Electroanalytical Methods Flashcards
Potentiometry, Coulometry, Electrogravimetry, and Voltammetry
Define potentiometric method of analysis
measuring galvanic cell potentials (electrochemical cells in the absence of appreciable current) to obtain chemical information (concentration)
Basic components of potentiometric analysis and usual arrangement
- Reference electrodes
- Indicator electrode
- Salt Bridge
- Potential Measuring Device
Ref Electrode | Salt Bridge | Analyte Solution | Indicator Electrode
Define (liquid) junction potential
Voltage difference (small voltage; mV; cation rich and anion rich) that develops at the interface when 2 dissimilar electrolyte solutions are in contact (found at each end of a salt bridge connecting 2 half-cells)
Define reference electrode and its desired characteristics (5)
- Known and stable potential
- Constant response
- Independent of the type of solution in which it is immersed
- Rugged and easy to assemble
- reversible and obey Nernst equation
Explain SHE equation and its component
Standard Hydrogen Electrode
2H+ (aq) + 2 e- <===> H2 (g)
- 1M HCl solution with Pt foil coated with black Pt
- H2 gas is passed through a tube and into the solution (bubbled continuously at 1 atm)
Why platinum is used in SHE?
Pt is inert and will not react much with hydrogen
Explain Calomel Reference Electrode and its equation
Consist of mercury in contact with a solution that is saturated with mercury (I) chloride (calomel) and contains a known concentration of potassium chloride.
Hg2Cl2 (s) + 2 e- <=> 2 Hg (l) + 2 Cl- (aq)
Explain Silver/Silver Chloride Reference Electrode and its equation
Silver electrode is immersed in a solution of potassium chloride that is saturated with silver chloride
AgCl(s) + e- <=> Ag (s) + Cl-
Advantage and disadvantage of silver-silver chloride electrodes vs calomel electrodes
silver-silver chloride electrodes can be used at temperatures greater than 60°C
mercury (II) ions can react with fewer sample components than silver ions which can react with proteins
At 25°C, the potential of the saturated calomel electrode versus the standard hydrogen electrode is ____. For the saturated silver/silver chloride electrode, it is ____.
calomel: 0.244 V (0.241 V in other sources)
Ag | AgCl: 0.199 V (0.197 V in other sources)
Define indicator electrode
Electrode that responds rapidly and reproducibly to variations in the activity/concentration of an analyte ion or molecule
Define electrodes of the first kind
Pure metal electrode that responds to its cation in the solution within a single reaction
Cu2+ (aq) + 2 e- <=> Cu(s)
Why electrodes of the first kind are not widely used for potentiometric determinations?
- Not very selective and specific
- Many metal electrodes can only be used in neutral or basic solutions (dissolved in presence of acids)
- Other metals are so easily to oxidized (deaerated solution is a must)
- Harder metals do not provide reproducible potentials
Define electrode of the second kind
Metal electrode that is used to determine the concentration of an anion that forms a precipitate or a stable complex with the cation of the electrode metal
Define electrode of the third kind
Metal electrode at which at certain circumstances can be made to respond to a different cation (involves secondary redox couple)
Define inert metallic indicator
Inert metal electrode (Au, Pt, Pd) which served as indicator electrode for redox systems
Define membrane indicator electrode
Also called p-ion electrodes (data is usually presented as p-functions) or ion-selective electrode (ISE)
Consists of thin membrane separating 2 solutions of different ions concentration
(2 solutions of different hydrogen concentrations for the determination of pH)
Two setups of glass pH electrodes
- Glass electrode (indicator) and SCE (reference) immersed in a solution of unknown pH
- Combination probe consisting of both an indicator glass electrode and a silver/silver chloride reference
Explain boundary potential
Develops when two side of the membrane are immersed in solutions having different ion concentration (e.g., H+ ions; outer and inner concentration)
Explain asymmetry potential
Develops when the concentration of the solution on both side of the membrane is the same and arises from dissimilarities between inner and outer surface of the membrane.
(usually due to imperfection in the manufacturing)
Explain glass indicator potential
Potential with the contribution of boundary, asymmetry, and internal reference electrode potential
What should be the slope in pH calibration curves?
0.0592 V
Differentiate Acid and Alkaline Error
Alkaline error - measured pH is low due to the presence of alkali ions at solutions with pH 10 to 12 or greater (glass surface become responsive to not only hydrogen ions but also alkali metal ions)
Acid error - measured pH is high when the pH of the solution is less than 0.5 due to saturation effect that occurs when all surface sites on glass are occupied with H+ ions.
Define solid state (crystalline) membrane electrodes
Has a membrane that consists of either a polycrystalline inorganic salt or a single crystal of an inorganic salt
Define liquid-based membrane electrodes
Has a hydrophobic liquid membrane impregnated with a hydrophobic ion exchanger that is selective for analyte ion
Define permeable membrane electrodes or gas sensing electrode
consists of a porous/permeable membrane that will selectively interact with specific ions in a solution
Explain enzyme electrode
A potentiometric biosensor that immobilizes an enzyme at the surface of a potentiometric electrode that will react selectively with analyte.
The analyte’s reaction with the enzyme produces a product whose concentration is monitored by the potentiometric electrode
Explain the titration curves (4) in potentiometric titration
- Normal titration - sigmoidal curve (pH vs volume); equivalence point at sudden change in pH
- First derivative - prominent peak (∆pH/∆V vs volume); equivalence point at prominent peak
- Second derivative - rational function {[∆(∆pH/∆V)]/∆volume vs volume}; equivalence point at the center of two curves
- Gran plot - volume of base x [H+] vs volume of base; equivalence point at the straight line after the slope
How to get the first derivative plot given the volume and pH
x values: get the average of the two consecutive volumes
y values: change in pH / change in volume
How to get the second derivative plot given the volume and pH?
x values: get the average of the two consecutive volumes
y values: [∆(∆pH/∆V)]/∆volume
Define electrolysis
Process in which a chemical reaction is forced to occur at an electrode by posed voltage
(forcing a nonspontaneous reaction to happen by applying voltage)
Define overpotential and its three factors
Additional voltage required to overcome the effects of kinetic barrier. Difference between theoretical and actual potential
- Ohmic Overpotential (IR)
- Activation Overpotential (AO)
- Concentration Overpotential (CP)
Define ohmic overpotential and formula
voltage needed to overcome internal resistance of the cell
E ohmic = IR
(obey Ohm’s law)