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Heyrovsky looked at the Dropping Mercury Electrode (DME) for measurement of surface tension:

Heyrovsky looked at the Dropping Mercury Electrode (DME) for measurement of surface tension:. System at hand. How did he measure g ??. By applying various Potentials to WE, he noted differences in drop rate of the Hg. He also found:. lifetime of drop. “Drop time”.

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Heyrovsky looked at the Dropping Mercury Electrode (DME) for measurement of surface tension:

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  1. Heyrovsky looked at the Dropping Mercury Electrode (DME) for measurement of surface tension: System at hand How did he measure g ?? By applying various Potentials to WE, he noted differences in drop rate of the Hg. He also found: lifetime of drop “Drop time” mass flow rate of Hg in mg/s gravitational acceleration http://chem.ch.huji.ac.il/instruments/electrochemical/polarographs_heyrovsky.htm

  2. Why is this so? At , the wt. of the drop is . At this point, the force of the drop is just balanced by the surface tension acting around the circumference of the capillary. Once then and the drop falls! fixed environment What affects this? -solvent properties -ionic strength -charge on metal drop (all electrostatic properties) ECM or PZC So, he (and others) found: How do we relate **Figure (12.2.1)**

  3. Well, let us think about this! What electrostatic forces are there in this situation? And what changes with Eapp? - - - - - C+ C+ C+ X- C+ C+ C+C+ X- C+ X- C+ C+ C+ X- X- C+ C+C+C+C+ X- - - - - X- C+ C+ X- X- C+ C+ X- X- C+ C+ X- Hg (metal) anion cation pure electrolyte phase (not ordered) Neutral Redox M? pure metal Phase Make - of PZC What about excess of H2O??? We have excess of C+ near (or at) the metal/solution interface. This means that or written as z=1

  4. Related through the Electrocapillary Equation: Must Ref H2O as solvent total differential activity of species i in phase of choice (must specify) all variables in given phase, held cst must be specified. ∂n is in # moles of i. std. chem. potential of species i. Across interface

  5. Chem. Rev.194741, 441 **Figure 12.2.2** E – Ez (V, vs. NaF) If we hold electrolyte type constant and neutral redox species M constant, we obtain: So we can get vs. (E vs PZC) plots.

  6. Chem. Rev.194741, 441 We can do this with each type of electrolyte at a fixed concentration. But how is plot to left related to CDL??? Take derivative wrt E. **Figure 12.2.3** E – Ez Can do two different ways: Differential Capacitance Integral Capacitance **Figure 12.2.4**

  7. What does the Double Layer look like? • Well, that is a very interesting question, particularly from • our previous discussions. • We need a good model that predicts • ion populations (surface xs concs, ) and • the field strength or electrostatic potential • as a function of distance. • This is because of the fact that the ions do the majority • of the screening of the applied potential. • Our original model was: • Helmholtz (Parallel Plate) Model GC+ - + - + - + - + - + - + - + Voltage drop over distance, d, is V is dielectric cst. of the medium is permitivity of free space. Neg. of PZC Ge- distance between plates is d or Dx; The plane is at x (OHP, IHP) metal conductor, cannot support electric fields within, so xs - or + at surface only

  8. Recall that , the differential capacitance So, This states that Cd is constant. We know this not to be true! Na I NaF - + E – Epzc (NaF) Helmholtz Model 0.1 M 0.001 M NaF Cd 0 E – Epzc (NaF)

  9. The Helmholtz Model says: • Charge in solution is fixed at the metal/solution • interface; • Conc. of electrolyte is ~ inconsequential. The electric field is ‘felt’ out in solution, thus leading to a diffuse layer of ions. - - - - - - - C+ X- C+ X- C+ C+ C+ C+ C+ X- X- C+ X- X- C+ C+ C+ C+ At high T, we will get randomized structure. What is distance? What affects distance? Thus, thermal excitation fights the electrostatic situation. As you move away from electrode, thermal processes win. Thus, we have two layers: 1. compact layer 2. diffuse layer of ions near the surface. x

  10. - - - - - Gouy – Chapman Model + electrostatics k T (Boltzman) Thermal Randomization The number of carriers in a given energy plane (distance away from electrode) is found to be: electrostatic thermal charge on e- Bulk carrier # concentration The potential profile is: In all cases For a 1:1 electrolyte (~e.g. NaF, CaSO4) is potential at electrode mol/L inverse thickness of diffuse layer

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