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Kinetic and Thermodynamic Study of the Reaction Between Bromocresol Green with Sodium Hydroxide in an Aqueous Solution

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Kinetic and Thermodynamic Study of the Reaction Between Bromocresol Green with Sodium Hydroxide in an Aqueous Solution JournalJournal of Chemical of Chemical Technology Technology and and Metallurgy, Metallurgy, 54, 54,1, 2019, 1, 2019 90-94 KINETIC AND THERMODYNAMIC STUDY OF THE REACTION BETWEEN BROMOCRESOL GREEN WITH SODIUM HYDROXIDE IN AN AQUEOUS SOLUTION Latona Dayo Felix1, Adejoro Ajibade2 1 Department of Chemical Sciences, Osun State University Received 15 November 2017 PMB 4494 Osogbo, Nigeria Accepted 06 March 2018 2 Department of Chemistry, University of Ibadan, Nigeria E-mail: [email protected] ABSTRACT The kinetics and the mechanism of the reaction of Bromocresol green and a hydroxyl ion in an aqueous solution is studied spectrophotometrically. The reaction is found of a first order in respect to both reactants. The Michaelis-Menten plot shows intermediate complex presence and reaction dependence on the solution ionic strength. The activation param- eters are evaluated. The negative ΔS‡ value obtained indicates an associative mechanism. Keywords: kinetics, thermodynamics, mechanism, Bromocresol green (BCG), sodium hydroxide and potassium nitrate. INTRODUCTION to emptying them into the water bodies as industrial ef- fluents is imminent in order to protect the aquatic organ- Bromocresol green belongs to the triphenylmethane isms [3]. Several methods of achieving this noble objec- dye family. It is an important industrial raw material tive are reported [4-7]. However, this research tends to in textile, leather, paper, printing, plastic and ceramic investigate the option of hydrolyzing Bromocresol green industries [1]. Bromocresol green is employed in col- in an alkaline medium with the view to ascertaining the orimetric detection technologies and for visualization of kinetics and the mechanism of the reaction, which has compounds of a functional group whose pKa is below hitherto not been reported in the literature. It is worth 5.0. Most importantly it is used in charge-transfer com- noting that Bromocresol green ionizes in an aqueous plexation processes [2]. The dyes have carcinogenic solution to give a monoanionic form (yellow), which properties and hence a concomitant effect on the aqua further deprotonates at higher pH to give a dianionic life. The need to find a way of removing these dyes prior form (blue) [8] as shown below: Protonation/deprotonation of Bromocresol green in an aqueous solution 90 Latona Dayo Felix, Adejoro Ajibade EXPERIMENTAL Analar grade Bromocresol green, NaOH, KNO3 were utilized and stock solutions were prepared with dis- tilled water. The reaction was investigated under pseudo- first order conditions by maintaining a large excess ( x10) of sodium hydroxide in respect to Bromocresol Effect of Bromocresol green concentration on the green dye. Appropriate quantities of Bromocresol green, observed rate constant potassium nitrate and sodium hydroxide were measured The effect of Bromocresol green concentration on from the stock solutions (kept in a water bath for 30 the apparent rate constant is determined studying reac- -5 min). The kinetic data was obtained by monitoring the tion mixtures containing [BCG]o ranging from 1x10 change of the absorbance of Bromocresol green with mol dm-3 to 8x10-5 mol dm-3, sodium hydroxide at a fixed time at 441nm using a UV-1800 Shimadzu spectropho- initial concentration of 1x 10-3 mol dm-3 and at ionic tometer. The latter had a thermostated cell interfaced to strength of 8.0 x 10-2 mol dm-3 at 298K. The reaction shows an increase of the apparent rate constant with a computer. The pseudo-first order rate constant (kapp) was obtained from a plot of ln A against time, while the increase of Bromocresol green concentration as shown reaction products were characterized by FTIR. in Fig. 1. The plot of ln kapp versus ln[BCG] has a slope of 1 implying a first order dependence with respect to RESULTS AND DISCUSSION Bromocresol green concentration. The stoichiometry of the reaction is determined via Effect of sodium hydroxide concentration on the ap- spectrophotometric titration. The absorbance at infinity parent rate constant time of solutions containing various concentrations of The effect of sodium hydroxide concentration sodium hydroxide in the range from 6.67 x 10-4 mol dm-3 on the apparent rate constant is determined at 298K to 4.00 x 10-3 mol dm-3, a fixed initial concentration of studying reaction mixtures containing BCG at a fixed Bromocresol green at 1 x 10-5 mol dm-3 and a constant concentration of 1 x 10-5 mol dm-3, sodium hydroxide -2 -3 with a concentration ranging from 6.70 x 10-4 mol dm-3 initial concentration of KNO3 at 5 x 10 mol dm are measured. The stoichiometry is evaluated from the plot to 4.00 x 10-3 mol dm-3 and a fixed ionic strength of 5 x of the absorbance versus [NaOH]. It is found to be 1:1. 