# Determining the Pka and Ka of Bromothymol Blue Using a Spectrophotometer

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Introduction Most acid-base reactions are reversible which means they can have a largely varied range of acidity. Bromothymol Blue is an indicator used in the experiment to signal a specific point in an acid-base reaction by changing color. In this experiment students learned how to analyze this information to calculate Ka and pKa of each solution, which varied in pH. Students used a spectrophotometer in the experiment to analyze the absorbance of different wavelengths of light passing through multiple solutions containing Bromothymol Blue. Materials and Methods The experiment was started by setting up a Spec 20 spectrophotometer and collecting materials such as glassware, stirring rods, and thermometers to mix solutions containing Bromothymol Blue. Approximately .416 grams of Na2HPO4 and .409 grams of NaH2PO4 were dissolved with 50mL of distilled water in a 150mL breaker to create a buffer solution. Twenty drops of 0.04% Bromothymol Blue were added to the solution and the pH was measured to be exactly 7 using a Vernier probe. 5mL of solution was then distributed to three 100mL beakers using a volumetric pipette. Each beaker was filled with either 1mL of 1.0 M HCl, 1mL of 1.0 M NaOH, or 1mL of water with a pipette, in order to create three solutions that had varying pH levels. Three cuvettes were filled with one of the solutions and a fourth cuvette was filled with distilled water to act as a blank. The cuvette was labeled “Yellow”, “Blue”, “Green”, or “Blank” according to its color, which was determined by either the mixture of HCl, NaOH, or distilled water. The second part of the experiment consisted of measuring the transmittance of each solution at numerous wavelengths using the spectrophotometer. The original setting of the Spec 20 was set to 360nm and the transmittance of the three colored solutions was taken. The Spec 20 was then recalibrated using the