Exp 10

Experiment 10 Determination of Dissociation Constant of Indicator Introduction: The equilibrium constant of a weak acid is called the acid dissociation constant: HA → A− + H+ Ka = [H+][A−]/[HA] The Ka can be treated more conveniently with the use of negative logarithms form: pKa = −log Ka pH = pKa +log ([A−]/[HA]) If we vary the ratio of [A−]/[HA] at different pH, plot log ([A−]/[HA]) against pH, this should be a straight line. The intercept of the line is pKa. We can then know the dissociation constant of the weak acid. Procedure: A. 1. Put 0.1 g of phthalic anhydride and 0.13 g of phenol in a 50-mL beaker, add 2 drops of concentrated H2SO4 and stir well. 2. Place the beaker on a hot plate in hood, heat it and stir with a stirring rod. 3. After 3~5 mins, move the beaker to the table, pour 10 mL of deionized water into it slowly and stir well. 4. Suction-filter the solution, wash the solid with deionized water. 5. Dissolve the solid with 5 mL of ethanol, add 5 mL of deionized water and stir well. B. 1. Measure out 0.5 mL of solution from A, add 3 mL of deionized water and 1 drop of NaOH(aq), mix well and pour it to a cell. 2. Within 400~600 nm, set zero scale of spectrophotometer with deionized water in a cell, then read the absorbance of spectrophotometer with solution from step 1 in a cell. 3. Plot the graph of “absorbance to wavelength”, point out the wavelength of the maximum absorbance. C. 1. Adjust the wavelength of spectrophotometer to the maximum absorbance, set zero scale with deionized water. 2. Pour 3 mL of buffer solution 1 in a small test tube, add 0.5 mL of solution from A and mix well. 3. Measure the pH value of the solution with a pH meter. 4. Put this solution into a cell and measure the absorbance. 5. Repeat steps 2~4 with buffer solutions 2~7 respectively.

Results: A. Wavelength (nm)

Absorption

λmax:

Wavelength (nm)

Absorption

nm

B. No.

1

2

3

4

5

6

7

pH Absorption [In−]/[HIn] log([In−]/[HIn])

Linear equation: pKa:

Questions: 1. Discuss the effect of the range of the color change by the dissociation constant of an indicator. 2. The indicators exhibit different colors in acidic and basic medium. How to choose the wavelength of the maximum absorbance if we want to measure the spectra of acidic (or basic) form of indicator? 3. The color change of an indicator happens when the concentrations of the basic form and the acidic form differs more than 10 times. Explain that’s why the range of color change of an indicator is about 2 pH values. 4. We can measure the dissociation constant as follows: Neutralize half of acid (or base), and measure the pH value of the mixture. Explain the reason. 5. Calculate the pH value of the buffer system: 0.2 M NH4Cl/0.2 M NH3.What pH value of the buffer

system will be if we add 10 mL of 0.1 M HCl to 65 mL of the buffer system. 6. How to choose and prepare a pH 9.0 buffer solution from the buffer pairs below? Acetic acid/Sodium acetate, Sodium bicarbonate/Sodium carbonate, Ammonium nitrate/Ammonia