J . S o p o u š e k ú l o h a
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1
1. Phase equilibria
1.a. Determination of the partition coefficient in the extraction
system
In the extraction systems the three components are the most important: the
solvent (mostly water), the solute, and the extractant (frequently organic solvent).
If the extraction is to be successful, the system must have appropriate phase behaviour.
The miscibility of the solvents and extractant must be negligible and the solubility of the
solute in the extractant as large as possible.
After the addition of the extractant to the solution, two liquid phases are formed: a phase
rich in the solvent (aqueous phase) and the extractant phase rich in the solute (organic
phase). The rate of the equilibrium reaching is proportional to the size of the interfacial
surface, which can be increased by intensive stirring. The extraction yield depends on
the composition of coexisting phases in the extraction equilibrium. The phase
compositions can be predicted from the phase diagram of the extraction system. In
practice, the solute contents are more simply described in both extractant and solvent
liquid phases by the partition coefficient of extraction:
Ex
V
v
ex
V
Ex
V
Ex
Ex
V
V
x
x
V
m
V
mm
c
c
K ⋅=
−
==
1
10
(4.1.)
where symbol Exc in g/l unit and Exx (molar ratio) means the equilibrium concentration
of the solute in the extractant phase, which has the volume ExV . Symbol Vc and Vx
means the equilibrium concentration of the solute in the solvent phase, which has the
volume VV . The m0 mass is equal to the amount of the solute that was originally in
solution (in the aqueous phase) before extraction. Concurrently, the m0 represents the
total mass of the solute in the extraction system. The m1 is the mass of the solute in the
solvent after extraction. The amount of the extracted solute substance transferred to the
organic phase is therefore consistent with the law of preservation of matter 10 mm − .
The partition coefficient is dependent on the temperature, pressure, and overall
composition of the extraction system. The dependence of the coefficient on the overall
composition of the system is neglected in practice for low contents of the solute
substance. However, this simplification does not apply to systems where components
are involved in the reaction equilibria (association, dissociation, dimerization, ...).
It follows from the expression (4.1.) that after extraction, the mass of the solute in the
separated solvent (in aqueous phase) is:
( )ExExV
V
VKV
Vm
m
+
= 0
1 (4.2.)
The separated solute-depleted aqueous phase may be subjected to further extraction.
The separated extractant phase rich in the solute can be used for another purpose.
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If the mutual miscibility of the aqueous and organic phases and the partial molar volume
of the solute in the phases mentioned are negligible, we can deduce the relation for the
n-stage extraction with the same amount ExV of organic solvent:
( ) ( )
n
ExExV
V
ExExV
Vn
n
VKV
V
m
VKV
Vm
m 





+
=
+
= −
0
1
(4.3.)
Yield after n-stage extraction is as follow:
( )
n
ExExV
Vn
n
VKV
V
m
mm






+
−=
−
= 1
0
0
π (4.4.)
Among other it follows that if the partition coefficient of extraction ExK is a constant,
then the n-stage extraction with addition of extractor volume 𝑉𝑉𝐸𝐸𝐸𝐸 leads to a greater yield
than a 1-stage extraction using extractor in an undivided amount 𝑛𝑛⋅𝑉𝑉𝐸𝐸𝐸𝐸. The extraction
yield is usually given as a percentage: π% = 100⋅π𝑛𝑛 .
TASK: Determine the partition coefficient of extraction ExK of the methylene blue
in extraction system water – methylene blue – amyl alcohol. Verify the relationship
(4.3.) for 2-stage extraction. Compare the yield (in %) of 1-stage extraction with 2-stage
extraction using the same total amount of extractor.
LABORATORY AIDS AND CHEMICALS: the stock solution 410­2
g/dm
3
of the solute –
methylene blue (CAS No 122965-43-9) dye in water, extractor – amyl alcohol
(CAS No 71-41-0), laboratory test tube rack, 9 teflon tubes with plugs, 4 scale pipettes
(1, 2, 5, and 10 ml), syringe with long needle or thin hosepipe, 1 volumetric flask (100
ml), 5 volumetric flasks (25 ml), spectrophotometer, wash bottle with ethanol, at least 3
pcs of conventional rod stirrers, or one stirrer with rotary disc on at least 10 test tubes.
