UNG Dansby Analytical Test 2

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Terms in this set (...)

Cyclic voltammetry
analyzing the oxidation and reduction of materials in solution
Explain voltametric diffusion
Diffusion to surface of electrode creates a gradient of concentration of pre-reduced species (becomes limiting). Smaller diffusion area = smaller gradient
Potentiometric measurement
Determines the voltage difference between 2 electrodes, but with no flowing current. Working electrode responds to analye concentration and reference electrode has a fixed potential
Working electrode
Interface of interest (analyte)
Reference electrode
High impedance/resistance
Auxiliary electrode
Current-carrying partner of the working electrode in an electrolysis, completes flow of charge to keep "sea level" the same
Calculate standard cell potential for:
Cu | Cu(NO3)2 .010 M || AgNO3 .50 M | Ag
Ered (Cd2+) = -.402
Ered (Ag+) = .799
Write out half reactions and balance
Use Nerst equation
Nerst
Calculates Ecell at non-standard conditions
Zero standard for electrode
Platinum in high concentration acid with H2 (flammable) gas bubbled in
Electrical potential
Measure of the tendency for the reaction to occur (how spontaneous)
How to balance a half rxn?
Split
Mass balance
Add H2O to balance O and H
Charge balance with E-
Make sure e- on both half rxns match
Add and cancel out
Oxidation
loss of electrons
Reduction
Gain of electrons
Concepts of electrochemistry
Analyte may participate in other reactions
Electrochem is a surface technique
Gradient of analyte between surface and bulk
Current is a measure of the rate of an analytes oxidation/reduction
Hydrodynamic flow
Friction on walls creates a faster middle flow and slower edge flow; more eddy diffusion (A)
Electroosmotic flow
Positive and negative ends pull sample through at the same rate, minimizes eddy diffusion
Efficiency of separation is controlled by?
Plate height, determined by Van Deemter
Eddy diffusion (A)
Want dense, uniform particles for column to minimize band broadening by minimizing path length
Longitudinal diffusion (B/mu)
Longer time on column broadens band. Apply pressure to increase flow rate and minimize time on column
Mass transfer (C)
Transfer of solute into/out of the stationary phase; more transfer in and less out if stationary phase is more viscous or thicker (think syrup pool). Faster flow leaves behind more solute in stationary as it can't difuse back into mobile in time. Increase temp to decrease viscosity, decrease surface area, and make stationary thinner
Van Deemter equation
Expresses how the column and flow rate affect plate height
How to optimize separations and get clearer data?
Minimize plate height, making narrower more distinct peaks
How can resolution (Rab) be improved?
Increase column length, decrease peak width (sharper), smaller plate height
Selectivity factor
Ratio of the adjusted retention times of two component peaks (larger alpha means greater separation)
Retention factor
Larger K means more time spent on column
Types of chromatography
Adsorption
Partition
Ion exchange
Molecular exclusion
Affinity
Affinity
Most selective (specific reaction, think enzymes)
Molecular exclusion
Gel permeation, separation based on size as smaller particles get caught in column (longer time)
Ion exchange
Stationary phase is an ion exchange resin with (SO3- AND N+R4 groups)
Partition
Solute partitions between stationary and mobile phases
WCOT= liquid
PLOT = particles
SCOT = mixture
Adsorption
Solid stationary phase, solute adsorbs to outer surface of stationary particles
Extraction
How solutes distribute between 2 immiscible liquids
partition coefficient
Distribution of solute between the mobile and stationary
Triplet phase
Phosphorescence is the slow relaxation from here
Transmittance
Amount of light not absorbed
Absorbance
Amount of light attenuated by solution
Transitions for molecular absorption
Electric
Vibrational
Rotational
Particle formation
1) Nucleation
2) Either more nucleation or particle growth (low RSS)
Molar solubility
molarity of a solute in a saturated solution
Activity coefficient
Effects of electrolytes on equilibrium
Single point calibration
We extrapolite beyond our standard and past its limits of calibration
Multipoint calibration
Range of known x's and predicted y's to predict x of analyte
Standard addition method
No matrix matching as standard and sample are in the same mix, meaning same interferents
Non-linear calibration
Acceptable but only 1 x-value allowed per y; sensitivity changes as concentration increases (gets worse)
Atomic absorption
Gases act independently, discrete sharp peaks
Molecular adsoprtion
In liquid phase, solvent interfence blurs transitions
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