workshop for secondary school teachers
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Dr. W M TSUI
Department of Chemistry, HKUST
Workshop for Secondary School Teachers
1
Experiments of Microscale Analytical Chemistry
Department of Chemistry
Outline2
Introduction: Analytical Chemistry Analysis of organic compounds
Qualitative Analysis ; Quantitative analysis Accuracy and precision: use of balance Instrumental quantitative analysis Introduction to data-logger Experiments highlight
Department of Chemistry
Introduction: Analytical Chemistry
3
What is chemical analysis?
Questions to answer: How much of substance X is in the sample? Does the sample contain substance X? What is the identity of the substance in the
sample? How can the species of interest be separated
from the sample matrix for better quantitation and identification?
Department of Chemistry
Introduction: Analytical Chemistry
4
“includes any aspect of the chemical characterization of a sample material.”
Quantitation; Detection; Identification; Separation
Department of Chemistry
Introduction: Analytical Chemistry
5
What is analytical chemistry?
“The science of chemical measurement…” “…Its object is the generation, treatment and
evaluation of signals from which information is obtained on the composition and structure of matter.”
Department of Chemistry
Qualitative analysis of organic compounds
6
“An analysis in which we determine the identity of the constituent species in a sample”
Does it contain …? Type? Binding state?
Department of Chemistry
Qualitative analysis of organic compounds
7
development of chemical tests to identify the presence of organic functional groups. >C=O + DNPH → orange precipitate >C=O + DNPH → orange precipitate
NH
NH2
NO2
NO2
tests for the presence of carbonyl compounds using 2,4-dinitrophenylhydrazine (DNPH) solution
NH
N
NO2
NO2
CCH3
CH3
OC
CH3
CH3
Department of Chemistry
Qualitative analysis of organic compounds
8
tests for the presence of >C=C< / -CΞC- using bromine solution
>C=C< + Br2 → rapid decolorization >C=C< + Br2 → rapid decolorization
Br BrCC
CH3
CH3
CH3
H
CC
CH3
CH3
CH3
HBr Br
→
Br BrCH3 C
H2
CH2
C C CH3 → CH3 CH2
CH2
C C CH3
Br
Br Br
Br
Department of Chemistry
Qualitative analysis of organic compounds
9
Qualitative analysis – chromatography and identification by measuring physical property (e.g. mass spectrometry, infra red spectroscopy)
Paper chromatography
Paper chromatogram
Analysis of components in ball-pen inks
Department of Chemistry
Qualitative analysis of organic compounds
10
Infra-red spectroscopy
infrared absorption
Tra
nsm
itta
nce
(%)
OHO
Department of Chemistry
Quantitative analysis of organic compounds
11
“An analysis in which we determine how much of a constituent species is present in a sample”
Developing methods to determine the concentration of targeted species in complex samples. OHO
O
O
CH3
Department of Chemistry
Quantitative analysis of organic compounds
e.g. measuring the amount of aspirin in analgesic tablets, by gravimetric method
e.g. measuring the amount of aspirin in analgesic tablets, by volumetric method
12
OHO
O
O
CH3
OHO
O
O
CH3
OO
O
O
CH3
Na+
NaOH(aq)+ H2O+
Department of Chemistry
Accuracy and Precision13
Measurement in Science: In science, we want measurements to
be both accurate and precise.
What is the difference between accuracy and precision?
Department of Chemistry
Accuracy and Precision14
Accuracy is a measure of how close a measured
value to the true value (is it the correct value?)
Precision is a measure of the reproducibility of a
result (is it exactly the value?)
Department of Chemistry
Accuracy and Precision15
What sort of measurements do we have?
Department of Chemistry
Accuracy and Precision16
Results may be reproducible, but wrong. 5.0 grams of sample
Balance “A” 5.2; 5.4; 5.3; 5.3
Balance “B” 5.0; 4.9; 5.1; 5.0
Balance “A”
Balance “B”
sample
Department of Chemistry
Accuracy and Precision
Systematic errors are constant and
always of the same sign and thus may not be reduced by averaging over a lot of data
Random errors produced by any
unpredictable & unknown variations in the experiment, e.g. fluctuations in room temperature, fluctuations in power supply voltage, mechanical vibrations etc.
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Weighing bottle
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Instrumental Quantitative Analysis
18
Employing modern instrumentations for determining how much of a constituent species is present in a sample
Modern instruments play a key role in chemical analysis nowadays
Quantitative - measuring property and determining relationship to concentration (e.g. UV and visible spectrophotometry)
Department of Chemistry
Datalogger – Introduction19
Datalogging device: also know as datalogger or data recorder
Datalogging: a process of measuring variables in a laboratory or outside using electronic sensors
Department of Chemistry
Datalogger – Introduction20
The measured data will be stored to computer/ hardwares
Data of experiment can be displayed in form of charts, graphs and tables
Department of Chemistry
Datalogger – components21
Electronic device that records data retrieved from electronic sensors
Department of Chemistry
Datalogger – components
Typical hardware setup
22
Department of Chemistry
Datalogger – components23
Electronic device that records data retrieved from electronic sensors
Stand-Alone datalogger
Department of Chemistry
Sensors24
Light sensor
Sound sensor Temperature
sensor
Various electronic sensors, e.g. Temperature sensor, humidity sensor, pressure sensor, & light sensor.
Department of Chemistry
Advantages25
Multi-functional; can be equipped with different types of sensors
Immediate feedback of results (Real Time!)
