Every PD formula I see in the literature indicates to use ferrous ammonium sulfate for the redox solution. Could someone explain to me whether using ferric ammonium sulfate would be acceptable as a replacement and why or why not? I have some ferric in the lab that we were considering trying due to running out of ferrous before our new order of chemicals comes in. Thanks in advance for your input.
Mark
Physical Developer Formulation
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David Fairhurst
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I'm not absolutely sure but I would suppose that the point of putting ferrous (fe2+) ammonium sulphate in a redox (reduction/oxidation) preparation would be that the iron oxidises forming ferric (fe3+) ammonium sulphate while reducing one of the other reagents, probably the silver.
ag+ plus fe2+ ----> ag(s) plus fe3+
Using ferric ammonium sulphate would make this reaction impossible and hence the reagent useless.
ag+ plus fe2+ ----> ag(s) plus fe3+
Using ferric ammonium sulphate would make this reaction impossible and hence the reagent useless.
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g.
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PD question
Definitely not. The Redox solution works the way it does specifically b/c you have Ferrous and Ferric Iron together. Ferrous represents Iron in +2 state (aq) and Ferric represents Iron in +3 state. This difference of a charge literally creates a "liquid battery" if you will, of freely available electrons, which are generated during what is call an "oxidation/reduction reaction"...or sometimes known as "redox".
If you replaced Ferrous Amm. Sulfate with Ferric Sulfate, you will have Fe+3 and Fe+3 from the Ferric Nitrate and thus no charge differential and hence no free electrons.
And it's the free electrons that work (rather complexly) to create the solid silver that precipitates out on your latent print residue.
Hope that helps and is not too chem-geeky.
The P.D. reaction is actually (chemically speaking) a fascinating and elegant reaction and there is quite a bit of literature on it and even some aspects that are not fully understood. It's also rather touchy and unstable as anyone who has "overused" a solution or let it stand too long or added things in the wrong order...there are all very important reasons chemically why we use it the way we do. For further info, I would reference the work from the US secret service and Home Office in Britian. Most recently though Dave Burow of USSS has done some excellent work in modifying the PD formulae (see JFI)
g.
If you replaced Ferrous Amm. Sulfate with Ferric Sulfate, you will have Fe+3 and Fe+3 from the Ferric Nitrate and thus no charge differential and hence no free electrons.
And it's the free electrons that work (rather complexly) to create the solid silver that precipitates out on your latent print residue.
Hope that helps and is not too chem-geeky.
The P.D. reaction is actually (chemically speaking) a fascinating and elegant reaction and there is quite a bit of literature on it and even some aspects that are not fully understood. It's also rather touchy and unstable as anyone who has "overused" a solution or let it stand too long or added things in the wrong order...there are all very important reasons chemically why we use it the way we do. For further info, I would reference the work from the US secret service and Home Office in Britian. Most recently though Dave Burow of USSS has done some excellent work in modifying the PD formulae (see JFI)
g.
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Guest
Physical Developer Formulation
Thank you both for your replies. I suspected obviously there had to be a reason, but just wasn't sure of why. Thanks again.
Mark
Mark
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David Fairhurst
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Can you guess which one is which????? 
