Doktor Klawonn
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How to prepare universal indicator - a recipie
Introduction
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
![](http://www.trinkwasserhilfe.de/klawonn/Chemie/weigh.jpg)
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.
![](http://www.trinkwasserhilfe.de/klawonn/Chemie/pHsolution.jpg)
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
![](http://www.trinkwasserhilfe.de/klawonn/Chemie/colorspH.jpg)
The universal indicator paper
![](http://www.trinkwasserhilfe.de/klawonn/Chemie/paperpH.jpg)
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.
links
- This procedure as youtube video
- simple experiments with multi plates on acid base reactions
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Magpie
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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?
The single most important condition for a successful synthesis is good mixing - Nicodem
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Doktor Klawonn
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Quote: Originally posted by Magpie ![](images/xpblue/lastpost.gif) | 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?
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It may not be very exact in the higher pH ranges. Near neutral the changes in color are quite pronounced.
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bquirky
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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 ?
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mewrox99
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It gets protonated or deprotonated. It's chemical
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