Recapitulation, factors affecting the solvent extraction of inorganic species,

1. Dnyanasadhana College, Thane.Department of ChemistryM.Sc. Analytical Chemistry Sem-I UNIT- 3.2 Solvent extractionDr.G.R.Bhgaure

2. 3.2 Solvent extraction Recapitulation, factors affecting the solvent extraction of inorganic species, Separation Of Metal Ions As Chelate, concept of [pH]1/2 and its significance, ion association, solvation with suitable examples, Craig’s counter current extraction: principles, apparatus and applications, Use of crown ethers in solvent extraction.

3. Terms • Chelates: When bidentate or polydentate ligand is attached through two or more doner atoms to the same central metal ion forming a ring structure ,the resulting complex is called as chelate and the ligand is called as chelating ligand. • Chelation: The process of formation of chelate is called as Chelation. • Partition: Transfer of solute from one phase to other phase is called as partition. • Distribution coefficient: It is an equilibrium constant obtained by applying law of mass action to the equilibrium reaction. • Solute(aq.)- --- Solute(org.) < -------- • [Solute] (org) • D=--------------------- • Solute(aq.)

4. Dielectric constant: The capacity of solvent to separate the oppositely charged particle of solute is called as called as Dielectric constant. • .Ex. Water is having High Dielectric constant (DH20=78.5) ,if any solute is added in water the amount of work required to separate the charged particle is less. • Similarly if any solute is added in acetic acid the amount of work required to separate the charged particle is more. • Ion Pair: An neutral species formed by electrostatic interaction of a cation and an anion in a solvent of low dielectric constant which does not encourages separation of ions as does water.

6. Solvent extraction • Solvent extraction is based on the principle that when a solute is brought in contact with two immiscible solvents, one of which is invariably water and the other organic, the solute distributes itself in a fixed ratio in the two solvents. • The technique finds application in separation, purification and enrichment. • There is another important aspect of solvent extraction in the form of extractive Spectrophotometry. • The absorbance of coloured metal complexes, particularly metal chelates, extracted in the organic phase is measured. • It is also used to concentrate the metal, separate it from interferences and develop the absorbing system in a single step.

8. Criteria For Selecting Organic Solvent For Solvent Extraction i) The organic solvent should be immiscible with the aqueous phase. ii) It should have good stability. This will amount to the fact that the organic phase should be capable of withstanding many recycling operations in a solvent extraction circuit without degeneration. iii) A difference in densities of the contacting phases is essential and should be as great as possible. iv) Solvent should have low viscosity . Hence, low viscosity is a desirable property. Dissolution of the Extractant in low viscosity diluents modifies this property to a favorable degree.

9. v) The interfacial tension should be high for rapid coalescence. vi) The solvent should cause no corrosion difficulties with common materials of construction to reduce the cost of equipment. vii) The solvent should have low toxicity, high boiling and flash points. These mainly avoid environmental pollution and fire hazards. viii) It should have high metal loading capacity. ix) It should be easily stripped of the loaded metal.

10. Factors Influencing Extraction • pH • Molarity of the Acid • Metal Ion Concentration • Presence of Salting out Agents • Presence of Masking (Sequestering) Agent • Concentration of the Extractant • Nature of Diluents

11. pH a) pH • In solvent extraction the equilibrium pH i.e., the pH attained after the two phases have been contacted to equilibrium is an important which affect on separation of extracting species. • The pH will be of great significance in extraction systems listed in the classification scheme under “Extraction by compound formation”. • The extraction by chelating agents, • Extraction by carboxylic and sulphonic acids and acidic organophosphorus compounds . • All the above process are susceptible to pH variation. • The effect of pH on extraction can been seen from the following diagram.

12. 0 1 2 3 4 5 6 7 8 9 10 Qualitative extraction curves for metal dithizonates

13. Molarity of the Acid • The study of the effect of Molarity of the acid on extraction is carried out on the extraction of metals by salvation or ion – pair formation. • These investigations are mainly related to the mineral acids and that too mostly to HCl and HNO3.

