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Chemistry Project Topics

Kinetics and Mechanisms of the Redox Reactions of Crystal Violet With Some Oxyanions in Aqueous Acidic Medium

Kinetics and Mechanisms of the Redox Reactions of Crystal Violet With Some Oxyanions in Aqueous Acidic Medium

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Kinetics and Mechanisms of the Redox Reactions of Crystal Violet With Some Oxyanions in Aqueous Acidic Medium

Chapter One

ย Aim

This research work aims to study the kinetics and mechanism of the redox reaction of crystal violet with oxidant BrO , IOย ,ย ClO , and S Oย 2- as oxyanions in an acid medium.

Objectives

ย Theย aboveย aimย wouldย beย achievedย throughย theย followingย objectives:

  1. determining the stoichiometry of the redox reaction,
  2. estimating the rate constant as well as obtain the order of the reaction,
  3. monitoring the effect of changes in acid concentration, ionic strength and added ions on the reaction rates,
  4. testing for intermediate complex formation and free radicals and
  5. deducing a plausible mechanism and assigning operative mechanism for the

CHAPTERย TWO

ย LITERATUREย REVIEW

ย Reactionsย ofย Crystalย Violet

ย Crystal violet (Gentian violet) with a molecular formular of C25H30N3Cl is a triphenylย methane dye that is antimicrobial (Hall et al., 1966). It is used as a bacteriostatic agent in medicalย solutionsย (Bale,ย 1981),ย andย toย treat skinย infectionsย byย Staphylococcusย aureusย (Sajiย etย al.,ย 1995).

Crystal violet is a cationic dye. Cationic or basic dyes are dyes in which the chromophoreย is on the cation (Parameswaran et al. 1974). It is used as a dye and also possesses antibacterial,ย antifungalย andย anthelminticย propertiesย whichย makeย itย usefulย inย dentistryย (Gorgasย etย al.ย 1901).ย The dyeย differs fromย otherย triphenylmethanesย in that the aminoย groups are methylated orย substituted.ย Crystal violet is readily soluble in water and highly stable, with the absorptionย maximum ofย maxย 590 nm. It is very important in biological stains for the study of bacteria andย related microorganisms. The dye is also used as a constituent of culture media, as indicator andย forย laboratoryย diagnosisย ofย diseaseย (Docampoย andย Moreno,ย 1990).

The kinetic studies of the reaction of crystal violet (CV+) and chlorate (ClO ) was carriedย out in aqueous acidic medium, at a temperature of 32 +ย 1oC; I = 0.50 mol dm-3ย (NaCl), [H+] = 1.00ย x 10-2ย mol dm-3ย (HCl) (Mohammed et al., 2011). In the stoichiometry, one mole of the dye wasย consumed by two moles of ClOย . The reaction was found to be first order in both the dye andย theย oxidant. Theย rateย ofย redoxย reactionย showedย dependence onย acid concentrations.

Rateย law forย theย reactionย hasย beenย proposedย as:

Theย secondย orderย rateย constantย forย theย crystalย violetย โ€“CIOย  reactionย wasย foundย toย be 3.88 dm3ย mol-1s-1. The rate of reaction displayed zero salt effect and was not affected by changesย in dielectric constant of the reaction medium. Added anions and cations catalysed the reaction.ย Results of the Michaelis-Menten analysis gave no evidence of intermediate complex formation.ย Basedย on theย results obtainedย experimentally, the outersphereย mechanismย wasย proposedย forย the reaction.

The kinetics and mechanism of the reaction of crystal violet by peroxomonosulphateย (oxone) has been reported by Kranti (2011). The reaction was studied under pseudo first orderย condition at constant temperature of 25+ย 0.1oC. The stoichiometry was found to be one mole ofย crystal violet to two moles of oxone. The reaction was not affected by changes in the dielectricย constant of the reaction medium and there was no evidence for the formation of free radicals inย theย mechanismย of theย reaction.

ย Reactionsย ofย metabisulphiteย ion

Metabisulphite ion or pyrosulphite ion is a good reducing agent whose compounds are used as preservative and antioxidant in food (EUFIC, 2007). Concentrated sodium metabisulphite can be used to remove tree stumps and it is a primary ingredient in campden tablets, used for wine and beer making to inhibit the growth of wild yeasts, bacteria and fungi (Miline, 2005). Some brands contain 98% sodium metabisulphite, and cause degradation of lignin in the stumps, facilitating its removal (OSHA, 2008).

