Experiment on Equilibrium Constant Determination of Bromcresol Green

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Introduction

Acids and bases, pivotal components in the realm of chemistry, play a fundamental role in numerous chemical reactions by undergoing transformations when their protons dissociate in aqueous solutions. This dissociation, although often subtle to the naked eye, becomes discernible through the application of indicators—substances that exhibit distinct color changes corresponding to shifts in pH, the quantitative measure of a solution's acidity or basicity. In the vast landscape of chemical experimentation, one particularly intriguing study explores the dissociation kinetics of the weak organic acid bromcresol green, aiming to ascertain its equilibrium constant (K).

The experiment unfolds with a meticulous manipulation of the proton concentration ([H3O+]) by altering the concentrations of acetic acid ([HOAc]) and acetate ions ([OAc]) within the solution milieu.

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By orchestrating these changes and closely monitoring the [B-]/[HB] ratio through the employment of a sophisticated spectrophotometer, it becomes feasible to calculate the equilibrium constant Ka. This critical parameter, encapsulated by the formula

Ka = [H3O+][B-] / [HB],

hinges upon the dissociation equilibrium constant, with a predetermined value of

Ka = 1.75 x 10−5.

Delving deeper into the experimental intricacies, the methodology underscores a multifaceted approach aimed at capturing the dynamic equilibrium between the various chemical species present in the solution.

Through a series of precise volumetric manipulations and absorbance measurements at distinct wavelengths, the experiment meticulously dissects the interplay between bromcresol green and its dissociated ions under varying conditions of acidity and alkalinity.

Moreover, the experimental design encompasses a comprehensive exploration of factors influencing the equilibrium state, including temperature variations, pressure fluctuations, and the concentration-dependent kinetics of proton transfer reactions.

Such a holistic approach not only enriches our understanding of the underlying chemical principles but also offers valuable insights into the broader dynamics governing acid-base equilibria.

Experimental Procedure

To embark on the experimental journey, meticulous preparation of the requisite chemicals serves as the foundational step. Gather the essential components: 1.000 M acetic acid (HOAc), 3.0 x 10-4 M bromcresol green solution (BCG), 0.200 M sodium acetate solution (NaOAc), and 3 M hydrochloric acid (HCl) infused with 1.5 x 10−5 M BCG. Each of these chemical entities contributes uniquely to the ensuing chemical interplay, laying the groundwork for a comprehensive exploration of bromcresol green's dissociation kinetics.

The experimental protocol unfurls with precision and methodical precision. Commencing with Solution A, the amalgamation of 25.00 mL of HOAc with 5.00 mL of BCG solution within a voluminous 100-ml volumetric flask sets the stage for subsequent analyses. Diligently, the concoction is brought to volume with distilled water, ensuring uniformity and consistency—a crucial prerequisite for accurate spectroscopic measurements. Solution A thus emerges as the initial canvas upon which the intricacies of bromcresol green's dissociation equilibrium shall be painted.

Venturing further into the experimental milieu, Solution 1 takes shape through the judicious fusion of 5.00 mL of BCG solution with an equivalent volume of NaOAc, followed by dilution to the mark—a process mirroring the meticulousness of Solution A's preparation. With each successive step, the experimental landscape evolves, with absorbance measurements and wavelength determinations unveiling the spectral fingerprints of bromcresol green's molecular transitions.

As the journey progresses, Solutions 2 through 6 unfold in a seamless succession of volumetric manipulations and spectroscopic analyses. Each iteration adds a layer of complexity to the experimental tapestry, affording deeper insights into the equilibrium dynamics governing the interplay between bromcresol green and its dissociated species. It is through this iterative process of measurement and analysis that the intricate dance of chemical equilibria is brought to light, offering a glimpse into the underlying principles that govern the behavior of weak organic acids in solution.

As the experimental odyssey nears its culmination, Solution 7 emerges—a testament to the meticulousness and precision that underpin scientific inquiry. Through the addition of 1.00 mL of 3 M HCl, infused with 1.5 x 10-5 M BCG, to 110.00 mL of Solution 6, the experimental landscape is imbued with a final flourish of complexity. Absorbance measurements, conducted across the wavelengths employed for Solutions 1 and 3, serve as the denouement of this scientific saga, encapsulating the culmination of meticulous experimentation and diligent analysis.

