Role of Aluminium Sulfate in Fixer

[geyes30]

I have a fixer that has the following composition:

Ammonium thiosulfate, sodium bisulfite, ammonium bisulfite, sodium tetraborate, aluminum sulfate.

My question is, is this a hardening fixer? If not, what is the role of aluminium sulfate? Thanks!

[Ian Grant]

It is the hardening agent.

Ian

[geyes30]

Hi Ian,

Thanks for your response. My follow-up question is, Why is the hardening agent not of the more common potassium/sodium alum form (i.e. potassium/sodium aluminium sulfate)? Do the components listed allow the formation of the more common alum form in the solution?

[Ian Grant]

It's going to depend on what else is in the fixer. Sometime a formula is made up from components that react to form what's needed, Rodinal uses Potassium Metabisulphte and Potassium Hydroxide to form Potassium Sulphite (and water).

So it's what the components form in equilibrium reactions in the final solution that's more important, without seening the entire list of components it's hard to be more precise.

Ian

[Gerald C Koch]

It is the Al+3 ion which does the hardening. It really doesn't matter what salt supplies it. It can come from aluminum sulfate or from sodium aluminum sulfate. I suspect that the bottom line determines which one is used. Sodium aluminum sulfate is one of a group of compounds known as alums which are double salts. For example sodium sulfate plus aluminum sulfate. They exist as definite compounds in the solid state but dissociate in water into the component ions.

[Photo Engineer]

Hi Ian,

Thanks for your response. My follow-up question is, Why is the hardening agent not of the more common potassium/sodium alum form (i.e. potassium/sodium aluminium sulfate)? Do the components listed allow the formation of the more common alum form in the solution?

Alum is Potassium Aluminum Sulfate.

http://en.wikipedia.org/wiki/Alum

The hardening action of Alum is reversible.

PE

[Ian Grant]

When Aluminium Sulphate is used instead of Potassium Aluminium Sulphate (often called Potassium Alum) the hardener often contains some Sulphuric acid.

It is possible as Gerald states to use an Alum like Sodium Aluminium Sulphate (Sodium Alum) in a harening fixer.

The term Alum itself is used in different trades to signify quite different compounds so is quite unspecific.

Ian