pH meter

Discussion in 'Soils, Fertilizers and Composting' started by Sunset Cycads, Jul 31, 2006.

  1. Sunset Cycads

    Sunset Cycads Active Member 10 Years

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    I need to measure the pH of many, many pots in my nursery which contain different potting soils and have concluded a pH meter is the way to go. Any suggestions on brand or method?
     
  2. jimmyq

    jimmyq Well-Known Member 10 Years

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    the cheapo ones from garden centers are good enough for a general idea, for a more accurate machine try a hydroponic supplier (western water farms), evergro/westgro or terralink.
     
  3. NiftyNiall

    NiftyNiall Active Member 10 Years

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    Accurate measurement of the pH of soils and solutions, in the field is difficult. I use two instruments a Oakton waterproof pockettester,(It floats!) & a; Hanna Pinpoint pH monitor, both work well for me,(It is IMPORTANT, to keep the electrodes wet, at ALL times). I also use fresh Ph paper to calibrate the instruments,(cheaper method), occasionally double-checking by using calibration solutions, sold for the purpose,(more expensive). The really cheap so-called pH meters are really only measuring electrical conduction,(EC), which can also be a usefull tool,(I use a SPER scientific EC meter, great tool, not waterproof, but it has recovered nicely when it has gone for a swim.) EC & Total Dissolved Solids,(TDS), are somewhat related terms, EC can be used to give a rough idea of nutrient levels in the solutions.
     
  4. GHGrower

    GHGrower Member

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    Where I work we use a Hanna Combo pH/EC meter. They sell for about $150 and are quick and easy to use. Also they are small enough to fit in your pocket. I've been using it for about two years and am very pleased with it.
    Hey, NiftyNiall, how do you use litmus paper to calibrate a tester. You're right, the calibration solutions are pricey. Fred
     
  5. jimmyq

    jimmyq Well-Known Member 10 Years

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    Ph paper, not litmus paper. Ph paper changes color ona grade vs pink or blue for litmus.
     
  6. Sunset Cycads

    Sunset Cycads Active Member 10 Years

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    Thank you for the advice. There are a plethora of meters out there and it is a bit daunting picking one. Now I have something to go on.
     
    Last edited by a moderator: Aug 1, 2006
  7. mandarin

    mandarin Active Member 10 Years

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    I am a chemist and have bought a few pH meters and electrodes (and worked with many more), maybe I can be of some help.

    1. Electrodes wear out, no matter how well you treat them. Some pH meters have their electrode permanently attached to it (but these are cheap, on the other hand), or has an odd connector which forces you to stick to the same brand. If you want to be able to replace the electrode, I believe that a BNC or DIN connector would be a good choice.

    2. Check that the electrode is of the gel-filled type, I don't think you want one with a liquid electrolyte.
     
  8. Mr.pH

    Mr.pH Member

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    Check out this site for more information on pH meters and pH measurements:

    www.ph-meter.info
     
  9. Thomas Anonymous

    Thomas Anonymous Active Member

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    I paid over $200 for an Oakton pHtestr1 and I think I paid way too much. I have found either identical models or very similar ones for less than half of that, so do shop around. The Oakton pHtestr1 is not very accurate. If you rinse it, calibrate it, test the pH of something, and then rinse it, and immediately put it back in the calibration solution it will tell you the pH is wildly different from what it is. Temperature drastically effects the readings it gives also, and this is a source of errors that the more expensive Oakton pH meters can automaticly compensate for, or so they say.
     
  10. GHGrower

    GHGrower Member

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    The Hanna meter I mentioned before does have a replaceable electrode and it automatically compensates for temperature differences.
     
  11. Mr.pH

    Mr.pH Member

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    Perhaps electrode has aged, they have limitied lifetime. You may try to look for replacement.

    pH(T2) = T2/T1 * pH(T1)

    Assuming you have measured neutral pH (7.00) in T1 (25 degC - but you must use absolute temperature in Kelvins here, so T1 = 25+273 = 298) and temperature dropped 5 degC (so the new absolute temperature is T2 = 20+273 = 293) new reading should be

    293/298 * 7.00 = 6.88

    So, lack of temperature calibration can give about 0.1 pH unit error per 5 deg C.

    Edit: most electrodes are made in such a way that near pH 7.00 their indications are (almost) not temperature dependent. But that's not a hard rule.
     
    Last edited: Nov 15, 2006
  12. Thomas Anonymous

    Thomas Anonymous Active Member

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    wow. now there's a guy who sounds like he knows what he's talking about.
    ;)
     
  13. Mr.pH

    Mr.pH Member

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    That's GenChem101 :) Most of the needed info is on the ph-meter.info site (see Nernst equation section).
     

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