[The_MD]

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. 

[The_MD]

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. 

[Borek]

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).

[The_MD]

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.

[Borek]

That would be some variant of Volhard method.

[The_MD]

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. 

[The_MD]

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.