Chemistry Reference
In-Depth Information
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Note
: Esters are hydrolyzed in the alkaline test solution, and the consumption of
hydroxide ions is indicated by phenolphthalein.
For the detection of ester groups in fat molecules, propanol is needed as a solvent
instead of ethanol.
Lactic acid gives a positive test result because of intermolecular esterification.
This is a nice exception, which can be used as a problem for inquiry learning.
This test was introduced by Carl August Rojahn (1889-1938) who was a
Professor for Pharmacy and Food Chemistry at the University of Halle (Germany).
1.4 Dinitrophenylhydrazine Test (DNPH Test)
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Reagent 1
: A mixture of 186 ml water, 33 ml hydrochloric acid (37%), and 1 g
2,4-dinitrophenyl-hydrazine are to be stirred well for 15 min. Undissolved
ingredients are then to be removed by filtration.
-
Procedure
: To 1 ml DNPH reagent in a test tube is to be added one drop of the
test substance. A stopper is to be put on the test tube which is to be shaken for
15 s and observed for another 45 s.
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Result
: A yellow or orange precipitate (sometimes only a milky turbidity)
indicates molecules with aldehyde or ketone groups.
-
Note
: the carbonyl groups of aldehydes or ketones react with the amino group of
the DNPH reagent and make an insoluble yellow or orange hydrazone.
For the detection of carbonyl groups in sugar molecules, heating with a boiling
water bath (5 min) is necessary.
Diethyl acetal is hydrolyzed (acid catalysis) to the ethanol and acetaldehyde; the
aldehyde makes the test positive.
1.5 Fehling Test
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Reagent 1
: 7 g CuSO
4
ยท
H
2
O are dissolved in 100 ml water.
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Reagent 2
: 35 g sodium potassium tartrate and 10 g sodium hydroxide are
dissolved in 100 ml water.
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Procedure
: 1 ml reagent 1 and 1 ml reagent 2 are mixed. The mixture is deep
blue or purple and clear. Then 1 ml test substance and a boiling chip are added.
After shaking the sample is put into a boiling water bath. The test can be stopped
when a precipitate or turbidity is observed. Otherwise it is observed for a
maximum of 5 min.
-
Result
: A red or reddish brown precipitate of copper oxide Cu
2
O at the bottom of
the test tube (often a small amount) indicates molecules with an Acetyl group.
