School of Chemistry, UNIVERSITY OF KWAZULU-NATAL, DURBAN CENTRE
CHEM340 : INSTRUMENTAL ANALYSIS
JUNE 2006 EXAMINATION
Section A: Prof Kindness
QUESTION ONE
The following calibration graphs are for the determination of iron in river water using
a spectrophotometric method.
Graph 2
Graph 1
0.9
y = 0.1949x - 0.0021
2
R = 0.9999
y = 0.078x - 0.0021
2
R = 0.9999
0.8
0.7
Absorbance
Absorbance
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3
4
0
2
4
Absorbance
Absorbance
0.5
0.4
0.3
0.2
0.1
0
1
2
8
10
Graph 4
Graph 3
1
0.9
0.8
0.7
0.6
0
6
Concentration Fe / mg L-1
-1
Concentration Fe / mg L
3
4
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
y = 0.1773x + 0.0137
R2 = 0.9882
0
5
1
2
3
4
-1
Concentration Fe / mg L
-1
Concnetration Fe / mg L
a) For the following questions, briefly state the reasons for your answer.
i)
Which calibration graph represents the represents the most sensitive
method?
ii)
(1)
Which calibration graph represents the represents the least sensitive
method?
(1)
iii) Which calibration graph has the smallest linear range?
(1)
iv) Which calibration graph has the largest linear range?
(1)
v) For calibration graphs 1 and 4, which graph would result in the highest
level of uncertainty in your analysis?
2
(1)
School of Chemistry, UNIVERSITY OF KWAZULU-NATAL, DURBAN CENTRE
CHEM340 : INSTRUMENTAL ANALYSIS
JUNE 2006 EXAMINATION
QUESTION ONE (Continued)
b) For calibration graph 1 above; the following data was obtained for the
measurement of the blank reagent; -0.005, -0.003, 0.000, 0.003, -0.007,
0.000. What is the limit of detection (99% confidence)? Express your answer
in mg L-1.
(3)
c) In HPLC, the stability of the signal is affected by two factors; noise and drift.
Define these two parameters and describe what effect they have on the
precision and accuracy of an analysis.
(4)
d) The following data was obtained for the analysis of xylene isomers by
capillary gas chromatography using a flame ionisation detector.
Standard
Sample
Isomer
v/v %
Peak area
Peak area
p- xylene
58.8%
401099
191311
m- xylene
29.4%
200182
74678
o- xylene
11.8%
80112
646228
What is the composition of the sample? Express you results in v/v%.
(5)
[17]
QUESTION TWO
a) State, in words and symbols, the Beer Lambert Law, give the units of each
variable.
(2)
b) The molar absorption coefficient (ε) of an organic analyte at 255 nm is
20300 L mol-1 cm-1. When light of that wavelength passes through a 15 mm
cell, containing a solution of that analyte, 44% of the light was absorbed.
i)
What is the concentration of the analyte solution?
(3)
ii) What type of cuvettes were used for recording the above spectrum? Give
your reasoning.
(2)
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School of Chemistry, UNIVERSITY OF KWAZULU-NATAL, DURBAN CENTRE
CHEM340 : INSTRUMENTAL ANALYSIS
JUNE 2006 EXAMINATION
QUESTION TWO (Continued)
iii) The following spectrum was obtained for a soft drink. The drink
contained both benzoic acid and caffeine. As can be seen, the absorption
spectrum of caffeine overlaps with that of the benzoic acid.
Absorbance
1.8
1.6
10ppm benzoic acid
1.4
12ppm caffeine
1.2
sample 1:25
1
0.8
0.6
0.4
0.2
0
200
220
240
λ (nm)
260
280
300
The concentration of benzoic acid and caffeine can be determined by
spectrophotometry. The absorption maximum for benzoic acid occurs at
230 nm, while that for caffeine is at 275 nm. The absorptivity data at
these wavelengths are as follows:
Absorptivity (a)
μg-1 mL cm-1
230 nm
275 nm
Benzoic acid
0.0927
0.00881
Caffeine
0.0268
0.0494
The absorbance measured at these two values (230 & 275 nm) for a × 25
(1:25) dilution was as follows, 272 nm = 0.343 and at 230 nm = 0.555
(1 cm cell)
What is the concentration of caffeine and benzoic acid in the drink?
4
(8)
School of Chemistry, UNIVERSITY OF KWAZULU-NATAL, DURBAN CENTRE
CHEM340 : INSTRUMENTAL ANALYSIS
JUNE 2006 EXAMINATION
QUESTION TWO (Continued)
c) Which will be the most sensitive method of spectrophotometric analysis for Fe:
a complex with dithizone (ε = 4.16×103 L cm-1 mol-1), thiocyanate
(ε = 7.90×103 L cm-1 mol-1) or o-phenanthroline (ε = 1.06×104 L cm-1 mol-1)?
Briefly explain why.
