| time
requ.: |
45 minutes. |
|
objective: |
detection
reaction |
| material: |
- 3 beakers 100ml
- magnetic stirrer
- magnetic stirring bar
- spatula 10mm for powders
- test tubes d=18mm
- test tube stand
- stopper
|
- graded cylinder 10ml
- 2-5 teat pipettes
- funnel, stand, clamp
- fluted filter paper
- molymod modelling kit for organic molecules
|
| chemicals: |
- sodium chloride solution
c=3mol/l
- bromothymol blue
|
- phenolphthalein
- distilled water
|
- lactic acid
|
- 2,4-dinitrophenylhydrazine
|
- sodium hydroxide
- hydrochloric acid w=37%
|
- ethanol
|
|
preparation
1: |
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 1: |
To 1ml DNPH reagent add 1 drop of the sample and
shake. |
|
observation 1: |
A yellow
precipitate or turbid is observed. |
|
interpretation 1: |
The reaction
mixture contains a carbonyl compound. |
|
preparation
2: |
Note: adapt
volume of stock solution to the estimated number of tests you plan. For
about 50 tests: Prepare stock solutions
of 0.05g phenolphthalein in 50ml ethanol and of 6g sodium
hydroxide in 50ml of water (c=3mol/L). |
|
procedure 2: |
To
1ml ethanol add 1ml 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
will
become colourless within some minutes. |
|
observation 2: |
The pink
solution becomes colourless within some minutes. |
|
interpretation 2: |
Lactic acid
contains small amounts of dimeric ester molecules. |
|
disposal of: |
Organic
solvents. |
|
procedure 3: |
Monomeric
lactic acid molecules exist in enantiomeric forms. Build mirror-inverted
forms with the modelling kit. |
|
source: |
Barke, H.-D. et al.: One Hundred
Chemistry-Experiments to Avoid Chalk and Talk; University of Muenster,
2004. |
