Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 70,901   Posts: 1,555,804   Online: 800
      
Page 1 of 3 123 LastLast
Results 1 to 10 of 27
  1. #1
    rmazzullo's Avatar
    Join Date
    Mar 2007
    Shooter
    Medium Format
    Posts
    263

    homebuilt vAg electrode?

    Hello PE,

    Has there been any progress on the idea of building a workable vAg electrode? Do you know of any references that may discuss how one would be constructed?

    Thanks,

    Bob M.

  2. #2
    Photo Engineer's Avatar
    Join Date
    Apr 2005
    Location
    Rochester, NY
    Shooter
    Multi Format
    Posts
    23,183
    Images
    65
    Bob;

    The starting point is a high accuracy volt meter with silver electrode(s). You will need more than that. A friend who has been helping me has been otherwise occupied. And, so have I.

    For your use though, 0.1 molar NaBr at 20 deg C will have a vAg of -61.48 mv and a pAg of 11.463

    That is, BTW, 10.289 g/l.

    So, here is a calibration point for you.

    PE

  3. #3
    kb3lms's Avatar
    Join Date
    Jun 2006
    Location
    Reading, PA USA
    Shooter
    35mm
    Posts
    707
    Images
    5
    While browsing the web at lunchtime, I found references to this article about constructing a reference electrode out of a used felt-tip pen. So I tracked down the article tonight and have posted it at the link below. The electrode described is for Ag/AgCl, but it seems like the basic design could be adapted by using KBr instead of KCl.

    http://dl.dropbox.com/u/4212013/electrode.pdf

    Any thoughts on it's usefulness for measuring vAg or at least pAg? The more I read I see how knowing vAg or pAg is important for making more advanced emulsions so it seems like finding an economical solution for the probes would be useful.

    -- Jason
    All this has happened before, and all this will happen again.

  4. #4
    Photo Engineer's Avatar
    Join Date
    Apr 2005
    Location
    Rochester, NY
    Shooter
    Multi Format
    Posts
    23,183
    Images
    65
    Jason;

    Thanks for the reference.

    Technically, an Ag/AgCl electrode is used for all vAg measurements. The plating on the other electrode is what changes depending on the salt used. So, this reference electrode looks just fine.

    PE

  5. #5
    kb3lms's Avatar
    Join Date
    Jun 2006
    Location
    Reading, PA USA
    Shooter
    35mm
    Posts
    707
    Images
    5
    Oh, I see. Well maybe that simplifies things.

    So the other electrode could be a silver wire plated with AgBr, for example, if you were working with a AgBrI emulsion? Or would it be plated with AgBrI?

    And the electrode should be solid silver, right?
    All this has happened before, and all this will happen again.

  6. #6
    Photo Engineer's Avatar
    Join Date
    Apr 2005
    Location
    Rochester, NY
    Shooter
    Multi Format
    Posts
    23,183
    Images
    65
    The silver electrode should be solid silver. I have a batch of them here that I made by hand. I used 12 gauge silver wire 99.99% pure. It should be plated with the salt which represents the major constituent of the emulsion. Use a reference electrode of Ag/AgCl in all cases. However, use of a bridge is advised due to Cl leakage from the electrode. The book contains photos and a description of the procedures. Much of that is posted here in less detail.

    PE

  7. #7
    kb3lms's Avatar
    Join Date
    Jun 2006
    Location
    Reading, PA USA
    Shooter
    35mm
    Posts
    707
    Images
    5
    Thanks, PE. I will check that out when it arrives - hopefully today!
    All this has happened before, and all this will happen again.

  8. #8

    Join Date
    Jun 2004
    Location
    Portland, OR
    Shooter
    4x5 Format
    Posts
    3,268
    Quote Originally Posted by kb3lms View Post
    While browsing the web at lunchtime, I found references to this article about constructing a reference electrode out of a used felt-tip pen. So I tracked down the article tonight and have posted it at the link below. The electrode described is for Ag/AgCl, but it seems like the basic design could be adapted by using KBr instead of KCl.

    http://dl.dropbox.com/u/4212013/electrode.pdf

    Any thoughts on it's usefulness for measuring vAg or at least pAg? The more I read I see how knowing vAg or pAg is important for making more advanced emulsions so it seems like finding an economical solution for the probes would be useful.

    -- Jason
    Note that what's described in that article is for a reference electrode.

    When making electrode measurements, you need to have both a reference electrode and what's called an ion specific or ion selective electrode.

    For pH measurements, you use a what's typically called a "pH" electrode. What's really being said there is that you need a glass electrode (which can simply be thought of as a hydrogen ion specific electrode) and a reference electrode.

    Most people never even mention the reference electrode when talking about pH measurements because most pH electrodes nowadays have the reverence electrode built right into the ion specific electrode. The reference electrodes are typically silver chloride electrodes or calomel electrodes. Calomel seems to have lost favor these days as they contain metallic mercury and mercury (I) chloride, both rather toxic compounds.

    The silver chloride referece electrode consists of a piece of silver metal that is coated with silver chloride, and it is placed into a concentrated solution of potassium chloride to stabilize the silver chloride that coats the silver wire. The potential that is generated by the silver|silver chloride junction is the "reference" voltage that this type of electrode produces. The standard electrode potential that the silver reference electrode produces against the standard hydrogen electrode (SHE) is 0.230V ± 10mV.

