Sodium metabisulphate is is used for loads of everyday things, however to reduce confusion I’m going to just straight to what I use it for within dyeing.
When reverse dyeing I use household bleach to remove the colour from fabrics, however bleach will continue to break down fibres even after it’s washed out. This causes fabrics to disintegrate. That’s where sodium metabisulphate flexes it’s muscles. A solution of sodium metabisulphate and water will neutralise the bleach and stop the chemical reaction protecting the fibres. Once the bleach has been rinsed from the garment soak in the solution for half an hour before washing as normal, and then you are good to go!