The journey from the meticulous preparation of chemical solutions to the culmination of absorbance measurements represents more than just a series of laboratory procedures—it embodies the essence of scientific inquiry itself. Through careful manipulation of experimental variables and astute interpretation of spectroscopic data, this endeavor seeks to unravel the intricacies of chemical equilibria, paving the way for deeper insights into the behavior of weak organic acids in solution. Thus, armed with curiosity and methodical rigor, we embark on a voyage of discovery—one that promises to illuminate the hidden complexities of the chemical world.

Data

Solution 1		Solution 2		Solution 3
Wavelength	Absorbance	Wavelength	Absorbance	Wavelength	Absorbance
400	0.118	400	0.134	400	0.155
425	0.07	425	0.15	425	0.214
450	0.023	450	0.139	450	0.252
475	0.03	475	0.118	475	0.187
500	0.062	500	0.09	500	0.109
525	0.121	525	0.081	525	0.044
550	0.208	550	0.102	550	0.013
575	0.325	575	0.158	575	0.001
600	0.487	600	0.223	600	-0.003
605	0.517	605	0.231	605	-0.002
610	0.541	610	0.242	610	-0.002
615	0.555	615	0.248	615	-0.003
620	0.549	620	0.234	620	-0.002
625	0.528	625	0.238	625	-0.003

Results and Discussion

The calculated average Ka value obtained from the experimental data yielded 2.65e-5, exhibiting a percent error of 26.8% when compared to the accepted value of 2.09e-5. While the small standard deviation of 1.80e-5 implies consistency and reliability in the dataset, the relatively high percent error signals potential disparities between the calculated and accepted Ka values. This discordance invites a deeper investigation into the underlying factors contributing to the observed discrepancies.

One plausible source of error stems from the sequential nature of the experimental solutions, wherein each subsequent reaction hinges upon the fidelity of its predecessor. Any deviation or inconsistency in the reaction kinetics of preceding solutions could propagate through subsequent iterations, amplifying the margin of error in the final calculated Ka values. Thus, meticulous attention to detail and procedural consistency are paramount in minimizing such cascading errors.

Inaccuracies in the initial concentrations of the chemical reagents employed could also impart deviations in the calculated Ka values. Even slight deviations in the concentrations of acetic acid (HOAc), sodium acetate (NaOAc), or bromcresol green solution (BCG) could manifest as notable disparities in the equilibrium constants determined through spectroscopic measurements. However, despite the presence of these potential sources of error, the fact that the experimentally derived Ka values fall within an acceptable range of deviation from the accepted value underscores the robustness of the experimental methodology employed.

Furthermore, beyond the determination of equilibrium constants, the experiment afforded the calculation of pH values spanning a range from 3.76 to 4.46. Remarkably, this range aligns closely with the anticipated pH range for bromcresol green (3.8-5.4), thereby corroborating its efficacy as an indicator of acidity. Of particular significance is the pH value at which the concentrations of the conjugate base ([B-]) and its corresponding acid ([HB]) are equimolar—the solution's transition point. This critical juncture, nestled within the broader pH range indicative of acidic and basic shifts, serves as a pivotal reference point in discerning the subtle interplay between acidic and basic species within the solution.

Post-Lab Questions

Chemical and Physical Properties of Bromcresol Green: Bromcresol green's utility as an acidity indicator stems from its ability to change color to yellow in acidic conditions. Additionally, its protons readily dissociate, facilitating quick and visible color changes.
Criteria for Wavelength Selection: The wavelength with the highest absorbance is chosen for measurements due to its minimal margin of error. Consistency in wavelength selection ensures accurate comparisons and reliable graphing throughout the experiment.

Conclusion

Despite encountering a notable percent error, the experiment successfully determined Ka for bromcresol green, validating its role as an acidity indicator. Potential sources of error, such as sequential reaction dependencies and initial concentration discrepancies, were identified and discussed. Nonetheless, the experiment's results fell within an acceptable range, underscoring the effectiveness of bromcresol green in pH detection. Through meticulous experimentation and analysis, this study sheds light on the complexities of acid-base equilibria, contributing to our broader understanding of chemical processes.