• British Pharmacopoeia Volume V
  • Appendices
  • Appendix I A. General Reagents

Thymol Blue Solution
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Thymol Blue Solution Dissolve 0.1 g of thymol blue in 2.15 mL of 0.1m sodium hydroxide and 20 mL of ethanol (96%) and add sufficient water to produce 100 mL.

Complies with the following tests.

Sensitivity A mixture of 0.1 mL and 100 mL of carbon dioxide-free water to which 0.2 mL of 0.02m sodium hydroxide VS has been added is blue. Not more than 0.1 mL of 0.02m hydrochloric acid VS is required to change the colour of the solution to yellow.

Colour change pH 1.2 (red) to pH 2.8 (yellow); pH 8.0 (olive green) to pH 9.6 (blue).