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

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]