Sunday, February 6, 2011

About Potassium bromide (KBr)

Just as a car needs both a speeder and a brak, a developer needs chemicals that acts as a starter and as a restrainer.

Usually we add a developing agent (metol, Hydroquinone or in our case Ascorbic acid (Vitamine C) and Kaffeic acid (from the instant coffee).

We add some chemical to make sure that the chemical reaction starts and goes exactly as we want it too. Since most developing agents, including ours, works only in a basic solution, we need to add some form of base, only a couple of the known developing agents will work in an acidic solution.

Most of the time we use, Lye (NAoh), Soda (Na2CO3) or Borax, in decreasing developing power. The more basic a solution is, the stronger and faster the reaction.

Now - the more active the solution is, the more vulnerable it is to a phenomenon called chemical fogging. That is something unexpected, the developer attacks silver grains in the emultion that has not been exposed! It leaves a residue of develpoed silver where there should have been nothing.

To get rid of this, we need a
retardant. Several are available, citric acid, and potassium bromide (KBr).
Potasssium bromide is better, and more active, it does not affect acidity in the solution, like citric acid, and works with halv the additive, compared to cittric acid. The downturn is that citric acid can be found at the mall, among bakery stuff, while KBr has to be bought from a chemical supplier.

Another difficulty is that we need just a few grams, which is hard to measure exactly.

This is what to do :

Dissolve 25 gram KBr in 250 ml of water. You now have a 10% stock solution. measure out 10ml for 1 gram of KBr. That is very accurate and very easy to do!

In the recipes given earlier we needed 0.24 gram of KBr. To measure out that from the stock solution, just measure 2,4 or roughly a little shy of 3ml, that is close enough, add this before topping up to full volume.

Simple and very effective.

Make a note : if you add too much KBr, ALL of the developing action will be retarded, you'll get thin, underdeveloped negatives with no contrast, and will have to increase developing timews considerably to compensate for that.

Therefore it is better to use too little, than too much, modern scanners will be able to look through the chemical fog, while thin negatives with nothing on them cannot be saved by any means.

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