You Asked: Why is sodium sulphite added to laundry and dish-washing detergents?

laundrySodium 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.

 

Joe Schwarcz

The Art of Mixing

cloroxChemistry is all about mixing things. But you have to know what you are doing.

A lady was complaining to a neighbor about an infestation of mice in her house. The well-meaning friend had a suggestion. Mix some toilet bowl cleaner with bleach in a container and leave the concoction in the house overnight. Guaranteed to get rid of the mice, she said. But she neglected to say that it could get rid of the human inhabitants as well. Permanently.

Chemically speaking, bleach is a solution of sodium or calcium hypochlorite. When mixed with any acid, it releases highly toxic chlorine gas. Most toilet bowl cleaners contain sodium hydrogen sulfate, an acid which will quickly liberate chlorine from bleach. The acrid fumes of chlorine can destroy lung tissue, cause the lungs to fill with water and in a sense cause death by drowning. Chlorine gas was of course used for this purpose in World War I. Our mouse-fearing lady almost suffered the same fate as did the French troops at Ypres at the hands of the Germans. Luckily her neighbor looked in to see how the experiment was going and saved her just as she was about to pass out.

Not every victim of this mixture turns out to be so lucky. Many who have poured bleach into a toilet bowl following an unsuccessful attempt to remove stains with a commercial cleaner have suffered permanent lung damage and some have died. No acid must ever be mixed with chlorine bleach. This includes acidic drain cleaners, rust removers and even vinegar.

 

Joe Schwarcz

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