| time
requ.: |
45 minutes. |
|
objective: |
|
| material: |
- 4 beaker 100ml
- magnetic stirrer
- magnetic stirring bar
- spatula 10mm for powders
- test tubes d=18mm
- test tube stand
- stopper
|
- graded cylinder 50ml
- graded cylinder 10ml
- 2-5 teat pipettes
- funnel, stand, clamp
- luted filter paper
- molymod model kit for organic molecules
|
| chemicals: |
- nitric acid c=2mol/l
- ceric ammonium nitrate
- sodium chloride solution
c=3mol/l
- bromothymol blue
- sodium hydroxide
|
- 2,4-dinitrophenylhydrazine,
- hydrochloric acid w=37%,
acetic acid
- phenolphthalein
- ethanol
- distilled water
|
|
preparation 1: |
Note: adapt
volume of stock solution to the estimated number of tests you plan. For
about 30 tests: Prepare a
stock solution of reagent by dissolving 10g ceric ammonium nitrate in 25ml
of the nitric acid. |
|
procedure 1: |
Dilute 1 ml reagent with 2 ml water. Add 5 drops
of the unknown sample and shake. |
|
observation 1: |
First the
colour changes from yellow to red, then gas bubbles rise up. A typcial
aldehyd smell and decolourization develope. |
|
interpretation 1: |
CH3CHOHCOOH
+ 2 Ce4+ ---> CH3COH + CO2+
2 Ce3+ + 2H+ |
|
preparation
2: |
Prepare a stock solution by
dissolving 0.02 g bromothymol blue and 0.6 g sodium hydroxide in 100 ml
ethanol. |
|
procedure 2: |
To
1 ml reagent add 1 ml of the unknown sample. |
|
observation 2: |
The colour
changes from blue to yellow/orange. |
|
interpretation 2: |
The sample
contains a carboxylic group. |
|
preparation
3: |
Note: adapt
volume of stock solution to the estimated number of tests you plan. For
about 50 tests: Prepare a
stock solution by dissolving 0,25g dinitrophenylhydrazine (DNPH) in 42ml water and
8ml hydrochloric acid. Stir well for 15 minutes and
then filtrate to get a clear yellow reagent solution. |
|
procedure 3: |
To 1ml DNPH reagent add 1 drop of the sample and
shake. |
|
observation 3: |
A yellow
precipitate or turbid is observed. |
|
interpretation 3: |
The reaction
mixture contains a carbonyl compound. |
|
preparation
4: |
Prepare stock solutions of 0.1 g
phenolphthalein in 100 ml ethanol and of 3 M NaOH in water (12g sodium
hydroxide in 100 ml solution). |
|
procedure 4: |
To
1 ml ethanol add 1 ml of the unknown sample and 3 drops of
phenolphthalein solution. Then add drop by drop NaOH solution until the
pink indicator colour just appears. (Shake thoroughly after any drop,
and avoid strictly to add more NaOH solution than is absolutely
necessary.) Put the reaction vessel into a warm water bath (40 °C) and
shake every minute.If the sample contains an ester, the pink solution
becomes colourless within some minutes. |
|
observation 4: |
The pink
solution becomes colourless within some minutes. |
|
interpretation 4: |
Lactic acid
contains small amounts of dimeric ester molecules.
CH3CHOHCOOH + CH3CHOHCOOH --->
CH3CHOHCOOCHCH3COOH + H20 |
|
procedure 5: |
Monomeric
lactic acid molecules exist in enantiomeric forms. Build mirror-inverted
forms with the model kit. |
|
disposal of: |
|
|
source: |
Barke, H.-D. et al.: One Hundred
Chemistry-Experiments to Avoid Chalk and Talk; University of Muenster,
2004. |
