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Journal of Chemical Technology and Metallurgy, 54, 1, 2019

90

KINETIC AND THERMODYNAMIC STUDY OF THE REACTION BETWEEN BROMOCRESOL GREEN WITH SODIUM HYDROXIDE IN AN AQUEOUS SOLUTION

Latona Dayo Felix1, Adejoro Ajibade2

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.

Received 15 November 2017Accepted 06 March 2018

Journal of Chemical Technology and Metallurgy, 54, 1, 2019, 90-94

1 Department of Chemical Sciences, Osun State University PMB 4494 Osogbo, Nigeria2 Department of Chemistry, University of Ibadan, Nigeria E-mail: [email protected]

INTRODUCTION

Bromocresol green belongs to the triphenylmethane dye family. It is an important industrial raw material in textile, leather, paper, printing, plastic and ceramic industries [1]. Bromocresol green is employed in col-orimetric detection technologies and for visualization of compounds of a functional group whose pKa is below 5.0. Most importantly it is used in charge-transfer com-plexation processes [2]. The dyes have carcinogenic properties and hence a concomitant effect on the aqua life. The need to find a way of removing these dyes prior

to emptying them into the water bodies as industrial ef-fluents is imminent in order to protect the aquatic organ-isms [3]. Several methods of achieving this noble objec-tive are reported [4-7]. However, this research tends to investigate the option of hydrolyzing Bromocresol green in an alkaline medium with the view to ascertaining the kinetics and the mechanism of the reaction, which has hitherto not been reported in the literature. It is worth noting that Bromocresol green ionizes in an aqueous solution to give a monoanionic form (yellow), which further deprotonates at higher pH to give a dianionic form (blue) [8] as shown below:

Protonation/deprotonation of Bromocresol green in an aqueous solution

Latona Dayo Felix, Adejoro Ajibade

91

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 green dye. Appropriate quantities of Bromocresol green, potassium nitrate and sodium hydroxide were measured from the stock solutions (kept in a water bath for 30 min). The kinetic data was obtained by monitoring the change of the absorbance of Bromocresol green with time at 441nm using a UV-1800 Shimadzu spectropho-tometer. The latter had a thermostated cell interfaced to a computer. The pseudo-first order rate constant (kapp) was obtained from a plot of ln A against time, while the reaction products were characterized by FTIR.

RESULTS AND DISCUSSION

The stoichiometry of the reaction is determined via spectrophotometric titration. The absorbance at infinity time of solutions containing various concentrations of sodium hydroxide in the range from 6.67 x 10-4 mol dm-3

to 4.00 x 10-3 mol dm-3, a fixed initial concentration of Bromocresol green at 1 x 10-5 mol dm-3 and a constant initial concentration of KNO3 at 5 x 10-2 mol dm-3 are measured. The stoichiometry is evaluated from the plot of the absorbance versus [NaOH]. It is found to be 1:1.

Therefore the overall reaction can be presented by:

Effect of Bromocresol green concentration on the observed rate constant

The effect of Bromocresol green concentration on the apparent rate constant is determined studying reac-tion mixtures containing [BCG]o ranging from 1x10-5 mol dm-3 to 8x10-5 mol dm-3, sodium hydroxide at a fixed initial concentration of 1x 10-3 mol dm-3 and at ionic strength of 8.0 x 10-2 mol dm-3 at 298K. The reaction shows an increase of the apparent rate constant with increase of Bromocresol green concentration as shown in Fig. 1. The plot of ln appk versus ln[BCG] has a slope of 1 implying a first order dependence with respect to Bromocresol green concentration.

Effect of sodium hydroxide concentration on the ap-parent rate constant

The effect of sodium hydroxide concentration on the apparent rate constant is determined at 298K studying reaction mixtures containing BCG at a fixed concentration of 1 x 10-5 mol dm-3, sodium hydroxide with a concentration ranging from 6.70 x 10-4 mol dm-3 to 4.00 x 10-3 mol dm-3 and a fixed ionic strength of 5 x 10-2 mol dm-3. An increase of the apparent rate constant ( appk ) with sodium hydroxide concentration increase is

Fig. 1. A plot of appk versus [BCG]. Fig. 2. Plot of 1/ appk versus 1/[NaOH].


