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Background

I'm building a watering robot for my household plants. For the purpose of measuring the moisture content of the plant I intend to use a resisitive approach where I pass an electric current through two electrodes in the soil.

I initially used a cheap off-the-shelf probe with a metal I don't know what it is but it corroded very quickly in the soil. I have learned since then that soil is quite corrosive to metals. I'm aware of a capacitive method of measurement that doesn't require the metal to be in contact with the soil but I haven't found a lot of information on how that works and I've already built most of the project around the resistive approach.

I'm not good with chemistry, and I'm trying to figure out what material I should use for these electrodes. As the plants I'm growing are eventually going to end up for personal consumption I am concerned with toxicity and if these materials will react with the soil and produce some thing that can be absorbed by the plant's leafs and fruit.

Research Done

I read that Graphite, Palladium, Platinum and Gold are the three best materials from a corrosion resistance point of view with Silver and Titanium coming in close. [1]

My understanding is that Platinum and Gold are noble metals and they are inert and are normally found in their pure form in the earth so they would not react with the soil at all under normal circumstances. So these metals would be my first choice if availability and cost was not a concern.

I managed to find some information on titanium since we use it in medical implants, it's not directly toxic invivo. Although I did find this site claiming you can have titanium poisoning (also claiming platinum poisoning). But to be honest I'm not sure if I consider that page a credible source, it seems to be fear mongering designed to sell 'home tests'. I found some research papers detailing that lab rats had adverse effects on lung development when exposed to TiO2 nano particles, but no studies to show any actual adverse effects on humans, some recommended caution however.

As for Graphite I found that it really depends on the alloy and basically I felt like it would be too hard for me to make an informed decision so I discounted that, also I'm kind of afraid that it might be too brittle for the application. But this may also be lack of knowledge on my part.

I know you can get platinum plated titanium mesh for electrolysis applications (but I only found alibaba online and that seemed sketchy) but they are too big and unsuitable for my application. And if I could find platinum coated titanium rods, I guess that would be optimal but I have had zero lucky finding such things (2-4mm thick, 7cm long or there about)

Questions

So I turn to you guys: Gold and Platinum are apparently the best choices. But would titanium be good enough? Is it safe? Should I still consider Graphite? Can the soil react with Titanium to produce TiO2? Do you know of a place online where I can purchase these things in Europe?

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  • $\begingroup$ I would search the patent and marketing literature, I would think this has been done many times before. Using AC excitation is a common way to avoid the galvanic corrosion. With chloride ions present in the soil it may be possible to electrolytically corrode even gold if the measurement voltage is high enough. $\endgroup$ – KalleMP May 13 '18 at 20:51
  • $\begingroup$ @KalleMP Yes, I'm doing AC excitation at 5V which helped a lot compared to the off-the-shelf thing that was always on DC 5V... Will try searching, can you suggest some keywords? $\endgroup$ – Emily L. May 13 '18 at 22:19
  • $\begingroup$ I would start with the search below and also check the image results for interesting ideas that might lead somewhere. - google.com/search?q=soil+moisture+electrodes+corrosion+patent $\endgroup$ – KalleMP May 14 '18 at 6:34
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    $\begingroup$ Unless you are using lead, cadmium etc. it is unlikely that your plants will take up toxic amounts of any electrode material, I expect the problem to be electrode lifetime and not soil toxicity. I would start testing with stainless steels unless literature has better ideas. Capacitive sensing is a common alternative in industry but not universal. $\endgroup$ – KalleMP May 14 '18 at 6:38

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