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Kirk Keyes
01-21-2009, 12:23 PM
I most certainly does change when you add in the halide!

Look up the Ksp of AgBr, and then look up the definition of Ksp (solubilty product constant - the Ksp is the product of the ions in solution). Try the math now with that added into the solution. The pAg will swing back into higher numbers (i.e. lower concentrations). Ammonia will mess with it a bit as it will increase the solubility of the silver by forming complexes.

drazak
01-21-2009, 12:24 PM
TA:

pAg is the -log of the molarity of Ag in the solution, molees/litres. The best way to calculate it is from vAg, which is an electrochemical measure of Ag vs. a standard cell. The pAg changes with times as the silver comes out of the solution.

Hope this helps

Ben

Kirk Keyes
01-21-2009, 12:26 PM
PS, you don't need to really calculate the number of moles of water, you just need the volume to get concentrations.

Photo Engineer
01-21-2009, 12:41 PM
It should be noted that pAg is meaningless if taken of a solution of Silver Nitrate as pAg must be in reference to something else! In this case it is halide. So, the initial pAg is taken of the HALIDE solution in the kettle and then is continued as Silver Nitrate is added.

Ammonia has a profound effect on vAg, but as of the last time I worked on the model there was no model for this effect which worked properly. Since SRADs were no longer made at EK, this was a moot point. Other methods were in use since the 70s or earlier.

PE

Kirk Keyes
01-21-2009, 12:49 PM
Here's a good reference, and from RIT!
people.rit.edu/lprsch/20042/SilverTitrationSheet2005solution.doc (http://people.rit.edu/lprsch/20042/SilverTitrationSheet2005solution.doc)

totalamateur
01-21-2009, 12:49 PM
So, firstly, my calculation of Pag w/o halides is jsut mathematically incorrect, and secondly, it appears to be just this side of impossible to calculate those values, which is why everyone in here is after the holy grail of a sensor to do it.

Thank you all for the chemistry lesson, I'm afraid it's been a decade or so since I tried to figure out a pH, and a lot of organic -OH groups have been through the brain since then.

I guess my last follow up question would be - lacking a sensor and computer controlled additions, is it best to give up trying to calculate pAg in the kettle and just use a trial and error approach to get what you're after (in terms of grain size and shape)? is it really as difficult as it appears, or would a non-chemist like myself have a prayer in doing it?

OH, and PE - what do you mean the pAg is in reference to another salt, I figured the oncentration is the same, regardless of what else is in the solution (at a finite point in time, naturally it will change as things react, I guess, at time zero, when the Ag hits the stirring rod, does the pAg change even though the Silver Nitrate hasn't reacted yet?

Thanks again.

Kirk Keyes
01-21-2009, 12:57 PM
So to calculate pAg, it's the negative log of the molar concentration of silver. In the SRAD formula posted, it's 130 g Ag to 500 ml water. That's .769 Mol Ag, and 27.7 Mol water, or a concentration of .027, for a pAg of 1.557?

You're calculating your concentration wrong. You want it in "M" units - that is molar concentration. Moles/liter.

So for your example above, you have 0.769 mol Ag, 500 ml water which is 0.5 litres, and that gives a conc of 1.531 M (mol/L). Then you take the negative log of that number to get pAg. You should get -0.185 or so depending on what you used for mol wt of AgNO3 (I used 169.83 for my calcs.)

Work through the RIT notes that I linked to above and see how that goes.

totalamateur
01-21-2009, 12:58 PM
Here's a good reference, and from RIT!
people.rit.edu/lprsch/20042/SilverTitrationSheet2005solution.doc

OK, I get it - possible, but pretty involved. That's a really good explanation, by the way.

Can we assume that the AgNO3 reacts instantaneously when dropped into the salt(or with a double run when ran into the salt w// the AgNo3)

Kirk Keyes
01-21-2009, 01:06 PM
The pAg of silver in silver nitrate solution being meaningless means that it has little to do with making an emulsion. Certainly, the silver in a silver nitrate solution has a certain concentration, and thus it does have a pAg.

When you mix your silver nitrate solution with halide in the kettle, the pAg is going to radically change, i.e. get much higher, as the solution will not be able to hold as much silver in solution as long as there is an excess of halide in the solution.

Kirk Keyes
01-21-2009, 01:11 PM
OK, I get it - possible, but pretty involved. That's a really good explanation, by the way.

Can we assume that the AgNO3 reacts instantaneously when dropped into the salt(or with a double run when ran into the salt w// the AgNo3)

Yeah, it is a good reference for doing the calculations!

I forget where I read it, but the precipitation of silver halides is a very quick reaction, on the order of milliseconds. You pretty much just have to get the silver ion and the halide ion to bump into each other and if the concentration of silver and halide already in the solution has a solubility product higher than the Ksp of that silver halide, it quickly drops out of solution.

totalamateur
01-21-2009, 01:15 PM
So, in a single run emulsion, pAg will start low in the kettle and increase as the excess of halide is exhausted, and also rise slightly because of the slight solubility of the AgBr.

In a double run, then, you can regulate the excess of halide and therefore control pAg, or at least keep it somewhat constant. (barring the sensor)

Then that's why you pretty much have to use a double run with specific concentrations of salt and silver to get a specific grain?

Photo Engineer
01-21-2009, 02:04 PM
At the start of a run, say with 0.1 m/l of NaBr at 40 deg C, the pAg will be about 10.5, the vAg will be about -50 mv, and the pBr will be about 1.1 So, in a single run, the pAg is high at the start, not low. It decreases as you add silver, so that if you add 0.05 m of silver to this kettle, the pAg will drop to about 10.2, the pBr will rise to about 1.4 and the vAg will be about -34 mv.

So, you have it pretty much backwards in your first paragraph.

A double run has two effects. It makes the size of the crystal more uniform, and it controls the type of crystal formed. A chart of the latter was posted here in another thread.

PE

totalamateur
01-21-2009, 02:09 PM
Had one of those light bulb moments... THanks.

JOSarff
03-16-2011, 03:30 PM
Ron:

IDK if anyone has asked but what form of gold should be used? I've seen HAuCl4 and a K2AuClsomething, plus a seller on ebay selling liquis gold chloride labeled AuCl3.

Joe

Photo Engineer
03-16-2011, 04:53 PM
Jim Browning has the exact amount and name on his web site. Do NOT try to use this on the Azo type emulsion.

PE