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observed as shown in Table 1. The slope of the plot of ln appk versus ln[NaOH] is unity indicating a first order dependence with respect to sodium hydroxide concen-tration. Moreover, a plot of 1/ appk versus 1/[NaOH] gives an intercept, which reveals the presence of an 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 appk versus [NaOH] is found equal to 1.80 mol-1dm3s-1.

Effect of ionic strength on the apparent rate constantThe dependence of the ionic strength on the apparent

rate constant is determined at 298K by studying a reac-tion mixture of [BCG]o of 1x10-5 mol dm-3, [NaOH]o of 1.00x10-3 mol dm-3 and an ionic strength within the range from 1x10-2 mol dm-3 to 7x10-2 mol dm-3. The apparent rate constant decreases with increase of the ionic strength (µ) of the solution. A plot of log appk versus √µ gives a straight line with slope equal to -1, which suggests the presence of -1 and +1 charges on the reactants at the rate determining step as shown in Fig. 3.

Temperature effect and activation parameters de-termination

The temperature effect on the apparent rate constant is determined by varying the temperature within the range of 298K- 313K at a fixed [BCG]o of 1.67x10-5 mol dm-3, [NaOH]o of 6.67x10-4 mol dm-3 and an ionic strength of 9.5x10-2 mol dm-3. There is rate constant increase with temperature increase (Fig. 4). The activa-tion energy (Ea) and the activation parameters (ΔH‡, ΔS‡, ΔG‡) of the reaction are obtained on the ground of the equations given below:

log 𝑘𝑘 = log𝐴𝐴 – 𝐸𝐸𝑎𝑎

2.303𝑅𝑅𝑅𝑅

ln �𝑘𝑘𝑅𝑅�

= −∆𝐻𝐻‡

𝑅𝑅𝑅𝑅 + ln�

𝑘𝑘/

ℎ� + �

∆𝑆𝑆‡

𝑅𝑅�

ln�𝑘𝑘/

ℎ� = 23.76

ΔG‡= ΔH‡ - TΔS‡

where k is the apparent rate constant, T is the tempera-ture, ΔH‡ is the enthalpy of activation, ΔS‡ is the entropy of activation, ΔG‡ is the free Gibb’s energy of activation, R is the the molar gas constant, k/ is the Boltzmann’s constant, while h is the Plank’s constant. The values obtained are listed in Table 2.

The negative value of the entropy of activation reveals an increase of the order or loss of freedom in the course of transition state complex formation. It is expected that a closely held solvation sphere exists in

103[OH-]/ mol dm-3 103 appk /s-1 0.67 1.00 1.33 1.67 2.00 2.50 3.00 3.50 4.00

1.36 2.05 2.56 3.25 3.67 4.68 5.43 6.05 6.98

Table 1. Effect of [OH-].

Fig. 3. Plot of log appk versus √µ. Fig. 4. Plot of appk versus temperature

Latona Dayo Felix, Adejoro Ajibade

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case of Bromocresol green cation. The high degree of order of this sphere is disrupted when it reacts with the hydroxide ion. Furthermore, the negative value of ΔS‡, which of course indicates entropy decrease upon achieving a transition state, often indicates an associa-tive mechanism. Furthermore, the relatively low values of ΔH‡ are attributed to a lack of involvement of high-energy free-radicals.

Products analysisSharp absorption at 1474.2 cm-1 signals the pres-

ence of C=C aromatic ring. The presence of “free” O-H is revealed by the sharp peak at 3650 cm-1-3600 cm-1. These results provide the suggestion that carbinol is the major reaction product.

Mechanism and a rate lawThe kinetic results obtained can be summarized as

follows: 1. The reaction order is unity with respect to Bro-

mocresol green and hydroxide concentrations.2. The plot of 1/ appk versus 1/[NaOH] gives an

intercept, which indicates the presence of an intermedi-ate complex.

3. The ionic strength dependence shows the pres-ence of -1 and +1 charges of the reactants at the rate determining step.

4. The negative value of ΔS‡ is indicative of an as-sociative mechanism.

Therefore, the below mechanism was proposed based on the kinetic data obtained:

Rate = k2[Complex] 𝑑𝑑[𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 ]

𝑑𝑑𝑑𝑑= 𝑘𝑘1[𝐵𝐵𝐶𝐶𝐵𝐵+][𝑂𝑂𝐻𝐻−] − 𝑘𝑘−1[𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶] − 𝑘𝑘2[𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶]

𝑑𝑑[𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶]

