Hello everyone! Today we are going to introduce the experiment Cyclic Voltammetry Method for Measuring Juice Concentration.
We know that for a reversible electrochemical system, the peak current can be described by the Randles–Sevcik equation, where the parameters include electrode area, diffusion coefficient, scan rate, and bulk concentration.
For an irreversible electrochemical system, the linear relationship between the peak current and various parameters remains similar. When n and A remain constant, the peak current can be described by this relationship.
Therefore, for an irreversible reaction, if we keep D and v constant, based on this linear relationship, by collecting sufficient data points for iₚ and C, a calibration curve can be established and used to measure the unknown concentration.
The premise of this linear model is that D and v are constant; otherwise, the variation of iₚ cannot be solely attributed to C, and thus the linear relationship will not hold.
To keep v constant, we maintain the same scan rate v for all solutions in the experiment.
To keep D constant, we must ensure that diffusion is the only mass transport mechanism determining the current magnitude.
Besides diffusion, the other two mass transport mechanisms are convection and migration.
To minimize convection, i.e., the macroscopic movement of the solution, we ensure the solution remains completely still.
To minimize migration, i.e., the directed motion of charged ions under the electric field, we use 0.1 M KCl as the supporting electrolyte, reducing potential loss caused by solution resistance and eliminating migration current as much as possible.
Due to the presence of the electrochemical double layer, the charge difference between the actual electrode surface and the adjacent solution behaves like a micro-capacitor, so the current term we focus on is the Faradaic current , which equals the measured peak current minus the capacitive current .
This means we need to perform a CV scan on a blank solution separately to obtain at each potential.
The oxidation of ascorbic acid (AA) in this experiment is also an irreversible process. Dehydroascorbic acid (DHA) cannot be effectively reduced back to AA during the reverse scan. Therefore, in its cyclic voltammogram, we observe only one oxidation peak; the absence of a reduction peak makes the AA voltammogram unlike the typical “duck-shaped” curve.
This is its Nernst equation in the oxidation direction. Under a fixed concentration condition, we can approximate that the concentrations of AA and DHA are equal, making the first two terms constants. Therefore, Eₚ can be approximated as linearly dependent on pH.
Since our potential window is fixed, we do not want variations in pH to cause Eₚ to drift outside the window. Thus, by assuming and maintaining the pH at neutral, Eₚ can remain stable within the scan range.
This is our Three-Electrode Setup used for Cyclic Voltammetry.
On the left is the Reference Electrode (RE), whose function is to provide an extremely stable and well-known potential reference throughout the potential scan, for calculating the potential difference with the Working Electrode. Ideally, no net current flows through the RE, so its potential remains constant.
In the middle is the Working Electrode (WE), where the actual redox reaction occurs. AA is oxidized at its surface. The Potentiostat precisely controls the potential of the WE and measures the current passing through it.
On the right is the Counter Electrode (CE), which forms the current loop with the WE and carries out charge balance conduction.
This setup separates the potential control and measurement function from the current conduction function, effectively eliminating the voltage loss caused by solution resistance (the iR drop), thus improving the precision and stability of WE potential control.
This is the Potentiostat we used.
Its principle is that, through a closed-loop feedback system, it continuously measures and compares the potential difference between the WE and RE with the set potential E_in, then adjusts the output current according to the error signal until the current exactly compensates this difference, maintaining the potential difference between WE and RE precisely at the set value E_in. In this way, the Potentiostat achieves accurate potential control of the WE and current measurement.
If the distance between the working electrode and counter electrode is reduced, since the solution resistance R_cell is proportional to d_WE/CE, this equivalently reduces R_cell and the ohmic drop |I·R_cell|. Because the output voltage V_potentiostat must be sufficient to drive the current I and overcome all potential differences, this means the required V_potentiostat also decreases, reducing the burden on the Potentiostat and ensuring stable operation under higher I or higher R_cell conditions.
This is our simplified procedure. After preparing all solutions, cyclic voltammetry measurements are carried out to construct the calibration curve and calculate the AA concentration in OJ.
The dilution of OJ is to make it fit within the calibration curve range and to reduce Analytical Matrix interference.
The Analytical Matrix refers to all other components in the sample except AA, which may affect peak potential and peak current by altering solution properties such as pH or viscosity, or may foul the electrode surface, causing the peak current to decay over time.
If other antioxidants are present, they may generate their own Faradaic currents.
These are the L-AA solid, orange juice, and prepared solutions in our experiment.