10-2 mol dm-3. An increase of the apparent rate constant Therefore the overall reaction can be presented by: ( kapp ) with sodium hydroxide concentration increase is Fig. 1. A plot of kapp versus [BCG]. Fig. 2. Plot of 1/ kapp versus 1/[NaOH]. 91 Journal of Chemical Technology and Metallurgy, 54, 1, 2019 Table 1. Effect of [OH-]. Temperature effect and activation parameters de- 103[OH-]/ mol dm-3 3 -1 termination 10 kapp /s 0.67 1.36 The temperature effect on the apparent rate constant 1.00 2.05 is determined by varying the temperature within the 1.33 2.56 -5 range of 298K- 313K at a fixed [BCG]o of 1.67x10 1.67 3.25 -3 -4 -3 2.00 3.67 mol dm , [NaOH]o of 6.67x10 mol dm and an ionic 2.50 4.68 strength of 9.5x10-2 mol dm-3. There is rate constant 3.00 5.43 increase with temperature increase (Fig. 4). The activa- 3.50 6.05 ‡ ‡ 4.00 6.98 tion energy (Ea) and the activation parameters (ΔH , ΔS , ΔG‡) of the reaction are obtained on the ground of the observed as shown in Table 1. The slope of the plot of equations given below: ln kapp versus ln[NaOH] is unity indicating a first order log = log – dependence with respect to sodium hydroxide concen- 2.303 tration. Moreover, a plot of 1/ versus 1/[NaOH] kapp ‡ / ‡ gives an intercept, which reveals the presence of an ln = + ln + intermediate complex during the course of the reaction −∆ ∆ as shown in Fig. 2. A second order rate constant obtained � �/ � � � � from the slope of a plot of k versus [NaOH] is found ℎ app ln = 23.76 -1 3 -1 equal to 1.80 mol dm s . �‡ � ‡ ‡ ΔG =ℎ ΔH - TΔS Effect of ionic strength on the apparent rate constant The dependence of the ionic strength on the apparent where k is the apparent rate constant, T is the tempera- ‡ ‡ rate constant is determined at 298K by studying a reac- ture, ΔH is the enthalpy of activation, ΔS is the entropy -5 -3 of activation, ΔG‡ is the free Gibb’s energy of activation, tion mixture of [BCG]o of 1x10 mol dm , [NaOH]o of / 1.00x10-3 mol dm-3 and an ionic strength within the range R is the the molar gas constant, k is the Boltzmann’s from 1x10-2 mol dm-3 to 7x10-2 mol dm-3. The apparent constant, while h is the Plank’s constant. The values rate constant decreases with increase of the ionic strength obtained are listed in Table 2. The negative value of the entropy of activation (µ) of the solution. A plot of log kapp versus √µ gives a straight line with slope equal to -1, which suggests the reveals an increase of the order or loss of freedom in presence of -1 and +1 charges on the reactants at the rate the course of transition state complex formation. It is determining step as shown in Fig. 3. expected that a closely held solvation sphere exists in Fig. 3. Plot of log kapp versus √µ. Fig. 4. Plot of kapp versus temperature 92 Latona Dayo Felix, Adejoro Ajibade Table 2. Activation parameters. -1 ‡ -1 ‡ -1 -1 ‡ -1 Dye Ea (kJ mol ) ΔH (kJ mol ) ΔS (kJK mol ) ΔG (kJmol ) Bromocresol 19.82 17.66 -0.24 89.18 green case of Bromocresol green cation. The high degree of Mechanism and a rate law order of this sphere is disrupted when it reacts with The kinetic results obtained can be summarized as the hydroxide ion. Furthermore, the negative value of follows: ΔS‡, which of course indicates entropy decrease upon 1. The reaction order is unity with respect to Bro- achieving a transition state, often indicates an associa- mocresol green and hydroxide concentrations. tive mechanism. Furthermore, the relatively low values 2. The plot of 1/ kapp versus 1/[NaOH] gives an of ΔH‡ are attributed to a lack of involvement of high- intercept, which indicates the presence of an intermedi- energy free-radicals. ate complex. 3. The ionic strength dependence shows the pres- Products analysis ence of -1 and +1 charges of the reactants at the rate Sharp absorption at 1474.2 cm-1 signals the pres- determining step. ence of C=C aromatic ring. The presence of “free” O-H 4. The negative value of ΔS‡ is indicative of an as- is revealed by the sharp peak at 3650 cm-1-3600 cm-1. sociative mechanism.Rate = k2[Complex] These results provide the suggestion that carbinol is the Therefore, the below mechanism was proposed major reaction product. based on the kinetic[ data] obtained: + = 1[ ][ ] 1[ ] 2[ ] − − − − [ ] + = 1[ ][ ] ( 1 + 2)[ ] − − − (1) Rate = k2[Complex] + 0 = 1[ ][ ] ( 1 + 2)[ ] − Rate = k [Complex] − 2 − [ ] + = 1[ ][ ] 1[ ] 2[ + ] (2) 1[ ][ ] = ( 1 + 2)[ ] [ ] + = 1[ ][ ] 1−[ ] 2[ ] − − − − − − + 1[ ][ ] [ ] − [ ] = −+ − − [ ] = [ ][ ] ( + )[ ] 1 + 2 (3) 1+ 1 2 = 1[ ][ ] ( 1 + 2)[ ] − − − − (6) (1) Applying steady state approximation − − − Substituting equation (6) in (1) [+][ ] 0= [ +][+] (+ )[ ] (4) = 1 2 0 = 1 1[ ][ ]1 (2 1 + 2) [ ] − 1 + 2 − − − − − − − 1 2 + = [ ][ ] = ( + )[ ] (5) 1 + 1 2 1 + 2 1[ ][ ] = ( 1 + 2)[ ] − − ℎ − [ − +][ ] (6) = [ +][ ] [ ] = 1 − −+ − 1[+ 2 ][ ] [ ]= = [ +] + − − 1 2 (6) (1) 93 − + (6) (1) 1 2[ ][ ] = − 1 + 2 + 1 2[ ][ ] = − 1 2 + − = 1 2 1 +2 − ℎ − [ +][ 1] 2 = = 1− + 2 = [ +] ℎ − + = [ ][ ] − = [ +] Journal of Chemical Technology and Metallurgy, 54, 1, 2019 CONCLUSIONS 3.
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