14. Effect of Metal Ion Concentration • The effect of the metal ion concentration on the distribution ratio of the metal is negligible. • This will mean that both tracer and macro amounts of metals may be expected to extracted to the same extent under similar equilibrium conditions provided the solubility of the extracting species in the organic phase is not exceeded. • The relationship between M(aq.) and M (org.) with the increasing metal ion concentration is used to find the loading capacity of the extractant. • These plots known as loading curves or extraction isotherms for the extraction of Ti (IV), V (IV), Fe (III), Cu (II) and Zn (II) in toluene solution of Cyanex 923 are shown in Fig. 3.8.

15. [Metal ion] (organic) -1 -2 -3 ---------Fe (III) --------Ti (IV) ---------V (IV) -3 -2 -1 [Metal ion ] (aq)

16. In all these plots, the linear part of the curve means that the extracting species does not change with the increasing metal ion concentrations, thereafter, at a certain point, • The loading condition sets in and no further increase in the metal content of the organic phase is observed. • From this, the amount of the metal ion that can be loaded on a particular amount of the extractant can be calculated and the results expressed in terms of molar ratio. • Sometimes from this, you can infer the Stoichiometry of the extracted species.

17. Presence of Salting out Agents • When inorganic salts are added more and more amount of solute transfer to organic phase this is called as salting and inorganic substances are called as Salting out Agents • The salting out effect is explained on the activity of the distributing species and strong ability of these ions to bind water ( hydration) thereby depleting aqueous phase of the water molecules to compete. • The magnitude of enhancement in extraction by the added salt depends upon the charge and ionic size of the cation for a given anion.

18. Aluminium and ferric salts are stronger salting out agents than ammonium salts but analytically, the ammonium salts are more convenient because it is easier to remove them in the aqueous phase by repeated evaporation with HNO3 and HCl. • Generally, large amounts of these salts are added.

19. Presence of Masking (Sequestering ) Agents • The substances which can form complex with a particular metal ion to prevent from taking part in their usual reactions are called Masking agents also known as sequestering agents. • They are mainly used to prevent particular metals from taking part in their usual reactions and therefore, the interference of the undesirable elements is removed without the actual separation step. • The masked metal forms a water soluble complex most often negatively charged. • In solvent extraction, the masking agents are used to prevent certain metal ions from forming extractable complexes and thus, they increase the selectivity. • The use of masking agents like cyanide, tartarate, citrate, fluoride and EDTA is restricted largely to metal chelate extraction systems. In highly acidic solutions encountered in many extraction systems, most of the masking agents do not function effectively.

20. Concentration of the Extractant • Extraction increases with the increasing Extractant concentration. • Invariably, a straight line is obtained in the plots of log [Extractant] vs log D. • The slope of the straight line corresponds to the number of Extractant molecules involved in the formation of the extracting species.

21. Log

22. Liquid -Liquid Extractions Involving Metal ChelatesOR Separation Of Metal Ions As Chelate, Principle:One of the most common applications of a liquid–liquid extraction is the selective extraction of metal ions using a chelating agent. • During the process of Chelation chelating agent undergoes ionization, the ionisable proton is displaced by the metal ion when the chelate is formed, and the charge on the organic compound neutralizes the charge on the metal ion. • Due formation of chelates size of the complex increases, this complex becomes hydrophobic in nature because of this more and more solute transferred in organic solvent. • As most of the chelating agents are less ionize in water for these reasons the chelating agent is added to the organic solvent instead of the aqueous phase.

23. Factors affecting chelate formation • Basic strength of chelating group • Electronegativity of donor group • Ring size • Nature of central metal ion • Resonance and steric effect

24. Basic strength of chelating group • The stability of chelate complex formed with the given metal ion generally increases with basic strength of the donating agent as measured by the pKa values. pKa values decides the stability of the chelate complex .

25. Electronegativity of donor group • Electronegativity of basic group (donor group)in the reagent is also important .For transition elements atoms of low electronegativity tends to form stronger bond. • For.Ex. Dithiozone with sulphur as doner atom forms a better chelate than diphenyl carbazone with oxygen atom as doner atom.

