Determining The Ka Of An Indicator Within An Unknown Solution

1098 Words Sep 28th, 2016 5 Pages
The purpose of this lab is to determine the Ka of an indicator within an unknown solution. This can be determined because acid-base indicators are weak bases or acids, and when neutralized they change colors. The color change is measured by the light absorption spectra in the compound which can be used to calculate the concentration. This experiment measures the concentrations by the different pH values which allows the pKa and Ka to be determined. Through the experiment, and the recorded color change with the use of the Spec 20 the unknown Ka of the solution was found.
As stated previously because acid-base indicators are weak bases or acids, they change colors. This color change will be used to determine the concentration because color change is a measurement of light absorption. The Spec 20 machine records percent transmittance, and absorbencies. Color changes will be observed as Ph values are observed. The Ka of the indicator can be measured if the Ph of the solution is known at the same time that the HIn and the In- are known. (See equation 1). Attempting to measure concentration by chemical reaction would shift the equilibrium, which in turn changes the pH and the concentrations without affecting the equilibrium. Intensity of color or absorbance of a substance is proportional to the concentration according to Beers Law (See equation 2). For this experiment the equilibrium constant of the weak acid indicator bromothymol is discovered, and used because the…

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