MadSci Network: Chemistry

Re: Recently I bought a pH meter for my garden soil. How does it work?

Date: Thu Jul 29 02:45:24 1999
Posted By: Charlie Crutchfield, , Retired, Retired
Area of science: Chemistry
ID: 931446061.Ch

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 
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 

                               Charlie Crutchfield

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