26. Ring Size • Size of chelate complex is also important. • 5 or 6 membered ring chelates are more stable as they have minimum steric hindrance and no strains. The functional group of ligand must be situated that they permit the formation of stable ring. The chelate stability increases with no. of rings

27. Nature of central metal ion • For metal ions charge to size ratio i.e. ionic radius decides the stability of the complex. • As this ratio increases more stable complex is formed.

28. Resonance and steric effect • The stability of chelate structure is enhanced by the contribution of resonance structures of the chelate rings. • Thus copper acetyl acelanto complex has greater stability than copper chelate of salicyldoxime.

30. Examples of metal chelates extractions

31. 8-Hydroxyquinoline (oxine) • Oxine is a versatile organic reagent and forms chelates with many metallic ions. The chelates of doubly and triply charged metal ions have the general formula • M (C9H6ON)2 , M (C9H6ON)3 , M (C9H6ON)4. • Oxine is generally used as a 1 per cent (0.07 M) solution in chloroform, but concentrations as high as 10 per cent are advantageous in some cases (e.g. for strontium). • 8-Hydroxyquinoline, having both a phenolic hydroxyl group and a basic nitrogen atom, is amphoteric in aqueous solution; it is completely extracted from aqueous solution by chloroform at pH < 5 and pH > 9; the distribution coefficient of the neutral compound between chloroform and water is 720 at 18 OC. • The usefulness of this sensitive reagent has been extended by the use of masking agents (cyanide, EDTA, citrate, tartrate, etc.) and by control of pH

32. Dimethylglyoxime. • Dimethylglyoxime: The complexes with nickel and with palladium are soluble in chloroform. The optimum pH range for extraction of the nickel complex is 4-12 in the presence of tartrate and • 7-12 in the presence of citrate (solubility 35-50 ug Ni mL-' at room temperature); if the amount of cobalt exceeds 5 mg some cobalt may be extracted from alkaline solution. Palladium(I1) may be extracted out of ca 1M-sulphuric acid solution.

33. 1-Nitroso-2-naphthol. • 1-Nitroso-2-naphthol reagent forms extractable complexes (chloroform) with • Co(III) in an acid medium and • with Fe(II) in a basic medium.

34. Cupferron (ammonium salt of N-nitroso-N-phenylhydroxylamine). • The reagent is used in cold aqueous solution (about 6 percent). Metal cupferrates are soluble • in diethyl ether and in chloroform, and so the reagent finds wide application in solvent-extraction separation schemes. • Ex. Fe(III), Ti, and Cu may be extracted from 1.2 M HC1 solution by chloroform: numerous other elements may be extracted largely in acidic solution.

35. Extraction by Solvation • In case of extraction by solvation the solvent it self participate in the extraction of complex. • The basic character of oxygen atom enables the incorporation of the solvent molecule in the coordination sphere of metal ion to form solvated complex which is extractable. • Ex.Fe(III) forms chelate complex of composition H[FeCl4] in HCl solution which can be extracted with verity of solvent possessing doner O atom such as diethyl ether, ethyl acetate, butyl acetate etc. The amount of metal extracted depends upon concentration of acid and passes through max. amount of 6M HCl. • Fe3+ HCl [HFeCl4] this complex formed is extracted by water molecule i.e. in aq.phase. Hence solvent like diethyl ether is added to water are replaced .Hence it passes into organic phase. The ability of oxygenated solvent complex with water molecule for acidic metal ion depends on electron donating ability that is bacicity of ‘O’ or any other doner atom. • Such bacicity in turn depends on steric availability of electrons. The comparative strength of water may be reduced by using high concentration, which by mass action to shift equilibrium .

