Introduction: Mini Electrolytic Cell
I have been working on this project for my Instrumental Chemistry course. My goal was to measure voltage detected by a cathode in salt water. I performed a standard addition of approximately 6.6 M salt water, with injections of 1 mL using a medicinal syringe.
- Graduated cylinder, volumetric pipet, micropipetter, etc. to measure volume. I used a medicine syringe with 0.2 mL markings.
- Microprocessor i.e. Arduino device
- assortment of male-to-male and female-to-male wires
- two alligator clips
- 10 kohm resistor or similar for voltage divider
- Vessel for electrolysis. I used an old spice jar and that worked pretty well
- Two paper clips to make the cathode and anode electrodes. I also cut a straw into sections just to hold my electrodes more securely in place, and prevent them from touching each other or the glass.
- Table salt (NaCl)
- Tap water
Step 1: Prepare Your Salt Solution.
I used tablespoons to measure amounts of salt and a measuring cup with 50 mL markings to measure water when making my salt solution. I used iodized salt from the brand Clover Valley. I measured out 3 tablespoons of salt, added the salt to a measuring cup and filled the measuring cup to 250 mL with tap water. 1 U.S. tablespoon is approximately 14.7868 mL, so 3 tbsp is approximately 44.3604 mL. The density of sodium chloride is 2.16 g/cm^3. I multiplied the volume and density to determine the mass of NaCl, which was 95.82 g. The molar mass of NaCl is 58.44 g/mol, so the moles of NaCl was 1.64 mol. 1.64 moles divided by the total volume of 250 mL or 0.250 L resulted in a 6.56 M NaCl solution. This is how I would go about finding the concentration of your salt sample if you don't have any fancy equipment at your disposal.
Step 2: Set Up the Electrochemical Cell
- As I said previously, I used a spice jar with wide enough holes at the top for me to inject salt water with a medicinal syringe. Any type of vessel should work, but it is best to be able to suspend your electrodes and solution and be able to position them where they do not touch each other or the walls of the container.
- I unfolded and straightened two paper clips to make my cathode and anode. I also polished them with sandpaper to make sure there was no coating that would act as an insulator. I made small tubes by cutting a straw in eighths. I used the straw tubes in the spice jar holes where the cathode and anode were placed to ensure they stayed in place when I attached the alligator clips. Hopefully the picture will help the visualization of this.
- It is best for the cathode and anode to be at a similar depth level in the solution.
- Add water to the spice jar to where electrodes are partially submerged in water, at least a cm in the water I would say. You want to leave some room in the vessel for when you inject salt solution into it.
Step 3: Set Up Your Circuitry
- I used an Adafruit Metro microprocessor, but most microprocessors on the market are similar as far as the different pin options.
- I set up the circuit as follows:
- Connect a wire to 5 V. Attach one side of an alligator clip to the other end. Attach the other side of the alligator clip to one of your electrodes. This will be your anode.
- Connect a wire to A0 and connect the other end to your board. Add another wire in line with the wire connected to A0 and your board.
- Connect a 10 kOhm resistor to this wire on your board. On the other end of the resistor, use a wire to connect the system to ground.
- Connect another wire to ground on your microprocessor and next to your other wire connected to ground on your breadboard.
- See photos for set up
Step 4: Compile/verify and Upload Code
I used the following code that is saved on the Arduino application under Examples --> Basics --> ReadAnalogVoltage. I hope this worked. The data wasn't as I expected, since the voltage decreased as more salt water was added. I thought about the purpose of the code some more and decided to do a corrected voltage by subtracting the output from the original 5 V added to the system. I then made a calibration curve using the concentration (calculated- I will talk about in the next step) and the corrected voltage, which now shows the voltage as increasing with addition of salt. If anybody has some advice on where I might've went wrong please let me know.
Interestingly, whenever I removed either the cathode or anode from solution the serial monitor read an output of 5.00 V.
Step 5: Analyzing the Data
- The concentration of salt added for each injection is found by multiplying the molarity of your salt solution by the injection volume (i.e. 1 mL=0.001 L), and then dividing by the total volume (so let's say you start out with 250 mL=0.250 L, the total volume for the first injection is 0.251 L). You would then calculate the concentration by dividing (0.001L*the molarity)/(total volume or 0.251 L)
- Calculate the concentration of the sample solution after each addition of salt solution.
- I corrected the voltage by subtracting the output voltage from the initial 5.00 V. This gave me the positive calibration curve of concentration vs. voltage that I was expecting, since addition of electrolyte in solution should decrease the resistance of the solution and allow current to flow more effectively.
- Note: for my graphs the linear range is horrible. I would highly recommend making a NaCl solution with a much smaller concentration or using smaller injection volumes. I maxed out detection early on in the experiment.
- Other ionic salts could be dissolved in water and used with this same procedure. I would have done trials with epsom salt if I had any.
These pages helped me understand how to expect the voltage to change when electricity was added to salt solution at increasing concentrations.