RAPID EXTRACTION OF COPPER (11) WITH l-PHENYL-3-METHYL-4-NITROBENZOYL- 5-PYRAZOLONE IN DIFFERENT ORGANIC DILUENTS

Abslmct Sobent extraction of Copper (11) using I-Phenyl-3-methyla-nitrobenzoyl-5-pyrazolone (HNPZ) in various organicsolvents has been studied. The extractablespecies has been identified by investigation of log-log plot of distribution ratios and reagent concentrations. Job's method and mole ratiomethd confirmed the mole ratio Cu:HNPZ to be 1:2. Maximum and reproducible results were obtained a?pH 4-6, and equilibration wasachieved within fewseconds. Wealso observed that thecapacity of the organicloading wassignificantly higherwhen chlorofoim and benzene were used assolvents. Equilibrium extraction constant, log Kex, yas detennined from experimental data as - 1.60.


INTRODUCTION
/9diketones have been extensively and successfully employed as chelating agents in the extraction of metals. But of these, thenoyltrifluoroacetone (=A)', acetylacetone (AA) and trifluor~acet~l-acetone (TFA) 2 ' 3 are more widely used.
Recent1 however, acyl pyrazolones and 4-benzoyl-3-methyl-1-phenyl-5-pyrazolone P (BMPP have been introduced. Much of the recent work reported in the literature has been done with BMPP rather than its derivatives. In this paper, the results of our study of solvent extraction of Cu(I1) with the nitro-derivative, l-phenyl-3-methyl-4nitrobenzoyl-5-pyrazolone is presented. -Rao et a15 have previously used this for the extraction of U(V1). The presence of a nitro group in the ligand is expected to enhance the acidic properties of the reagent and hence its extractive property. Since the pKa values of BMPP and HNP are comparable, HNPz is expected to be equally good in solvent extraction. The liquid-liquid extraction behaviour of copper (11) with the ligand, and the dependency of the extraction processes on pH, ligand concentration and contact time are reported here.

Apparatus and Reagents
The intermediate compound, 3-methyl-1-phenyl-5-pyrazolone (MPP) was synthesized from analytical grade ethylacetoacetate and phenylhydrazine. HNFz was prepared subsequently from MPP and high grade p-nitrobenzoylchloride (BDH) and purified as described elsewhere for the acyl pyrazolones. Stock solutions of Cu(I1) were prepared by dissolving O.098g of analytical grade CuS045H20 in 250mlO.lM HCl or 0.1M HNOJ solution. Further dilutions were made as required. Solutions of 0.02M HNFz were prepared in different organic solvents, and further dilutions were made as required. Standard buffer solutions of HCI/KCI, HAcINaAc and NaOHboric acid were used in the pH range 1 -8. Apart from the pH variation studies, all experiments were conducted at pH 4.0. In carrying out the studies, the aqueous phase was prepared in each case to maintain constant ionic strength of 0.10 usingsodium sulphate (Na2S04.10H20) solution. The ionic strength is considered to have negligible effect on the extraction equilibrium constant, GX in the pH range and total ionic concentrations studied.
The ligand has been characterized by elemental analysis, IR, UV-Visible and melting point measurements. The cream coloured pure product was found to have a melting point of 164 + 0.5O~. . . PYE Unicam SP6-400 and Shimadzu-408 IR Spectrophotometers were used for spectrophotometric measurements. 1n all cases, the initial and equilibrium pH were recorded using a pH meter standardized with a buffer solution of pH 4.0.
Extraction Procedure 5ml aliquot of Cu(I1) solution (50ppm) and similar volume of 0.02MHNFz solution in chloroform, benzene, toluene or ethylacetate were mixed and agitated by means of a mechanical stirrer for 10 mins. However, equilibration was a&ieved within a few seconds. After allowing for the separation of the two phases, the amount of copper remaining in the aqueous phase was determined spectrophotometrically at 440 nm using benzoin-a-oxime? The concentration of copper extracted into the organic phase was obtained by material balance. The distribution ratio, D, of copper was determined as the total concentration of it in the organic phase divided by the total concentration in the aqueous phase of two mutually equilibrated sensibly immiscible liquid phases.

Absorption Curves
In order to determine the molar absorptivities and wavelengths of maximum absorption for HNPz and Cu(I1)-nitrophyrazolonate complex, the latter was isolated in its solid from mixing 0.808g (0.0025mol) of CuS04.5H20 in 50ml of distilled water and 1.6166g (0.005mol) of HNP, in 50ml of ethanol. The appearance of yellow-green precipitate was instantaneous. Figure 1 shows the absorption spectra of HNPz and Cu(1I) complexes. The appearance of maximum absorption at similar wave lengths ruled out the possibility of spectrophotometric studies of the equilibrated organic phase for copper concentrations. Hence the extraction studies were spectrophotometrically done in the aqueous phase. The molar absor tivities for the ligand and copper complex at 360nm and 370nm P -1 -1 were 5 . 4~1 0~ and 3.3~10 lmol crn respectively.

