Sil Horwitz (silh@iag.net)
Mon, 22 Mar 1999 16:47:56 -0500
At 99/03/22 11:18 AM -0700, Richard Sullivan wrote:
>I believe the old formulas called for sodium borate decahydrate which has a
>molecular weight of 381.37 where the (dry) fused version has a molecular
>weight of 201.22. None of the historical formulas specify which hydrate to
>use but I strongly suspect that it is the decahydrate. I think most people
>will end up today with the dry version. Thus if the old formula calls for
>500 grams you would use only 260 gms of the dry stuff. This accounts for
>why you have all that sludge in your tray when you try to mix the old
>formulas.
Most of the old formulas called for plain old grocery/hardware store borax,
which is the decahydrate of sodium borate. This effloresces, so the stuff in
the paperboard boxes is always of indeterminate hydration. The dry sodium
borate sold by scientific supply houses is uniform. Use of the commercial borax
in formulas like D-76 is one of the reasons for variability, and is also why
Kodak developed the "Balanced Alkali" which is sodium metaborate, a very stable
chemical which doesn't have the hydration problem. (Sodium carbonate also has
the hydration problem, but the photographic item is the monohydrate - one
molecule of water for each molecule of carbonate - which is very stable.)
<<sil>>
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Sil Horwitz, FPSA
Technical Editor, PSA Journal
check out <http://www.psa-photo.org>
personal page: http://www.iag.net/~silh/
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