Suppression by bromide

[mike_scarpitti]

Does anyone know the mechanism by which bromide releaed during

development suppresses development? Is it acidity? Is the bromide

acidic, and does that inhibit the development?

[jorge_oliveira2]

See:

 

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&selm=8u7ktl%24h46%241%40freenet9.carleton.ca&rnum=2

 

You may have to copy and paste this long lik in your browser.

[michael_graziano]

<P>

Jorge you evil, Evil, EVIL man! You have no idea how much reading that chemobabble made my brain hurt!<br>

For the copy/paste phobic, the link: <A HREF="http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&selm=8u7ktl%24h46%241%40freenet9.carleton.ca&rnum=2">ChemoBabble (informative)</A><br>

<br>

Distilling it down for the chemically disinclined, the bromide liberated in development seems to slightly reduce the solubility of silver ions (at least at low bromide concentrations - the post doesn't say what would happen if you dumped an obscene amount of bromide into your dev, and I wouldn't reccomend trying).<br>

The post also asserts that the presence of bromide reduces the rate of the redox reaction<br>

<br><PRE>

M A G I C

(Silver Halide) -------------> (metallic silver grain)

</PRE>><br>

which makes sense, since it restrains (slows down) development.<br>

<br>

<br>

For the even less chemically inclined, just go with my standard explanation for such things:<br>

<br>

IT'S MAGIC!!!! (now stop asking before it stops working!)

<br><br><br>

:)

[chris_waller]

It's just due to concentration. As the developing agent reduces

the silver hailde to silver it pushes bromide out into solution. As

this builds up so the reacton slows down . If my memory serves

me correctly it's called the 'Law of Mass Action". Some

developing agents, e.g. metol, are more susceptible to the

restraining efect of bromide whereas phenidone isn't. Thus

metol-only developers are 'soft-working' and control highlight

densities.

[jorge_oliveira2]

Michael Graziano

 

Another question about magic: How does one post magical links as you've done?

 

About development chemical magic - just get a PhD as Michael G.(can't ever spell it - have to copy and paste) did and it will be very simple, almost silly ((-:

[lex_jenkins]

1. Jorge, click on the "Photo.net FAQ" at the bottom of every page.

 

2. Then scroll down to the section on "How do I place hyperlinks in my posts?"

 

3. Copy and paste this HTML tag into your message:

 

<a href="xxxx">yyyy</a>

 

4. Modify it to something like this:

 

<a href=www.target_url.com>Go here!</a>

 

5. Change "The above text is:" from Plain Text to HTML.

 

6. Hit "Submit" and examine formatting before confirming. If you've done it right the actual URL will be invisible and the viewer will see something like:

 

"Go here! to see my new portfolio."

 

...with the words "Go here!" highlighted.

 

Hope that helps.

[jorge_oliveira2]

Thanks, Lex

 

Damm, the same old thing, and one never learns - RTFM!

[chris_waller]

There used to be a book entitled simply 'Developing' by Jacobson and Jacobson. This is an excellent volume on the chemistry and physics of developers. It has been out of print for some years but it can probably still be found.

[mike_scarpitti]

I own that book. The bromide supression is not discussed to the extent that I am asking about. I'm asking about not the effects, but the causes.

[Bill C]

Mike (Scarpitti), regarding the why, my understanding as a non-chemist (but with more than a few years in the technical side of photography) is along these lines: there are two main mechanisms and here are simplified explanations of them.

 

One mechanism slows down what they call the induction periond. The induction periond is the time it takes before development actually starts; sometimes it is defined as the intial stage of development where the rate is very slow. It apparently has a lot to do with halides being adsorbed to the silver bromide grains; the grains are thus surrounded by a negative charge; the so called "charge barrier" or "bromide ion shield". If a developing agent has a negative charge, it tends to be repelled; thus there is some delay before development actually gets started. Chris Waller indicated that phenidone is not affected; this makes sense as phenidone is reported to have zero charge. But Metol, with a charge of -1 should be affected.

 

The second mechanism is along the lines of what Michael Gudzinowicz indicated in Jorge's link; that is, bromide ion in the developer reduces the solubility of silver ions. Haist, in Modern Photographic Processing, says "Fewer silver ions are available for the building of fog centers, and potassium bromide thus acts to restrain the development of nonimage silver." There is more to it than that, but it gets beyond my grasp so I can't comment much on it.

 

Two sources where you can find more info are (coincidentally?) from the same people referenced in Jorge's link. See Haist's Modern Photographic Processing, Vol I, page 260. In The Theory of the Photographic Process, fourth edition, look for Chapter 14, section 4, called "Effect of Halide Ion And Antifoggants In The Developer". T.H. James, the editor as well as chapter author also talks about the effects of higher concentrations of bromide in the developer; it's probably not what you expect and is somewhat contrary to Chris Waller's assertion "It's just due to concentration".

 

I have a question in turn; sometimes when I answer a question like this I wonder if it's worth the time, so my question; do any of the people in this forum really want to hear about things like this ?

[lex_jenkins]

<i>"...do any of the people in this forum really want to hear about things like this?</i><p>

 

I believe that some do, Bill. At the very least it's worthwhile to mention some reference sources where the fundamentals have already been documented.<p>

 

There's some discussion of the effects of bromide on development in my reference book but, frankly, I've been both too lazy and too busy to reread that section.<p>

 

Besides, not being a chemist likely as not I'd simply garble the translation and make matters worse.

[mike_scarpitti]

Bill C:

 

I won't be asking anything but rather advanced questions. If others share any interest, fine.

[johndc]

"do any of the people in this forum really want to hear about things like this ?"

 

i do. while i may not understand everything in the suggested sources, it never hurts to plant the seeds in the back of your brain. when you least expect it, the fully realised idea will take up residence in the grey matter.