𝑑𝑑𝑑𝑑= 𝑘𝑘1[𝐵𝐵𝐶𝐶𝐵𝐵+][𝑂𝑂𝐻𝐻−] − (𝑘𝑘−1 + 𝑘𝑘2)[𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶]

𝐴𝐴𝐶𝐶𝐶𝐶𝐶𝐶𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑠𝑠𝑑𝑑𝐶𝐶𝑎𝑎𝑑𝑑𝐴𝐴 𝑠𝑠𝑑𝑑𝑎𝑎𝑑𝑑𝐶𝐶 𝑎𝑎𝐶𝐶𝐶𝐶𝑎𝑎𝐶𝐶𝐶𝐶𝐴𝐴𝐶𝐶𝑎𝑎𝑑𝑑𝐴𝐴𝐶𝐶𝐴𝐴 0 = 𝑘𝑘1[𝐵𝐵𝐶𝐶𝐵𝐵+][𝑂𝑂𝐻𝐻−] − (𝑘𝑘−1 + 𝑘𝑘2)[𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶] 𝑘𝑘1[𝐵𝐵𝐶𝐶𝐵𝐵+][𝑂𝑂𝐻𝐻−] = (𝑘𝑘−1 + 𝑘𝑘2)[𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶]

[𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶] =𝑘𝑘1[𝐵𝐵𝐶𝐶𝐵𝐵+][𝑂𝑂𝐻𝐻−]

𝑘𝑘−1 + 𝑘𝑘2

𝑆𝑆𝑆𝑆𝑆𝑆𝑠𝑠𝑑𝑑𝐴𝐴𝑑𝑑𝑆𝑆𝑑𝑑𝐴𝐴𝐴𝐴𝐴𝐴 𝐶𝐶𝑒𝑒𝑆𝑆𝑎𝑎𝑑𝑑𝐴𝐴𝐶𝐶𝐴𝐴 (6) 𝐴𝐴𝐴𝐴 (1)

𝑅𝑅𝑎𝑎𝑑𝑑𝐶𝐶 = 𝑘𝑘1𝑘𝑘2[𝐵𝐵𝐶𝐶𝐵𝐵+][𝑂𝑂𝐻𝐻−]


𝑊𝑊ℎ𝐶𝐶𝑎𝑎𝐶𝐶 𝑘𝑘 = 𝑘𝑘1𝑘𝑘2


𝑅𝑅𝑎𝑎𝑑𝑑𝐶𝐶 = 𝑘𝑘[𝐵𝐵𝐶𝐶𝐵𝐵+][𝑂𝑂𝐻𝐻−]

𝑅𝑅𝑎𝑎𝑑𝑑𝐶𝐶 = 𝑘𝑘𝑎𝑎𝐶𝐶𝐶𝐶 [𝐵𝐵𝐶𝐶𝐵𝐵+]

(1)

(2)

(3)















Applying steady state approximation















Substituting equation (6) in (1)

(4)

(5)

(6)

Dye Ea (kJ mol-1) ΔH‡ (kJ mol-1) ΔS‡(kJK-1 mol-1) ΔG‡ (kJmol-1)

Bromocresol

green

19.82 17.66 -0.24 89.18

Table 2. Activation parameters.


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CONCLUSIONS

Alkaline hydrolysis of Bromocresol green is first order in respect to Bromocresol green and the hydroxyl ion participating. The hydrolysis of Bromocresol green involves an attack of the hydroxyl ion at the carbon within the dye planar ring. This results in destruction of the conjugation configuration of the dye. Carbinol is the resultant product. Bromocresol green dye is removed from industrial effluents by a hydrolytic reaction as this is the fastest and the least cost effective technique compared to the photolytic and the adsorption methods.

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5. I. Alamddine, M.M. El Jamal, Effect of supporting electrolyte and substitution of electrochemical treat-ment of monoazo benzene dyes, J. Univ. Chem. Technol. Met., 44, 2, 2009, 127-132.

6. R. Rammel, Zatiti, M.M. El Jamal, Biosorption of crystal violet by chaetophora Elegas Alga, J. Univ. Chem. Technol. Met., 46, 3, 2011, 283-292.

7. M. Ozdemir, O. Durmus, O. Sahin, C. Saka, Removal of Methylene blue, Methyl violet, Rhodamine B, Alizarin and Bromocresol green Dyes from Aqueous Solution on Activated Cotton Stalks, Journal Desalination and Water Treatment, 57, 38, 2016, 18038-18048.

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