A quality-control chemist adds two different acid–base indicators to the same aqueous…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A quality-control chemist adds two different acid–base indicators to the same aqueous solution so that color changes can be monitored across a wider pH window. Details for the two indicators are provided below.
Indicator X: pKa=4.5 acid form = yellow base form = blue
Indicator Y: pKa=5.8 acid form = red base form = green
Assume that equal molar amounts of each indicator are present and that the optical intensity of each color is the same per mole of indicator.
The solution’s measured pH is 5.2.
(1) For each indicator, calculate the ratio [acid form][base form] at pH 5.2 and identify which colored form of that indicator predominates.
(2) Taking your answers from part (a) into account, describe the composite color that the chemist will observe for the solution. Justify your description qualitatively, referring to the relative contributions of all colors present.