Verification of sulfite with formaldehyde
formaldehyde solution 1% Xn R 40-43 S 23-37
sodium sulphite Xi R 31
hydrochloric acid 7%
it is basically advantageous to use still less concentrated acids
- 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.
DisposalThe diluted solution can be disposed of into the drainage.
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-
Dropwise added formaldehyde solution causes the indicator to change.
Derived from experiment 6.,"Nachweis mit Formaldehyd", p.122,