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The pKa of an Unknown Acid-Base Indicator

By: Josephine Hong

Lab Instructor: Yue Zhang

Due: April 5, 2013
Submitted: April 5, 2013

In this lab, the pKa of the unknown indicator of expression was determined both qualitatively and quantitatively. To verify our procedures, the experiment was tested using a known indicator, bromocresol green. Qualitatively, we used the color change of the solution with indicator to obtain the pKa value solely using a pH meter. Quantitatively, we used a pH meter and the spectrophotometer with varying concentrations of the acid and base. The maximum wavelengths of absorbance used to quantify the dissociated and undissociated forms of the bromocresol green were 440 nm (yellow, undissociated) and 616 nm (blue dissociated). For the unknown indicator, the wavelengths were 505.96 nm (red, undissociated) and 601.66 nm (blue, dissociated). For bromocresol green, the experimentally obtained pKa value measured qualitatively was 4.04. Quantitatively, the pKa came out to be 4.16. The percentage error (10.47%) obtained was within reasonable range, allowing the same procedures to be used to determine the pKa value of the unknown indicator, Indicator of Freedom. Indicator was Freedom was found to have a pKa value of 5.32 qualitatively. On the other hand, the quantitative pKa was measure to be 4.265. Introduction:

This lab applies the concept of an indicator dye, which is principally a weak acid that changes colors when reacting with the amount of hydronium ion in a solution. Thus, the qualitative part of the lab assumes that at the point where the solution changes color into an intermediate shade of the two, the concentrations of [HIn] and [In-] is approximately equal. Using the Henderson-Hasselbalch equation for them:

the pH equals pKa when the ratio between the indicator and its conjugate base is 1. Subsequently, the quantitative portion of the lab deals with the relationship between absorbance and concentration: A1(λ)A2(λ)= c1c2

Thus, a certain absorbance of a solution can provide the concentration of the indicator and its conjugate base when taking into consideration the maximum wavelength of the two colors. Experimental Section:

To reduce the volumes of acid and base needed and the amount of time to complete the lab, the stock solutions of strong acid and base were diluted to at least one-fourth their starting amount. Thus, 60 mL of deionized water was added to both stock solutions of 20 mL HCl and NaOH to create 2 new 80 mL diluted solutions. Part 1: Finding pKa Qualitatively

25 mL solution of the diluted NaOH with 3 drops of indicator was titrated against the diluted HCl drop wise until a color change occurred. To determine the pKa of the indicators, the pH of the solutions were taken with a pH meter when there was a color change. Determining the color of bromocresol green to be yellow when acidic and blue when basic, the point of color change to measure pH was when the solution turned green. Likewise, the indicator of Freedom was red when acidic and blue when basic, which established the point of color change to be purple. These steps were performed three times for both the known and unknown indicators to find an average pKa value. Part 2: Finding pKa Quantitatively

For the quantitative determination of pKa, multiple solutions of varying acid/ base concentration were prepared with 3 drops of indicator. Despite the varying concentrations of each solution, the total volume stayed constant at 20 mL. Before anything, the wavelengths of maximum absorbance were determined first by using solutions of pure acid and base with three drops of indicator. For each solution, the pH was measured before placing them in cuvettes to measure their absorbance values using the spectrophotometer. These steps were performed for both the known and unknown indicator. Results:

Table 1: Qualitative Data for Bromocresol Green

Trial| pH|
1| 3.82|
2| 3.93|
3| 4.36|...
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