Verification of sulfite with formaldehyde
Chemicals needed
distilled water
formaldehyde solution 1% Xn R 40-43 S 23-37
sodium sulphite Xi R 31
phenolphtalein solution
hydrochloric acid 7%
it is basically advantageous to use still less concentrated acids
Equipment needed
test tube
Test procedure
- One adds a small spatula-tipful of sodium sulphite into a test tube and dissolves this with about 1 ml water.
- Now one adds 1-2 drops of the phenolphthalein solution. If the solution thereby becomes pink-violet, one adds further drops of diluted hydrochloric acid until decolorized (but no more than necessary!).
- Now one adds dropwise the formaldehye solution, a pink-violet colorization develops, the quicker the more neutral the solution is.
Disposal
The diluted solution can be disposed of into the drainage.
Elucidation
Sodium hydroxymethyl sulfite(bisulfite adduct) and sodium hydroxide are formed. The latter is strongly alkaline and therefore causes the indicator to change.
HCHO + H2O + Na2SO3 --> HOCH2OSOO- + 2 Na+ + OH-
Photos
Dropwise added formaldehyde solution causes the indicator to change.
Literature
Derived from experiment 6.,"Nachweis mit Formaldehyd", p.122,[7]