[Kate]

Hi. My teacher told me that both methyl orange and methylene blue are acid-base indicators. Basically, I need to answer a question and in order to answer it I need to know the pH range of both indicators. I've already researched it but I only found the pH range for methyl orange (which is 3.2 - 4.4). Does someone know the pH range for methylene blue ?

[Borek]

Methylene blue? Not sure I have ever heard about this one.

http://www.ph-meter.info/pH-measurements-indicators

[Kate]

I just looked it up on wikipedia and it says methylene blue is used as a redox indicator.

Here's the thing: In my chemistry class I did the thin-layer chromatography and column chromatography. The point was to seperate the mixture containing methyl orange and methylene blue. In the thin-layer chromatography we used silica as adsorvent in order to seperate methyl orange and in the column chromatography we used alumina (adsorvent) to separate methylene blue. In both chromatographies the eluent is ethanol.
The question I need to answer is why did we use silica to separate methyl orange and why did we use alumina to separate methylene blue. Tips ?
 

[ARGOS++]

Dear Kate;

You are absolutely right!

“Methylene Blue” is normally only known as a RedOx Indicator (+0.011Volt pH 7.0; +0.582Volt pH 0.0), and I’m not sure that someone knows a real situation where it can be used as a pure Acid/Base Indicator.
(But I’m sure it will show you a certain dependency on the pH because it is a dye, and a basic group is part of its Chromophor. Compare the data above!)

But why the different “Stationary Phases”?:
You have only to remember, that both Indicators are Salts, but one of an Acid and one of a Base!
If you also remember, that Alumina “reacts” basic and Silica on the opposite, you are very close to the solution.
Keep also in mind that for this type of Chromatography it is not very useful to have the Indicators in their Salt Form.

Now it should not be very difficult for you to find the final conclusion.
I hope it is of help to you.

Good Luck!
                   ARGOS⁺⁺

[Kate]

Thanks for answering.

I didn't really get this part in your post though:

"Alumina “reacts” basic and Silica on the opposite".

Could you tell me more about it ?

[ARGOS++]

Dear Kate;

Both Alumina and Silica are “Anhydrides” of their corresponding Base or Acid and there wile don’t loss the character of their “Father-House”.

For Example:
         Alumina:     2Al(OH)_3(aq.)    <------>   Al₂O_3(s)  + 3H₂O
( Al(OH)₃ is also an “Amphoter” but with “Equivalence point” away from pH 7 on the base side.)

Similar is true for Silica with its starting point of  Si(OH)₄ and its different partial “Anhydrides” ( (Si(OH)₃)₂O, etc.). You may also think on the “Silicates” and their nature.

Good Luck!
                   ARGOS⁺⁺

[Kate]

Thanks for answering again.

About the column chromatography method, I have a question. Imagine you want to separate 2 components of a mixture. The more affinity one component has with the adsorbent the slower it'll migrate to the bottom of the column (compared to the component with lesser affinity), right ?

[ARGOS++]

Dear Kate;

As a basic Idea/Principal of Chromatography you are right.
(Depending however you define these terms for each of all different Chromatography types.)

Good Luck!
                   ARGOS⁺⁺