Midterm 3 Flashcards
Potentiometry def
Use of electrodes to measure voltages that provide chemical information
Following a potentiometric titration
A pair of electrodes, a saturated calomel electrode and a Pt indicator electrode, is inserted into the reaction mixture.
experimental setup for potentiometry
Calomel reference electrode and the Pt wire are submerged in the Fe 2+ solution. A buret containing Ce4+ is suspended on top of the beaker. There is a circuit between the anode and the cathode that measure potential for the reduction of Ce4+ or Fe3+ at the Pt electrode (cathode).
Conditions before the equivalence point for Fe 2+ titrated with Ce4+
Ce4+ + Fe2+ –> Ce3+ + Fe3+
However much Ce4+ is added = How much Fe3+ is formed
Use reduction of Fe:
Fe3+ + e- –> Fe2+ at the cathode
Calculations of cell voltage at the equivalence point
[Ce3+]=[Fe3+] and
[Ce4+]=[Fe2+]
Uses both half-reactions at the cathode. Want average potentials for half-reactions at Pt electrode.
Calculation of cell voltage after the equivalence point
All Iron are in Fe3+, the product form.
Moles of Ce3+=Moles Fe3+. We know the excess Ce4+
[Ce3+] and [Ce4+] are known.
Use the reduction reaction of Ce4+ at the cathode
Ion selective electrodes
- Different from metal electrodes
-Do not depend on redox processes - Key feature: thin membrane, allowing the passage of only one ion
Mechanism of the electrode prior to migration
A Ca2+ binding ligand that is soluble in the membrane is inserted. One side of the membrane is high concentration and the other is low.
Mechanism of electrode after migration
See Ca2+ that migrated from high to low concentration. The amount that migrated is subtracted from the high side, and added to the low side. Concentration of Cl- stays the same because it cant migrate. A voltage develops across the membrane and the voltage is proportional to the ratio of Ca2+ on each side of the membrane
What can prevent further migration of Ca2+?
-Excess charges prevent further migration of Ca2+
- Electric potential difference developed across membrane prevent further migration of Ca2+.
What is the steady state situation?
It is where the decrease in free energy change due to activity difference is balanced by increase in free energy due to charge imbalance (repulsion of like charges)
Steady state situation equation
-RTln(A1/A2) = -nFE
F- faraday constant
n- charge of the ion
E- electric potential difference across the membrane.
A2 is high concentration, (activty) and A1 is low.
Activity, Ac
Ac=[c]Yc
Yc- activity coefficient of C (measures deviation of behavior from ideality) If it were ideal behavior, it would be 1, but its usually less than
Why use activity instead of concentration?
To account for effect of ionic strength (u or I)
What is ionic strength, u?
A measure of total concentration of ions in solution
u= (1/2) the sum of the concentration of each species multiplied by its charge squared.
Effect of ionic strength on solubility of salts
- Add KNO3 to solution: solubility of Hg(IO3)2 (s) increases to 1.0 x10^-6
KNO3 is inert and has no interaction with Hg2(IO3)2
-Solubility increases due to increase in ionic strength of solution
How does adding KNO3 to a solution of Hg2(IO3)2 make it more soluble in water?
The NO3- surrounds the Hg and makes it less positive
The K+ surrounds the IO3- and makes it less positively charges, and decreases their attraction allowing them to dissolve in water.
Dybye length
The higher the ionic strength- the shorter the debye length.
Debye length equation
Dont need to know the equation but parameters:
E0: permittivity of free space (F/m or C^2/Jm)
Er: Relative permittivity (dielectric constant) of water
kB: Boltzann constant (J/K)
T: Absolute temperature (K)
NA: Avogadros number (mol^-1)
e: Elementary charge (C)
I: Ionic strength (mol/L)
How does ionic strength relate to dissociation?
Increasing ionic strength promotes dissociation of compounds into ions
Extended Debye-Huckel equation for aqueous solutions at 25 degree
Detailed treatment of ionic sphere model.
This equation can only be used when the ionic strength is less than 0.1M
Equation has the variables:
Y: activity coefficient
Z: charge on ion
a: effective hydrated radium of ion and tightly bound water molecules.
Effective hydrated radius (a)
- Small, highly charged ions (bond solvent molecules tightly) have large a
_ larger, less highly charged ions have a smaller a.
F- has a larger hydrated radius than I-.
The glass electrode
The most widely used ion selective electrode.
Order of the glass electrode line diagram
Outer reference electrode, salt bridge, H+ outside glass electrode, glass membrane that selectively binds H+, H+ inside glass electrode, inner reference electrode.
The glass electrode diagram
Outer reference electrode is a Ag wire suspended in the analyte solution.
Inner reference electrode contains a tube with an Ag wire and an AgCl paste suspended between two sides of folded Ag wire. A glass membrane at the bottom V
Contains HCl saturated with AgCl.
Combination electrode def
Combines both glass and reference electrodes in one body.
Combination electrode diagram
The electrode is suspended in the analyte solution. Inner the electrode, there is an aqueous filling solution saturated with AgCl and KCl. Ag wire is suspended in the middle and there is an AgCl paste suspended between two sides of folded Ag wire.
STructure of silicate glass.
