CHEM2110 Applied Analytical Chemistry

Introduction

Solvent extraction, also known as liquid-liquid extraction, as the name suggests is a method of separation utilized in industrial scale that is dependent on transfer of components under separation from one liquid phase to another liquid given it is immiscible with the former. At equilibrium alongside constant temperatures, the solute concentration ratio of the two immiscible liquid is constant.

Metal ions separation through solvent extraction, where they are moved from an aqueous solution and deposited in an immiscible solvent, only occur if uncharged, hence there is formation of organic-soluble species. This may arise from two perspectives, formation of a neutral molecule through complexation or formation of strong ion-pair. Complexation is an essential aspect metal ions as well as strongly coordinating ligands. Highlighted below are the equilibria involved:

Moreover, because D is associated with a term in [H⁺], hence D is expected to vary with pH. Therefore, ions of the involved metal at one pH are completely extracted, and not at all another. The most suitable range of pH vary with solvents used, ligand, and metal ion. Below is an exhibition in a graph of how   and  extraction from water by 8-hydroxyquinoline should behave.

In this experiment, we demonstrate extraction of  and by solvent extraction method from an aqueous solution into an organic immiscible liquid by forming a complex metal with 8-hydroxyquinoline. A colored complex is formed when the particular ions of the metal combine with 2 or 3 8-hydroxyquiline, which is detectable by UV-visible spectrophotometer.

Procedure

Note to take: to achieve excellent results in this experiment, careful analytic skills are needed.  The analytic glassware used should be clean. Prior pipetting, the solution should be decanted into a clean beaker, avoid contamination of the reagent bottles.

Safety precaution: caution must be taken when handling dichloromethane since it is very toxic. Separation must be done a fume hood while wearing gloves. Proper disposal of dichloromethane residue into halogenated residue bottle.

Extracting and detecting

From the figure above,  extraction should be done completely with pH under 4 under fume-hood. Take 20 milliliters of 0.0002 molar of   in 3.6 pH buffer and 20 milliliters of 0.002 molar 8-hydroxyquinoline in dichloromethane, put them in 100 milliliters seperatory funnel and shake. Leave out the mixture to settle and separate then take out the lower layer, which is blue-green, and place it in a flask which is covered to prevent dichloromethane solvent evaporation.

With the help of quartz, record electronic spectrum measurements from 300-500nm against 8-hydroxyquinoline solution baseline scan. Here we use UV/Vis spectrometer with a lid cover on the cell as soon as the solution is placed inside to prevent dichloromethane layer evaporation. The task is to record the maximum wavelength and the corresponding absorbance. Due to unbound ligand, massive spike may be seen on the spectrum, after the spike record the wavelength with the highest absorbance.  The same cell as well as the spectrometer used here should be used throughout the remaining portions experiment.

Extracting and detecting

From the figure above, extraction should be done completely with pH of 7.0. Take 20 milliliters of 0.0002 molar of  in 7.0 pH buffer and 20 milliliters of 0.002 molar 8-hydroxyquinoline. Dichloromethane layer is collected as in part A, the color will be distinct from the observed with the case of. Taking out of measurements and recording are done as in part A again.

Ligand concentration effects on  extraction

Take a 50 milliliters volumetric flask, pipette 25 milliliters of provided 0.002 molar solution and fill to the mark with dichloromethane to make a dilute solution of 8-hydroxyquinoline. With this dilute solution, the whole procedure from extraction to measuring absorbance in part A is repeated.

From the provided stock solution 0.004 molar , make a 100 milliliters 0.0004 solution. Using pipette, mix 10 milliliters of  solution with 10 milliliters of buffer each of the provided buffers pH from 3.6-7.0. In a 100-milliliter separatory funnel, mix the resulting 20-milliliter  solution with 20 milliliter of 0.002 molar 8-hydroxyquinoline.

Allow some time for the layers to separate, and then dichloromethane layer is collected in a covered conical flask. The absorbance for each particular solution is measured using maximum absorbance wavelength determined in part B.

PH is dependent on solvent extraction. In this process,  is exchanged with hydrogen ion. The aqueous phase is turned back to its original acidity and as well recycled to the step of leaching in the process. In the meantime, copper carrying organic phase is stripped of the Cu through conduction of with strong acidic aqueous solution where Cu is transferred to aqueous phase, where else organic phase reconstitution to hydrogen form occurs. 

Reference

De, A. K., Khopkar, S. M., & Chalmers, R. A. (1970). 

Rydberg, J. (Ed.). (2004). Solvent extraction principles and practice, revised and expanded. CRC press.

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