Basic Question about Washing
Here I go again! At the risk of looking dumb (again) I ahve a question that has been bothering me for some time now.
a) The purpose of washing is to get rid of exesive halide salts.
b) The moler equivelent of halide salts is usualy in exess of silver(up to 2.5 x acording to baker)
c) Therefor the weight of bromide and iodide salts removed from the emulsion will be proportionately greater than the silver halide,gelatin,ect.
My question is: When we weigh the emulsion after washing, by what "standard" are we weighing it against?
It seems to me that the only standard that can be usied is previously prepared, identical emulsions.
If I am Wrong, please explane.
Here are some answers Bill.
a. Washing also removes Nitrates.
b. Baker is wrong. It is not always that high especially after wash when there should be virtually no halide left. Just a tad must remain to prevent fog.
c. The weight is against the Silver Mole or about 108 grams of Silver in 1 mole.
If you used 1 mole of silver to make 1 kg of emulsion and end up with 1.5 kg, your emulsion is thin in silver and gelatin. It must be thickened with add in gelatin and coated heavier to compensate. If it weighs in at 0.9 kg and you used noodle wash, you lost some , but if you used ISO or UF washing, then you concentrated it and need to dilute it to 1 kg to get back where you started.
Washing also removes ammonia, if you used it.
But I think Bill is trying to figure out a way to calculate what wieght one would expect based on what one added. It's a question I've wondered about myself.
So trying to use nice whole numbers, if you take 1 mole silver nitrate (170 g), dissolve it in 250 ml water and then precipitate and redissolve it with 150 ml 28-29% ammonium hydroxide (1.067 mole NH4OH).
Then you add it to a solution of 500 ml water, 1.5 mole of KBr (178.5 g) and 0.03 mole of KI (5 g), and say 50
grams of gelatin.
These all add up to 1187.5 grams.
It give you about 1.0 moles of AgBr with 3% AgI (or about 0.84 mole Ag/kg emulsion), with 4.2% gelatin. We'll probably want to add more gelatin to raise that conc so the emulsion sets up firmly if we are noodle washing.
If I remember right, 1.0 mole Ag/kg emulsion is a good target concentration, so we are a little low here. And I suspect, with noodle washing (at least from my experience) we will end up even more weight when we are done with that, dropping our silver level even more. We can always add more gelatin when needed to bring it up, but it's hard to remove water when noodle washing. That's why isowashing is nice.
So back to Bill's question - Before washing, we now have about 1.0 moles of KNO3 (101 g) in the emulsion, about 1.0 mole ammonium ion (NH4+) at 18 g, as well as 0.5 mole of excess KBr (59.5 g). That totals to 179.5 g of stuff to wash out of the emulsion.
Removing 179.5 g from 1187.5 g leaves 1008.5 g. That puts us right at 1.0 mole Ag/kg emulsion. But like I said, I think we we'll be high from retained water left over from the noodle washing.
So Bill, I guess you could try to do a calculation like this to determine a target weight for your emulsion.
I think the targets you want to set are based on mole Ag/kg emulsion, and then % gelatin so that you don't end up with runny emulsion.
Last edited by Kirk Keyes; 01-04-2009 at 08:00 PM. Click to view previous post history.
You did exactly what I said but ran around the bush, so to speak. If you base it on Silver moles, and assume no loss (Silver Halide doesn't wash out), then keep track of the weight at the end, you know the exact concentration in moles / kg.
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And as a reminder - it is 1.0 mole Ag/kg emulsion that should generally be our target?
Not necessarily, but that is a good one. Many of the ones I make are 50% more dilute or 1 mole / 2 kg.
Thank you P.E. and kirk,
This is now clear to me. I will apply it to my next emulsion.
BTW,I made my "cleaneSt" emulsion yet today. A fogging test shows no fog whatsoever in the onexposed portion,while 60 seconds under room leval incandecent light gose black,black,black.
The developer is undiluted D19.
Bill - congrats on the emulsion. I've been meaning to work this out myself for some time!