The acid-base indicator HIn undergoes the following reaction
in dilute aqueous solution:
The following absorbance data were obtained for a 5.00 3 10-4
M solution of HIn in 0.1 M NaOH and 0.1 M HCl. Measurements were made at
wavelengths of 485 nm and 625 nm with 1.00-cm cells.
In the NaOH solution, essentially the entire indicator is
present as In2; in the acidic solution, it is essentially all in the form of
(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 485 nm 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 0.245 at
635 nm (1.00-cm cells)?
(d) A 25.00-mL aliquot of a solution of purified weak
organic acid HX required exactly 24.20 mL 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 of a solution at 485 and
625 nm (1.50-cm cells) that was 2.00 10-4 M
in the indicator and was buffered to a pH of 6.000?