(2)
[17]
[34]
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School of Chemistry, UNIVERSITY OF KWAZULU-NATAL, DURBAN CENTRE
CHEM340 : INSTRUMENTAL ANALYSIS
JUNE 2006 EXAMINATION
Section B: Ms. G. Dawson
QUESTION THREE
Atomic Spectroscopy
a)
Atomic spectroscopy can be divided into two processes: atomic absorption and
atomic emission spectroscopy.
Briefly discuss the theoretical and practical
(instrumental) differences between absorption and emission, with respect to
Flame-AAS/AES, and ICP-OES.
(6)
b)
With respect to graphite furnace AAS:
i)
The atomisation process occurs in three steps. Name these three steps,
ii)
c)
and briefly explain what is happening at each of these steps.
(3)
Briefly discuss an advantage of GF–AAS over flame AAS.
(1)
“Non specific absorption” is one of the interferences in electrothermal
atomic spectroscopy.
One of the correction methods used to overcome it is by using a deuterium
lamp. Briefly discuss how this correction method works.
(3)
d)
Briefly discuss which atomic spectroscopy method you would use for the
analysis of the following samples, and why.
i)
trace amounts of cadmium in soil
(2)
ii)
arsenic in hair
(2)
[17]
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School of Chemistry, UNIVERSITY OF KWAZULU-NATAL, DURBAN CENTRE
CHEM340 : INSTRUMENTAL ANALYSIS
JUNE 2006 EXAMINATION
QUESTION FOUR
Chromatography
a)
Define the following terms:
i)
Elution
(1)
ii)
Retention time
(1)
iii)
Capacity factor
(2)
b) The two main modes of separation in chromatography are adsorption and
partition. Briefly discuss them.
c)
(4)
Discuss briefly, the difference between normal and reversed phase
chromatography.
d)
(4)
Briefly discuss how you would analyse the following solutions. Give, explaining
your reasons, the type of chromatography, the stationary phase and the
detector.
i)
the amount of ethylene glycol in wine
(2)
H H
HO
C
C OH
H H
ii)
the amount of phosgene (COCl2) in chloroform (CHCl3)
oxidised into phosgene in the presence of air.)
Cl
Cl
(Chloroform is
(2)
Cl
C
O
H
C
Cl
Cl
[16]
[33]
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School of Chemistry, UNIVERSITY OF KWAZULU-NATAL, DURBAN CENTRE
CHEM340 : INSTRUMENTAL ANALYSIS
JUNE 2006 EXAMINATION
Section C Mr Bissessur
QUESTION FIVE
a)
Consider the following diagram and answer the questions that follow:
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School of Chemistry, UNIVERSITY OF KWAZULU-NATAL, DURBAN CENTRE
CHEM340 : INSTRUMENTAL ANALYSIS
JUNE 2006 EXAMINATION
QUESTION FIVE (Continued)
2H+ + 2e → H2(g) Eo H+/H2 = 0.000 V
AgCl(s) + e- → Ag(s) + Cl- Eo AgCl/Ag = 0.222 V
i)
Calculate the thermodynamic potential of this cell.
(5)
ii)
Why is it not necessary to have salt bridge in this set-up?.
(1)
b)
Name one important criterion that must be satisfied for the application
of potentiometric measurements using electrochemical cells.
(1)
c)
Explain why the Standard Hydrogen Electrode (SHE) is seldom used
as a reference electrode in potentiometric measurements.
(2)
d)
Explain how the silver/silver chloride electrode functions as an electrode
of the second kind. Indicate the importance of the plot of Eind vs pCl.
(6)
e)
Explain how a glass membrane functions in the determination of the
pH of a solution.
(4)
f)
The amount of chloride in tomato sauce was evaluated by a potentiometric
titration. Indicate, using illustrations, what data was obtained from the
experiment and how the amount of chloride was obtained.
(4)
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School of Chemistry, UNIVERSITY OF KWAZULU-NATAL, DURBAN CENTRE
CHEM340 : INSTRUMENTAL ANALYSIS
JUNE 2006 EXAMINATION
QUESTION FIVE (Continued)
g)
An excess of Sn(II) was electrolytically generated at a constant
current of 1.06 mA for 8.30 min according to the following equation:
Sn4+ + 2e- ↔ Sn2+
i)
Calculate the amount (in moles) of Sn(II) produced.
(Faraday constant = 96485 C)
(2)
ii)
The Sn(II) generated in (i) above was used to reduce quinone
to hydroquinone according to the following equation:
OH
O
+ Sn4+
+ Sn2+
OH
Hydroquinone
O
Quinone
After the reduction of all the quinone, the excess Sn2+ was oxidized to
Sn4+ with electrolytically generated Br2 in a coulometric titration at a
constant current of 1.06 mA for 0.7 min according to the following
equation:
Sn2+ + Br2 →
Sn4+ + Br- (unbalanced)
Calculate the excess moles of Sn2+ and hence the mass of Quinone
(C6H4O2, MW = 108 g mol-1) in the sample using data from part (i).
(5)
h)
Explain, using a diagram, what information can be derived from a
current-potential curve for the following linear sweep voltammetric
reduction of Cd2+(aq) + 2e- → Cd(s) at a glassy carbon electrode
versus a SCE.
(3)
[33]
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