Boiling silver nitrate/excess iodides

Discussion in 'Alternative Processes' started by DonF, Jan 9, 2019.

  1. DonF

    DonF Member
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    Several guides on the web, including an old Kodak wet plate manual from the 1930s, recommend adding sodium bicarbonate or sodium carbonate to the silver nitrate solution used to sensitize plates to raise the ph above neutral, boiling the solution to some fraction of its original volume, then adding additional distilled water to precipitate out the excess iodides.

    I tried this last week and it seemed to work out fine, getting rid of the alcohol and ether smell as a beneficial side effect. A few things about the process are puzzling to me:

    -Why does adding water after evaporating by boiling precipitate out excess iodides? It seems adding water should allow MORE iodides to stay in solution, not less.

    - When I added a small amount of sodium bicarbonate to the silver nitrate, it raised the ph to about 6.0, but also caused fizzing and a yellow color that faded after a short period of heating. A large amount of black sludge precipitated out, several times what would be seen with normal sunning. Was this organic contamination, or was some other reaction between the silver nitrate and sodium bicarbonate occurring? I had previously added a few drops of dilute nitric acid to slightly acidify the silver nitrate.

    Any insights are appreciated.

    Best,

    Don.
     
  2. Photo Engineer

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    Too many variables here to give a definite answer, but anything black in that mix could be Silver metal, Silver Oxide or Silver Hydroxide. I'm not quite sure what you started with and how much light and heat it was exposed to, nor am I sure exactly what you expected to get.

    PE
     
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    DonF

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    I started with 600ml of 9% silver nitrate solution that was heavily contaminated with ether and alcohol. I expected to remove the ether and alcohol through evaporation by heating, as well as the excess iodides in solution that were causing black specks on plates and difficulty with developer spreading smoothly over the plate. I also intended to remove any organic contamination.

    I added about 1/2 teaspoon of sodium bicarbonate to raise the ph of the boiling solution, which resulted in the yellow color, fading quickly and the precipitation of the black solids, similar to what is produced by normal sunning. Boiling the solution down to 400ml., cooling and filtering resulted in a very clear and concentrated solution with a high specific gravity. Adding cool distilled water resulted in a bluish-white cloudy solution. Exposure to a UV lamp turned the cloudiness black almost immediately. Several hours exposure and the fine black precipitate settled out overnight. Additional filtering resulted in a clear solution requiring a small amount of additional silver nitrate to bring the SG back to 1.09.
     
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    DonF

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    Here's the relevant text from the 1930s Kodak manual. Pardon any OCR errors. It seems the instructions distinguish between tray and tank sensitization, but implies the excess iodide treatment applies only to silver nitrate used in a tray and the organic contamination (sunning/UV) treatment only to tank sensitization. In fact, both issues can occur regardless of the container. Note that they recommend neutralizing the acidity with sodium carbonate (rather than sodium bicarbonate, as I used) before sunning as well. I believe the Osterman wet plate guide advocates a similar process:

    "If a tray bath is used. it is possible to so maintain the solution
    that the only trouble to be corrected is ovcr-iodization. When the
    bath becomes over-iodized. pour it into a clean, clear glass bottle
    containing about one-third water. This will precipitate the excess of
    iodides. Then test it with the hydrometer, adding silver nitrate until
    the hydrometer reads 35 (or 40, if that is the strength to be used)."

    "Neutralize the acidity by putting in sufficient sodium carbonate.
    Then set the bath in the light until all the precipitate has settled,
    Filter, re-acidify with pure nitric acid. and it is ready for use."

    "A dipping bath, besides becoming over-iodized. becomes overloaded
    with alcohol from the collodion plates sensitized in it. An
    alcoholic condition can be detected by slow sensitization, the emulsion
    appearing uneven or streaked. It presents a generally ''greasy"
    appearance, especially when the plate is being flowed with developer.
    The bath solution will smell strongly of alcohol. When this occurs
    pour the solution into a porcelain evaporating dish and boil it down.
    When nothing else is wrong with the bath, only about one-third
    need be boiled away, but if the bath is giving fog because of the
    presence of other organic impurities, it should be boiled down until it
    has formed a pasty mass and has become liquid again. It is then
    allowed to cool. The resulting fused silver nitrate is dissolved in distilled
    water, and this is poured into the quantity required to bring the
    bath to its original volume. It is then tested with the hydrometer,
    brought up to strength with silver nitrate, and set in the sun for a few
    days. After being filtered and acidified, the bath is again ready for use.
    By setting a silver bath in the sun, all organic mauer which it
    contains will be oxidized, and a dark precipitate will settle out. It
    is therefore advisable to make enough solution for two baths, so that
    one may be out in the light while the other is in use. The baths
    must be filtered at least once a day through absorbent cotton."
     
