Assuming the resisitivity is constant in the subsurface, give the formula for th

Question

Assuming the resisitivity is constant in the subsurface, give the formula for the measured potential difference in case the distance between all electrodes is the same. (3 points) Explain this measurement technique and explain the meaning of the lines with arrow points and the dashed lines. (3 points) The sensitivity of the resistivity measurement is determined by the electric current strength along the path from current electrode to a subsurface location where the resitivity is different than that of the surrounding medium, combined with the reverse path from the subsurface point to the potential electrode. The formula of the sensitivity F3D for a measurement in which the electrodes C1 en P1 are used is given a. b. c. y: (R3)3(R+)3 Explain where the subsurface point of contrasting resistivity is located relative to the electrode locations (this can be done by a sktech). (3 points) d. Give the formula for the measured potential at P1 if the electrical resistivity of the subsurface is constant and given by . (4 points)

Explanation / Answer

a)

Potentiometry
Principle
1. Potentiometry measures the difference in electrode potential between the reference electrode and indicator electrode.
2. Reference electrode has a constant potential and indicator electrode potential changes with the composition of the sample.
3. Difference of potential between the electrodes give the composition of the unknown sample.
Advantages
1. Non destructive measurement.
2. Good selectivity
3. High sensitivity
Dis advantages
1. Establishing equilibrium is very time consuming.

Coulometry
Principle
1. Coulometry measures the current of the cell over time for the conversion of the analyte from one oxidation state to another oxidation state.
2. The measured current helps to determine the number of electrons passed which indicate the concentration of the analyte.

Advantages
1. Unknown concentration of the analyte can be determined.
2. Number of electrons involved in the redox reaction can be estimated.
Dis advantages
1. Only redox substances can be estimated.

Voltammetry
Principle
1. Voltammetry applies a constant and/or varying potential at an electrode's surface and measures the resulting current with a three electrode system.
2. This method can reveal the reduction potential of an analyte and its electrochemical reactivity.
Advantages
1. Nondestructive
2. Very small amount of the analyte is consumed
Dis advantages
1. Separation of the analyte from the bulk solution is difficult for estimation.

b)

Differences between potentiometry and voltammetry/coulometry
Electrochemical cell for potentiometry:
1. Potentiometry is based on the measurement of the potential of an electrode system (e.g. electrochemical cell).
2. Potentiometric measurement system consists of two electrodes called reference and indicator electrode, potentiometer and a solution of analyte
3. Reference electrode is an electrode with potential which is a) independent of concentration of analyte (or other) ions in solution; b) independent of temperature.
4. Potential of an indicator electrode depends mainly on the concentration of the analyte ions (in this case hydrogen ions).
5. Potentiometric measurements enable selective detection of ions in presence of multitude of other substances.
6. The potential of the indicator electrode is sensitive to hydrogen ions. In a system like this, the potential is measured in reference to a calomel electrode, e.g. calomel electrode functions as the reference electrode.

Electrochemical cell for voltametry:
1. A typical arrangement for a voltammetric electrochemical cell consists of the working electrode, the reference electrode, and the auxiliary electrode
2. The working electrode, which typically consists of a chemically inert conductive material acts as a donor or acceptor of electrons participating in the general electrode reaction.
3. The reference electrode keeps the potential between itself and the working electrode constant.
4. The potential is measured between the reference and working electrodes, and the current is measured between the working and counter electrodes.
5. A counter electrode is employed to allow for accurate measurements to be made between the working and reference electrodes.
6. The counter electrode's role is essentially to ensure that current does not run through the reference electrode, since such a flow would change the reference electrodes potential.

c)

• The working electrode, which typically consists of a chemically inert conductive material acts as a donor or acceptor of electrons participating in the general electrode reaction.
• The reference electrode keeps the potential between itself and the working electrode constant.
• The potential is measured between the reference and working electrodes, and the current is measured between the working and counter electrodes.
• A counter electrode is employed to allow for accurate measurements to be made between the working and reference electrodes.
• The counter electrode's role is

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