Homework 11: Chapter 23 Potentiometry, Chapter 24 Coulometry, Chapter 25
Voltammetry, Chapter 26 Chromatographic separations, Chapter 27 Gas
Chromatography, Chapter 28 HPLC, Chapter 29 SCFE, Chapter 30 Capillary
Electrophoresis, Chapter 31 Thermal Methods
Assigned 26 April; Due 1 May 2006
1. What are suitable properties of a reference electrodes?
 Known half-cell so comparisons with measured potential can be made
 Insensitive to solution under examination. The electrode response should be
reversible and obey Nernst equation. The potential should be constant under
the examined condition and return to the original potential.
2. In equations for measuring ion concentration by specific electrode how is the ion
concentration evaluated?
The response is based on the Nernst equation so is temperature dependent and a
function of the number of electrons involved in the reaction. From the Nernst
equation the log of the concentration impacts the potential. From this the
negative of the log of the concentration (pX) is evaluated.
3. What are some ions that can interfere with a pH measurement?
Monovalent cations can interfere: Na+, K+, NH4+, Cs+, Rb+, Li+, Ag+
4. What is a liquid membrane electrode?
The electrode has a porous membrane that is used to separate liquids. The
electrode response is based on the potential that develops across two immiscible
liquids with different affinities for the analyte, ideally with selective bonding of
the analyte. An example is calcium dialkyl phosphate which is insoluble in
water, but binds Ca2+ strongly. This can be used for a Ca2+ specific electrode.
5. What is the basis of coulometry and what are 2 coulometric methods?
Coulometry is the quantitative conversion of ion to new oxidation state. The
amount of electrons needed provides information that can be used to determine
the ion concentration. Two methods are constant potential coulometry
(Potentiostatic coulometry) and constant current coulometry (Coulometric
titration).
6. A Cu2+ solution is exposed to a current of 0.5 A for 500 seconds. How much Cu is
deposited on the cathode?
Current = 0.5 A, t= 500 s
Q= 0.5(500) =250 C
2 electron reduction, n=2 and F=96485 C/mole eN = 250/(2*96485) = 1.30E-3 moles Cu2+
7. How can the Nernst equation be used to determine the number of electrons involved
in a redox reaction?
The potential plotted against the log of the redox concentrations should yield a
slope of -0.0592/n where n is the number of electrons.
8. How can the half-cell potential be determined from a voltammogram?
The half-wave potential can be evaluated to determine the half-cell potential.
On a plot of current versus applied potential the potential at half the limiting
current height is the half-wave potential. The standard potential is determined
by normalizing the reference used in the voltammogram to SHE.
9. In the CV below label the cathodic peak potential and the anodic peak potential.
The cathodic peak potential is due to the reduction of the compound and is
labeled D in the figure. The anodic peak potential is driven by the oxidation
of the sample and is point B in the figure. The difference between the peak
potentials in volts is 0.0592/n where n is the number of electrons involved in
the half-reaction.
10. What terms are responsible for zone broadening in chromatography?
Multipath term where analyte molecules can take different paths, diffusion
which is a function of time on the column, and a mass transfer is related to the time
it takes for the analyte to reach equilibrium on the column.
11. You have a 10 cm liquid chromatography column. You have the following data
Compound tr
Width of Peak Base (Wt) (min)
N
(minutes)
A
B
C
D
3.2
6.7
8.9
25.3
0.18
0.37
0.48
1.4
5056.79
5246.457
5500.694
5225.224
5257.292
A. Calculate the theoretical plates for each peak and the column average
B. The average plate height for the column
N=16(tr/Wt)2
H=L/N= 10 cm/5257 =0.0019 cm
12. What instrumentation is used in gas chromatography?
A carrier gas for the analyte is needed. These gases include He, N2, and H2. A
column with silica beads or functionalize surfaces is required. An oven to keep the
operating temperature appropriate is needed. Finally a method to detect the
analyte is required.
13. What physical properties of molecules are exploited in HPLC
Size and polarity are used to achieve separations in HPLC. Solvent polarity can
be varied to exploit differences in analyte polarity. The column particle size is
used for size exclusion and hence separation based on molecular size
14. What is a supercritical fluid?
A supercritical fluid has physical and thermal properties that are between those
of the pure liquid and gas. The fluid density is a strong function of the
temperature and pressure of the state for creating the SCF. The SCF has a
diffusivity that is radically different from a liquid and the SCF readilypenetrates
porous and fibrous solids.
15. What is needed for capillary electrophoresis?
The set up includes a power supply for the potential, two electrodes, and a
medium for the analytes to traverse.
16. What is the basis of thermogravimetric analysis?
TGA involves monitoring the weight loss of the sample in a given atmosphere as
a function of temperature. The components of the molecule can be identified by
the mass loss at a given temperature or through the analysis of the off-gas.