Easy analysis of results
Easy for the repetition of experiments
Department of Chemistry
Advantages26
Suitable for both prolonged & short experiments.
How temperature, particle size, and concentration affect the rate of reaction between magnesium metal and HCl(aq)?
Department of Chemistry
Applications of Datalogging Experiments
27
Measurements in: pH change caused by chemical reactions Temperature change caused by chemical
reactions Color change caused by chemical
reactions (colorimetry) Pressure change caused by chemical
reactions etc …
Department of Chemistry
Datalogger Set up
1. Log onto the computer2. Choose an electronic
detector 3. Connect it with interface
hardware (USB-link)4. Connect the assembly into
the USB port on computer5. Start the software in the
computer
28
Department of Chemistry
Neutralization Reaction29
pH measurement:1. Transfer 30 mL of
HCl(aq) into a beaker2. Put the probe into the
solution3. Start collecting the
data4. Add equimolar
NaOH(aq) slowly until you have added 35 mL
5. Stop collecting the data6. Print your graph
Department of Chemistry
Neutralization Reaction Mark in red where the graph is acidic. Mark in blue where the graph is basic. Make in green where neutralisation
happens.
30
Time
pHpH
Time
Department of Chemistry
Spectrophotometry31
Intensity of electromagnetic radiation:
IR;Visible; UV;X-ray.
Department of Chemistry
UV-Vis spectrophotometry32
Corresponds to EM radiation in the ultraviolet (UV) region---100-400 nm
Visible (Vis) regions--- 400-800 nm Suitable for organic compounds Unsaturated (conjugated) ~ 180 nm Carbonyl ~ 300 nm
C-C=C-C
C=C-C=C
C=C-C-C=C
Department of Chemistry
UV-Vis spectrophotometry33
Department of Chemistry
UV-Vis spectroscopy
Two sources are required to scan the entire UV-VIS band:
Deuterium (D2) lamp – covers UV: 200-400 nm
Tungsten (W) lamp – covers visible: 400-800 nm
34
Department of Chemistry
Beer-Lambert Law35
Also known as Beer’s Law: A = -log10(I1/I0) = ε c l where ε = molar absorptivity; or
molar extinction coefficient [cm-1·M-1 or cm-1·mol-1·dm3] c = concentration [mol-1·dm3]
l = path length [cm]
Department of Chemistry
Transmittance and Absorbance
36
Transmittance (T) = I1/I0
(i.e. %T = T 100) Absorbance = -log10(%T/100)
= -log10(I1/I0)
= -log10 T
T ≠ 1 - A
Department of Chemistry
Instrumentation37
UV spectra are recorded in solution Cells (cuvette) can be made of plastic, glass
or quartz Quartz - transparent in 200-700 nm Plastic and glass - visible spectra ONLY
Department of Chemistry
Instrumentation
Colorimeter
38
Department of Chemistry
Experiment: 1
Determine the order of reaction of phenolphthalein in alkaline solution
39
Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution
40
Acid base titration Phenolphthalein in alkaline solution
C
C
O
O
HO OH
2 OH -
- H2OC
COO
O
O
(fast)
(Colourless)H2Ph Ph2-
(Pink)
(pH = 0 - 8.2) (pH = 8.2 - 12)
C
O O
OH
COO
OH -
(slow)
POH3-
(Colourless)(pH > 12)
Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution
41
Reaction: Ph2- + OH- POH3-
Rate law: Rate = k [OH-]m [Ph2-]n
The concentration of OH- is largely excess, therefore, can be assumed to remain essentially constant
Rate = k1 [Ph2-]n , where k1 = k [OH-]m
ln[Ph2-] = -k1t + In[Ph2-]oAccording to Beer’s Law, A
= bcA plot of In A against t
Department of Chemistry
UV-Vis Spectrophotometry42
Color of a solution:
Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution
43
UV spectrum of phenolphthalein
Department of Chemistry
Colorimeter44
measure the absorbance at wavelength of 565nm (green) for each coloured solution using your colorimeter.
The measured absorbance is a direct measure of the intensity of the solution’s colour
Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution
45
A plot of In A against t
Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution
46
Half-life (t1/2) determination
Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution
47
To determine the reaction order with respect to [NaOH] k1 = k [OH-]m
ln k1 = ln k + m ln[OH-]
Slope = reaction order with respect to [NaOH]
Department of Chemistry Determine the order of reaction of phenolphthalein in alkaline solution
48
Data logger – for repeating measurements– for storage of the change of absorbance against time
Department of Chemistry
Procedure of using colorimeter49
Select a wavelength filter- Red (660nm), orange (610nm), green (565nm) or blue (468nm)
Record the Absorbance of sample at different time
Plot graph of ln A vs time Plot graph of In k1 vs In [OH-]
Department of Chemistry
50
OHO
O
O
CH3
Experiment: 2
Thin layer chromatography analysis and purification of aspirin by recrystallization
Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization51
Recrystallization For purification of solid samples
solvent,supersaturated, heating
Cooling
Dissolution Filter Crystallization Collection Drying
Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization
Thin layer chromatography (TLC) For determination of purify
52
Chromatography: Separation of compounds by the distribution between two phases – Mobile phase & Stationary phase
Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization53
stationary phase(TLC plate: Silica gel)
Sample mixture
mobile phase (solvent)
Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization54
Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization55
TLC visualization UV lamp (254 nm)
Department of ChemistryThin Layer Chromatography Analysis and Purification of Aspirin by Recrystallization56
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