A potentiostat is a device that allows one to control the electrochemical potential of a conducting cell. With the correct code, potentiostats can also be used to measure the current across a cell and predict the solution's concentration. Potentiostats can vary in electrode count, the one is this instructable has three electrodes. Shown above is the circuit schematic of the potentiostat I created. I obtained the schematic from the Journal of Chemical Education from an article written by Gabriel N. Meloni.
Step 1: Gather Materials
-DC Power Supply
-Electrodes (I used one zinc and two copper)
-Wires and cables
-5 op-amps (I used LM741's)
~2 x 100nF
(Zener diode with LED and second 200kΩ resistor are pictured yet not necessary)
Step 2: Circuit Construction Pt.1
I find it easiest to add the op-amps evenly space first. Next, connect each pin 7 (this will be your +6.5V). Connect each pin 4 (this will be your -6.5V). Va and Vb will represent these (+) and (-) voltages while ground will still be ground.
Step 3: Circuit Construction Pt. 2
Fill in the remaining capacitors, resistors, and jumper wires. You will want to make the (+) red rows of the breadboard -5V and the (-) blue row ground. Attach the cables from the function generator and power supply to the board.
Side note: You may notice I have many resistors. This is due to me not having the exact values of the required resistors. I added multiple resistors in series to make the desired resistances.
Step 4: Conducting Cell
Create a conducting cell of your desired solution. Place the electrodes in the solution, measuring their distances apart and always keeping the distances consistent
Step 5: Arduino
Connect your circuit to input into pin A0 of the Arduino. Also connect your Arduino to ground. Once plugged in to the computer, you can run the Arduino code. Changes to the above code can be made depending on how many terms you would like to average together.
Step 6: Excel
In order to get the potentiostat to report the value of the concentration of the solution, you must generate a calibration curve first. Make multiple solutions (the more the better) and record their currents from the potentiostat reading. Make the curve in Excel. The curve will have concentration on the x-axis with current on the y-axis. Most likely the trendline will be logarithmic. Use this equation in the second-to-last line of code. The potentiostat should now be able to determine the concentration of whatever solution was being used. (Keep solution type constant. The potentiostat will not work if you decide to change from NaCl to KCl.)