Chemistry Reference
In-Depth Information
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Procedure
: To 10 drops of the test substance and 2 drops phenolphthalein,
sodium hydroxide solution is to be added drop by drop until the phenolphthalein
indicator just changes to pink. Then the solution is discolored without delay by
adding one drop of hydrochloric acid (or a few, if necessary). If the solution
warmed up, it has to be cooled down to room temperature.
Now, 2 drops of iron chloride solution and 1.5 ml amyl alcohol are to be added.
The stoppered test tube has to be shaken vigorously for 5 s. Wait until the two
phases separate.
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Result
: The test is positive if an orange color can be observed in the upper or
lower phase. This indicates carboxylic acid molecules with short chains. With
formic acid and acetic acid, the lower phase is orange; with propanoic acid and
butanoic acid, the upper phase is orange.
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Note
: The iron chloride test does not work directly with the carboxylic acids, but
with their salts. The purpose of the first step of the procedure is therefore to
neutralize the carboxylic acids. (This step is not necessary for the salts, of
course.) The acid ions form orange complexes with the iron ions. The complexes
with propanoate and butanoate prefer the unpolar amyl alcohol (upper phase),
whereas the more polar complexes with formiate and acetate prefer the lower
water phase.
The iron chloride test is negative with mineral acids, in contrast to the BTB test.
However, it is limited to short-chain carboxylic acids.
1.10 Dichromate Test
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Reagent
: 2.35 g potassium dichromate are to be dissolved in 150 ml water. Then
8 ml concentrated sulfuric acid have to be added. After stirring, fill up with water
to 200 ml.
This reagent is poisonous. Use gloves. Pay special attention to solid dichromate,
because it is carcinogen. Do not inhale dichromate dust.
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Procedure
: To 2 ml dichromate reagent are added 4 drops test substance and a
boiling chip. The sample is to be put into a boiling water bath for 5 min.
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Result
: A color change from orange to green or brown indicates molecules which
are oxidizable (e.g., primary and secondary alcohols, aldehydes, esters, and
other compounds) under the test conditions. Alkanes, ketones, and mono car-
boxylic acids (exception: formic acid) are not oxidizable.
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Note
: The dichromate test should be used only by experienced students, who are
aware of the necessary safety conditions. Another possibility is to restrict
this test to teachers' demonstrations, or even omit it because it is not strictly
necessary as an analytical tool. (Note that for synthesis the dichromate reagent
can be substituted by potassium permanganate.)
