PH Meters

Discussion in 'B&W: Film, Paper, Chemistry' started by albada, Aug 8, 2012.

  1. albada

    albada Member

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    After 6 months of moderate use, my Hanna pHep-5 meter developed a problem in its electronics making the display hard to read. Hanna has a couple of aspects of poor design, so I've decided to buy another meter from a different maker. I'll first buy a new meter, and then return the Hanna for warranty-replacement so I won't lack a meter during the return process. I'll end up with two meters.

    I'd like a meter that has an accuracy of +/-0.02 pH, and that uses a standard BNC connector. Price around US$200-300. Any recommendations?
    There are many meters on the market, and here are a couple that appeal to me:

    Cole-Parmer meter: http://www.coleparmer.com/Product/Cole_Parmer_laboratory_pH_mV_meter_digital/WU-05996-60
    This meter lacks bells and whistles, but looks solid with good resolution. Probe must be purchased separately.

    Sper 850056 kit: http://www.sperdirect.com/cgi-bin/item/850056/-Advanced-pH-Meter-Kit
    More features than the Cole-Parmer, and includes a probe, for a lower price. But I wonder if the quality of this is good enough. I'm leery of anything that looks like mass-produced junk.

    Oakton sells many lower priced meters, but my concern as always is the quality.

    Opinions are encouraged!

    Mark Overton
     
  2. RattyMouse

    RattyMouse Subscriber

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    Why in the world do you need that extreme level of accuracy?
     
  3. albada

    albada Member

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    Because I'm creating new developers, and I need better accuracy than +/-0.1 pH to insure good consistency and repeatability.
    When I mix developers, I am consistently within +/-0.02 pH. When the difference reaches 0.05 pH, I know something went wrong. But that requires a two-digit meter.

    Mark Overton
     
  4. jochen

    jochen Member

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    Hello,
    if you really want to use the extreme accuracy of +/- 0.01 pH-values, you have to use the best possible electrode for your work. You should contact a laboratory pH-meter manufacturer or a supplier for laboratory equipment for the best recommendation for your work. Only professional laboratory pH-meters (several haundred $ or €) can use different types of dedicated electrodes. Furthermore it is important that your meter is temperature-compensated and can be calibrated by at least two, better three calibration buffer solutions. The shelf-life of these calibration standards is limited. When not in use the electrode has to be stored in a specified storage-solution in order not to get dried.
     
  5. albada

    albada Member

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    The cheap Hanna meter broke! I wanted to be able to run experiments this weekend, so I built my own based on this web site that describes a simple DIY meter. Its heart is a TL082 op amp. After spending a few USDs at Radio Shack, I built this (inside view):

    OpAmpInside1.jpg OpAmpInside2.jpg

    Here it is taking a measurement of a developer. Connect a voltmeter to it, and then calculate pH based on millivolts.

    OpAmpUsing.jpg

    You calibrate it any way you'd like, which gives you a slope and offset. Having those, the actual pH calculations are quick; much quicker than the time it takes to measure and mix the chemicals and wait for the probe to settle.

    QUESTION

    I want a better probe than the one that came in the Hanna (pictured above). Do you suggest single- or double-junction? Refillable versus gel? I've heard that refillable probes are faster than gel-electrodes. Mine takes about 5 minutes for the tenths of millivolts to stabilise -- that's slow! jochen pointed out that good probes are important. Any opinions about these two:

    http://www.amazon.com/Thermo-Scientific-Orion-Combination-Waterproof/dp/B004PYDYCS/ref=sr_1_8?ie=UTF8&qid=1344744238&sr=8-8&keywords=pH+probe+thermo

    http://www.amazon.com/dp/B004G8PUGG/ref=biss_dp_t_asn

    Mark Overton
     
  6. Mr Bill

    Mr Bill Member

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    I don't have any personal knowledge about the items you linked, but I read quite a few photo chemical samples in my younger days - beyond 20 years ago. We used info from Kodak for our guidelines. Similar to what Kodak's motion picture group has available here (if you haven't already seen it): http://motion.kodak.com/motion/uplo...n_motion_support_processing_h243_ulm191-2.pdf

    I think you'd be fine with a single junction reference. Double junctions are mainly for special cases where something you're doing will cause interference. We typically used separate electrodes - this lets you rotate out the pH electrodes. And if something goes bad, you only replace that electrode.

