vibbzlab
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Methyl orange - indicator/Azo dye synthesis writeup and video
In this video we will be synthesising methyl orange indicator dye
If you want to directly watch the video
link here
https://youtu.be/dRKUbgBTMu8
otherwise continue.
Methyl Orange is an azo dye formed by the classic azo coupling reaction between diazotised sulfanilic acid with NN dimethyl aniline.
Methyl orange is actually an acid indicator which shows a yellow color in pH more than and red color in pH less than .
Materials required
Sodium Carbonate anhydrous - 3.6g
Sulfanilic acid - 10.8g
Sodium Nitrite - 4.5g
Concentrated 37% HCl - 15ml
NN - dimethylaniline - 7.43ml
10% Sodium hydroxide soln - 90ml
Sodium chloride - 30g
Glacial acetic acid 6ml
Synthesis can be achieved by following various steps
1: Prediazotisation step
This step involves converting sulfanilic acid into sodium sulfanilate
For this purpose 150ml distilled water was taken in a 250ml Erlenmyer flask and 3.6g sodium carbonate was dissolved in it.
Next we add 10.8g Sulfanilic acid into this solution
On adding ,you see an effervescence and bubbles are seen. This is because
Sodium carbonate reacts with sulfanilic acid to form Sodium sulfanilate and CO2 is released.
Gently warm the solution to ensure complete reaction(The solution turns clear)
Step 2: Diazotisation
Add 4.5g sodium nitrite into filtrate, Stir to dissolve it.
Now keep it in ice bath to cool.
Keep that thing aside.
Now take a large 400ml beaker and add 75 ml of crushed ice water.
Add 15ml conc.HCl into it with stirring.
Be careful with concentrated acid.
Make sure that the temperature of this solution is less than 5°C.(Use a thermometer)
Now transfer the prepared diazotised sulfanilic acid solution into this slowly with stirring.
After stirring for some while we start seeing a white precipitatw which is so fine that it almost looks like a suspension.
This is the Diazonium chloride suspension.
We dont need to isolate the compound from this because we are directly going to use it as such.
Now we move into coupling reaction.
We transfer the 7.4ml NN- dimethylaniline into the solution present in it 400ml beaker.Wash the container in which it was taken with
6ml of glacial acetic acid and pour that too into the beaker.
Stir for 15minutes.
Since NN dimethylaniline is an organic compound and oily in nature is it not soluble in water ,this necessitates stirring which allows the particles
to react.
On stirring for sometime the color of the compound turns darker and darker and the consistenct becomes thicker and thicker.
Finally a thick red precipitate is formed.
Now on addinng acetic acid ,dimethylaniline acetate is formed which is a good nucleophile and attack on the diazonium compound occurs in the para
position due to hindrance at the ortho position due to bulky Dimethylaniline substituent.
Now we start adding the 90ml of 10% Sodium hydroxide slowly with constant stirring. Continue adding the Sodium hydroxide with stirring until the
solution becomes basic.
Thus completes the reaction of synthesis of methyl orange. The next step is purification. For that we need to boil the ppt in hot water.Methyl orange
is soluble in hot water. So adjust the volume of hot water to completely dissolve the precipitate.
Now we can use the 30g Sodium chloride to help induce precipitation of methyl orange.
Allow it to cool and we get beautiful golden orange flakes of Methyl orange which can be dried and collected.
ok thankyou for patiently reading
#vibzzlab
 
[Edited on 23-4-2020 by vibbzlab]
Amateur chemist. Doctor by profession
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