Pyro staining developers

Dear Sandy,
In your very interesting article :"An introduction to pyro staining
developers,with special attention to the Pyrocat-HD formula "
(at www.unblinkingeye.com/Articles/PCat/)
you give the formula at PCat2, page 2 :
STOCK SOLUTION A :
Distilled water 750 ml
Sodium Bisulfite 10 g
etc.....
etc.....

Why do you advise sodium bisulfite ?
Sodium bisulfite ( NaHSO3 . CAS : 7631-90-5 ) is very unstable.
When you buy the expensive puriss.quality ( ~99 % )
and store it in the best possible way (brown glass bottle with a cut
glass stopper ) you will have after a year only ~ 20 % NaHSO3,
the rest is Na2SO3.
  ( 2 NaHSO3 --> Na2SO3 + H2O + SO2 )
Much better is Potassium metabisulfite ( K2S2O5 .
CAS 16731-55-8 ).
You can buy the puriss.quality ( ~96 % ) for ~$ 16/kg, it is stable
when you store it dry ( after two years you will still have ~90 % )
and dissolved in water it gives KHSO3.
  ( K2S2O5 + H2O --> 2 KHSO3 )
Theoretically you need 11 g K2S2O5 instead of 10 g NaHSO3.

PS.:B & S sells "sodium bisulfite " but that isn't NaHSO3 .It is a
mixture of Na2S2O5 and NaHSO3 Melody Bostick ( what a
lovely given name!) told me.
In their price-list February 2002 they use the CAS nr :
7681-57-4. But that is Na2S2O5 ! She also told me that the
number in the price-list is wrong , I don't believe that.
I don't know,but I think that they sell : Na2S2O5 ( with H2O
it will give NaHSO3 ) and that their supplier told them it is sodium
bisulfite.

PS 2 : On that same page there are more stange things :
They sell "pyrogallic acid " CAS 87-66-1 .But it is PYROGALLOL.
( 1,2,3 trihydroxybenzene )
( Read " The Book of Pyro ".aut. Gordon Hutchings : Introduction ,
page 3 ).
They sell Sodium citrate CAS 6132-04-3 with the formula
C6H5Na3 .2H2O but that must be C6H5Na3O7 . 2 H2O with
the name Sodium citrate tribasic,dihydrate and
Sodium borate,decahydrate CAS 1330-43-4 is Sodium
tetraborate with the formula Na2B4O7 . 10 H2O instead of
NaB4O7 .
Chemistry isn't difficult but you must work accurate !

PS 3 : What means the value "7" in the black box at PCat3 ,page 6,
fig. 3 ?
What are BTZS type tubes ? and at page 8 : what means "SBR"
 of 7 (equivalent to N development in Zone speak ) ?
I hope you know the answers on my questions .

Kind regards

Erich
Received on Wed Nov 19 00:56:19 2003