Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: The_MD on May 09, 2011, 10:29:06 AM
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Good afternoon,
I am trying to assay undeveloped photographic film. Normally I would do a cyanide digestion and run through the ICP, but the instrument is down, and I am currently pretty much limited to titrations. Which I don't currently have a method for.
The film is essentially silver chloride, gelatin and assorted dyes on a cellulose back. I cannot currently give a breakdown of the dyes. It normally assays out as 2 to 2.5g/kg silver in the film.
My current thought is that I might be able to react Sodium Hydroxide with silver chloride to produce Silver Oxide, Sodium Chloride and Water. I then might be able to run a back titration of the sodium hydroxide against nitric acid to determine the NaOH used, and therefore possibly the silver.
So that would be:-
AgCl + NaOH -> NaCl + AgOH
2AgOH -> Ag2O +H2O (apparently this is fairly reliable)
That sounds like it might work, but I've got a horrible feeling that it isn't that simple. Anyone care to poke holes in the plan? I need to test to +/- 0.1g/kg, but don't currently have a decent standard to check against.
Alternatively I could try some form of Tollens test, but I am uncertain about quantifying that. Any suggestions would be extremely welcome - I rely overly on the ICP and frankly feel a bit lost without it.
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Please ignore my previous thoughts on a sodium hydroxide titration. I did a trial run and the results were ridiculous - overreading by approximately a factor of 5. Clearly this isn't the answer.
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Perhaps dissolving silver chloride in thiosulfate and using some method for silver determination in the spent fixing solution will work? That's just a general idea, I don't have any ready method at hand (which is slightly surprising, I am sure there was one described in the books I have here - but I can't find it).
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I am being told to do thiocyanate titrations using ferric ammonium sulfate indicators.
I am not entirely convinced that will work, but given that it's the company owner telling me to do so, I'll try.
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That would be some variant of Volhard method (http://www.titrations.info/precipitation-titration-argentometry-chlorides-Volhard).
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Indeed. The trouble is getting the silver chloride into solution without adding ions that might interfere with the titration. I'm suspicious that thiosulfate might cause issues.
We shall see.
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hmm
Slight update on this one.
It appears fixing solution inhibits the formation of the ferric thiocyanate precipitates.
Might try and get hold of some tartrazine, or just have a general rethink.