You Asked: Why is sodium sulphite added to laundry and dish-washing detergents?
Sodium sulphite is not added for cleaning purposes, at least not as far as the laundry is concerned. It is added to protect the washing machine or dishwasher from corrosion. Iron reacts with oxygen to form ferric oxide which is better known as rust. This reaction proceeds more readily at high temperatures, as found in washing machines. Where does the oxygen come from? It is dissolved in water. The surface of water is in contact with air, so some oxygen can always dissolve. Oxygen also is a byproduct of photosynthesis which of course occurs as aquatic plants grow. The amount of oxygen that dissolves depends on the temperature (less dissolves as the temperature increases), the pressure (less dissolves at higher altitudes) and the amount of other substances already dissolved in the water (freshwater holds more oxygen than salt water). Oxygen in water is a good thing for fish who fulfil their oxygen needs by extracting the gas as water passes through their gills, but it is not so good for metals in washing machines which will corrode. Sodium sulphite is an oxygen scavenger. It reacts with oxygen to form sodium sulphate and effectively lowers the dissolved oxygen content thereby protecting the insides of washing machines from rusting.