Answer to Question #116442 in Chemistry for Areeb

Question

Indicator - Thymol Blue, Phenol Red, Alizarin Yellow
Colour Observed - Blue, Red, Yellow

Select two indicators that, when combined, will produce a solution which is yellow at pH 3.0, green at pH 5.8, and blue at pH 10.0.

Answer 1

These indicators are normally used over a fairly short range of pH values (indicated in the table above). As a pH indicator, bromothymol blue, for example, would be useful between from about pH 6.0 to pH 7.6. Other indicators work in different pH ranges.

Using bromothymol blue you wouldn't be able to distinquish between an acid with pH 4 from an acid with pH 5. They would both appear yellow. Similarly solutions with pH 9 and pH 10 would both appear blue (vivid blue). With bromothymol blue you could hope to identify solutions with pH 6.5 (yellowish green), pH 7 (green) and, and pH 7.5 (bluish green).