Coulometry Tiltration Method Statement.ppt
- 1. A.) Introduction: 1.) Coulometry: electrochemical method based on the quantitative oxidation or reduction of analyte - measure amount of analyte by measuring amount of current and time required to complete reaction charge = current (i) x time in coulombs - electrolytic method external power added to system 2.) Example: - Coulometric Titration of Cl- - use Ag electrode to produce Ag+ Ag (s) ↔ Ag+ + e- Ag+ + Cl- ↔AgCl (ppt.) - measure Ag+ in solution by 2nd electrode - only get complete circuit when Ag+ exists in solution - only occurs after all Cl- is consumed Coulometric Methods
- 2. Typical coulometric titration cell. e.g. At the generator electrode (anode) Ag (s) Ag+ + e- (oxidation of silver to silver ion) At the cathode: Possible reaction 2H+ + 2e- H2 (g) (hydrogen evolution) Therefore, need sintered glass to separate the species generated in the other electrode (e.g. cathode, hydrogen gas) to prevent reactions with the “titration species” such as Ag+ . cathode anode
- 3. 3.) Based on Measurement of Amount of Electricity (or charge, in coulombs) Required to Convert Analyte to Different Oxidation State - Q = It for constant current with time where: Q = charge required (coulombs = amp . sec) I = current (amp.) t = time of current (sec) for variable current with time: Q = Idt Relate charge (coulombs, C) to moles of e- passing electrode by Faraday constant Faraday (F) = 96,485 Coulombs (C)/mole e- F = 6.022 x 1023 e- / mole e- x 1.60218 x 10-19 C/ e- = 96,485 Coulombs/mole e- If know moles of e- produced and stoichiometry of ½ cell reaction: Ag (s) ↔ Ag+ + e- (1:1 Ag+ /e- ) 0 t
- 4. Example: Constant current of 0.800 A (amps.) used to deposit Cu at the cathode and O2 at anode of an electrolytic cell for 15.2 minutes. What quantity in grams is formed for each product? ½ cell reactions: Cu2+ + 2e- Cu (s) (cathode) 2H2O 4e- + O2 + 4H+ (anode) To solve: Q = i. t Q = (0.800 A)(15.2 min) (60 sec/min) Q = 729.6 C (amp. sec) amount Cu produced: =(729.6 C)(1 mole e- /96,485 C)(1 mole Cu/2 mole e- )(63.5g Cu/mole Cu) = 0.240 g Cu amount of O2 produced: =(729.6 C)(1 mole e-/96,485 C)(1 mole O2/4 mole e-)(32.0g O2/mole O2)
- 5. 4.) Two Types of Coulometric Methods a) amperostatic (coulmetric titration) - most common of two b) potentiostatic Fundamental requirement for both methods is 100% current efficiency - all e- go to participate in the desired electrochemical process - If not, then takes more current over-estimate amount of analyte B) Amperostatic Methods (Coulometric Titrations) 1.) Basics: titration of analyte in solution by using coulometry at constant current to generate a known quantity of titrant electrochemically - potential set by contents of cell - Example: Ag (s) ↔ Ag+ + e- for precipitation titration of Cl- - To detect endpoint, use 2nd electrode to detect buildup of titrant after endpoint.
- 6. 2.) Applications a) Can be used for Acid-Base Titrations - Acid titration 2H2O + 2e- ↔ 2OH- + H2 titrant generation reaction - Base titration H2O ↔ 2H+ + ½ O2 + 2e- titrant generation reaction b.) Can be used for Complexation Titrations (EDTA) HgNH3Y2- + NH4 + + 2e- ↔ Hg + 2NH3 +HY3- HY3- ↔ H+ + Y4- c.) Can be used for Redox Titrations Ce3+ ↔ Ce4+ + e- Ce4+ + Fe2+ ↔ Ce3+ + Fe3+
- 7. 3.) Comparison of Coulometric and Volumetric Titration a) Both Have Observable Endpoint - Current (e- generation) serves same function as a standard titrant solution -Time serves same function as volume delivered - amount of analyte determined by combining capacity - reactions must be rapid, essentially complete and free of side reactions b.) Advantages of Coulometry - Both time and current easy to measure to a high accuracy - Don’t have to worry about titrant stability - easier and more accurate for small quantities of reagent small volumes of dilute solutions problem with volumetric - used for precipitation, complex formation oxidation/reduction or neutralization reactions - readily automated c) Sources of Error - variation of current during electrolysis - departure from 100% current efficiency - error in measurement of current - error in measurement of time - titration error (difference in equivalence point and end point)
- 8. 4.) Change in Potential During Amperostatic Methods a) In constant current system, potential of cell will vary with time as analyte is consumed. - Cell “seeks out” electrochemical reactions capable of carrying the supplied current Cu2+ + 2e- ↔ Cu (s) initial reaction -Nernst Equation Ecathode = Eo Cu2+/Cu – 0.0592/2 log (1/aCu2+) Note: Ecathode depends on aCu2+. As aCu2+ decreases (deposited by reaction) Ecathode decreases.
- 9. - When all Cu2+ is consumed, current is carried by another electrochemical reaction generation of H2 (g) if reduction 2H+ + 2e- ↔ H2 (g) breakdown of water if oxidation 2H2O ↔ H2O2 + 2H+ + 2e- H2O2 ↔ O2 + 2H+ + 2e- M2+ + 2 e- M(s) (co-deposition)
- 10. C) Potentiostatic Coulometry 1.) Basics: -detection of analyte in solution by using Coulometry at fixed potential to quantitatively convert analyte to a given form current controlled by contents of cell. 2.) Instrumentation requirements: - electrochemical/electrolysis cell - a potentiostat (apply the required potential/voltage to the system) - an integrator (analog or digital) for determination of the charged consumed Electrochemical cell Equivalent circuit Practical Circuit of a Potentiostat and an Electrochemical/Electrolysis
- 11. 2) Advantages: - more specific than amperostatic coulometry avoids redox of species that may interfere with constant current coulometry - can be used for over 55 elements without major interference 3) Disadvantages - does take longer than amperostatic titration current (i) decreases with time conversion becomes slower as less analyte around to oxidize or reduce It = Ioe-kt k = 25.8 DA/V where: It = current at time t (A) I0 = initial current (A) t = time (sec) D = diffusion coefficient (cm2 /s) A = electrode surface area (cm2 ) V = volume of solution (cm3 ) = thickness of the surface layer where concentration gradient exists (cm) *** typical values of D are in the range of 10-5 cm2 /s *** typical values of d is 2 x 10-3 cm
- 12. 4) Other Applications of constant potential coulometry - electroplating, apply the correct potential, the “metal of interest” will be deposited. e.g. gold plated onto silver (vermeil) as jewelry e.g. zinc plated onto steel for anti-corrosion (zinc as “sacrificial cathodic coating”) The term "vermeil" refers to a silver item, containing no less than 92.5% silver, that has been plated with a gold or gold alloy that is no less than 10 karat, to a thickness of not less than 2.5 microns. Example (using the two equations): Deposition of Copper: Cu2+ + 2e- Cu (s) After 30 min, current decreases from the initial 1.5 A to 0.08A By this time, approx. 96% of the copper has been deposited.