This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: TITLE: SPECTROPHOTOMETRIC DETERMINATION OF DISSOCIATION CONSTANT USING AN ACID-BASE INDICATOR Abstract: The purpose of this lab was to demonstrate the link between pH and absorbance measurements by performing an acid-base titration using a weakly acidic indicator that exhibit different colors depending on whether or not it was present in the solution as its basic or acidic form. By determining the pH of the vary samples, the K a was determined and compared to the literature value. The pH of the unknown was 4.6. The average K a , 1.8x10-5 , was the exact same as the literature value of 1.8x10-5 . Introduction: The purpose of this experiment was to determine the K a of an organic acid. An acid-base indicator is commonly used in chemical analysis to estimate the pH of a solution. In order to determine the K a of an organic acid the indicator, bromocresol green, was used. The indicator itself is a weak acid that changes color in response to changes in hydrogen ion concentrations, and therefore has a K a . For strong acidic solutions, the indicator was in the HIn form and showed a color change of yellow. In strong basic solutions, the indicator was in the In- form and showed a color change of blue. Lastly at intermediate pH values, the solutions were a mixture of both, the HIn and In- forms, depending on their relative amounts, and showed a color change of green. Changes in pH in accordance with the K a for the indicator resulted in changes in the absorbance for each form in the various solutions. Because the two forms absorb differently, the compound can be analyzed to obtain the pH to determine the K a for the organic acid. The reaction for this experiment is given by Reaction 1: (1) Brown 10/4/2010 The experiment was broken into two parts. In the first part, a series of buffers solutions were prepared and characterized. The buffer series were used as a reference point for the second half of the experiment. In the second part, the dissociation constant of the indicator was determined by first obtaining the molar absorptivity calculated for samples 1 and 9. Once the molar absorptivity was calucalated the concentration of [In- ]/[HIn] in each solution was then obtained. The calculated pKa from all the samples then used to calculate the K a, thus determining the K a of the unknown. DATA: Table 1. Raw data collected for the buffer solutions diluted to 100mL Samples A B C D E 1.05M HA mL 1.0 1.0 5.0 4.0 0.0 0.997M (8.201g)A- mL 1.0 5.0 1.0 4.0 0.0 pH measured 4.64 5.35 3.96 4.63 5.27 # drops of 1M NaOH 6 8 34 26 1 Table 2. Raw data collected for the determination of the pKa of BCG Soln # HCl/NaOH mL BCG mL HOAc mL OAc- mL pH measured Wavelength nm 1 5.0 (HCl) 5.0-- 1.18 449.984 2- 5.0 10.0 1.0 3.63- 3- 5.0 5.0 1.0 3.93- 4- 5.0 10.0 5.0 4.29- 5- 5.0 5.0 5.0 4.59- 6- 5.0 5.0 10.0 4.90- 7- 5.0 1.01....
View Full Document
This note was uploaded on 02/03/2011 for the course CHEM 3000 taught by Professor Staff during the Fall '08 term at Kennesaw.
- Fall '08