Neutral FeCl3 Test

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.




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:


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


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



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


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


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


2. 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.


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


3. 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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