Structure of glass consists of an irregular network of SiO4 tetrahedra connected through oxygen atoms. Cations such as Li+, Na+, K+ and Ca2+ are coordinated to the oxygen atoms. The silicate network is not planar. This diagram is a projection of each tetrahedron onto the plane of the page.
Silicate is negatively charged
What is measured with the two reference electrodes?
The electrical potential across the membrane
What is responsible for the conduction within the glass membrane?
Singly charged ions
Hydration reaction of the electrode.
Before using the electrode, it must be hydrated.
H+ + NaGl (glass) -> Na+ + HGl (glass)
The glass now has the H+
H+ from solution goes in and Na+ goes out.
Schematic diagram of hydrated glass membrane
There is a dry interior, movement of Na+, hydrated parts, one on each side. then on the other side of the hydrated parts, the gel region, the movement of H+.
On the left is the Internal solution , 0.1M and on the right is the external solution , variable.
What are the parts of the hydrated glass membrane? and what is being conducted, moved?
Gel region- H+ is the only mobile cation
Hydrated parts- Consists of H+ and Na+ ions, both moving.
Dry interior part- Movement of only Na+
How are the equilibrium positions of the analyte and reference side ps determined?
Equilibrium positions of the analyte side and the reference side are determined by [H+] on both sides of the membrane. Greater dissociation on analyte side due to lower [H+].
What happens if the position of equilibria differ?
One surface will be more negative with respect to the other surface (surface where greater dissociation occurs => analyte side
Devepoment of boundary potential (Eb) across membrane
Boundary potential Eb
Magnitude of Eb is a function of the ratio A1/A2 where A1 is the activity of analyte solution and A2 is activity of internal solution (const) A1<A2.
Look at the equilibrium at the analyte side and the ref. side:
H+Gl- -> H+ + Gl-
E1; potential at surface of analyte side- more negative
E2; potential at surface of ref side- less negative
Equation for boundary potential
Eb= E1- E2 = 0.0592 log(A1/A2)
Eb is a measure of activity of analyte solution (A1)
What are the categories of ion selective electrodes?
-Glass membranes for H+
-Glass membranes for certain monovalent cations: for Li+ selectivity the composition of the glass membrane 65% SiO2, 25% B2O3, 5% Li2O, 5% Rb2O. Favorable environment for Li+ ion transport; permeability of H+ and Na+ minimized due to the larger size of Rb+.
-Solid State electrode based on inorganic crystals.(ex F- electrode)
- Liquid based electrodes
Liquid-based electrodes
Use a hydrophobic polymer membrane (PVC), a hydrophobic liquid ion exchanger/ionophore (valinomycin), along with a plasticizer (DOS). Valinomycin selectively bonds to the target ion (K+)from analyte solution
Examples are specific for K+.
What does a plasticizer do?
It gets in between the polymer chain and makes it more flexible, softens the polymer. Example for K+-DOS
How does the liquid-based electrode work?
Dissolve PVC, valinomycin and DOS in anhydrous THF, and cast the solution as a thin film on a substrate. Thin film can then be peeled,
Glass electrode properties
- Most ion selective electrode
- Na+ is the principal interfering species
-Interference is significant when [H+] < 10^-12 M; [Na+] < 10^-2 M
Dont want low H+ of high Na+ concentrations
Selectivity coefficient
Ka,x
Relative response of the electrode to different species of the charge
Ka,x= response to interfering species/ response to desired species
Want a Ka,x values of less than 1.
Preparation of Eu doped LaF3
LaF3 is mixed with EuF2 in a 19:1 ration , in an Argon filled dry box. BaF2 is used in 2-fold excess in the reaction as a flux.
Quantitatively transfer the mixture into a cylindrical shaped Pt crucible containing a seed crystal of La(0.95)Eu(0.05)F(2.95) previously grown along the 111 plane. Seed crystal serve as a template to produce high-purity, defect-free and well-oriented single crystal in (111) plane. F- ionic conductivity is enhanced along the 111 plane.
Place the crucible in a vertical Bridgman furnace under Ar purge. Temperature at the top is 1300 and the bottom is 600.
The crucible slowly moves down the furnace to ensure high quality, large, and highly-ordered single crystal formation along the 111 plane.
At 600 degrees, its held for 4 hours.
Programmed cooling to RT
BaF2 is removed by suspending the crystal in water at 50 degrees with ultrasonication.
How do F- ions migrate through LaF3 doped with EuF2?
Because Eu2+ has less charge than La3+, an anion vacancy occurs for every Eu2+. A neighboring F- can jump into the vacancy, thereby moving the vacancy to another site. Repetition of this process moves F- through the lattice.
Response of F- electrode
F- electrode is more responsive than other ions.
Interfering species: OH
At low pH, F- converts to HF, which cannot migrate through the crystal since its neutral.
Procedure for measuring F-
Dilute unknown and standard solutions in a high ionic strength buffer containing CH3COOH, sodium citrate, NaCl, and NaOH to adjust pH to about 5.5
Keep all standards and unknowns at constant ionic strength
Hat pH is best for measuring F-?
5.5 because OH doesnt interfere at this pH and there is little conversion of F- to HF
WHat is the citrate for in the buffer?
citrate complexes Fe3+ and Al 3+, otherwise these ions interfere with F-.