Effect of pH
The results are given in Figure 3. Experiments conducted to study the effect of variation of pH on the extraction of copper at constant excess ligand concentration, HNPz = 0.02M, indicates that between pH 4.0 and 6.0, copper is completely extracted and the slope of the linear portion of the plot of log D vs pH is approximately two, indicating , that two moles of protons are liberated during complex formation; The result shows that the extraction of copper. using this reagent is hydrogen ion concentration dependent up to pH of 4.0, and followed by a non-dependent horizontal region. Within this buffer region the slope is equal to zero up to pH of 6.0 when the extraction process seems to have reversed as the aqueous phase becomes more basic.
Preliminary investigation on the effect of complexing agents on the extraction of Cu(I1) showed that at the concentrations used in preparing the aqueous-buffer media, the components of the buffer solutions had no effect on the general extraction processes. But in the presence of diverse ions, Cu(I1) is found to be quantitatively extracted with Fe (111) between pH 3.0 to 4.0. However, Fe (III) is completely masked by EDTA (< 1 0 -'~) and oxalate.ions ( < IO-~M), while at the same masking-agent concentrations, more than 99% of Cu(I1) is extracted when oxalate ions are used as masking agents. Nickel (11) is negligibly extracted with chloroform solution of HNPz at the pH and equilibration time used for this study, while Co (11) is only extracted from pH 6.0 aqueous solutions. With proper choice of extraction chemical parameters, HNPz could be agood selective reagent for copper. The plot also shows that at lower ligand concentrations, Cu:HNPz mole ratio of 1:l is formed. That means that at lower ligand concentrations up to 0.0016M, copper forms a 1:l complex with the reagent, while at higher ligand concentrations up to 0.0125M, a 1:2 complex is formed, from which point further increase in ligand concentration produced no more effect in the degree of extraction (Table 1). Since the slopes of Figures. 2 and  3 are two and it has been accepted that two moles of the ligand are used up with subsequent release of two moles of H+ ions in the formation of extractable complex, it can therefore be reasonably assumed that the extraction process for copper follows the following extraction equilibrium equation.

Effect of Ligand Concentration
with the equilibrium extraction constant, Kx given by:

D = Kex [~~p i l t o r g )
[H ](aq) a n d .  The log K , found using the above relationship is -1.60; It has been known that log &increases with decrease in pKa value of ligand in the extraction of a number of metals. The introduction of a nitro group is ewected to enhance the acid properties of the reagent, decrease the pKa value of BMPP from 4.11, and increase the GX for copper.

Extraction of Copper Ions
The value of -1.60 obtained in this experiment and the inconsistent results of other researchers for BMPP point to some unidentified inherent chemical and structural factors that tend to dominate the proton release during the extraction process, such as steric hindrance, oxidation characteristics of the ligand, mesomeric effects, etc. These factors may perhaps tend to increase the pKa value of the nitropyrazolone, and therefore, decrease the GX for the pyrazolonate com lex. ~i r z a has found log GX for extraction J' of Cu(1I) using BMPP as -0.77, and Arora 0.70 and ~olotov'O 1.36. The value of -1.40 has also been reported for Cu(I1) using TTA'. Experimental data show that 0.0125M HNPz is sufficient for 100% extraction of copper when all other factors are kept constant.

The Composition of the Complex
Essentially, the determination of the formula of a complex involves the measurement of the number of ligands complexing with one mole of metal ion. Investigation of composition of extractable copper nitropyrazolonate were carried out using Job's method and the mole ratio method at pH 4.0. The results obtained are given in Figures 4 and 5 respectively. In the Job's method, equimolar solutions of the metal salt and HNPz were prepared-such that Cc, = C H N P~ = 1 . 6~1 0 -~~ Their concentrations were varied by varying their volumes from lml to lOml and mixed in such a way that the total volume of each mixture is the same (10ml). The concentrations of copper in the organic phase were plotted against the molar fractions (deduced from volume fractions) of Cu(I1) in the aqueous phase as shown in figure 5.
The maximum concentration of Cu(I1) species in the organic phase appears at the molar fraction of Cu(1I) of 0.33. In Figure 4, the results obtained in the extraction of copper with various amounts of HNPzgave straight lines with points of intersection at the mole ratio of HNPz to Cu(I1) of 2.0. It is obvious from these data that the concentration of copper in the organic phase is maximum for those mixtures in which the molar ratios (or volumes of the equi-molar solutions) of Cu(I1) and HNPZ are in the ratio 1:2. The results obtained from Figures 2 to 5 show that extractable species is present as a bis (nitropyrazolonato) copper (II), Cu(NPz)* complex.

Effects of Solvents
Data on the effect of various organic diluents on Cu(1T) extraction are shown Figure 6. Maximum extraction is acliieved using chloroform and benzene. Wider pH range within which copper can be extracted using our reagent is possible with chloroform and reverse is the case with benzene. HNPz and C U ( N P~)~ are found to have limited solubility in carbon tetrachloride, ethers, cyclohexane, n-hexane, and many other common organic solvents. Because of the more favourable physical and chemical characteristics of chloroform compared to other organic solvents, it was chosen and used for the rest of the studies.

Effect of Time
Equilibration was found to be established bclow 10 seconds, and further increase in time had no further effect on the cxtraction. The rate of attainment of extraction equilibrium has been known to be slow or very slow, for several extraction reagents, most notably, thep-diketones. This feature is contrasted by the present study.

DISCUSSION
We have studied spectropliotometricaIly the behaviour of Cu(I1) with HNPz and found that copper forms a bis-chelate with HNPz. Extraction is rapid as equilibrium is established within seconds. The degree of cxtraction was second-power dependent on pH and ligand concentration. As with most other acyl pyrazolones, HNPz has several advantages over others-diketones, such as benzoylacetone (BA, AA, TFA, ITA, etc., is being easily prepared and purified by recrystallization from alcohol) from readily available chemicals. It is characterized by good chemical stability and reasonable solubility h organic solvents. HNPz is relatively cheap compared to fluorinated @.

Ejrtractio?~ of Copper Ions
diketones (BMPP is about six times cheaper than TTA) . Other conditions remaining the same, the more acidic character of the reagent should permit extraction of a considerable number of metals in more acidic solutions. HNPz is recommended as a rapid extraction reagent for copper. Chloroform is also recommended, among others, as a suitable organic solvent.