Spectrophotometry with indicators. Acid-base indicators are themselves acids or bases. Consider an indicator, HIn, which dissociates according to the equation The molar absorptivity, , is

Spectrophotometry with indicators.† Acid-base indicators are themselves acids or bases. Consider an indicator, HIn, which dissociates according to the equation

The molar absorptivity, ε, is 2 080 M^-1 cm^-1 for HIn and 14 200 M^-1 cm^-1 for In^-, at a wavelength of 440 nm.

(a) Write an expression for the absorbance of a solution containing HIn at a concentration [HIn] and In^- at a concentration [In^-] in a cell of pathlength 1.00 cm. The total absorbance is the sum of absorbances of each component.

(b) A solution containing indicator at a formal concentration of 1.84 × 10^-4 M is adjusted to pH 6.23 and found to exhibit an absorbance of 0.868 at 440 nm. Calculate pK_a for this indicator.

HIn H* + In