From: Ryuji Suzuki (rs@silvergrain.org)
Date: 06/30/03-09:31:48 AM Z
From: Sil Horwitz <silh@earthlink.net>
Subject: Re: Sodium carbonate "Washing soda"
Date: Mon, 30 Jun 2003 10:23:52 -0400
> "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.
I wonder if you are talking about general term washing soda, not the
Arm & Hammer Washing Soda product. I did assay about two years ago,
and at that time, it was sodium carbonate monohydrate. Two methods I
used were measurement of loss of weight at 250+C in crucible furnace
and that of total alkalinity through titration.
The following information is based on a fairly detailed article by
Christian Thieme, Solvay Alkali GmbH, Solingen, Germany.
Commercial sodium carbonate monohydrate technical grade comes in
several physical forms, "light ash," "dense ash," "granulated," etc.,
varying in bulk density and appearance. These come as a mixture of
different particle sizes, but in these cases the powders do not
necessarily mean that they contain any less water, unlike less stable
decahydrate case.
> 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).
In my understanding, crude sodium carbonate is extracted from Solvay
or modified Solvay method plants after calcining sodium bicarbonate.
Therefore, the crude sodium carbonate should be mostly anhydrous form,
from which variety of monohydrate forms like "light ash" are produced
and shipped. (This crude stock is already comparable to technical
grade in terms of its purity.)
The decahydrated form is made from the crude stock by dissolving it in
water, then bring the solution to crystal precipitation. In order to
make hard crystals, some sodium sulfate needs to be present. Even
after crystallization, the mole ratio of sodium carbonate to sulfate
impurity is much higher in decahydrate form.
-- Ryuji Suzuki "Reality has always had too many heads." (Bob Dylan, Cold Irons Bound, 1997)
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