In the study of the kinetics of reduction of rosaniline with sodium metabisulphite, oneย mole of rosaniline was consumed by one mole of the metabisulphite ion (Onu and Iyun, 2001).ย Second order rate constant was determined and theย effect of acid on the rate of reactionย showedย two pathways,ย oneย whichย isย acidย dependentย andย the otherย whichย isย acidย independent.

 

CHAPTERย THREE

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ย MATERIALSย ANDย METHODS

ย Materials

All solutions were prepared with distilled water except otherwise stated. Analytical grade reagents were used throughout this work without further purification. Hydrochloric acid was used to investigate the effect of hydrogen ion on the reaction for the crystal violet- metabisulphite, crystal violet-bromate and crystal violet-periodate systems, while perchloric acid was used for crystal violet-hypochlorite system. Sodium chloride was used to maintain a constant ionic strength for each run in the crystal violet- metabisulphite, crystal violet-bromate and crystal violet-periodate systems, while sodium perchlorate was used for crystal violet- hypochloriteย system. Sodiumย metabisulphite,ย potassiumย bromate,ย sodiumย periodateย andย sodium hypochloriteย wereย the redox reagentsย used.

CHAPTERย FOUR

ย RESULTS

Stoichiometry

The results of the stoichiometric investigations indicate that one mole of crystal violet is consumed by one mole of metabisulphite ion, two moles of crystal violet consumed three moles of bromate ion, while one mole of crystal violet consumed two moles each of periodate ion and hypochloriteย ionย respectively.ย Theย titrationย curvesย fromย which theย stoichiometriesย ofย eachย ofย theย systemsย wereย determinedย are presented inย Figuresย (4.1-4.4).

CHAPTERย FIVE

ย ย DISCUSSION

Crystalย violet-ย metabisulphiteย ionย system

The results of the stoichiometric investigations indicated that one mole of crystal violetย was consumed by one mole of metabisulphite. This agrees with the stoichiometries observed inย the reactions of S2O 2-ย with triphenylmethane dye (Onu and Iyun, 2001) and basic fuchsin (Lawal,ย 1997). However, stoichiometry of 1:3 was established for the reduction of methylene blue withย metabisulphite (Babatundeย etย al.,ย 2013).

The order ofย theย reaction wasย foundย toย be oneย in both the oxidant andย reductantย respectively. Thus the reaction is second order overall and the second order rateย constantย k2ย was determined to be k2ย = (23 ยฑ 0.45) dm3ย mol-1ย s-1. Similar order was obtained with respect toย the reductant concentration by earlier workers (Gupta et al., 1987, Lawal, 1997, Onu and Iyunย 2001ย andย Babatundeย etย al., 2013). The order of zero in the [reductant] was observed in the reaction of Fe2O4+ย and S2O 2-(Idris et al., 2005), thus the k1ย evaluated from the slopes of theย pseudo-firstย orderย plotsย wereย constant irrespectiveย ofย the concentrationย ofย S2Oย 2-.

The studies on the effect of hydrogen ion concentration, [H+], on the rate of the reactionย showedย thatย increaseย inย hydrogenย ionย concentrationย decreasedย theย rateย ofย reactionย withย negative slope of 0.97 (Figure 4.15). Least square plot of k2ย versus [1/H+] had intercept (Figureย 4.19), this showed two parallel reaction pathways: the acid independent and the inverse acidย dependentย pathways.ย The inverseย acidย pathwayย showsย thatย thereย isย aย pre-equilibriumย stepย before theย rateย determiningย stepย inย whichย a protonย isย lost. This means that theย twoย rate-ย controlling steps are preceeded by a rapid equilibrium for which the equilibrium constant isย small, and both the forms, protonated and deprotonated, are reactive (Gupta and Gupta 1984).ย Similar acid dependence pathway has been reported on the oxidation of S2O 2-ย by basic fuchsinย (Lawal, 1997) and Reduction of triphenylmethane dye by S2O 2-(Onu and Iyun, 2001). The resultย in this differs from Babatunde et al., 2013 where enhancement in the rate was obtained byย increaseย in hydrogen ionย inย theย reductionย ofย methylene blueย with S2Oย 2-.

whereย a = 2.27 dm3ย mol-1ย s-1, b = 0.86 s-1, c = 9.22 dm3ย mol-1ย s-1ย d = 1.16 dm9ย mol-3ย s-1,ย eย =ย 1.49ย s-1,ย fย =ย 1.8ย x 10-3ย dm3ย mol-1ย s-1ย andย g =ย 6.16ย dm6ย mol-2ย s-1.

