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Electrogravimetry
Potential is applied to cause a soluble species to be reduced or deposited on a solid electrode e.g., reduce Cu2+ onto Pt cathode Cu2+ (aq) + 2 e Cu (metal on Pt) Change in weight of dried cathode before & after deposition = amount of Cu in sample Assumptions: All Cu is plated out Nothing else plates out
Cu2+(aq) + 2 e Cu(s) Eo = 0.34 v O2 + 2 H+ + 2 e H2O Eo = 1.23 v Cu2+ + H2O O2 + 2 H+ + Cu(s) For zero current, Ecell = ECu EO2,H2O Use Nernst Equation with Eos & concentrations Ecell = 0.34 (0.0592/2)log (1/[Cu2+]) 1.23 (0.0592/2)log{1/(PO2)[H+]2 = 0.91 v
Therefore, apply potential more negative than 0.91 v to force system to reach an equilibrium where [Cu2+] is small (like 99.9% lower than the approximate starting concentration) Choose cathode potential to reduce equilibrium [Cu2+] to any desired value Must be cautious not to set potential too far negative to make sure nothing else is reduced Normally set conditions so that reduction is complete in a reasonably short period of time
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Electrogravimetry Cell
Pt electrodes
Problem with simple electrogravimetry system Is that the potential of the cathode is not accurately known Eapplied = Ecathode - Eanode - EIR Eovervoltage Ej is insignificant and can be ignored here Magnitude of changes in EIR or Eovervoltage & EIR throughout experiment are not known A similar problem exists at the anode due to H2O O2 + 2 H+ + Since and H & O2 change during electrolysis, anode potential also changes
Much of this problem can be eliminated by using a three electrode system
Use potentiometer to measure potential of cathode relative to reference electrode then manually adjust slidewire contact to hold Ecathode at desired value. This operation must be repeated at intervals during electrodeposition because the current changes with conc. Bulk current still flows from cathode to anode, but anode potential is no longer important.
Electrochemical cells resist the flow of current Effect of resistance on magnitude of current E = IR To generate a current of I amperes in the cell, a potential IR that I more negative than the thermodynamic potential (Ecell) must be applied Eapplied = Ecell IR IR drop in the cell can be minimized by having a very small cell resistance (high ionic strength) or using a 3electrode cell Only a very small current passes between the working electrode and the reference electrode
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Three Electrode Configuration 1) Reference electrode maintains fixed potential despite changes in solution composition 2) Working electrode electrode of interest which is the cathode in this system 3) Counter electrode (Auxiliary electrode) third electrode taking most of current flow (acts as current sink)
Advantages of 3 electrode system 1) Changes in concentration at counter electrode are not important, there is no effect on working electrode potential 2) If no current flows through reference, there is no IR drop (its potential is constant)
Applications of Electrogravimetry 1) Quantitative analysis (electrogravimetry)
a. very accurate & precise, b. only measurement operation is weighing, c. Can get deposition reaction to go to any desired
degree of completion by proper choice of potential, d. some degree of selectivity using potential 2) Separations separate one species from another in solution by selectively plating it out and removing it from solution. This method is used to remove interferences particularly in electrochemical methods
3) Preconcentration plating out metals from a large volume of solution (> 1 L) onto a small electrode effectively increases their concentration. Can also reoxidize metals back into a small volume of solution (e.g., 1 mL) the ratio of volumes gives preconcentration factor (i.e., 1000). Can also use other methods for analysis of metal on solid electrode surface (e.g., AA, ICP, XRF, XPS).
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4) Electrosynthesis (controlled potential electrolysis) used by organic chemists to perform oxidation or reduction reactions at bench scale. Takes advantage of ability to control potential & produce any oxidizing or reducing strength desired.
5)Purification to remove trace metals from reagents by plating them out of solution onto a large Hg pool electrode. This cell can also be used for electrosynthesis, etc.
Coulometry
Methods based on counting coulombs (C), the basic unit of electrical charge (Q) Faradays Law QM m = nF Where: M = molar mass (g/mole) m = mass (g) n = number of electrons (unitless) F = Faradays constant (96,500 C/mol)
Fundamental assumption is that reaction is 100 % current efficient i.e, all coulombs go to oxidizing or reducing species of interest Kinds of coulometry 1) Controlled Potential Coulometry t Q = i dt 0
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2) Constant Current Coulometry Q=it Involves nothing more than integrating area under the curve in chronoamperometry Can be referred to as chronocoulometry Care must be taken so that there is enough stuff to carry the current at electrode surface Rarely used anymore
Major application is coulometric titrations where titrant is prepared electrochemically and standardized by counting/measuring coulombs e.g. bromine, Br2, as titrant 2 Br Br2 + 2e 1) Useful for titrants that cant be stored as stable solutions 2) Small currents can be measured accurately so even very dilute titrants can be used 3) Theoretically, the number of coulombs can be counted for any method where current is measured by integrating
Examples of Coulometric Titrations
Assayed Substance Br Fe2+ H2O Organic acids Bases Ca2+, Zn2+ Olefins Reagent Generated Ag+ Cl2 I2, I3 OH H+ HEDTA3 Br2 Precursor Titration Type
Ag anode HCl KI(pH<7)
Precipitation Redox KarlFischer reagent H2O Neutralization H2O Neutralization HgNH3EDTA Compleximetric KBr (pH < 5) Olefin addition (redox)
