Universal indicator is widely used in school and university labs. Surprisingly little information on its composition can be found on the internet
except that its a mixture of indicators. I found one recipie eventually and tested it.
chemicals
- phenolphthalein
- methyl red
- methyl orange
- bromothymol blue
- thymol blue
- ethanole
- water
experimental
0.1 g phenolphthalein, 0.2 g methyl red, 0.3 g methyl orange, 0.4 g bromthymol blue and 0.5 g thymol were weighed and dissolved in a mixture of 250 mL
ethanole and 250 mL water.
The resulting red solution was tested with buffers of different pH. Also, filter paper was soaked in the solution to produce pH indicator paper.
results
The universal indicator on a multi plate at different pH
The universal indicator paper
discussion
This indicator gives a nice color shift through the pH scale. I use it in my chemistry lessons. The colors are slightly different to the universal
indicator bought from merck.
This recipie gives a quite concentrated indicator solution. Some more solvent was necessary to completely dissolve all the indicator dyes.
Nice! Do you feel that there is adequate discrimination so that each integral value of pH can be read from say, pH 2 to pH 13?Doktor Klawonn - 21-11-2010 at 04:24
Nice! Do you feel that there is adequate discrimination so that each integral value of pH can be read from say, pH 2 to pH 13?
It may not be very exact in the higher pH ranges. Near neutral the changes in color are quite pronounced.
bquirky - 21-11-2010 at 05:20
prehehaps a basic question but.
What change happens to the indicator molecules that alters its optical property's ? is it a chemical or physical change ?
