Stabilised hydrosulphite

Reductive bleaching with non-stabilised sodium hydrosulphite [98], sodium hydrosulphite-based sulphoxylate formaldehyde [99] and sodium and zinc hydroxymethanesulphinate [100] have been shown to provide wool excellent whiteness, with a lower cystine content than in the case of bleaching wool with other reducing agents. The result obtained is permanent, in contrast to the unstable peroxide white. 

The goods are treated with a solution containing sodium hydrosulphite (4-8 g/1) or stabilised sodium hydrosulphite (5-10 g/1) and pyrophosphate or buffer (3-4 g/1) at 50-55°C for 1-2 h. Sodium hydrosulphite in aqueous solution is converted into sodium bisulphite when applied at 50 C at pH around 7.0. After bleaching the... 

Reduction bleaching of silk is generally carried out with sodium hydrosulphite or appropriate stabilised commercial products on this basis. The silk fibrion is... 

As peroxide-bleached goods tend to show reversion to a creamy colour, it is usual to follow with a treatment in reducing agent to stabilise the bleach in a bath containing 3-4 g/1 stabilised hydrosulphite and 1 g/1 synthetic detergent at 50 C for 30-40 min, rinsed and dried. 

For acrylic/wool blends, the acrylic portion is brightened in the normal way and then the wool is bleached with hydrogen peroxide and subsequent reduction bleaching is done in a bath containing suitable brightener (1-2%) and stabilised sodium hydrosulphite (3-5 g/1) at 85 C for 30 min. The fabric is then rinsed and dried. 

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