HCO2- as a Photo-dopant
I tumbled to 2 papers from Jacqueline Belloni
Radiation Physics and Chemistry, 67:291
Photography: enhancing sensitivity by silver-halide crystal doping
The paper describe the use of HCO2- (10e-6 Mol / 1 AgBr Mol) to cover holes and increase film sensitivity (by over 5 times)
It is also in nature:
Belloni, J., Tréguer, M., Remita, H. and de Keyzer, R., 1999. Enhanced yield of photoinduced electrons in doped silver halide crystals. Nature 402, pp. 865–867.
I've read the 1st one, pretty interesting. If you cant access these journals, you can email me in
narigas-2006 AT bol DOT com DOT br
(no spaces, just to avoid spam...)
I've read most all of that.
The methodology was sold to Agfa but never was commercialized.
I have been told that Ms Belloni has taken out a law suit against Kodak over the 2 electron sensitization, but since the Kodak method uses an Osmium salt, I cannot see how she can compare the two.
It is my understanding that there are practical problems preventing the use of her method. At least, Agfa was never able to make it work.
What you have described seems different than what she used. I thought she used formalin which is CH2O. Could you clarify this or is my memory failing? What is the chemical name.
Formate ion. "is introduced as Silver formate" (AgHCO2)."
Originally Posted by narigas2006
That from Dr. Chapman in an article in Photo Techniques,
Nov/Dec 2001. Good explanation but no methods of
application. Agfa had it and did nothing. I Wonder
who now holds the patents? Dan
the dopant is HCO2, from that article:
'When the dopant is included in AgBr at the relative concentration of 10−6 mol HCO2−per mol Ag+, the emulsion is completely stable in the dark. When illuminated, its absorbance immediately at the end of a 2 s exposure is five times that of the undoped emulsion where the yield is Φeff=0.2 atom/photon absorbed. Then it increases slowly up to a plateau and after 15 min the absorbance is twice that just after the exposure (Fig. 3) (Belloni et al., 1999). The first step is assigned to the fast hole scavenging by formate during the exposure ( Fig. 2b). In the second step, each formyl radical CO2− resulting from the hole scavenging transfers slowly an additional electron to a silver cation, so doubling the gain (Fig. 2c). This photoinduced bielectronic transfer is strictly proportional to the number of photons absorbed down to the shortest exposure times.'
I think I'll try to put a bit of neutralised formic acid on it, what do yous think?...
Also, sorry to get too late on the matter but I am a novice'...
Last edited by narigas2006; 02-07-2007 at 07:20 PM. Click to view previous post history.
Well, as you note, it is silver formate or formic acid.
Either would work, but sodium formate, being slightly alkaline might form silver hydroxide thereby neutralizing the effect. This is, BTW, 2 electron sensitization. The problem is this; why did Agfa not bring it to fruition, and I feel that this may be due to inherent problems.
Kodak had enough problems getting their version to work. It is tough science. I'm sorry but I rememberd it as formalin, but I should have known better because it is obvious that some sort of silver interaction is needed. It is as you say, formic acid.
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The 'problem' as I heard it is that the formate ion won't stay put. Usable perhaps for a home emulsion maker, but not for commercial films that have to trickle through a dealer network to the end user.
Originally Posted by Photo Engineer
Ten to the minus 6. That is very nearly None at all.
Originally Posted by narigas2006
Maybe a few drops of 10% in a liter of solution. How
about soaking a sheet of print paper and see what
I wouldn't worry about ph at that dilution. Silver
formate may be less soluble than one of the silver
halides thus making conversion an easy matter. Dan
If I wanna try it, Would yous think that i should soak some pure silver in formic acid than applying it to the gelatin or should just add the formic acid straight?
The formic acid should be added to the salt + gelatin before addition of silver when you make the emulsion. This is the only way to truly incorporate the silver formate into the emulsion.
And, ten to the minus 6 (10^-6) is common for many dopants. That concentration range is used for most dopants, as it is just right for causing the 'effect' desired on the overall grain.
The actual range is usually expressed as 1 * 10^-6 to 8 * 10^6 which gives a 'large' concentration range. If the figure used is incorrect, you can actually supress the effect. So, 10^-6 does not tell the whole story.
For example, I know of a chemical used at 3 - 6 * 10^-6 which varies contrasat, but at 10 * 10^-6 causes a huge speed loss and contrast loss. So, this is *critical* in most cases.