Potentiometry And Electrodes Flashcards
What is Q in potentiometry
Electric charge
What are the electrodes in potentiometry
A reference electrode
A indicator (working) electrode
What is the reference electrode
A half cell with a constant potential (as long as temp stays constant, and concentration)
What is the indicator(working) electrode
The electrode that responds to analyte activity, has a pt wire that measures charge
What does the salt bridge have and what does it do
Has gel that has electrolyte (kno3)
Separates reactants into two half cells so redox happens in two separate containers, ground the solutions so that measured potential is right
The k+ moves into Cathode
The no3- moves into anode
This replenishes the lost charges that flow though the light bulb
How is the potential staying constant in reference electrode
The dissolving and precipitating equilibrium (Ksp) in a saturated solution keeps the concentration of dissolved ions constant
What makes up the single junction reference electrode
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What is a saturated calomel electrode
SCE
A liquid mercury electrode saturated with kcl
Subtract potentials
Idk
What are the two classes of indicator electrodes
Metal electrode: create a electric potential response to a redox reaction at the surface of the metal
Ion selective electrode: binding one specific type of ion to a membrane makes a electric potential (instead of redox signal it make junction potential)
What is a junction potential
How does it develop
Why is it a problem
The voltage difference that’s in between two different electrolyte solutions
Is made because of opposite charged ions having different mobilities (ionic strengths)
Limits the accuracy of measurements because we don’t know the size of the junction potential
Give junction potential example
In nacl the cl ions diffuse into water faster than the na.
This makes a charge separation where the nacl salt bridge touches water
How do we minimize Ejinction
Kcl salt bridge
Ionic liquid based salt bridge
What are porous glass plugs for in a slat bridge
To keep the salt in the beige while still letting ions current in the half cell to flow through
What are nanoporous glass plugs
4-20nm pore size
Smaller pores which help more specific ions to pass
Less stable junction potentials
What are microporous glass plugs
100-3000nm
Best for measurement
Larger pores so less selectivity
More stable junction potentials
Ion selective diagram
Ok
What 3 things does the ion selective glass electrode consist of
Outer ag|agcl electrode (has solid kcl and solid agcl)
Selective h+ glass membrane
Inner ag|agcl electrode (has buffered kcl with agcl)
What is the glass membrane of the glass ph combo electrode made of
SiOH (silica)
Pores that are coated with negative charged O- atoms
Na+ ion that move through the pores (slowly)
It’s slightly conductive
How are hydrated gel layers formed
Two surfaces of the ph sensing glass membrane absorb water which froms it (basicallymelted glass)
What is the cell notation of the ph sensing glass electrode
Check pics
What are the 10 errors in glass ph measurements
- Standard buffer uncertainty (standards are only accurate to +/- 0.01
- Junction potential uncertainty
- Junction potential drift error
- Concentrated Sodium error
- Strong acid error
- Equilibrium time error
- Hydration of glass initialization error
- Temperature error
- Dirty electrode
- Stirring error
How do you minimize junction potential uncertainty
By calibration the same ionic strength as the analyte solution
How do you minimize junction potential drift error
By calibrating every two hours
How do you minimize concentrated sodium error
Avoid using NAOH because glass electrodes respond to na+
How do you minimize dirty electrodes
Avoid hydrophobic substances
What are the advantages of ion selective electrodes
Less expensive
Respond linearly in log form
Wide range
Non destructive
Short reposnse time
Unaffected by colour or clarity of liquid (turbidity)
What are the disadvantages of ion selective electrodes
Fragile
Small shelf life
Need constant ionic strength
Uncomplexed ions only
Interference
Can be fouled by protiens
Does the ISE respond to a specific ion or it is selective
Selective
What are the types of ion selective electrodes and what are each for
Glass membranes (h and Monovalent cations)
Solid state electrodes (inorganic crystals or conductive polymers)
Liquid based electrodes (hydrophobic polymer membrane with a hydrophobic liquid ion exchanger)
Compound electrodes (electrodes enclosed by a membrane the separates analyte from other species or genterate and analyte)
