I am looking for a (colored) pH indicator covering three ranges. 0-6,5, 6,5 -7,5 (the most important) and 7,5 — 14. The first and third steps may involve the same colour.
Among the numerous indicators which are applicable in principle, there are only a few which change at the exact values given but a large number which approximate to the requirements. The following are applicable for the pH ranges given: Reassuring (up to 6,5); pink/blue), methyl-, ethyl- or propyl- red (up to 6,3; red/yellow) and chlorophenol red (up to 6,4; yellow/red). For the middle range the following are possible bromothymol blue (6,0 to 7,6; yellow/blue), aurin (6,2 to 7,0; yellow/pink) and brilliant yellow (6,7 to 7,9; yellow/red). Finally for the upper range the following can be used: chinolin blue (from 7,0; colourless/violet), alphanaphtholphtalein (from 7,4; colourless/green-blue), alpha-naphtholsulfonphthalein (from 7,5; yellow/blue and metacresol red), (from 7,6; yellow/purple).
There are also several fluorescence indicators which it might also be possible to use. In the lower pH range the following are available: dichlorofluorescein (up to 6,0; green), beta-methylesculetin (up to 6,2; colourless/blue) or resorufin (up to 6,4;colourless/orange). The middle range would be exactly covered by umbelliferon (6,5 -7,5; colourless/blue) while cumaric acid (from7,2; colourless/green) and aridin orange (from 8,4; colourless/yellow-green) could be used for the upper range.
pH Indicators
Blog, Updated at: 8:31 PM
