6 Facts about Fehling test
Fehling solution is prepared by mixing Fehling's A and Fehling's B in equal volume. Both of these solutions are placed in reagent bottles in the laboratory.
Fehling's A is the blue-colored solution (It is an aqueous solution of Copper sulphate) but Fehling's B is colorless (It is the solution of Rochelle salt “potassium sodium tartrate) prepared in NaOH solution). Upon mixing produces a deep blue solution which is Fehling’s solution
Fehling's A (Bule color)+ Fehling's B (Colorless)=Fehling Solution (deep blue solution)
Fact 1: Fehling solution is an equivolume mixture of Fehling A and
Fehling B. Only Cu++ & OH-
takes part in chemical change during the Fehling test.
When Aldehyde except benzaldehyde is warmed with Fehling
solution, a brick red precipitate of cuprous oxide is formed.
Fact 2: Benzaldehyde although aldehyde doesn’t give Tollen’s test.
Fact 3: It can be used to distinguish aldehyde from ketone.
In this test, Cu++ gets reduced to Cuproup oxide whereas aldehyde gets oxidized to the carboxylic acid.
Fact 4: Fehling test is a redox reaction.
When Formic acid (Methanoic) acid is warmed with
Fehling’s solution brick red precipitate of cuprous oxide forms. Whereas other
members of this family doesn’t give this test. This is because Formic acid has –CHO
group whereas other carboxylic acid doesn’t have.
Fact 5: It can be used as a chemical test to distinguish Formic acid
from other carboxylic acids.
Glucose on warming with Fehling’s solution forms Brick
red precipitate of Cuprous oxide. Glucose gives this test because it has –CHO group.
Fact 6: Fehling test can be used to diagnose glucose/sugar in the urine.
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