Final Exam Flashcards
Standard Hydrogen Electrode
- reference electrode for standard reduction potentials
- Difficult to use bc requires H2 gas, freshly prepared catalytic Pt surface which is easily poisoned
– chemicals in electrode set to standard states - Pt surface in contact with 1.0M H+ (pH=0) through which H2 is bubbled to maintain pressure of 1 bar.
- Acidic solution is in contact with analyte through salt bridge/porous plug
S-S Chloride Reference Electrode
- comprised of silver wire in loop in which there is AgCl paste
- placed in sat KCl solution
- solid KCL in sat solution is mixed with small amount of AgCl and acts as part of salt bridge b/w red electrode and analyte solution via porous plug
- not in standard conditions, [Cl-] for sat KCl is 4.2M rather than 1.0M
- ionic strength of KCl is high enough to use activity rather than conc
Nernst:
E=E*-(0.05916/n)log A(Cl-) = 0.222 - (0.05916/1)log ([Cl-]y(Cl-))
y(Cl=) is activity of coefficient of Cl- at ionic strength of 4.2KCl solution
S.H.E Potential
0.000V
S-S Electrode Potential
0.197V
S.H.E Reaction
H+ + e- -> 1/2 H2
Pt(s)/H2(g)/H+(aq)
S-S Reaction
Based on : AgCl(s) + e -> Ag(s) + Cl-(aq)
Ag(s)/AgCl(s)/Cl-(aq)
Van Deemter Equation
H = A + B/Ux + Cux
H
Theoretical plate height
A
Multipath term.
- Multiple paths possible, however average route is usually taken.
- Small particles preferred ( most efficient ) as it ensures all solute particles travel similar distances. Bigger particles lead to routes that are longer or shorter than the mean -> Higher degree of band broadening is observed
- Property of all packed columns bc open tubular columns do not have a multipath term (they aren’t packed).
- Independent of flow rate
B
Longitudinal Diffusion
- observed when molecule moves randomly form region of high conc to low conc, or diffuses
- measure of thermal energy of a solute molecule and the friction it encounters as it moves through the mobile phase.
- Thermal energy is a function of temperature.
- High temp = high kinetic energy
- Friction term = viscosity of the mobile phase (inversely proportional to temperature)
- When solute introduced to column, it is the most concentrated (very narrow band)
- d starts immediately
- The observation is that as time progresses, the number of molecules at the center of the band decreases as they diffuse to other regions of the band and broaden it, and they do so in a Gaussian distribution
- inversely proportional to linear velocity of mobile phase therefore faster it moves, the less diffusion there is
Ux
Linear velocity of the mobile phase
Why do bands broaden as they pass through a mobile phase?
Due to H = o^2/L , we can discuss solute band width (amount by which solute is broadened) in terms of theoretical plate height
C
Mass transfer term
- movement of mass from one region to another
- in order for solute to separate, it has to move through mobile phase and come into contact with stationary phase (does so by diffusion)
Purnell
- want baseline resolution
- improve sep factor and selectivity factor to increase number of plates
^ closely related, and can be done by changing solvent (which changes eq b/w mobile and stationary phase), change flow rate, temp (which is inversely proportional to viscosity therefore less friction).
can also be done by changing theoretical plates (van deemter)
Van Deemter What we want
as narrow as possible = resolution as high as possible
