The acid-base indicator Hln undergoes the following reaction in dilute aqueous solution:
The following absorbance data were obtained for a 5.00 X 10-4 M solution of HIn in 0.1 M NaOH and 0.1 M HCI. Measurements were made at wavelengths of 485 nm and 625 nm with 1.00·cm cells.
In the NaOH solution, essentially all of the indicator is present as In; in the acidic solution, it is essentially all in the form of HIn.
(a) Calculate molar absorptivities for In- and HIn at 485 and 625 nm.
(b) Calculate the acid dissociation constant for the indicator if a pH 5.00 buffer containing a small amount of the indicator exhibits an absorbance of 0.567 at .+85nm and 0.395 at 625 nm (1.00-cm cells).
(c) What is the pH of a solution containing a small amount of the indicator that exhibits an absorbance of 0.492 at 485 nm and 0.2'+5 at 635 nm (1.00-cm cells)?
(d) A 25.00-mL aliquot of a solution of purified weak organic acid HX required exactly 24.20mL of a standard solution of a strong base to reach a phenolphthalein end point. When exactly 12.10 mL of the base was added to a second 25.00-mL aliquot of the acid, which contained a small amount of the indicator under consideration, the absorbance was found to be 0.333 at 485 nm and 0.655 at 625 nm (1.00-cm cells). Calculate the pH of the solution and Ka for the weak acid.
(e) What would be the absorbance o[ a solution at 485 and 625 nm (1.50-cm cells) that was 2.00 x 10-4 M in the indicator and was buffered to a pH of 6.000?