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Litmus on Aldehydes,Ketones,Alcohols,Amines.

  1. Jan 16, 2015 #1
    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?
     
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  3. Jan 16, 2015 #2

    Borek

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    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.
     
  4. Jan 16, 2015 #3
    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: Jan 16, 2015
  5. Jan 16, 2015 #4

    Borek

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    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.
     
  6. Jan 16, 2015 #5
    Yeah, got it , aniline at room temperature is in liquid form.That earlier was confusing me.
    Okay, I guess they are a bit more neutral to these tests.

    The only query remains
    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?
     
  7. Jan 16, 2015 #6

    Borek

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    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.

    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.

    Most of the solutions you use contain mostly water. Adding some more won't change the situation much.
     
  8. Jan 17, 2015 #7
    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?
     
  9. Jan 17, 2015 #8

    Borek

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    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.
     
  10. Jan 17, 2015 #9
    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?
     
  11. Jan 17, 2015 #10

    Borek

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    That would be my understanding.
     
  12. Jan 17, 2015 #11
    Yeah okay, By the way thanks for explaining little concepts.
     
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