INSTRUCTIONS: For time reasons, select this work schedule:
1. Equilibration. Pipette gradually 0.5; 0.75; 1.0; 1.25; 1.5; 1.75; 2.0; 2.5 and 5.0 cm3
of
methylene blue stock solution into 9 tubes (or separating funnels) and add water to
make up total volume 5 cm3
. Add 5.0 cm3
of extractor (amyl alcohol) to each tube. I.e.
each tube will involve a final volume of 10 cm3
(resulting in the extraction systems No
1-9) Pipette 5.0 cm3
of methylene blue stock solution and 2.5 cm3
of extractor into
10th tube (extraction system No n2). Stir the contents of the tubes for at least 20
minutes. Ensure extraction takes place at a constant temperature.
2. Phase separation. After stirring phases in the tubes, let both the clear organic and
aqueous phases separate from the extraction mixtures. Use syringe with pipehose
and suck the upper phase (rich in extractor). Discard the contents of the syringe into
organic waste bottle despite upper phase contains most of the solute. Use the water
rich phase (bottom phase) inside tubes No 1-9 for analysis.
3. Two-stage extraction. Suck the upper organic phase through a syringe from the
extraction system No n2 and pour it into the organic waste bottle. Add again 2.5 cm3
of the extractor and repeat the extraction and phase separation under the same
conditions as done before.
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4. Measurement of calibration dependence. Pipette the volume 50ml of the dye stock
solution into 100ml volumetric flask and make up with water. Use this new dye
solution, pipette successively volumes 2.5; 5; 7.5; 10; 12.5; and 15 cm3
into six 25
cm3
volumetric flasks and fill them with water up to the marks. Measure the
absorbance of the new stock solution and the prepared solutions by a
spectrophotometer at a wavelength corresponding to the maximum absorbance value
of the aqueous extract solution (methylene blue: 660 nm).
5. Measurement of the aqueous phase absorption after extraction. Dilute all water
phases in the tubes after extraction (No 1-9 and n2) with water in a 1:1 ratio (ie
dilution factor = 2). The dilution should remove a possible turbidity in the water
phase. Measure the absorbance of the solute in all samples with a spectrometer. Do
the same for sample No n2 after the second equilibration.
Wash the dishes after experiment with water and rinse them by ethanol.
DATA ANALYSIS: For all experiments No 1-9: Calculate the weights 0m and 1m of
the solute from the concentrations of the dye 0c and Vc in the water-rich phases
before and after extractions. Use the calculated weights ( 0m , 1m ), used volumes ( ExV ,
VV ), and calculate the equilibrium concentrations of the solute Exc in the phases rich in
the extractant. Evaluate the partition coefficients of the extractions ExK for all samples
according to equation (4.1.).
Calculate the weight value 2m using the average partition extraction coefficient obtained
from experiments No 1-9 and the volumes used in the extraction system No n2
according to the relationship (4.3.). Use the calculated value 2m to calculate the residual
concentration of the solute 2c in the water rich phase. Compare the calculated value 2c
with the experimentally measured value for extraction No n2.
Calculate the yield π𝑛𝑛 of two-stage extraction (𝑛𝑛 = 2) according to the relationship (4.4.)
using average partition coefficient of extraction and compare it with the extraction yield
that we would get if we used the same amount of extractor for one-stage extraction
(𝑛𝑛 = 1).
REPORT: Calibration table 1: for all dye solution: pipetted volume, concentration,
absorption. Calibration graph 1: Dependence of the absorption on the solute
concentration in water. Table 2: for each experiment No 1-9 and No n2: the initial
concentration of the extractant 0c and its weight m0 in the aqueous phase prior to the
addition of the extractant, dilution factor of the aqueous phase after extraction,
absorbance after extraction, concentration of the extractant in the water phase
subtracted from the calibration graph, concentration Vc after extraction, weight of the
solute in the water phase m1 and in the organic phase (m0 - m1) after extraction,
experimental yield π1, value of partition coefficient of extraction ExK , average of ExK
from experiment No 1-9. In addition, for experiment No n2 (use average of ExK ):
calculated mass 𝑚𝑚2 and concentration 𝑐𝑐2. Theoretical yield calculation for one-stage
extraction with 5 ml and for two-stage extraction with use of 2-times 2.5 ml of the
extractor. Finally, compare the calculated yield of the 2-stage extraction with the yield of
the experiment No 9.
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