SC VI C. Indicator Colour Changes

This Supplementary Chapter provides a list of colour change intervals for some common indicators, with approximate pKa values and colours. It is provided for the convenience of the analyst in selecting appropriate indicators for their applications. Values given are approximate and are the modal or median averages of several reported figures. Where “not reported” is stated the value has not been reported in the sources consulted in the compilation of the list.

Alizarin S

Approximate pKa: not reportedApproximate pH range for colour change: 3.7-5.2 Colour of acid form: yellow Colour of base form: red

Alizarin yellow GG

Approximate pKa: 11.0 Approximate pH range for colour change: 10.0-12.0 Colour of acid form: clear Colour of base form: yellow

Alizarin yellow R

Approximate pKa: 11.0Approximate pH range for colour change: 10.1-12.0 Colour of acid form: yellow Colour of base form: red

Brilliant yellow

Approximate pKa: not reportedApproximate pH range for colour change: 6.6-7.8 Colour of acid form: yellow Colour of base form: orange

Bromocresol green

Approximate pKa: 4.7 Approximate pH range for colour change: 3.8-5.4 Colour of acid form: yellow Colour of base form: blue

Bromophenol blue

Approximate pKa: 4.0 Approximate pH range for colour change: 3.0-4.6 Colour of acid form: yellow Colour of base form: blue

Bromothymol blue

Approximate pKa: 7.1 Approximate pH range for colour change: 6.0-7.6 Colour of acid form: yellow Colour of base form: blue

Chlorophenol red

Approximate pKa: 6.0 Approximate pH range for colour change: 4.8-6.4 Colour of acid form: yellow Colour of base form: red

Clayton yellow

Approximate pKa: not reportedApproximate pH range for colour change: 12.2-13.2 Colour of acid form: yellow Colour of base form: amber

Congo red

Approximate pKa: not reportedApproximate pH range for colour change: 3.0-5.0 Colour of acid form: blue Colour of base form: red

Cresol red (in acid solution)

Approximate pKa: not reportedApproximate pH range for colour change: 0.0-1.0 Colour of acid form: red Colour of base form: yellow

Cresol red (in basic solution)

Approximate pKa: not reportedApproximate pH range for colour change: 7.2-8.8 Colour of acid form: yellow Colour of base form: red

Crystal violet

Approximate pKa: 0.8Approximate pH range for colour change: 0.0-2.0 Colour of acid form: yellow-green Colour of base form: blue

Dimethyl yellow

Approximate pKa: 3.3 Approximate pH range for colour change: 2.9-4.0 Colour of acid form: red Colour of base form: yellow

Erythrosine (disodium salt)

Approximate pKa: not reportedApproximate pH range for colour change: 2.2-3.6 Colour of acid form: orange Colour of base form: red

Malachite green

Approximate pKa: 1.0 Approximate pH range for colour change: 0.2-1.8 Colour of acid form: yellow Colour of base form: blue-green

Metacresol purple (in acid solution)

Approximate pKa: 2.0 Approximate pH range for colour change: 1.2-2.8 Colour of acid form: red Colour of base form: yellow

Metacresol purple (in basic solution)

Approximate pKa: 8.3 Approximate pH range for colour change: 7.4-9.0 Colour of acid form: yellow Colour of base form: purple

Metanil yellow

Approximate pKa: not reportedApproximate pH range for colour change: 1.2-2.4 Colour of acid form: red Colour of base form: yellow

Methyl green

Approximate pKa: not reportedApproximate pH range for colour change: 0.2-1.8 Colour of acid form: yellow Colour of base form: blue

Methyl orange

Approximate pKa: 3.8 Approximate pH range for colour change: 3.0-4.4 Colour of acid form: red Colour of base form: yellow

Methyl red

Approximate pKa: 5.0 Approximate pH range for colour change: 4.4-6.0 Colour of acid form: red Colour of base form: yellow

Phenolphthalein

Approximate pKa: 9.7 Approximate pH range for colour change: 8.3-10.0 Colour of acid form: clear Colour of base form: red-violet

Phenol red

Approximate pKa: 7.4 Approximate pH range for colour change: 6.8-8.4 Colour of acid form: yellow Colour of base form: red

Quinaldine red

Approximate pKa: not reportedApproximate pH range for colour change: 1.0-2.2 Colour of acid form: clear Colour of base form: red

Resazurin

Approximate pKa: not reportedApproximate pH range for colour change: 3.8-6.4 Colour of acid form: orange Colour of base form: violet

Thymolphthalein

Approximate pKa: 9.9 Approximate pH range for colour change: 9.3-10.5 Colour of acid form: colourless Colour of base form: blue

Thymol blue (in acid solution)

Approximate pKa: 1.7 Approximate pH range for colour change: 1.2-2.8 Colour of acid form: red Colour of base form: yellow

Thymol blue (in basic solution)

Approximate pKa: 8.9 Approximate pH range for colour change: 8.0-9.6 Colour of acid form: yellow Colour of base form: blue