Analytical Chemistry II
SEPARATION METHODS
Ideal analytical methods would enable species to bedetermined directly in various matrices. Only few analytical measurements are specific for a single species. The major problem in quantitative analysis is the elimination of interferences and these can be eliminated by:
Use of masking agents
Isolation of the analyte in separate phase from the interfering species using various techniques
SOLVENT EXTRACTION
Solvent extraction or liquid –liquid extraction is a technique in which a solution( usually aqueous) brought into contact with a second solvent (usually organic) essentially immiscible with the first in order to bring about the transfer of one or more solutes into the second solvent. Whether the organic liquid is the upper or lower layer depends on the density of the liquid. Solvent extraction is usually carried out in a separatory funnel.
For a solute A distributed between two immiscible phases a and b the distribution law (Partition) states that , provided its molecular state is the same in both liquids and the temperature is constant
KD = Concn of solute in solvent a/Concn of solute in solvent b
=[A]a/[A]b
=distribution or partition coefficient.
The law does not apply when the species undergoes association or dissociation in either phase.
In solvent extraction the interest is the fraction of the total solute in one phase regardless of its mode of association ,dissociation and interaction with other dissolved species.
Hence
D =(CA)a/CA)b =Distribution ratio.
Where CA denotes the concentration of A in all its forms analytically determined
Common problem in solvent extraction is the number extractions to be carried out in order to quantitatively extract a given analyte.
Suppose V cm3 of an aqueous phase contains Xo g of a solute to be extracted n times with v cm3 portions of a given solvent , the then weight of solute Xn remaining in the aqueous layer is given by the expression
Xn =Xo(DVDV+v )n
D =Distribution ratio
% E =( 100DD+V/v)
%E=percent extraction.
Example
Suppose 0.1g of iodine in 50 cm3 of water is shaken with 25 cm3 of CCl4. The distribution coefficient of iodine between water and CCl4 at ordinary temperature is 1/85. Compute the weight of iodine left in the aqueous layer after one extraction with 25 cm3 and after three extractions with 8.33 cm3 of CCl4
Solution
Suppose x1 g remains in the water layer
Concn of solute in water =x1/50 g cm3
Concn inof solute in CCl4 =0.1-x125 g cm3
D =Concn in waterConcn in CCl4 =x1/500.1-x1/25=1/85
X1=0.00230g
Concn after 3 extractions
X3 =0.1(185x505085+8.33)3 =0.000029
Factors that affect solvent extraction
Selectivity of the reagent
The pH of the system
The solvent.
Applications of solvent extraction
e.g
1.determination of Pb in vegetation
ash the vegetation in a furnace and dissolve ash in acid or mixture of acids
alternatively carry out acid digestion
adjust the pH of the system with masking agents e.g EDTA or thiocyanide to remove interferences
add about 5cm3 of dithizone (about 0.05%in CHCl3)
add about 15cm3 CHCl3
shake for 1 minute in a separatory funnel
separate layers and measure the absorbance of theCHCl3 layer in UV-Visible spectrometer at 510nm
carry out same procedure for standards to obtain a calibration curve.
From the curve and the absorbance of the unknown the concentration of Pb in the unknown is determined.
2. Ni can be determined by extraction with dimethylglyoxime(DMG) in alkaline medium. The nickel –DMG complex can be extracted and its absorbance measured at 366nm .
3. Copper can be determined in environmental samples using Neo-cuproin (2,9-dimethyl-1,10-phenanthroline) into chloroform and then measured at 457nm.
When the distribution coefficient is very low batch extraction becomes ineffective and therefore a continuous extraction is used e.g
a)Use of soxhlet extractor for continuous extraction of a
Solid
b)The use of countercurrent extraction
GAS CHROMATOGRPHY
Gas –liquid chromatography accomplishes a separation by partitioning a sample between a mobile gas phase and a thin layer of nonvolatile liquid held on a solid support.
Gas –solid chromatography employs a solid adsorbent as the stationary phase.
Gas chromatography differs from other forms of chromatography in that the mobile phase is a gas and the components are separated as vapours.
It is thus used to separate and detect small molecular weight compounds in the gas phase.
The sample is either a gas or a liquid that is vapourized in the injection port.
The mobile phase for GC is a carrier gas ,typically He,N or H. The mobile phase is usually of low molecular weight and chemically inert.
The pressure applied forces the mobile phase to move through the column and separation is accomplished using a column coated with a stationary phase.
The equilibrium for GC is partitioning and the components of the sample will partition or distribute between the stationary phase the mobile phase.
Compounds that have affinity for the stationary phase spend more time in the column and this elutes later and have a longer retention time (tR) than samples that have a higher affinity for the mobile phase.
Affinity for the stationary phase is driven mainly by intermolecular interactions and the polarity of the stationary phase can be chosen to maximize interactions and thus the separation.
he pressure is applied and the mobile phase moves the analyte through the column. The separation is accomplished using a column coated with a stationary phase.
Image Source: Bitesize Bio.
Instrumentation
A GC consists of basically ;
a carrier gas in a high pressure cylinder
sample injection port/system-liquid samples are injected with a microsyringe. Volume of samples is the order 0.1-0.2 ml.
column : this is known as the heart of the GC as the separation occurs here. Usually made of metal or glass. Could be U-shaped, coiled or spiral in shape and the length could be 1m or more for capillary columns. The column contains the stationary phase which is a liquid and could be: (a) non-polar such as paraffin, squalene, silicone greases etc. these help separate components based on their boiling points.
(b) intermediate polarity-these contain a polar or polarizable group on a long non-polar skeleton which can dissolve both polar and non-polar solutes e.g diethyl hexyl phthalate is used for the separation of high boiling alcohols
(c) polar such as carbowaxes . Used for separating polar and non-polar substances.
Solid supports: these should be inert but capable of immobilizing a large volume of liquid phase as a thin film over its surface.
Large surface area is required to ensur rapid attainment of equilibrium between stationary and mobile phases
Should be strong to resist breakdown in handling e.g Diatomaceous earth, Kieselguhr etc.
Glass beads with low surface area and low porosity an be used.
Detector : could be (a) thermal conductivity cell
(b) flame ionization detector (c) electron capture detector.etc.
(v) Recorder or printer: records peaks of the elution of the components of the mixture.
The separation in GC can be compared to fractional distillation in which it is assumed that the distillation process occurs in stages along the length of the distillation column .
At each stage equilibrium is reached between high and low-vapour pressure components.