Chemistry Reference
In-Depth Information
-
Note
: Alcohol molecules make red colored complexes with hexanitratocerate
ions.
- Diethyl acetal is hydrolyzed (acid catalysis) to ethanol and acetaldehyde; the
alcohol makes the test positive.
Sometimes, the red color, indicating an alcohol group, disappears immediately
or slowly, and the solution becomes colorless. This indicates that cerium ions Ce
4+
have been reduced to Ce
3+
, and that the test substance has been oxidized.
For example, ethanol is oxidized to acetaldehyde (after heating), 2-propanol to
acetone, lactic acid to acetaldehyde and carbon dioxide, citric acid to acetone and
carbon dioxide. These reactions can be discovered by the students when they make
the cerium nitrate test with these substances in a larger scale in a synthesis
apparatus (Fig.
9.3
) and investigate the distillates with the reactions.
1.2 Bromothymol Blue Test (BTB test)
-
Reagent
: 0.02 g bromothymol blue and 0.6 g sodium hydroxide have to be
dissolved in 100 ml ethanol.
-
Procedure
: 1-ml test substance is to be added to 1-ml BTB reagent. Mix it well
and observe.
-
Result
: A color change from blue to yellow/orange indicates molecules with
carboxylic acid groups.
-
Note
: The test is also positive with inorganic acids. Therefore, the detection of
carboxylic acid groups is only conclusive in the absence of inorganic acids.
- If the test substance is not soluble (e.g., stearic acid), heating is helpful for
detecting the carboxylic acid groups.
1.3 Phenolphthalein Test (Rojahn test)
-
Reagent 1
: 0.1 g phenolphthalein is dissolved in 100 ml ethanol.
-
Reagent 2
: Sodium hydroxide solution (
c
3 mol/l).
-
Procedure
: 1 ml of the test substance and 3 drops phenolphthalein solution are
added to 1 ml ethanol. Sodium hydroxide solution is then to be added drop wise
(constant shaking) until a permanent pink color is observed which even resists
vigorous shaking. (It is very important to only add as much sodium hydroxide
solution as needed to change the indicator to pink.) The test tube with the
solution is then to be put into a water bath (40
C). It needs to be taken out
after every minute to shake it. The test can be stopped, when a discoloration is
observed. Otherwise, the test tube has to be observed for a maximum of 10 min.
-
Result
: Discoloration of the pink phenolphthalein solution indicates molecules
ΒΌ
with ester groups.
