Conductivity Probe Project

One of the core requirements of the RET program at NYU Poly is the creation of a lesson or activity derived from the experience.  My original idea dealt with the construction and testing of a conductivity probe.  Ideally, I would work through the kinks in the fabrication, coding and calibration of the probe during the summer.  There is a ton of chemistry involved in the project, and I feel like I keep discovering more as I get further into the construction of the prototype.

Current conductivity probe project circuitry

Current conductivity probe project circuitry

I had initially felt that my biggest stumbling block would be the code and circuitry for the probe.  The code portion is turning out to be one of the easiest components.  When I add a function, I reference the function via Arduino forums and insert the relevant code.  The more difficult task deals with the complexity of powering a simple conductivity sensor.

You might think you could just use two wires, separated by a known distance connected to a voltage, and place them into a salt solution to be measured…but this would be wrong, and would result in a constantly varying resistance measurement.  The reason? The voltage you apply will likely cause an electrochemical reaction to occur.  Depending upon the voltage applied, you would likely split water, while also off-gassing chlorine gas (from the Chloride ions in the salt solution).  The formation of bubbles on the probe surface would result in constantly varying resistances…not good when you want to perform a conductometric titration.  The solution? You must use a low alternating current, which requires a substantial set of components to convert the 5V dc signal from the Arduino into a viable AC signal.  The alternating current prevents electrolysis from occuring as long as the signal oscillates fast enough (the value I’ve found online suggests a frequency of 1 kHz or greater).  This site has a wealth of general info on the subject, as well as a link to molecular workbench software which can be used to illustrate this nanoscale phenomena.

The good news? I know what needs to be added into my system from a circuitry vantage point.  Plus, I was able to get the LCD to display varying values by taking my prototype probe and substituting a variable resistor for the original button switch.  Both of these components will not be a part of my final prototype, and serve as surrogates for the to-be-constructed conductivity probe.  Below is a screenshot of the current code, and a brief YouTube video showing the prototype in action.

Current code for Arduino conductivity probe project

Current code for Arduino conductivity probe project

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