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What are the specific conditions for a compound to have in order to show positive $\ce{FeCl3}$ test? I got this doubt, when I took 2 different test papers having different key for $\ce{HCOOH}$ and $\ce{CH3COOH}$. I never found anywhere, except for the classical example of $\ce{-OH}$ group attached to $\ce{sp^2}$ carbon, which gives $\ce{FeCl3}$ test.

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    $\begingroup$ What do you call the $\ce{FeCl3}$ test ? Is it the colored reaction with phenols ? Is it the red reaction with thiocyanate ions ? $\endgroup$
    – Maurice
    Dec 31, 2020 at 17:45
  • $\begingroup$ For the ferric chloride test the gist is that the subject molecule has to form a complex with $\ce{Fe^{3+}}$. $\endgroup$
    – MaxW
    Dec 31, 2020 at 19:09
  • $\begingroup$ Thanks, but could you give me common feature of all the compounds which give (+)ve FeCl3 test? Like, why that compound forms a complex with Fe3+ ion... $\endgroup$
    – Math_Whiz
    Jan 1, 2021 at 4:09
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    $\begingroup$ Related for phenols: chemistry.stackexchange.com/questions/43187/… .... chemistry.stackexchange.com/questions/61926/… $\endgroup$ Jan 1, 2021 at 6:54

1 Answer 1

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The test used to detect carboxylic acid is called neutral $\ce{FeCl3}$ test. It is called neutral because the pH value should be maintained at 7. This is achieved by adding very dilute ammonia solution drop by drop while shaking the solution. If the characteristic odor of ammonia becomes predominant, the solution has become basic (you can also check with litmus paper or phenolphthalein indicator). It can be made neutral by gently heating to remove excess ammonia (the odor will disappear). Cool the solution and then add few drops of ferric chloride solution to get different colors of precipitates. The reactions are:

$$\ce{RCOOH + \underset{aq. ammonia}{NH4OH} -> \underset{neutral}{RCOONH4} + H2O}$$

$$\ce{3RCOONH4 + FeCl3 -> \underset{colored salt}{(RCOO)3Fe} + 3NH4Cl}$$

Why reaction condition should be neutral/Why pH should be at 7?

  1. If the solution is basic:

Presence of excess ammonia leads to formation of brown-orange precipitate of ferric hydroxide which interferes with the color of the ferric carboxylate salt resulting in a false result.

$$\ce{FeCl3 + \underset{excess aq. ammnonia}{3NH4OH} -> \underset{brown orange ppt.}{Fe(OH)3} \downarrow+ 3NH4Cl}$$

$$\ce{FeCl3 + 3RCOOH ->[excess NH4OH] (RCOO)3Fe + Fe(OH)3 \downarrow}$$

  1. If the solution is acidic

If the solution is acidic (little ammonia is added), colorless complexes can form between the acid and ferric ions leading to a false negative result.

$$\ce{FeCl3 + 3RCOOH ->[acidic medium] (RCOO)3Fe + other colorless complexes}$$

  1. Neutral pH

At right pH, you will get colored precipitate. For formic and acetic acid, you will get red colored solution indicating positive results.

$$\ce{3RCOOH + FeCl3 -> \underset{red}{(RCOO)3Fe} + 3HCl~~~~~R= H, CH3}$$

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  • $\begingroup$ Do formic acid and acetic acid both give FeCl3 test ? since this test is used to distinguish both of them, it doesn't make sense to use the test if both are giving positive result... $\endgroup$
    – Math_Whiz
    Jan 1, 2021 at 9:45
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    $\begingroup$ @Math_Whiz Yes both give positive FeCl3 test. If you do want to distinguish between them, try Tollen's test. Formic acid give positive result while acetic acid do not. See: vedantu.com/question-answer/… $\endgroup$ Jan 1, 2021 at 11:05

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