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Chem 362
February Midterm
2012
Smith-Palmer
Name:
The overall principle behind chromatography is differential hold up.
[2]
1. Name 5 different ways to achieve this.
Adsorption
Partition
Affinity
Ion exchange
Molecular exclusion
[1]
2. Which of these ways is important in reverse phase HPLC and in gas liquid
chromatography?
[2]
3. Draw a labeled block diagram for an HPLC.
loop injector
solvent
reservoirs
[2]
pumps
column
UV-vis
absorption
detector
4. Draw a labeled block diagram for a GLC.
oven
gas tank &
regulator
column
injector &
vaporizer
FID
2
[4]
5. In gas chromatography, the parameter that is changed during a gradient run is
temperature…………………………
Should we increase or decrease it during a run?
Increase
In liquid chromatography with a reverse phase column, the parameter that is changed
during a gradient run is
Polarity
Should we increase or decrease it during a run?
Decrease
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Is it easier/more convenient to run a gradient in gas chromatography or liquid
chromatography.
Why? HPLC columns take longer to equilibrate
When should you use a gradient?
When you have a mixture of very different compounds – and there are some later ones
that are all well separated
If its a long run for HPLC so equilibration time doesn’t matter
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6. Why do bands get broader in a chromatogram as retention time increases?
More diffusion because of the time the analyte spends on the column
Why is this worse in gas chromatography than in liquid chromatography?
Sample is a gas, high temperatures
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7. Calculate N and H using the second peak of the following chromatogram.
The column used was 12 m in length.
First peak: retention time = 60 s, half width = 6.0 s
Second peak: retention time = 100 s, half width = 10 s
N = 5.55 × 1002 = 5.55 × 102 (5.611 × 102)
102
H = 1200 cm = 2.2 cm (2.1 cm)
555
What is the resolution between the two peaks?
Rs = 0.589 × 40 s = 2.945
8s
= 2.9
[1]
Suggest what mobile phase might have been used.
N2 gas
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What instrument settings could be changed to decrease the resolution?
Increase flow rate, increase temperature
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8.
With respect to an ion selective electrode, what does the selectivity coefficient tell us?
Tells us how much other ions interfere
Is it better to have a large or a small selectivity coefficient?
Explain
Small – as in way less than 1 This means that the response to another ion is way less
[4]
[1]
Draw a labeled diagram to show how an ISE can be modified and used to detect
carbon dioxide.
Which is more sensitive – an ISE for calcium ions or an ISE for lithium ions?
Lithium ions
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[5]
9. Calculate the cathodic potential when a dilute sodium sulfate solution , pH 8.0, is
electrolyzed. The cathode is a smooth platinum electrode with a surface area of
1 x 10-3 m2. The current is 0.100 A and the product is hydrogen gas at 1.00 bar.
TABLE: Overpotential (V) for hydrogen evolution at various current densities at 25oC
Electrode
Current density (A/m2)
10
100
1000
10000
Smooth Pt
0.024
0.068
0.288
0.676
2H+ + 2e– → H2
Ec = 0 = 0.059 V log 1
2
(10–8)2
= 0 – 0.059 V × 8 = –0.47 V
Overall Ec = –0.47(2) – 0.068
= – 0.54 V
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10. Name four different parameters that are important in determining the size of an
overpotential.
Current density
Gas production
Number of e– transferred
Nature of metal
Temperature
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[2]
11. Calculate the ionic strength of a solution containing 0.3 M aluminum chloride.
= ½ (0.3 × 9 + 0.9)
= ½ × 3.6
= 1.8 M
=2M
[1]
Will the activities of these solutions be larger or smaller than their concentrations?
Smaller
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12. In polarography, what produces the charging current? (What causes it?)
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What does it mean if we say an electrode is partially polarized?
Current is not linearly dependant on V
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What does it mean if we say an electrode is completely polarized?
i doesn’t depend on V
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Why is complete polarization of the mercury electrode important in polarography?
id is proportional to [analyte]
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[3]
[2]
Draw the voltage versus time plot for the voltage that is applied in square wave
polarogaphy.
What is the advantage of square wave polarography as compared to differential pulse
polarography?
Fast, better sensitivity
[2]
Draw the shape you would expect to see for the cyclic voltammogram of a reversible
reaction.
Label the axes. I like ducks.
i
v