The Equilibrium Constant of Bromocresol Green Purpose of the Experiment To determine the equilibrium constant of bromocresol green using spectrophotometric techniques. Equipment SpectroVis spectrophotometer and LabQuest, cuvette, beakers: 50-, 100- and 250-mL beakers, 50-mL graduated cylinder, 10-mL graduated pipette, stir rod. Reagents Bromocresol green (BCG) and sodium acetate (NaAc) mixture (6.0 x 10M BCG and 0.020 M NaAc), acetic acid, a 0.25 M solution, KCl, a 1.0M solution. INTRODUCTION The indicator bromocresol green is a weak acid that dissociates according to the following equation: (I1.1) (yellow) (blue). Hence, when [H] is large, i.e., highly acidic solutions, this equilibrium is shifted far to the left (by Le Chatelier’s Principle) and the indicator is largely in the HB yellow form. Likewise, when [H] is very small, i.e., basic solutions, the above equilibrium is shifted far to the right and the indicator is largely in the B blue form. The equilibrium constant expression for the reaction is: H L21.
The Equilibrium Constant of Bromocresol Green Purpose of the Experiment To determine the equilibrium constant of bromocresol green using spectrophotometric techniques. Equipment SpectroVis spectrophotometer and LabQuest, cuvette, beakers: 50-, 100- and 250-mL beakers, 50-mL graduated cylinder, 10-mL graduated pipette, stir rod. Reagents Bromocresol green (BCG) and sodium acetate (NaAc) mixture (6.0 x 10M BCG and 0.020 M NaAc), acetic acid, a 0.25 M solution, KCl, a 1.0M solution. INTRODUCTION The indicator bromocresol green is a weak acid that dissociates according to the following equation: (I1.1) (yellow) (blue). Hence, when [H] is large, i.e., highly acidic solutions, this equilibrium is shifted far to the left (by Le Chatelier’s Principle) and the indicator is largely in the HB yellow form. Likewise, when [H] is very small, i.e., basic solutions, the above equilibrium is shifted far to the right and the indicator is largely in the B blue form. The equilibrium constant expression for the reaction is: H L21.
The Correct Answer and Explanation is:
Correct Answer:
The equilibrium constant expression for the dissociation of bromocresol green (BCG), a weak acid indicator, is: HB⇌H++B−\text{HB} \rightleftharpoons \text{H}^+ + \text{B}^- K=[H+][B−][HB]K = \frac{[\text{H}^+][\text{B}^-]}{[\text{HB}]}
Explanation
Bromocresol green (BCG) is a pH-sensitive indicator that exhibits different colors depending on the pH of the solution. It behaves as a weak acid (HB) that dissociates in water to form its conjugate base (B⁻) and a hydrogen ion (H⁺), following this equilibrium: HB (yellow)⇌H++B−(blue)\text{HB (yellow)} \rightleftharpoons \text{H}^+ + \text{B}^- \text{(blue)}
In acidic solutions where [H+][H^+] is high, Le Chatelier’s Principle dictates that the equilibrium shifts to the left, favoring the undissociated yellow form, HB. In basic solutions where [H+][H^+] is low, the equilibrium shifts to the right, producing more B⁻ (blue). This color change is the basis for its use as a pH indicator.
The equilibrium constant KK for this dissociation describes the ratio of the product concentrations to the reactant concentration at equilibrium: K=[H+][B−][HB]K = \frac{[\text{H}^+][\text{B}^-]}{[\text{HB}]}
This expression allows us to determine KK spectrophotometrically by measuring the concentrations of HB and B⁻. Since these species have different colors, their absorbances at specific wavelengths can be measured using a spectrophotometer. The molar absorptivity (ε) values for HB and B⁻ allow us to use Beer’s Law (A=ε⋅l⋅cA = \varepsilon \cdot l \cdot c) to calculate their respective concentrations from their absorbances.
By preparing a solution of BCG in a buffered environment with known pH (and hence known [H⁺]), and measuring the absorbance, the concentrations of B⁻ and HB can be calculated, and subsequently the equilibrium constant KK can be determined. This experiment is a clear demonstration of how spectrophotometry and chemical equilibria can be combined to analyze acid-base behavior quantitatively.
