as an inhibitor on the thermal decomposition; while mercuric chloride exhibits a catalyzing influence on the decomposition.


Introduction
The thermal decomposition of potassium peroxydisulphate was the subject of study of many workers (Morgan & Christ, 1927;Fronaeus & Ostman, 1955;Kolthoff & Miller, 1951), all they suggested that the decomposition follows a first order kinetics.Early workers (Green & Mason, 1910) studying the thermal decomposition of potassium peroxydisulphate found that the decomposition in aqueous solution was accelerated by rise of temperature and depends on the concentration of solution.Levi and Migliorini (Levi, Migliorini, & Gazz, 1963) observed that peroxydisulphate solutions which were stable at 35 O C are decompose catalytically by the hydrogen and the hydroxyl ions as well as platinum black and lead.Fronaeus and Ostman (Fronaeus & Ostman, 1955) and Kolthoff and Miller (Kolthoff & Miller, 1951) showed that the rate constant of the decomposition was independent of the ionic strength but in acid solution there is a negative salt effect.Vasudeva (Vasudeva, 1969) noticed that in aqueous solution potassium peroxydisulphate decomposes slowly at 60 O C and the decomposition shows auto-inhibition in the beginning and then follows first order kinetics.Bartlett and Cottman (1949) suggested for the uncatalyzed thermal decomposition the following chain mechanism: Deionized water was used in all kinetic runs.Since the reaction does not take place to any measurable extent at room temperature (t 1/ 2 = one month) (Vasudeva, 1969), the temperature used was70 O C. The iodometric method was used for the analysis and estimation of unreacted peroxydisulphate [S 2 O = 8 ] which is a modification of the method used by Bartlett and Cottman (Bartlett & Cotman, 1949) and Rosin (1946).

Results and Discussion
Tables (1) and (2) show the inhibitory effect of the metal halides, potassium chloride and potassium bromide on the thermal decomposition of potassium peroxydisulphate at 70 O C in which the concentration of the metal halides were 0.1 and 0.2 mole/l while that of peroxydisulphate was kept constant at 0.01 mole/l.Results were represented graphically in Figure ( 1).The inhibitory effect of iodide ion could not be investigated because even at very low concentration of KI a reaction between KI and K 2 S 2 O 8 took place, in the case of KBr also a concentration equal to that of KCl could not be used as bromine was liberated due to the interaction of KBr with K 2 S 2 O 8 , this leads to the conclusion that the specific inhibitory effect of Br ion is greater than that of Cl ion.
Comparing the curves in Figure ( 1) corresponding to the four different initial concentrations of halides indicate that the specific inhibitory effect of halides ions is in the order F< Cl < Br < I and the specific inhibitory increased with increasing concentration.So we can conclude that the inhibitory effect of halides ions is not a proper salt effect and it is an instance of specific ionic effect, because when we used a very high concentration of potassium sulphate the decrease in the rate constant is much less than in the case of any halide ion.
Tables (3) and ( 4 ) show the influence of the mineral salt potassium sulphate on the thermal decomposition of 0.01 mole/l potassium peroxydisulphate at 70 o C. Results were represented graphically in Figure (2).

Figure 2 .
Figure 2. The effect of mineral salts on the thermal decomposition of peroxydisulphate at 70 O C

Figure 3 .
Figure 3.The effect of electrolytes and non-electrolytes on the thermal decomposition of peroxydisulphate at 70 O C Tables (7) and (8) show the influence of mercuric chloride on the thermal decomposition of 0.01 mole/l potassium peroxydisulphate at 70 O C. Results were represented graphically in Figure (4).

Table 1 .
The inhibitory effect of the metal halides, potassium chloride and potassium bromide on the thermal decomposition of potassium peroxydisulphate at 70 o C

Table 2 .
Summary of the rate (R) and the observed rate constant k O of the inhibitory effect of halides ions

Table 3 .
Figure 1.The effect of halides ions on the thermal decomposition of peroxydisulphate at 70 O C The influence of the mineral salt potassium sulphate on the thermal decomposition of potassium peroxydisulphate at 70 O C

Table 4 .
Summary of the rate (R) and the observed rate constant k O due to the influence of the mineral salt potassium sulphate on the thermal decomposition of potassium peroxydisulphate at 70 O C

Table 5 .
The effect of electrolytes and non-electrolyte on the thermal decomposition of potassium peroxydisulphate at 70 O C

Table 6 .
Summary of the rate (R) and the observed rate constant k O of the effect of electrolytes and nonelectrolytes

Table 7 .
The influence of mercuric chloride on the thermal decomposition of potassium peroxydisulphate at 70 O C

Table 8 .
Summary of the rate (R) and the observed rate constant k O due to the influence of mercuric chloride on the thermal decomposition of potassium peroxydisulphate at 70 O C