[paula]

A good way to neutralize the damaging effects of chlorine bleach (5% sodium hypochlorite) on fabric is supposed to be to wash the fabric and then soak it in a solution containing sodium metabisulfite, which is sold as "Anti-Chlor". This treatment is said to stop further damage to the fiber.

What is happening here? How many metabisulfite molecules it takes to neutralize a molecule of sodium hypochlorite? What is produced?

              NaOCl + Na2S2O5 -> ??

Can anyone help by explaining what happens, or by telling me where to look for the answer?

Sodium metabisulfite seems to be very efficient for this purpose, because the common recipes for neutralizing bleach on fabric call for making a solution of 2.2 grams in 10 liters of water, as compared to 30 grams of sodium thiosulfate in 4 liters of water.

[ajkoer]

2 HSO (3−) <==> (aq) S2O5 (2−) (aq) + H2O (l)
 
NaOCl +  NaHSO3 --> Na2SO3 + HOCl

Sodium bisufite is a weak acid salt and the action of a weak acid on a hypochlorite forms Hypochlorous acid.

The reaction proceeds with the oxidation of the sulfite to sulfate by the HOCl:

 Na2SO3 + 2 HOCl --> 2 HCl + Na2SO4

Per Watt's Dictionary Chemistry, HOCl can oxidize aqueous suspension of S to H2SO4!

[ajkoer]

Balance Correction: The reaction proceeds with the oxidation of the sulfite to sulfate by the HOCl:

2 Na2SO3 +  HOCl -->   HCl + 2 Na2SO4

The net reaction is:

Na2S2O5 + H2O + 2 NaOCl --> 2 HCl + 2 Na2SO4