Litmus on Aldehydes,Ketones,Alcohols,Amines.

  • #1
1,007
76

Main Question or Discussion Point

What happens if we put blue litmus or red litmus on alcohol.Though alcohol is acidic it does not show any change in litmus solution? What about ketones and aldehydes? At home I have not this materials to see for myself at the time. About primary amines I found something interesting. I took a test tube, poured some solution of amine in it. The amine was dark red in colour. As I had little amount of solution I poured some distilled water and it was like amine was not soluble in it and when poured blue litmus solution 2-3 drops, the solution turned blue and when poured red litmus solution no change. Why , isn't amine basic?
 

Answers and Replies

  • #2
Borek
Mentor
28,353
2,742
Alcohol in water solution is not acidic enough to change teh solution pH.

What is this particular amine solubility? Remember, if the litmus solution is red, it means it already contains some amount of acid, which has to be neutralized first.
 
  • #3
1,007
76
Alcohol in water solution is not acidic enough to change teh solution pH.

What is this particular amine solubility? Remember, if the litmus solution is red, it means it already contains some amount of acid, which has to be neutralized first.
I don't know the numerical value for solubility of amine but when I poured some water on it in test tube, the amine was not soluble and was forming some blob like structure beneath some water. I forgot to mention the amine is aniline. So it's molecular mass is quite high and solubility quite low.

Does water has a role here you mean? What if I poured no water, although amine already is in aqueous state and directly poured red litmus. Then would the litmus turn blue?

We were given a set of organic compounds to analyse which I have listed in the thread topic.
What about litmus test on ketones and aldehydes. Would they act as same as you have told about alcohol?
 
Last edited:
  • #4
Borek
Mentor
28,353
2,742
Amine is not in aqueous, but in a liquid state. That's something completely different. Litmus tells you what is pH of the solution, so it requires the substance to dissolve, otherwise it won't change the pH. Low solubility can mean small changes in pH - but using red/blue litmus here can be misguiding, as these solution are already basic/acidic and they can react with whatever is dissolved.

Ketones and aldehydes don't change pH of the solution either.
 
  • #5
1,007
76
Amine is not in aqueous, but in a liquid state. That's something completely different.
Yeah, got it , aniline at room temperature is in liquid form.That earlier was confusing me.
Ketones and aldehydes don't change pH of the solution either.
Okay, I guess they are a bit more neutral to these tests.

The only query remains
Litmus tells you what is pH of the solution, so it requires the substance to dissolve, otherwise it won't change the pH. Low solubility can mean small changes in pH - but using red/blue litmus here can be misguiding, as these solution are already basic/acidic and they can react with whatever is dissolved.
I didn't understand it fully. You are saying low solubility can mean small changes in pH. But amine(aniline) is having low solubility in water. How it can have low solubility in litmus solution?
My practical journal was saying that aniline will be in blue in both red litmus and blue litmus solution pouring.
When I did the experiment blue colour was coming with blue litmus and red with red litmus.
I guess, the problem arises in mixing some distilled water. Is that true?
 
  • #6
Borek
Mentor
28,353
2,742
I didn't understand it fully. You are saying low solubility can mean small changes in pH. But amine(aniline) is having low solubility in water. How it can have low solubility in litmus solution?
Litmus solution is not much different from just water. Anything that ghas low solubility in water won't magically dissolve in large quantities just because of traces of litmus present.

My practical journal was saying that aniline will be in blue in both red litmus and blue litmus solution pouring.
When I did the experiment blue colour was coming with blue litmus and red with red litmus.
As I tried to explain it can be a problem with the amount of acid added to the red litmus solution. Litmus changes its color around pH 7, so to make its solution purely red you need to acidify it. If there were too much acid added, aniline (because of its low solubility) can be not able to neutralize it fully.

I am not saying that's what is happening, that's just my guess.

I guess, the problem arises in mixing some distilled water. Is that true?
Most of the solutions you use contain mostly water. Adding some more won't change the situation much.
 
  • #7
1,007
76
But our lab uses standard litmus solutions. Other people were getting a blue colour when they were mixing red litmus in aniline. When I poured it was just red, no change. Because of this I rejected that the organic compound may be aniline. What I should have done?
 
  • #8
Borek
Mentor
28,353
2,742
If others were getting correct results, you did something wrong. Sorry, I wasn't there, I haven't seen what you did, so I have no way of knowing what it was.
 
  • #9
1,007
76
If others were getting correct results, you did something wrong. Sorry, I wasn't there, I haven't seen what you did, so I have no way of knowing what it was.
Well a last thing. In contrast to red litmus which I poured on aniline which was red in colour, when I poured blue litmus the solution turned quite dark bluish. So is it a signal determining basic nature?
 
  • #10
Borek
Mentor
28,353
2,742
That would be my understanding.
 
  • #11
1,007
76
Yeah okay, By the way thanks for explaining little concepts.
 

Related Threads on Litmus on Aldehydes,Ketones,Alcohols,Amines.

Replies
8
Views
7K
Replies
17
Views
18K
  • Last Post
Replies
3
Views
3K
Replies
3
Views
5K
  • Last Post
Replies
9
Views
2K
  • Last Post
Replies
1
Views
4K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
5
Views
4K
  • Last Post
Replies
1
Views
17K
Replies
9
Views
2K
Top