From: Sil Horwitz (silh@earthlink.net)
Date: 06/30/03-08:23:52 AM Z
At 2003-06-30 03:01 PM +0900, you wrote:
>I am trying to find out if it contains the anhydrous (zero
>H2O), the monohydrate (1 H2O) or the decahydrate (10 H2O)
>form of sodium carbonate.
"Washing Soda" has indeterminate amounts of H20. As manufactured, it is a
mixture of all three forms, mostly the decahydrate, but during storage it
loses some of the water, and the monohydrate is the stable form. That is
why photographic sodium carbonate is normally the monohydrate as it is the
form that does not change over time. That there is also some anhydrous in
the washing soda can be seen by the white powder on the outside of the
crystals, so if you want to use the soda for formulary purposes, it should
be assayed to determine the percentage of water contained in it!
>Along the same lines, Does anyone know, if for the purpose
>of washing and cleaning glass, if they are
>interchangable... (I imagine they are, but can find only
>the decahydrate mentioned in descriptions of washing
>soda.)
Makes no difference at all. In solution, the pH should be between 10 and
11, and both do a good job of removing greasy residue. Be careful, though,
with plain glass, as the high alkalinity can etch glass over periods of
time (though not as fast as sodium hydroxide).
>Does anyone know their comparative solubility? Or why only
>the decahydrate form is referred to as 'washing soda'?
They are all equally soluble (though the large decahydrate crystals may
take longer because of less surface area). As noted above, as manufactured
(non-purified for laboratory use), the decahydrate is the product. Other
forms are produced by further processing (desiccating or heating).
---sil
Sil Horwitz, FPSA
silh@earthlink.net
personal page: http://www.silphoto.us
This archive was generated by hypermail 2.1.5 : 07/09/03-08:31:14 AM Z CST