Ph testers

Discussion in 'Pesticide & Herbicide Application' started by wcb607a, Jan 22, 2009.

  1. wcb607a

    wcb607a LawnSite Member
    Posts: 132

    Have any of you guys used the ph testers from Gemplars. The ones that you stick in the ground and wait 1 minute. What do you guys think about them and are they any good for accuracy?
     
  2. mngrassguy

    mngrassguy LawnSite Silver Member
    Posts: 2,167

    Yes, I have used one. No, they are not accurate.

    A soil test is the only good way to measure Ph. jmo
     
  3. Whitey4

    Whitey4 LawnSite Silver Member
    Posts: 2,448

    Hanna Instruments have a good accurate pH meter for under $100. Only meters with a glass electrode are accurate, the rest are trash.
     
  4. RigglePLC

    RigglePLC LawnSite Fanatic
    Posts: 12,111

    I have three cheap meters--cost about $15, each. Soil test on my lawn by a major laborotory was at pH 6.8.
    J&M Instruments meter reading averaged 6.5
    Rapidtest meter 7.1
    Ferry Morse meter 7.03
    The variation was that they were off by up to 6 tenths of a point.
    I also tested mine using Diet Coke as a test: found pH of 5.8.
    It is best to compare it with a known soil test, before you begin to use it. Accuracy only fair. Even if you follow the directions carefully.
     
  5. Ric

    Ric LawnSite Fanatic
    Posts: 11,956

    Riggle

    I have an older Hanna pH/EC meter and even the good ones need to be calibrated regularly to be correct. Fact is by the time you buy the calibrating solutions you have close to the cost of the meter. But even brand new meters right out of the box are not always calibrated correctly. BTW I also had a $ 15 Rapidtest that wasn't to far off.
     
  6. phasthound

    phasthound LawnSite Platinum Member
    Posts: 4,576

    For those who don't know, it is important to remember that a change of one pH unit represents a tenfold change in hydrogen ion concentration; for example, a solution with a pH of 6 has 10 times the hydrogen ions as one of pH 7, and pH 5 has 100 times the hydrogen ions of pH 7.
     
  7. ted putnam

    ted putnam LawnSite Platinum Member
    Posts: 4,539

    I have a Kelway in each of my trucks. I always thought I had a decent tool(even though not a lab test) but a quick way to get a general idea of pH. I guess maybe I have a piece of crap and didn't know it:confused: I guess I need to do some research on it....
     
  8. Whitey4

    Whitey4 LawnSite Silver Member
    Posts: 2,448

    I have cross checked quite a few meters. My findings where that any pH meter that did not use a glass electrode were simply measuring the pH of the distilled water used to make the slurry. Sure, a drop of lemon juice would change the reading, but the metalic sensors only measure the pH of the liquid, not of the soil. I found the same results with litmus paper testing too.

    I started with a RapidTest meter, but the results were always the same, which made me rather suspicious. Then I started adding lemon juice, or epsom salt... and that changed the reading, but only because it was dissolved in the liquid.

    A meter tests the conductivity of the solution.


    How a pH meter works
    When one metal is brought in contact with another, a voltage difference occurs due to their differences in electron mobility. When a metal is brought in contact with a solution of salts or acids, a similar electric potential is caused, which has led to the invention of batteries. Similarly, an electric potential develops when one liquid is brought in contact with another one, but a membrane is needed to keep such liquids apart.
    A pH meter measures essentially the electro-chemical potential between a known liquid inside the glass electrode (membrane) and an unknown liquid outside. Because the thin glass bulb allows mainly the agile and small hydrogen ions to interact with the glass, the glass electrode measures the electro-chemical potential of hydrogen ions or the potential of hydrogen. To complete the electrical circuit, also a reference electrode is needed. Note that the instrument does not measure a current but only an electrical voltage, yet a small leakage of ions from the reference electrode is needed, forming a conducting bridge to the glass electrode. A pH meter must thus not be used in moving liquids of low conductivity (thus measuring inside small containers is preferable).
    The pH meter measures the electrical potential (follow the drawing clock-wise from the meter) between the mercuric chloride of the reference electrode and its potassium chloride liquid, the unknown liquid, the solution inside the glass electrode, and the potential between that solution and the silver electrode. But only the potential between the unknown liquid and the solution inside the glass electrode change from sample to sample. So all other potentials can be calibrated out of the equation.


    http://www.seafriends.org.nz/dda/ph.htm

    Metalic electrodes are too conductive. Only a high impedance glass electrode measured against the reference elctrode are accurate.
     
  9. Whitey4

    Whitey4 LawnSite Silver Member
    Posts: 2,448

    Not sure if this is clear, but bottom line... any accurate pH meter has to use a glass electrode, be battery powered and must be calibrated with reference solutions on a regular basis. The elctrodes must also be stored wet, in a special storage solution. This helps maintain the impedance of the electrode. If allowed to dry out, it must be soaked in storage solution for 24 hours before used for making measurements.
     
  10. Whitey4

    Whitey4 LawnSite Silver Member
    Posts: 2,448

    This is the key part of the whole thing, excepted from above:

    Because the thin glass bulb allows mainly the agile and small hydrogen ions to interact with the glass, the glass electrode measures the electro-chemical potential of hydrogen ions or the potential of hydrogen.

    ONLY the hydrogen ions penetrate the high imedance of the glass... while a metalic probe will measure many more larger free ions, which skews the measurement. Thus, the metalic probe measures all ions, not only the hydrogen ions as a glass electrode does. That is why it measures the pH of the solution used to make the slurry, not of the soil itself.
     

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