[alt-photo] Re: Stoichiometry for the nonscientist

EJ Photo ejnphoto at sbcglobal.net
Tue Aug 3 13:42:02 GMT 2010

David and Etienne, In the case of potassium oxalate, where the solution is
in may liters and not a few ml, I see no reason to get too complicated here.
And since it is also so very cheap to make yourself from potassium carbonate
and oxalic acid, if one can get them, I say use them to make it. 

As far as where to stop with the mixing, I do not agree with Loris that many
go to pH7 with this. I keep my solution slightly acid, around pH5 and allow
for a little excess oxalic acid to help keep it that way after several
sheets of buffered papers is run through it. 

It doesn't hurt to know though which kind of oxalic acid you are getting. I
won't name names, but a reputable supply with a short three character name,
could tell me when I asked which kind of oxalic acid they sold. Why did I
need to know? well, 18 gm per unit will be water in one, and a source for

The formula is 2 lbs of potassium carbonate with 1.75 lbs oxalic acid to
make 1 gallon of developer. I make 2 gallons at a time in a 5 gallon bucket.
Why such a big bucket? I start with mixing the potassium carbonate in warm
distilled water. Then slowly add the ox acid to the solution. Why slowly?
This is when the CO2 is released and it is vigorous but not explosive, so it
bubbles quite rapidly. The more you've added the less vigorous the reaction.
Depending on the source of your chemicals the solution should be clear with
no crap floating around. I sourced some of the chemicals from another well
known alt supplier only to have black and brown bits of stuff floating all
over. I buy the potassium in 50 lbs bag and oxalic acid in 55 lbs bags from
a chemical supplier about 4 miles from home; clean and pure.

Mix responsibly


Eric Neilsen
Eric Neilsen Photography
4101 Commerce Street, Suite 9
Dallas, TX 75226
skype me with ejprinter
Let's Talk Photography

-----Original Message-----
From: alt-photo-process-list-bounces at lists.altphotolist.org
[mailto:alt-photo-process-list-bounces at lists.altphotolist.org] On Behalf Of
etienne garbaux
Sent: Monday, August 02, 2010 10:35 AM
To: The alternative photographic processes mailing list
Subject: [alt-photo] Re: Stoichiometry for the nonscientist

David wrote:

>It seems this hole is much deeper than what was represented
>originally.  I can see why prepackaged kits are so popular.
>You went from gms to ml?

To get the right amounts of the reactants, you necessarily must mess 
with the molecular weights of the compounds involved.  You can do 
this every time you have a reaction to conduct, or you can do it 
beforehand by making up solutions that are based not on absolute 
weight but rather on molecular weight.  That's all.  If you follow 
along, I believe the previous e-mail explained clearly how to get 
from grams to ml.

And yes, mixing from a kit is easier than calculating reactions, and 
you don't have to worry about fizzing (or worse) from evolved gasses 
or the need for ice baths to keep temperatures from running 
away.  With potassium oxalate, you don't even need a kit -- just buy 
potassium oxalate (preferably, from a reputable scientific supplier 
like Malinckrodt Baker, so you know exactly what you have).

Best regards,


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