From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Sodium
Date: 5 Nov 1995 22:13:47 GMT
Organization: Consulting Chemist
Lines: 42

In article <47h766$ghi@classic.iinet.com.au>, daveb@iinet.net.au (David
R. Brooks) says:

[Re: Why manufacture NaOH using a mercury cathode]

>  I assume 1. To ensure purity of the NaOH (no residual chloride
>present), and 2. to enable the process to run continuously, rather
>than on a batch basis.
>  In my school chemistry book, it showed just such a process, where
>the Hg was pumped from the electrolysis cell (where it picked up the
>Na) into a water cell where the Na came out, and back around.
>
>  I don't know if this process is obsolete now (that book was quite
>old), but it was described as a standard method.

David, you are quite right. My foggy old memory now recalls that 
there are two basic industrial electrolytic processes, the 
diaphragm cell and the mercury cell. The latter is more expensive 
but, as you say, produces a more chloride-free product. Product 
from the diaphragm cells can be purified by crystallization or by 
extraction with strong ammonia water, in the latter case 
to purity comparable to that of mercury cell product. There is 
also an older chemical process in which NaOH is made from soda ash 
and lime.

I understand from looking in my not-very-up-to-date references that
the use of the mercury cell is actually increasing - probably
because of demand for pure caustic.


Jerry