[ctgscott]

Hi all,

I beg forgiveness for my general chemistry ignorance.  I am guessing my question is quite elementary for you so I have placed it in the HS forum.  I have a project that involves measuring soil pH and a traditional glass electrode presents practical problems when used repeatedly in hard, perhaps rocky soil (damage to the glass).  I know that pH probes measure the concentration of H ions across a thin layer of glass in the form of charge potential, and I am wondering if there is another way to accomplish the same measurement?  Would two dissimilar metals work... if so which metals? 

And, yes... I have seen the cheap, widely available metal probe pH meters online and at the local gardening supply shop.  I am just curious what principle is at work with these and what trade-off is being made as all of the industrial, or lab grade probes use glass electrodes.

If you feel this is the wrong forum for my question, please accept my apologies.  Any redirect suggestions would be greatly appreciated.

Sincerely,
Scott

[Borek]

http://www.ph-meter.info/pH-measurements-other-procedures

Those two metal devices do exist, but due to the way they are made they measure two things at the same time - pH and resistance, so the result will depend on the humidity. Results of measurements taken in the same place will differ depending on when it was raining for the last time.

[ctgscott]

Thank you, Borek.  The ph-meter.info site is very good.  I suspected the two-metal meters were not accurate and I have since read quite a bit indicating this.  I still have not found, however, a thorough break-down of the inaccuracies of these second-rate meters.  I am curious whether one would be able to improve results by correcting for moisture in a metal probe pH reading, much like the glass electrode meters compensate for temperature. 

If I got down to a point where I could expect two standard deviations within around .5 pH using a metal probe, I think I could be satisfied.

Any thoughts?

[Borek]

My bet is that you will get better reproducibility with an old school method - using pH strips and soil mixed with water. I remember special 'devices' made for that, there was a spoon to measure amount of soil and a porcelain cup (more like plate with a dent - not sure how to name it properly in English) were you mixed soil with water. In earlier versions indicator was added as a solution (hence white porcelain, so that the color was easier to read), but later it was replaced by pH strips.

[ctgscott]

I completely agree.  The soil suspension in water test also would be much more accurate, particularly as it would allow the use of either strips or the standard glass electrode meter.  My circumstance however requires numerous tests and sometimes probes left in situ, but is tolerant of a relatively low degree of individual accuracy (for pH anyway).  I was just hoping I could use (and maybe tweak a little) a more sturdy and convenient meter.