Conductance of Solutions

Calibrate the system

1. Prepare KCl calibration solutions with different concentrations, for which you found κ in your preparations. Prepare your solutions using a 1M stock solution. In this experiment you will use DDI (Distilled De-ionized) water tanks for all solutions preparation. Explain why.
2. Measure the resistor's resistance.
3. Connect the 4-electrode cell setup.
4. Wash the cell with deionized water and with the one of the KCl solution you prepared (think which one). Pour the solution into the cell so that it covers all the electrodes.
5. Find the optimal AC frequency:
   1. Set the AC frequency to 100Hz.
   2. Calculate the current that flows in the solution and from that calculate the resistance of the solution.
   3. Repeat the procedure for increasing AC frequencies (up to 20000Hz). The optimal frequency is the one where the system's impedance is minimal.
   4. Set the system to the optimal frequency you found. The frequency should not be changed from now on.
6. Find the optimal combination of the four electrodes:
   1. Measure the resistance of the solution with the 3 electrode combinations.
   2. Set the system to the optimal position, which should not be changed from now on. Consult before with your instructor which cell configuration is optimal and why.
7. Find the cell constant:
   1. Measure the resistance of each KCL solution.
   2. Use equation 2 to calculate the cell constant at each concentration. Are they consistent?
   3. Compare the geometrical estimate of the cell constant you made in your preparation and your measured value.
   4. Connect the 2-electrode cell instead of the 4-electrode cell, and measure the resistor's resistance. Pay attention! the resistor in this case is external!
   5. Measure the resistance of each KCl solution in the 2-electrode cell, and calculate its cell constant.
   6. The cell constant of the 2-electrode cell is often dependent on concentration, while that of the 4-electrode cell is independent of it. Can you explain why?

Pay attention! The cell constant may change slightly with time! On the second week, it is important to recalculate the cell constant using 1-2 solutions. You may store the remains of the solutions you prepared on the first week for that purpose.

Equivalent conductance of strong electrolytes

1. Prepare 4 NaCl and HCl solutions with different concentrations by diluting the available 1M stock solutions. Prepare a fresh Sodium acetate stock solution with concentration of 1M and then prepare 4 solutions with different concentrations using your fresh stock solution.
2. Connect the 4-electrode cell.
3. Determine the equivalent conductance at infinite dilution, Λ₀, for each substance:
   1. Wash the cell with deionized water and the lowest concentration solution.
   2. Find the resistance of the solution.
   3. Calculate the equivalent conductance Λ, using equations 2 and 3 and the cell constant you found.
   4. repeat the procedure for each concentration of the measured substance.
   5. Plot Λ vs. c^½, and find Λ₀ according to equation 4.
4. Calculate Λ₀ of acetic acid using equation 7.

Dissociation constant of acetic acid

1. Prepare 4 solutions of acetic acid with different concentrations.
2. Determine the equivalent conductance at infinite dilution, Λ₀, of acetic acid as you did for the strong electrolytes.
3. Find the dissociation level of acetic acid using equation (10) (Ostwald formulation).
4. Find the ionization constant, α, of acetic acid using equation 8.

Walden's rule and ionic radius

1. Connect the 2-electrode cell.
2. Find the optimal AC frequency. What is the resistance depended on the frequency? Do the measurements agree with the comparable measurements for the 4-electrode cell? If not, why?
3. Measure the cell constant.
4. Prepare a dilute solution of NaCl and heat it to ~70°C. Allow the solution to cool, and calculate the equivalent conductance, Λ, at different temperatures as you did in the previous sections.
5. Find the viscosity of water, η, at the temperatures you used. You can use the CRC book (page 6-186). A link to the CRC book can be found in the recommended literature section. Calculate $Λ_0η$. Is it constant?
6. Why were you allowed to use Λ instead of $Λ_0$ and η of water instead of that of the solution? Explain.
7. Calculate the specific conductance at infinite dilution of $Na^+$, $λ_0$, using equation (5). Assume that $λ_0(Cl^-) = 76.4 cm^2 Ω^{-1} eq^{-1}$.
8. Use the relation you found during your preparation between $λ_0$ and the ionic radius to calculate the radius of Na+. Compare your results to the literature.