|MadSci Network: Chemistry|
Dear Jim This looks funny to me too. From your description, this resembles a gadget I once used to see if the potted plants needed watering. It was essentially a conductivity meter. On close inspection, at the tip of the metal rod there was a small part of a second metal. Without actually examining your device, my opinions are of little value and I can be of very little help. A soil of pH = 1 would be found only near a volcanic fumarole, with much sulfuric acid present. Good for growing some wierd algae and bacteria as in Yellowstone Nat. Park, but no good for roses. Similarly for highly alkaline soils. I know little about gardening, but I feel that most soils would be between pH=5 and pH=9. A soil pH meter with 1 - 14 pH range is to me a bit silly and pointless, like a kitchen oven thermometer reading up to 1500 C. Some pH electrodes are no longer the well-known glass tubes, some "combination" electrodes are so small it is hard to see that there are actually two electrodes present. Sometimes even a minute thermistor is also present. In past years, before the glass elctrode became common, the "Antimony Metal" electrode was used. Your device might use something similar to this. Information on this could be found in older [1930 - 1950]texts on analytical or electro- chemistry. Alas,I have disposed of my old books, so I have no more info. on this. You state that the device is accurate, so I presume that you have checked it against solutions of known pH or a laboratory type pH meter. Any metal or alloy in contact with an electrolyte will generate a potential [the EMF]. For some metals this EMF will be strongly dependent on the pH of the electrolyte, while for another metal the EMF will be much less dependent. Perhaps this is what is utilized in this device. Why not contact the manufacturer, they are usually very helpful to students. Charlie Crutchfield
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