Physical Sciences chemistry part PYT32ES - Lesson 10_2023.pdf

Chemistry notes on electrogravimetry and coulumetry

1. PYT32ES –Chemistry Theory
Thinking Beyond
Lecturer –Dr. Gumede

2. Lesson 10
Faculty of Humanities
Electrogravimetry and Coulometry

3. Learning Outcomes
Faculty of Humanities
•Distinguishbetweenelectrogravimetryandcoulometry
•Distinguishbetweentheconcentrationandkineticpolarization

4. Comparison of the two methods
Faculty of Humanities
•Inelectrogravimetry,theproductisweighedasadepositontheworkingor
indicatorelectrode.
•Incoulometry,thequantityofelectronsrequiredtocompletetheelectrolysisis
measuredanditisconnectedinserieswiththeelectrolyticcell.

5. Coulometry
Faculty of Humanities
Involvemeasuringthequantityofelectricalcharge(electrons)neededto
convertananalytequantitativelytoadifferentstate.
Incoulometry,thequantityofelectronrequiredtocompletetheelectrolysisis
measured(controlledcurrent).
Advantages
•Calibrationstandardsarenotrequired.
•Rapidandproductofelectrochemicalreactionneednotbeasolid.
•Readilyautomated.

6. Coulometry
Faculty of Humanities
Factorsthatinfluencethephysicalcharacteristicsofelectrolyticdeposits
include:
•Temperature
•Currentdensity
•Thepresenceofcomplexingagent

7. Coulometry
Faculty of Humanities
Typesofcoulometers(asadevice):
•Colorimetric coulometers
•Gasometric coulometers
•Gravimetric coulometers
•Radioactivity coulometers
•Titrimetric coulometers

8. Coulometry
Faculty of Humanities
Twotypesofcoulometry
•Controlled-currentcoulometry
•Potentiostaticorcontrolledpotentialcoulometry
•Firstly,usingaconstantcurrent,leadtomorerapidanalysissincethecurrent
doesnotdecreaseovertime.Thus,atypicalanalysistimeforcontrolled
currentcoulometryislessthan10min,asopposedtoapproximately30-60
minforcontrolledpotentialcoulometry.
•Secondly,withaconstantcurrentthetotalchargeissimplytheproductof
currentandtime.

9. Coulometry
Faculty of Humanities
•It is necessary to isolate the working electrode from the reference electrode in
a controlled potential coulometric analysis because,
•the species produced at the reference electrode are a potential interference
by reacting with the product at the working electrode.
•So isolation of the one from the other is generally necessary.

10. Electrogravimetry
Faculty of Humanities
ThemetalisdepositedonaweighedPtcathodeandtheincreaseinweightis
obtained.
Twotypesofelectrogravimetry
•Withoutpotentialcontroloftheworking/indicatorelectrode
•Constant-cathodepotentialgravimetry

11. Advantages of bothMethods
Faculty of Humanities
•Moderatelysensitiveandrapid.
•Amongthemostaccurateandprecisetechniques.
•Nopreliminarycalibrationagainststandardsrequired.(absolutemethods,not
relative,thusnostandards).

12. Effect of Current on Cell Potential
Faculty of Humanities
•Asignificantcurrentisrequiredthroughouttheprocess.
•Thisresultsinadditionalphenomenawhichneedstheapplicationofpotentials
greaterthantheoreticalforelectrolyticcellsandpotentialssmallerthan
theoreticalforgalvaniccells.
•ThetwophenomenaareOhmicPotential(IRdrop)andpolarizationeffects.
•Theelectrodeatwhichreductionoccurs(cathode)iscalledtheworking/
indicatorelectrode,andtheoneatwhichoxidationtakesplace(anode)is
calledthereferenceelectrode.

13. OhmicPotential (IR drop)
Faculty of Humanities
•OhmicpotentialistheproductofresistanceRofacellinohms(Ω)andthe
currentIinamperes(A).TogeneratecurrentofIamperes,apotentialofIR
voltsismorenegativethanthethermodynamiccellpotential(E
cell=E
cathode–
E
anode)mustbeapplied.
•ThatisE
applied=E
cell-IR.

14. Polarization Effects
Faculty of Humanities

15. Polarization Effects
Faculty of Humanities
Twocategoriesofpolarization:
•Concentrationpolarization
•Kineticpolarization

16. Concentration polarization
Faculty of Humanities
•Itoccurswhenreactantspeciesdonotarriveatthesurfaceoftheelectrode
(cathode)orwhenproductspeciesdonotleavethesurfaceoftheelectrode
(anode)fastenoughtomaintaindesiredcurrent.
•Reactantsaretransportedtotheelectrodebythreemechanisms:
•Diffusion
•Migration
•Convection

17. Concentration polarization
Faculty of Humanities
•Diffusion:Whenthereisaconcentrationdifferencebetweentworegionsofa
solution,ionsormoleculesmovefromthemoreconcentratedregiontothe
moredilute.
•Migration:Theprocessbywhichionsmoveundertheinfluenceofanelectric
field.
•Convection:Reactantscanalsobetransferredtoorfromanelectrodeby
mechanicalmeans.

18. Kinetic polarization
Faculty of Humanities
•Themagnitudeofthecurrentislimitedbytherateofoneorbothelectrode
reactions.
•Offsetbyusingadditionalpotentialorovervoltagetoovercomeenergybarrier
oractivationenergyofthehalfreaction.
•Significantinelectrodeprocesseswhichproducegaseousproducts.

19. GET IN TOUCH:
Dr. T Gumede
Department of Life Sciences
Faculty of Health & Environmental
Sciences
[phone]
[email protected]
Thank
you
Thinking Beyond