    The silver chloride reference electrode that is discussed in the article linked to above is this type of electrode, not an "silver" electrode that is used for making vAg measurements.

    The vAg electrode that PE talks about for making vAg measurements, is simply a piece of high purity silver wire/rod that has been coated with the dominant halide for the emulsion that is to be measured. Like PE mentioned, somewhere here on APUG are a series of post where he and I discussed the construction of a home-made vAg electrode. Basically, it's the silver wire that PE mentioned, I think what I found was 12 or 14 guage - which is nice and sturdy and resists being bent from use. That silver wire was then soldered to a length of coax cable with a BNC connector so it could be directly connected to the "pH" meters that we both had. The silver wire/coax assembly was then inserted into some fire-polished glass tubing with bare silver wire outside the glass and the silver wire solder joint inside the glass and then 5-minute epoxy was then used to set the wire in place in one end of the glass tube and set the cable into the other end.

    For use, this vAg electrode was cleaned with fine wet-dry sandpaper and then placed into a solution with the halide of choice, and a small current run through the wire and solution to drive the halides to the vAg electrode to plate it. (As they mention in the article, the vAg electrode can be connected to the positive terminal of a 3-volt battery and then current run through for 10 minutes using a second vAg electrode or a simple siver wire in circuit with the plating solution.) Once this had been done, the vAg electrode was ready for use. This treatment was good for several days if stored in solution and kept in the dark.

    Now this vAg electrode would have to be used with a reference electrode - just like the one in the article linked to above, so that you had both an ion specific electrode and a reference electrode.

    And actually, the idea of a felt-tip pen as the body of a reference electrode is pretty good, and great for the home lab, as silver chloride reference electrodes are expensive new and hard to find used. Note that the pen-based reference electrode has a "bridge" like PE mentioned above built into it - that's what the agar gel is doing. It's slowing down the mixing of the reference solution (the potassium chloride that surrounds the silver|silver chloride electrode). The felt tip is merely acting as the junction that allows the interior solutions to come into contact with the solution to be tested.

    PE also suggests that you want to use a double bridge to minimize the migration of non-chloride halides from the test solution to the reference solution. If bromide or iodide get into the reference solution that surrounds the silver|silver chloride reference electrode, then it will be poisoned and it will no longer serve as a "reference" electrode.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  9. #9
    kb3lms's Avatar
    Join Date
    Jun 2006
    Location
    Reading, PA USA
    Shooter
    35mm
    Posts
    707
    Images
    5
    Kirk, thanks for the great explanation! I had understood that the article was describing the reference electrode and you needed a separate one for the salt but didn't quite understand the role of the Ag/AgCl inside the reference electrode. You have helped to clear this up.

    Now the Ion-Specific Electrode (ISE) is the next part to investigate. Soldering silver wire to a piece of coax should be simple enough. What is the purpose of the glass? Sounds to me like it is to give a bit of stiffness and durability to the assembly plus protect the soldered connection and prevent it from contaminating the emulsion? I don't know anything about glass tubing but I suppose you can get it from a chemistry supplier or the big auction site.

    I had found the other posts you mentioned and along with PE's book have started to fill in the shopping list for the two electrodes plus the salt bridge. This has all been very, very helpful.

    -- Jason
    All this has happened before, and all this will happen again.

  10. #10

    Join Date
    Jun 2004
    Location
    Portland, OR
    Shooter
    4x5 Format
    Posts
    3,268
    Glass was chosen for several reasons - to give the electrode some rigidity, to keep the solder joint clean, to minimize the length of silver wire needed, to have something that you can insert into the solution that will not contaminate it, it's easy to clean, and can be supported in the solution using a ring stand clamp.

    I think PE recommended around 2 cm of silver wire in the solution and I wanted all the wire under the liquid level so there's no possible changes as the solution level rises, so I wanted something that I could insert into the solution so I immediately thought of glass tubing. Commercial electrodes used to use glass, but now they often use epoxy for the electrode bodies. That was where I figured that 5-minute epoxy would be great for sealing the silver wire into the glass tubing. And it's much easier to use the epoxy and I think more water-tight than trying to seal the silver wire by blowing the glass to make a glass seal.

    I wanted a technique that people without much glass-blowing skills could use. As it is, you'll need to practice cutting the glass, or get someone at a glass shop to do it. After cutting it, you'll probably want to fire polish the ends - Watch some online youtubes for basic glass-blowing techniques. None of this glass working is difficult and all you need is a glass triangle file to make a nick in the glass to "cut" it, and then a propane torch that you may have around the house already.

    Oh, and when you connect the coax, just solder the signal wire of the coax and not the braided outer wire to the silver. Strip the braided part back a bit to it doesn't make contact with the center wire. It's going to take a fairly large piece of glass tubing to get the coax into the glass. Like 12 or 14 mm or so diameter tubing. Those are a bit hard to cut, so maybe a glass shop would be good if you've no experience cutting glass tubing.

    I was also going to try making one out of acrylic tubing as it can be cut with a hacksaw - I bought the tubing but I never got around to making the electrode. You might want to git that a try. Other than the siver wire/rod, nothing it really expensive here.

    And the coax I used was from Radio Shack - just a 2 ft coax with BNC connectors on both ends. Clip on BNC off and then you have a nice 2 ft lead on your electrode.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

Page 1 of 3 123 LastLast


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin