HCO2- as a Photo-dopant

Discussion in 'Silver Gelatin Based Emulsion Making & Coating' started by narigas2006, Feb 7, 2007.

  1. narigas2006

    narigas2006 Member

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    Hi all,

    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...)

    cheers
     
  2. Photo Engineer

    Photo Engineer Subscriber

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    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.

    Thanks.

    PE
     
  3. dancqu

    dancqu Member

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    Formate ion. "is introduced as Silver formate" (AgHCO2)."
    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
     
  4. narigas2006

    narigas2006 Member

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    Hi PE,

    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'...

    Many thanks.
     
    Last edited by a moderator: Feb 7, 2007
  5. Photo Engineer

    Photo Engineer Subscriber

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    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.

    PE
     
  6. Struan Gray

    Struan Gray Member

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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.
     
  7. Hologram

    Hologram Member

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    See also
    US 6,277,549.
     
  8. dancqu

    dancqu Member

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    Ten to the minus 6. That is very nearly None at all.
    Maybe a few drops of 10% in a liter of solution. How
    about soaking a sheet of print paper and see what
    happens?

    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
     
  9. narigas2006

    narigas2006 Member

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    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?

    cheers
     
  10. Photo Engineer

    Photo Engineer Subscriber

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    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.

    PE
     
  11. totalamateur

    totalamateur Member

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    I ran across this a while back, but I was (and am) still learning, so I'm not sure how to use this in my fledgling emulsion recipe. It appears that Kodak tried to add the formate at the end of the proccess, and that it was only effective at higher pBr (which I must assume is the negative log of the concentration of bromine ions?)

    See
    http://www.freepatentsonline.com/EP1271233.html
     
  12. Photo Engineer

    Photo Engineer Subscriber

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    Kodak measures pAg or vAg as it is more accurate, and is more indicative of the actual conditions.

    In any case, this method did not work out well and was abandoned AFAIK. Kodak uses an Osmate complelx.

    PE
     
  13. dyetransfer

    dyetransfer Member

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    This would be just the thing for astro-photography. I currently bake the tech-pan film in a hydrogen atmosphere for 2 days to limit reciprocity failure. Even an emulsion which only lasted a few days would be useful as you would coat plates just prior to use. It sounds like this formate emulsion would be competitive with CCD technology, but much cheaper and MUCH higher resolution.

    Regards - Jim Browning
     
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  15. Photo Engineer

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    There is a French patent on this that was leased to Agfa by the inventors. It was intended to improve speed IIRC. Jon Eikenberry, the inventor, was a respected Kodak Engineer and did fine emulsion work, but I don't remember if this was ever adopted. All I can say is - try it.

    PE
     
  16. Hans2008

    Hans2008 Member

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    How do I measure pAg? I would like to be able to do that.
     
  17. Photo Engineer

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    It is generally measured using a conductivity meter and silver or silver halide plated electrodes. A normal junction cannot be used because the chemistry involved affects the emulsion adversely. You have to calibrate the meter in pAg units as the result is expressed in mV. That is why we use vAg as it is a direct and more accurate measure.

    The vAg scale goes from about -250 mV to about + 200 mV.

    PE
     
  18. Kirk Keyes

    Kirk Keyes Member

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    A conductivity meter does not work for this.

    You need a pH meter.

    A silver ion specific electrode is the other item needed. You can do with with:
    • one silver ion electrode half cell and
    • one reference electrode
    or
    • one combination silver electrod which has both the silver ion electrode with a reference electrode in one physical package.

    As far as electrode go, they will generally be expensive, and you need to maintain them well to keep them functional.

    You can buy a used digital pH meter for not a lot if you are lucky, but I would not buy a used silver ion electrode unless it was really cheap and then I would expect that it would not work... Now a silver ion electrode is pretty spendy - $500 or so.
     
  19. Photo Engineer

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    Kirk;

    We used to measure pH with a conductivity meter. In fact, we used to use a Wheatstone bridge. :D Yes, I'm that old. It works. And the method you describe might work, but the electrodes are critical to avoid poisoning the emulsion.

    When you get down to it, a pH meter is really a tricked up conductivity meter with a pair of fancy electrodes.

    PE
     
  20. Kirk Keyes

    Kirk Keyes Member

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    Well when you break it down that way, sure. but if someone is to go and buy a conductivity meter and a silver electrode, they will be disappointed as they will not work together.
     
  21. Photo Engineer

    Photo Engineer Subscriber

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    Well, I am working on a method that uses a VOM and Silver electrode with a friend. We are trying to devise an inexpensive method. And a VOM can be used as a conductivity meter too.

    The biggest problem is not building a meter setup, it is the calculations involved in calibrating it. Someday I may publish a table on this. It is incredibly complex.

    PE
     
  22. Hans2008

    Hans2008 Member

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    So, if I had two silver electrodes and a constant current source and then just measure the voltage between the two electrodes... Would that be a reasonable indication of pAg? I suppose I could calibrate that by measuring differing concentrations of AgNO3. But how will the KBr and KI that are also in solution affect the readings
     
  23. Kirk Keyes

    Kirk Keyes Member

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    No, you need two electrodes - a silver electrode, which can be as simple as a silver billet/rod or a rod of silver that's coated with bromide or even sulfide. For the second electrode, you need a reference electrode. Both of these are called "half-cell" electrodes, and the two together make a single silver ion specific electrode. The two half-cell electrodes can be as individual physical electrodes so that there are two electrodes that you put into the solution, or they can be combined into one physical "combination" electrode.

    And then you do measure the millivolt difference between the electrodes.

    Calibration will have to be in a solution containing the halides at the concentration range that you will be operating the electrode. You can do additions of small amounts of silver to the solutions for calibration.
     
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  24. Kirk Keyes

    Kirk Keyes Member

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    A Wheatstone bridge is how they did it in the REALLY old days - like pre 1930s. Then Beckman used a two vaccuum tube amplifiers in his design. You need to not have a big current draw off the silver (or pH electrode, as Beckman was working on) in order to get accurate measurements. Modern designs use solid state devices with have very, very low current draw.
     
    Last edited by a moderator: Dec 7, 2008
  25. Kirk Keyes

    Kirk Keyes Member

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    You know, I think a lot of digital meters from pre-2000 use simple point-to-point calibrations between each calibration point. That makes calibration much simpler. But it means you need to check your calibration with standards that fall between your standardization points to ensure accuracy.

    The math is really simple when you do it that way, just simple slope calculations...
     
  26. AgX

    AgX Member

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    VOM = (I guess) multi purpose meter

    Dictionary says: Volt-Ohm-Milliameter
    but the term Milliameter makes me wonder...