36. Crown ethers • Crown ethers are cyclic  chemical compounds that consist of a ring containing several ether groups. • Crown ethers find the application to form stable complexes with a number of metal ions, particularly the alkali metal ions. • These crown ethers are macrocyclic compounds containing 9-60 atoms, including 3-20 oxygen atoms, in the ring. • Complexation is considered to result mainly from electrostatic ion-dipole attraction between the metal ion situated in the cavity of the ring and the oxygen atoms surrounding it. • The ion-pair extraction of Na+, K+ and Ca2+ with some organic counter-ions and dicyclohexyl-18-crown-6 as complex-forming reagent has been described.

37. Crown ethers

38. The first number in a crown ether's name refers to the number of atoms in the cycle, and the Second Number refers to the number of oxygen atom. 12-crown-4 18-crown-6 Dibenzo-18-crown-6 15-crown-5 diaza-18-crown-6

39. Affinity for Cations • Crown ethers strongly bind certain cations, forming complexes. The oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic. The resulting cations often form salts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in phase transfer catalyst. • The denticity of the polyether influences the affinity of the crown ether for various cations. • For example, 18-crown-6 has high affinity for potassium cation, 15-crown-5 for sodium cation, and 12-crown-4 for lithium cation. The high affinity of 18-crown-6 for potassium ions contributes to its toxicity.

40. Affinity for amines • Apart from its high affinity for potassium cations, 18-crown-6 can also bind to protonated amines and form very stable complexes in both solution and the gas phase. • Some amino acids, such as lysine, contain a primary amine  on their side chains. • Those protonated amino groups can bind to the cavity of 18-crown-6 and form stable complexes in the gas phase. Hydrogen-bonds are formed between the three hydrogen atoms of protonated amines and three oxygen atoms of 18-crown-6. • These hydrogen-bonds make the complex a stable adduct.

41. Concept of [pH]1/2 and its significance, ion association, solvation with suitable examples

42. 2. Extraction by ION ASSOCIATION COMPLEXES • An alternative to the formation of neutral metal chelates for solvent extraction is that in which the species of analytical interest associates with oppositely charged ions to form a neutral extractable species. • Such complexes may form clusters with increasing concentration which are larger than just simple ion pairs, particularly in organic solvents of low dielectric constant. The following types of ion association complexes can be discussed .

43. The ion pair formation is the process in which uncharged species are formed which can be easily extracted by organic solvents. • Ion pair formation process can be represented as ; • A++B- A+B- --- The value of ion pair formation constant is related to dielectric constant. Where :K= Boltzman constant a-= empirical parameter e= charge T= Temperature €= dielectric constant The decrease in die electric constant of aqueous phase by addition of salt fevours the ion pair formation. The important step in mechanism of extraction process is distribution of unextractable species in organic phase.

44. Factors affecting extraction by ion pair association • Salting effect • Dielectric constant of solvent and temperature • Size of anion • Nature of doner atom

45. Salting effect • Low dielectric constant of a solution fevours the ion pair association. Hence during extraction salt is added to aqueous phase which decreases the dielectric constant of aq. Phase which in turn facilitate the ion pair association this effect is called as salting effect

46. Dielectric constant of solvent and Temperature • In case of solvents with high dielectric constant ion association increases with increase in temperature. While in case of solvents with low dielectric constant ion pair formation decreases with increase in temperature.

47. Size of anion • The size of anion plays an important role in ion pair formation. As the size of anion increases the ion pair formation process also increases. • The ion association increases with anion as per the following order. • [Na+ NH2-]< [Na+NH-C6H5] < [Na+ N(C6H5)2-]

48. Nature of doner atom • The extent of ion pair formation also changes with nature of doner atom. • [Na+OC6H5] < [Na+NHC6H5] < [Na+SC6H5] • The order of doner atom for association is • O<N<S

49. Craig’s counter current extraction: principles, apparatus and applications, • Lyman C.Craig • Counter Current Extraction: • A solvent extraction technique in which two immiscible liquids move in opposite directions in continuous contact with each other with a resultant separation of solutes.

50. In this extraction process one solvent, generally lighter one ( low density) moves by steps through a series of equilibration vessels in which it comes in contact with stationary portions of the other heavier portion. • Solutes separates on the basis of differences in their partitioning between two solvents