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  1. #21

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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

  2. #22

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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.
    Last edited by Kirk Keyes; 12-07-2008 at 05:44 PM. Click to view previous post history.

  3. #23

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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 Kirk Keyes; 12-07-2008 at 06:39 PM. Click to view previous post history.

  4. #24

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    Quote Originally Posted by Photo Engineer View Post
    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.
    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...

  5. #25
    AgX
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    VOM = (I guess) multi purpose meter

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

  6. #26

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    You can wonder, but you want to measure millivolts for pH or pAg.

    I have a VOM that measure to 10 Amps. And I have to remember not to have it on that setting when I try to measure house VOLTAGE. I don't mind it tripping the 15 amp circuit breaker, it's the popping sound of the tip of the meter probe vaporizing off that scares me...

  7. #27

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    PE - care to go into your pAg electrode design more? How are you doing the reference cell? The rest it not too tough, as I see it.

  8. #28
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    Quote Originally Posted by Kirk Keyes View Post
    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...
    Well, VOM is as described by AgX.

    Kirk, the calibration I meant is turning resistance or conductivity if you will into vAg or pAg. After all a value expressed in mV is not easily related to vAg. That is the complex calculation.

    Old formulas gave values in ohms of resistance if they gave any measurement at all. The salt measured total conductivity including nitrates present and anything else. Current meters include the nitrate but the calibration software can get at concentration even in the presence of nitrate by cancelling it out.

    There is a considerable difference between 1 M AgNO3 with and without 1 M of NaNO3 as a "side salt" produced by pptn.

    PE

  9. #29

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    Can you give an example of "an old formula" for this?

    For vAg you would want to use the mV setting on the VOM, and then we are working directly in vAg, right? Forget pAg as you say. Graph it on semi-log paper, I beleive.

    For conductivity, you use one of the ohm scales, as appropriate for the electrode you have. Then there are formulas for that, based on your conductivity probe. Are you using a commercial conductivity probe or making your own.

    Maybe this talk should be in it's own thread?

  10. #30
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    Well, Kirk, I don't have an "old" formula that gives readings in resistance (or conductance) that I can publish but they exist.

    As for current mV readings, at 40 C, 0.1 m/l NaBr should read a vAg of -50.11 mv with a pKsp of 11.61, pAg of 10.49, pX of 1.11, Activity Coefficient of 0.6616 nd Junction Potential of 1.78. This was in a previously deleted post about a year ago. Another APUG member disagreed with me on some of the "data" and the entire exchange was deleted by the moderators.

    If this can be reconciled with your data, then by all means use it, but it depends a lot on your setup as we discovered at EK. If you want more information, then by all means call me. You have my #. This talk is getting to the limit of what I can put in print.

    PE

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