  5. Photo Engineer

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    At this point, your best recourse would be to contact Mark. He is the true expert in this field. He is quite open to this sort of contact. You can call him or write him at GEM (George Eastman Museum) or you can contact him here on Phototrio or on Facebook.

    PE
     
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    DonF

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    Thanks, I will do that! The process I followed is one the Mark promotes and was illustrated on a YouTube video (not Mark's). I actually finished filtering, replenishing the silver nitrate, and adjusting the ph this evening. I was afraid all of the black sludge after bicarbonate and boiling was a silver compound that had depleted my silver nitrate concentration to nothing, but this was not the case. After all the treatment and filtering, the SG dropped from my normal 1.07 to 1.055, not bad at all. 18 grams of silver nitrate added to my 650ml volume brought me to about 1.08. The ph after the sodium bicarbonate was about 6.0. I used 9% nitric acid to bring that down to 4.0. Perfect! Plates look amazing. The replenished silver nitrate is very active, sensitizing a plate in 2 minutes. The developer seems to flow over the plate much more smoothly.

    I was just curious about the chemistry involved.
     
  7. TheMissingLink

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    Hi Don,

    I prefer it to keep it simple: No addition of anything to change the ph.

    Since some years I use 2 Liter of silver nitrate. From time to time sun bathing, filtering after that. After measuring the SG I fill up some distilled water or, from time to time adding some silver nitrate.

    In this way I never had a problem with the ph. I always try not to change to many variables.

    horst
     
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    DonF

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    I did find this info in Bostick and Sullivan's wet plate kit instruction file.

    Apparently, the lower ph allows the iodides in the silver nitrate solution to remain in solution rather than depositing on the plate. I'm guessing that when the silver nitrate gets near saturation level so that black specks begin to appear, raising the ph from 4 to 6 with the sodium bicarbonate is enough to cause the excess iodides to precipitate out when the solution is further concentrated by boiling.

    From B&S:
    "Adding a small amount of Nitric Acid to a fresh Silver Nitrate bath will help extend the useable life by dozens of 8x10 plates. As contaminants from the salted collodion build up in the Silver Nitrate bath, the Nitric Acid will prevent them from creating unwanted Silver compounds that can fog the image. To 1000ml Silver Nitrate 10% Solution add: 16 drops Nitric Acid 7% Before sensitizing plates in the Silver Nitrate 10% Solution for the first time, you need to make sure the pH is within acceptable limits. Using the pH test strips from your kit, test the pH of the Silver Nitrate 10% solution. Typically, a new Silver Nitrate bath will be in the pH 3 to 5 range. For maintenance, test the pH of your Silver Nitrate after 30-40 8”x10” plates. Our experience shows that the pH should remain fairly constant around pH 4 even after making sixty or seventy 8”x10” plates."
     
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    DonF

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    Quite a few places recommend just allowing the sun to evaporate off the ether, alcohol, and a portion of the water. Adding additional distilled water to the concentrated silver nitrate solution seems to precipitate out the iodides in a manner similar to artificially raising the ph, boiling and then adding additional distilled water. I have seen the cloudy and extremely fine iodide cloudiness form after only sunning and adding water. It looks like a sort of bluish-cloudy plume that forms where the poured water enters the beaker of silver nitrate. Many times, though, it seems to re-dissolve and clear after mixing without throwing off any black precipitate.

    Adding the small amount of sodium bicarbonate and boiling threw off a large quantity of black precipitate. After filtering and cooling, then adding distilled water to reconstitute, I saw a similar reaction to that of adding distilled water to the sunned silver nitrate, but the reaction was much stronger and the bluish cloudiness persisted rather than vanishing after stirring. Hitting the cloudy solution with a strong 20 watt UV LED lamp (I use it for exposing salt prints), the precipitate turned black in seconds. I exposed it for another few hours, then allowed the extremely fine black precipitate to settle a few days before filtering.

    My boiled, filtered, and reconstituted (to the original volume) silver nitrate read 1.055 on the hydrometer, down from the original 1.07 (9%). Some of the silver loss was from the 40-50 plates that had been run through the bath, the rest from the precipitates formation with organic contamination and the excess halides. I needed only 18 grams of additional silver nitrate to restore the solution to an SG of 1.08.

    Of course, I'm assuming the majority of dissolved substance accounting for that SG is silver nitrate after all of the treatments.

    The treated silver nitrate works great with very fast (< 3 minutes) plate sensitization time, no fogging, brighter more metallic looking "highlights, and better spreading of the developer over the plate.

    I think sunning alone is sufficient if done frequently and if enough water is allowed to evaporate naturally from the solution. Boiling seems to offer faster and more complete results.

    Don
     
  10. Photo Engineer

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    Don, my problem from the start was that the only Iodide that could precipitate out, IMHO, was Silver Iodide and this would lose you some of your silver.

    Also, boiling anything with ether in it is dangerous due to flammability and the potential formation of explosive peroxides.

    PE
     
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    DonF

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    Point taken on the formation of peroxides. The amount of ether in solution is very small, though. I'm not sure if it would be cause for concern.
     
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    DonF

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    I exchanged some email with Mark Osterman on what exactly is happening here. He was unsure of the exact chemical reactions, so I did a bit of research. What I found lines up perfectly with what is happening in the boiling process , confirmed by my own experiences.

    I am not a chemist, but still wanted to have some high level idea of what is going on chemically.

    I did find some chemistry references, unrelated to photography, on the synthesis of silver iodide.

    https://en.wikipedia.org/wiki/Silver_iodide
    and
    http://www.softschools.com/formulas/chemistry/silver_iodide_formula/326/

    The method is to add potassium iodide to concentrated silver nitrate solution. The silver iodide then forms. It is normally insoluble in water, but the silver nitrate keeps it in solution. An important point is that the silver iodide is not photo-sensitive at this point. The text states it is necessary to add distilled water to force the silver iodide out of solution as a yellowish precipitate that rapidly turns dark if exposed to light, as the precipitate is very photo-sensitive and identical to what is formed on the salted collodion plate when dipped in the silver nitrate solution. This dark precipitate contains metallic silver produced by the action of light upon the silver iodide precipitate.

    This entire process is pretty much what is being seen in the purification process. Potassium iodide and the other halide salts are leached from the salted collodion on the plate into the silver nitrate solution, where silver iodide is formed. The silver iodide is held in solution by the acidified silver nitrate and is not photo-sensitive at that point, so the solution remains clear.
    The chemical reaction in the silver nitrate must be ph-related as well, with a lower ph helping to keep the dissolved silver halide from precipitating out onto the plate during sensitization, where it would cause black spots and other issues. The Bostick and Sullivan wet plate manual mention this as the reason for acidifying the silver nitrate bath when making ambrotypes and tintypes, at the cost of reduced plate sensitivity.

    During the silver nitrate boiling process, adding the sodium bicarbonate shifts the ph to 6.0 from 4.0. I saw an immediate bright yellow color that rapidly changed to grey-black as it was heated and exposed to light. The sludge I saw after boiling was certainly the silver iodide/metallic silver being forced from the solution. This did take silver from the bath, but that silver was already unavailable for sensitizing plates as it had already converted to soluble silver iodide, which can't sensitize the potassium iodide in the collodion on the plate. Therefore, little additional silver was lost during this step.

    After boiling the solution further to concentrate the silver nitrate, then filtering to remove the silver iodide sludge, the solution appears clear. The addition of distilled water to reconstitute the solution back to its original volume precipitates out any remaining silver iodide (as described in the chemistry text), which appears initially as a very fine bluish-yellow cloudiness. I saw this turn black almost immediately when exposed to UV light, just as the precipitated silver iodide had done when adding sodium bicarbonates. After filtering, and re-acidifying with nitric acid, most of the dissolved silver iodide is likely gone.

    Anyway, this seems a pretty good description of what is happening, chemically.

    Best,

    Don
     
    Last edited: Jan 12, 2019
  13. Photo Engineer

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    Silver Iodide is not really soluble. I'll have to think more about this and talk to Mark.

    PE
     
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