5.29      describe  the  manufacture  of  sodium  hydroxide  and  chlorine  by  the  electrolysis  of concentrated sodium chloride solution (brine) in a diaphragm cell

Remember that “brine” is a name for sodium chloride solution and that sodium chloride is common salt.  This can be obtained from underground rock salt deposits or from sea water.

Also remember that the electrolysis of solutions of compounds of reactive metals like sodium produces hydrogen at the negative electrode (see Section 1.54).

Note the porous diaphragm that is used to separate the two halves of the cell. This acts as a semi-permeable membrane to keep the products apart, but allows sodium ions in the solution to move from one side to the other.  It is important to keep chlorine away from sodium hydroxide.  The two react together to make bleach (sodium chlorate (I) formula NaClO).  Whilst we want some of the chlorine and sodium hydroxide to make bleach (see Section 5.31 below) we don’t want all of it to do so!  It is also important to keep hydrogen and chlorine apart.  Mixtures of these gases can be explosive in sunlight.