Transcript ert207 analytical chemistry

ERT207
ANALYTICAL
CHEMISTRY
CHAPTER 3 : Preparing
Samples for Analysis
Pn Syazni Zainul Kamal
CO2 : Ability to classify and use
separation techniques and gravimetric
methods for mass determination
What is Analytical Chemistry?
• Concerned with the chemical characterization
of matter and the answer of :
1) What is it (Qualitative) –identification of
elements, ions or compound
2) How much is it (Quantitative)
“everything is made of chemicals. Analytical
chemists determine what and how much”
Separation process
• almost every element or compound
naturally found in an impure state
• these impure raw materials must be
separated into their purified components
before they can be put to productive use
• a separation process is used to transform a
mixture of substances into two or more
distinct products.
• Important in synthesis, chemical industries,
biomedical
• The separated products could differ in
chemical or physical properties, such as
size, or crystal modification or other
separation into different components
• The mixture at hand could exist as a
combination of any two or more states:
solid-solid, solid-liquid, solid-gas, liquidliquid, liquid-gas, gas-gas, solid-liquid-gas
mixture, etc.
Petroleum refinery plant
1st step – petroleum separation into fraction on the
basis of boiling point
• Interferent – chemical sp. that causes a
systematic error in an analysis
• Separation isolate the analyte from
interfering constituents
• Separation can be either complete or
partial
Complete separation
A
B
C
Mixture ABC
Partial separation
Mixture ABC
Mixture AB
C
Classifying separation techniques
Separation based on ….
• Size – filtration, dialysis, ion-exchange
chromatography
• Mass/density – (if there is diff. in the
mass/density) centrifuge
• Complexation reaction – masking
• Change of state – (if an analyte and
interferent in the same phase) – distillation,
sublimation, recrystallization, precipitation
• Partitioning between phase – (analyte and
interferent are in two immiscible phases) –
extraction, chromatography
Filtration – (size)
Centrifuge (mass/density)
• Centrifugation is used when we want to
separate small amounts of suspension.
• The suspension of solid in liquid is poured into a
centrifuge tube, then spin around very fast in a
centrifuge. The spinning motion forces the solid
to the bottom of the tube. Then the liquid can be
poured off from the solid.
• Centrifugation is commonly used in dairies to
separate milk from cream to make skimmed
milk. It is possible because milk has less density
than cream.
Masking – (complexation reaction)
• Used a masking agent which reacts
with chemical species that may interfere
in the analysis.
• Masking agent react rapidly to form
strong complexes with the ion to be
masked
Sublimation – (change of state)
• Some solids can change to vapor state without
melting when heated. When the vapor is
cooled, the solid forms again.
• Eg iodine from a mixture of sand.
• When heated, only iodine changes to vapor.
The vapor changes back to solid on the side of
the funnel.
• Substances which sublime include anhydrous
aluminum chloride, iodine and benzoic acid,
anhydrous iron (III) chloride and anhydrous
aluminum chloride.
Distillation – (change of state)
• When a solution of solid in liquid is heated, the
liquid will evaporates. The hot vapor that formed
can be condensed back to liquid again on a cold
surface.
• A way is to recover water from a salt solution.
The solution is heated and the stream is to be
condensed back to water. The solute and
solvent can both be collected.
Evaporation + Condensation = DISTILLATION
Separation based
on partitioning
between phase
•
2 techniques can be use to separate
analyte and interferent:
1. Extraction (is which a solute is
transferred from one phase to a new
phase)
2. Chromatography (will be discussed in
chapter 6)
Type of extraction
•
•
•
Liquid-liquid extraction
Solid-phase extraction
Continuous extraction
Liquid-liquid extraction
• Very useful for isolate analytes from complex
sample matrices prior to chromatographic
analysis and spectrophotometric determination.
• Method to separate compounds based on their
relative solubility in two different immiscible
liquids. (water & solvent)
• Liquid-liquid extraction involve distribution of a
solute between two immiscible liquid phases
• ‘Solvent extraction’ – perfume, biodiesel, vege
oil
Distribution coefficient
• If a solute is in aqueous phase and is extracted into
an organic phase:
• A solute S will distribute itself between two phases
(after shaking/mixing).
• Ratio of [S] in the two phases will be constant.
• KD – distribution coefficient
KD = [S]1
[S]2
solvent 1 eg. organic solvent
solvent 2 eg. water
Distribution process in liquidliquid extraction
• Using separatory funnel
• The mixture is put into a separatory funnel,
• The mixture is shaken for about a minute, and
the phases are allowed to separate.
• A solute is extracted from an aqueous solution
into an immiscible organic solvent.
• A solute is an organic compound, will distribute
from water into organic solvents.
• The principle is “like dissolves like”.
• at bottom layer, the solvent is denser and will be
drawn off after the separation is completed.
Liquid-liquid extraction
The distribution ratio
• Many substances, partially ionized in aqueous
layer as weak acid
• Eg extraction of benzoic acid from an aqueous
solution. Part of HBz in aqueous layer exist as
Bz-.