The second order rate constants for the various systems were observed as follows:ย Crystalย violet-S2Oย 2-ย reaction,ย (kย = 23.57ย ยฑย 0.45 dm3ย mol-1ย s-1)

Crystal violet-BrO ย reaction, (kย 38.47 ยฑ 0.26 dm3ย mol-1ย s-1)ย Crystal violet-ClOย reaction, (k2ย = 52.53 ยฑ 0.41 dm3ย mol-1ย s-1)ย Crystalย violet-IOย ย reaction,ย (kย =ย (6.89 ยฑย 0.032)ย xย 10-2ย s-1)

All reactions were sensitive to change in ionic strength except the crystal violet-ClOย which shows no dependent on ionic strength. Except in crystal violet-ClOย system where addedย anionsย increase theย rate of reactionย and crystal violet-IO ย whereย anionsย had noย effect on theย rateย of reaction, addedย anions inhibited theย rateย of reactionsย forย theย restย ofย theย systems.

Evidence of intermediate complex formation was suggested for the crystal violet-IO ย reactions by the shift in ฮปmaxย from 585-620 nm and no shift was observed for the other systems.ย Free radicals were not observed for all the systems. Thus the results obtained in this study showย that crystal violet-S2O 2-, crystal violet-BrO ย and crystal violet-ClOย reactions could be said to beย occurring through the outersphere mechanism, while the crystal violet-IO ย reaction occurs viaย theย innersphereย mechanism.ย Thereforeย suchย mechanismsย wereย proposedย forย theย reactionsย respectively.

CHAPTERย SIX

ย ย SUMMARY,ย CONCLUSIONย ANDย RECOMMENDATION

ย Summary

The kinetics study of the redox reactions of crystal violet with some oxyanions (S2O 2-,ย BrO , IO ย and ClO) in aqueous acid solution was carried out. The stoichiometry of 1:1 wasย observedย forย CV+-S2Oย 2-,ย 2:3ย forย CV+-BrOย ย andย 1:2ย forย CV+-IOย ย andย CV+-ClOย systemsย respectively.

The reactions were second order at constant [H+] for all the systems except that ofย crystal violet with IO ย which was first order. Apart from the crystal violet-S O 2- system whichย showedย inverseย firstย orderย hydrogenย ionย dependenceย pathways,ย acidย dependentย pathwaysย was observed for the crystal violet ClO,ย whereasย acidย dependentย andย acidย independentย pathwaysย wereย observedย forย crystalย violetโ€“IOย ย andย crystalย violet-BrOย ย systemsย respectively.

Theย reactionsย areย thereforeย in conformityย withย theย followingย rateย equations:

Conclusion

On the basis of the results of these investigations, absence of and/or presence of kinetic and/or spectroscopic evidence for complex formation prior to electron transfer and non- conformity of the results with Michaelis-Menten plots, rationalization of previous results, outersphere mechanism is postulated for the CV+-S2O52-,ย CV+-BrO ย andย CV+-ClOย reactions,ย whereas,ย innersphereย mechanism is postulated forย CV+-IOย ย reaction.

Recommendation

ย It is recommendedย that:

  1. further studies should be carried out on the activated parameters of these reactions,
  2. thorough analysis to identify the organic product should be made, and
  3. more systems involving other oxidants and reductants should be studied for more information on the chemical characteristics of crystal

REFERENCES

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  • Abdel-Khalek, A.A. Sayyah, S.M. and Abdel Hamed, F.F. (1994). Kinetics and mechanism of oxidation of chromium(III)-tetraoxalurea complex by periodate. Transitionย Metalย Chemistry 19:108-110.
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