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Author: Subject: How to prepare universal indicator - a recipie
Doktor Klawonn
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[*] posted on 20-11-2010 at 09:51
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



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.

links

- This procedure as youtube video
- simple experiments with multi plates on acid base reactions
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Magpie
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[*] posted on 20-11-2010 at 10:14


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?



Knowing that I can buy good quality NaOH, HCl, and H2SO4 locally gives me great peace of mind.
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Doktor Klawonn
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[*] posted on 21-11-2010 at 04:24


Quote: Originally posted by Magpie  
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.

colorspHnumbers2.jpg - 20kB
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bquirky
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[*] posted on 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 ?

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mewrox99
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[*] posted on 21-11-2010 at 16:49


It gets protonated or deprotonated. It's chemical
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