The pKa of an Unknown Acid-Base Indicator

By: Josephine Hong

Lab Instructor: Yue Zhang

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

Abstract: 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,
Because the experimentally determined pKa was less than the actual pKa for both cases, strong enough color change were probably not observed. Since the titration was performed roughly using drops, the drops could have added more acid than needed, resulting in a smaller pKa..
Quantitatively, the pKa can be determined by plotting the data, with absorbance as a function of pH. There will be two separate lines, one for each wavelength of the two colors. The intersection of these two lines will indicate the point at which the pH should be equal to the pKa. This works because at the intersection point: pH=pKa+logIn-HIn It is known that A1A2= c1c2. Since A1= A2 at the intersection point of the graph, then 1= c1c2= In-HIn.
Thus,pH=pKa+log1
pH=pKa at the intersection of both curves The procedure for determination of bromocresol green pKa appeared to have worked, because the percent error was roughly around 10%. Although slightly high, these errors are unpreventable. For example, cuvettes with smudges on the sides would have increased the absorbance readings of the solutions. Moreover, the maximum wavelength is crucial because it is used as a standard to know where to record the absorbance levels of the other solutions. This is because it is where the maximum absorbance occurs for the particular color produced by the solution is. It is important to measure the absorbance levels at these standardized wavelengths to keep the