    I never personally used gel electrodes; they were either frit or anular (sleeve) junctions. A frit junction is harder to troubleshoot for clogging; with sleeve junctions, you can just open them up to drain, then refill. So if there are any clogging issues, a sleeve junction is good. Note that we were testing replenished and regenerated solutions; if you only test scratch-mixed B&W developers, I suspect that these are less problematic.

    Whatever you use, you'll probably get a little faster response by gently stirring the sample while reading. It should be a constant speed, so a small magnetic stirrer works great. Calibrating (with buffers) should be done the same way.
     
  7. michael_r

    michael_r Subscriber

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    Mark, are you taking requests? :smile:
     
  8. albada

    albada Member

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    At least you didn't ask me to build a pH meter for you. :smile:
     
  9. kreeger

    kreeger Subscriber

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    Try looking for Beckman Zeromatic model. Again the accuracy as someone mentioned is in the probe itself. Get your buffer solutions to calibrate and you're set.
    When I was doing photo chemical analysis in the 80s under military spec requirements, we used Beckman pH meters as the standard. Lots of Beckman stuff on eBay cheap.

    http://www.ebay.com/itm/Beckman-Zer...659?pt=LH_DefaultDomain_0&hash=item3cba92e07b

    And, in best practice you leave them running all the time with the probe in a proper buffer solution.
     
    Last edited by a moderator: Aug 18, 2012
  10. albada

    albada Member

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    Thanks for the suggestion. I'm realizing that I can't go cheap on the probe, so I'm looking at a refillable model from Thermo Scientific. Interesting that the Beckmans are cheap on eBay. I'd guess those meters were well north of US$1000 when new.

    Mark Overton
     
  11. Kirk Keyes

    Kirk Keyes Member

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    I'm partial to Orion (now owned by Thermo) pH meters. Beckman makes fine meters as well. For Orion, look for 520, 720, 920 models for desktop, or 290 for handheld. If you look long enough, you can get a nice digital model for cheap.

    Get a digital, as I can tell from your past posts you will not be happy with a swing needle readout. And if you're fine with a needle readout, I've got a beautiful Corning Model 10 I'll sell you at a good price! It was the state of the art in 1980 before digital readouts started to take over the market. It has a needle swing of 10" so it is pretty precise.

    Get a probe that is refillable, and get a temp compensation probe either separate from the pH probe or built in. The built in ones are sometimes called triodes as they have a pH electrode, reference electrode, and temp sensor all in one.

    Also look for a double junction electrode at the minimum as it has the reference electrode built in, with an extra junction built in to help with longer life.

    I like Ross double junction electrode, again look for a refillable one, not a gel-filled one.
     
  12. Kirk Keyes

    Kirk Keyes Member

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    Also, I recommend stirring the solution when taking pH readings.
     
  13. Kirk Keyes

    Kirk Keyes Member

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    The Themo electrode you linked to on Amazon above if fine but I'd suggest finding a surplus one on the bay.
     
  14. albada

    albada Member

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    For now, I can get by with my homemade circuit connected to a Fluke DVM, which buys me time to think about what meter to get. One thing I like about Beckman meters is they use the more standard phone plug for the ATC connection, instead of Orion's 6-pin connector.

    You're right about that. I have a nice swing needle voltmeter that I bought around 1973 from Lafayette Radio Electronics, before anyone heard of digital. Now I use the Fluke instead.

    Anyway, I bought a Thermo probe last night, so thanks for your posting.

    Mark Overton