• HBz is a weak acid in water with ionization
constant Ka
• Distribution ratio: is the ratio of the
concentrations of all the species of the solute in
each phase.
D=
[HBz]e
Organic solvent
[HBz]a + [Bz-]a
Aqueous solvent
• Ka = [H+]a[Bz-]a
[HBz]a
• Equation that relates D, KD and Ka is
D=
KD
1 + Ka/[H+]a
• the extraction efficiency will be
independent of the original concentrations
of the solute.
Percent extracted
• Fraction of the solute extracted will depend on
the volume ratio of the two solvents
• Fraction of solute extracted is equal to
milimoles of solute in the organic layer divided
by the total number of milimoles of solute.
• The milimoles are the molarity times the
mililiters.
• Therefore, the percent extracted is given by,
%E =
[S]oVo
X 100%
[S]oVo + [S]aVa
• Vo and Va are the volumes of the organic and
aqueous phases, respectively.
• Relation between %E and D is given by,
%E = 100D
D + (Va/Vo)
If Vo = Va, then
%E = 100D
D+1
• If D is less than 0.001, the solute can be
considered quantitatively retained.
• The percent extracted changes only from
99.5% to 99.9% when D is increased from 200
to 1000.
Example 1
20 ml aqueous solution of 0.10 M butyric
acid is shaken with 10 ml ether. After the
layers are separated, it is determined by
titration that 0.5 mmol butyric acid remains
in the aqueous layer. What is the
distribution ratio, and what is the percent
extracted?
Solution
Starting in aqueous = 20 ml x 0.10 M BA = 2.0 mmol BA
Left in aqueous sol.
= 0.5 mmol BA
Extracted in ether
= 1.5 mmol BA
[BA]e = 1.5 mmol/10ml
= 0.15M
[BA]a = 0.5 mmol/20ml
= 0.025M..therefore
D = 0.15
0.025
%E =
= 6.0
100D
= 100 x 6.0
= 75%
D + (Va/Vo)
6.0 + (20/10)
Solid-phase extraction
• Liquid-liquid extraction – extracting solvents
used limits to those that are water immiscible,
Large volume of solvents used – waste
disposal problem
• SPE – become widely used for sample cleanup
prior chromatographic analysis
• compounds that are dissolved or suspended in
a liquid mixture are separated from other
compounds in the mixture according to their
physical and chemical properties.
• SPE uses the affinity of solutes dissolved
or suspended in a liquid (mobile phase) for
a solid through which the sample is
passed (stationary phase) to separate a
mixture into desired and undesired
components.
• The result either - desired analytes of
interest or undesired impurities in the
sample are retained on the stationary
phase.
• The stationary phase comes in the form
of a packed syringe-shaped cartridge
• Solid phase extraction cartridges and
disks are available with a variety of
stationary phases, each of which can
separate analytes according to different
chemical properties.
• Most stationary phases are based on
silica that has been bonded to a specific
functional group.
• Some of these functional groups include
hydrocarbon chains of variable length
(for reversed phase SPE), quaternary
ammonium or amino groups (for anion
exchange), and sulfonic acid or carboxyl
groups (for cation exchange)
Working principle
Conditioning
• Powdered phase (sorbent extraction bed)
placed in a small cartridge
• Powdered phase eg silica bonded with
hydrophobic phase, must be conditioned in
order to interact with aqueous sample
• Accomplish by passing methanol through the
sorbent bed.
• This penetrate into bonded layer and permit
water mol. and analyte to diffuse into bonded
phase
• Water is passed to remove excess solvent prior
to adding sample
Loading sample/retention
• The sample (dissolve in liquid) is passed
through sorbent extraction bed to separate a
mixture into desired and undesired
components.
• Analytes adsorbed to sorbent extraction bed
Rinsing/washing
• Rinse the column to remove undesired
components
Elution
• Remove desired analytes
Continuous extraction
• For component of interest that has
unfavorable partition coefficient (desired
compound insoluble in solvent).
• Extraction is accomplished by continuously
passing the extracting phase through the
sample until a quantitative extraction is
achieved
• Involving solid samples are carried out with
a Soxhlet extractor
Working principle
• solid material containing some of the desired
compound is placed inside a thimble made
from thick filter paper, which is loaded into the
main chamber of the Soxhlet extractor.
• The Soxhlet extractor is placed onto a flask
containing the extraction solvent.
• The Soxhlet is then equipped with a
condenser.
• The solvent is heated to reflux.
• The solvent vapour travels up a
distillation arm, and floods into the
chamber housing the thimble of solid.
• The condenser ensures that any solvent
vapour cools, and drips back down into
the chamber housing the solid material.
• The chamber containing the solid
material slowly fills with warm solvent.
• Some of the desired compound will then
dissolve in the warm solvent.
• When the Soxhlet chamber is almost full,
the chamber is automatically emptied by
a siphon side arm, with the solvent
running back down to the distillation
flask.
• This cycle may be allowed to repeat
many times, over hours or days.
• After extraction the solvent is removed,
typically by means of a rotary
evaporator, yielding the extracted
compound.
• The non-soluble portion of the extracted
solid remains in the thimble, and is
usually discarded.