I don't think it's as off topic as Bob said.
Odorless stop bath formulae are published in many forms. Many of them
are pretty bad, amateurish concoctions.
Stop bath should have a pH in the range of 3 to 6, preferrably 4 to 5.5,
and it should be preferrably buffered. Acetic acid is actually a very
good choice for this pH range, buffering capacity, cost and
availability. The disadvantage is the odor. The odor is most noticeable
when plain acetic acid stop bath is prepared fresh. The odor is less if
the stop bath is buffered at pH of 5. Such a stop bath is commercially
available, for example, Sprint stop bath (did they call it Block?).
Alternative method is to use odorless acid with suitable dissociation
constant. There are many acids suitable for this, such as citric acid
and glycolic acid. Disadvantage is cost.
Some old formulae called for sulfuric acid and other mineral acids. They
are not very good formulae and I wouldn't recommend them.
Bill Troop recommended 5% boric acid solution. While it works ok when it
is fresh, the pH is not really buffered (boric acid is a good buffering
agent at pH of 9.2 but not in acid region) and the processing capacity
is low, considering the amount of agents in the solution.
Metabisulfite (bisulfite) is used in some formulae, but this is also not
very good option, especially if the solution is preapred in advance.
I think Ilford makes two versions of stop baths. One is purely citric
acid based, and another a mix of acetic and citric acids. The citric one
is for small scale tray use and mix acid for large scale processing.
On Mon, 20 Mar 2006 15:16:50 -0400, "BOB KISS" <bobkiss@caribsurf.com>
said:
> DEAR LIST,
> I keep hearing about “odorless stop bath” which I have heard
> might be some form of tri-acetate solution but I don’t believe everything
> I
> hear.
> Does someone have the formula? Please send me the formula,
> or
> the URL where I can find it, directly to me at bobkiss@caribsurf.com
> <mailto:bobkiss@caribsurf.com>
> Thanks!
> CHEERS!
> BOB
>
> Please check my website: http://www.bobkiss.com/
> <http://www.bobkiss.com/>
>
>
Received on Mon Mar 20 14:00:44 2006
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