Octafunctionalized Calix [ 4 ] resorcinarene-N-fenil-acetohydroxamic acid for the Separation , Preconcentration and Transport Studies of Cerium ( IV )

Um novo método para extração líquido-líquido, separação e pré-concentração de cério(IV) com um derivado ácido do calix[4]resorcinareno [C4RAHA] é descrito. Cério foi extraído em pH 9 com etilacetato, e recuperado de areias monazíticas, na presença de tório, urânio, lantânio e um grande número de cátions e ânions. Cério(IV) forma um complexo colorido, amarelo, λ max 381 nm, absortividade molar de 1,06 × 10 L molcm e obedece a lei de Beer no intervalo entre 0,4 e 15 mg mL. A razão metal-ligante encontrada para cério(IV) foi de 4:1 (M:L). Estudos de transporte foram realizados em um sistema de três fases, através de uma membrana líquida contendo C4RAHA. Para medidas por ICP OES, o extrato pode ser diretamente aspirado, o que aperfeiçoa a sensibilidade em 25 vezes e os limites de estimativa são 5-150 μg L de cério. O método foi aplicado para a determinação de cério em materiais geológicos de referência, areia monazítica e amostras de água de mar.


Introduction
The chemistry of bowl shaped macrocyclic compounds that can complex organic and inorganic guests has been receiving significant attention in the field of supramolecular chemistry. 1Among the different macrocyclic compounds, calix [4]resorcinarenes and calixarenes 2 have been widely employed as changeable units with a well defined structure and easy accessibility.The calixresorcinarenes are unique three-dimensional cyclic tetramer and hexamer that belong to a class of metacyclophanes in which resorcinol units are linked by methylene bridges. 3he interest towards this class of compounds which are capable of forming host guest complexes is stimulated by the development of supramolecular chemistry which uses them for the possible creation of synthetic receptors and carriers of cations, anions and neutral molecules. 4he host properties of calix [4]resorcinarenes continue to draw research interests due to its potential application as macrocyclic receptors, 5 as dendrimers in biological systems, 6 nano-capsule, 7 nanoparticles, 8 optical chemosensors, 9 supramolecular tectons, 10 host molecules, 11 as components in liquid crystals, 12 photoresists, 13 selective membranes, 14 surface reforming agents, 15 HPLC stationary phases, 16 as ion channel mimics, 17 and metal ion extraction agents. 18igh purity cerium is required for nuclear power production and as catalyst for automobile and nuclear industries.The analysis of environmental samples, like monazite sand, for rare earth elements is an important topic for geochemical and geological research, which allows solving the problems of genesis of ore deposits. 19erium is the major element present in monazite sands associated with lanthanum, thorium and uranium.Generally monazite sand is treated with an alkali followed by acid digestion and extraction with tetra butyl phosphate for the separation of lanthanides.However, the process is time consuming and needs further purification for isolation of high purity elements.1][22][23][24] Highly versatile techniques like atomic absorption spectrometry (AAS) and also inductively coupled plasma optical emission spectrometry (ICP OES) cannot be applied directly for the trace determination of cerium as they have high detection limits of 10 μg mL -1 , 25,26 and a prior separation is mandatory. 27,28Hydroxamic acids are well known to be efficient extractants that find wide applications in analytical, 29 and agricultural fields. 302][33][34][35] Thus still remains a need of a specific ligand that discriminate Ce(IV) from La(III), Th(IV), and U(VI) from other metal ions, present in great excess in sea water, monazite sand, and other environmental samples.
Since we had recently prepared a series of novel Calix [4]resorcinarene-N-fenil-acetohydroxamic acid derivatives to study the extraction of uranium and thorium from acidic media, 36,37 we thought to extend our investigations to include the extraction of cerium under similar conditions.In the present study we have used Calix [4]resorcinarene-Nfenil-acetohydroxamic acid (C4RAHA) for the liquid-liquid extraction, preconcentration and transport of cerium across a liquid membrane.The extraction removes the bulk of the elements and at the same time preconcentrates Ce(IV) into small volumes.The extract is directly aspirated into ICP OES improving the sensitivity and detection limit.To check the applicability of the proposed method, cerium was determined in monazite sand, standard geological materials and sea water samples.

Instrumentation and measurements
Spectrophotometric determinations were done with a Hitachi 3210 UV/Vis spectrophotometer using 10 mm quartz cells.Plasma scan model 710 sequential inductively coupled plasma optical emission spectrometer with plasma scan multitasking computer and peristaltic pump was used for emission measurements applying the following conditions.

Chemicals and solutions
All chemicals used were of analytical reagent grade of BDH, E-Merck or Qualigens unless otherwise specified.All aqueous solutions were prepared with quartz distilled deionized water, which was further purified by a Millipore Milli-Q plus water purification system.The standard stock solution of cerium(IV) (1000 μg mL -1 ) was prepared by dissolving 4.511 g of (NH 4 ) 4 Ce(SO 4 ) 4 .2H 2 O in water containing 2 mL H 2 SO 4 in a 1000 mL volumetric flask and standardized spectrophotometrically. 38 The buffer solutions in the pH range 3-10 were prepared from 0.2 mol L -1 ethanoic acid/ 0.2 mol L -1 sodium ethanoate or triethanolamine buffer (mix 200 mL of 15% v/v triethanolamine, 160 mL of 1 mol L -1 HNO 3 , and 40 mL of water).The C4RAHA (Figure 1) was synthesized as reported earlier. 36,37Its 7.14 × 10 -5 mol L -1 solution was prepared in ethyl acetate.

Liquid-liquid extraction of cerium(IV)
An aliquot of sample solution containing 4.0-150.0μg of Ce(IV) was transferred into a 25 mL separatory funnel.pH was adjusted between 8-9 by adding the buffer solution.The mixture was shaken with 3 mL of 7.14 × 10 -5 mol L -1 C4RAHA in ethyl acetate for 8-9 min.The organic extract was separated, dried over anhydrous sodium sulphate and transferred into a 10 mL volumetric flask.To ensure complete recovery, the extraction was repeated with 1 mL of C4RAHA.The combined extracts and washings were diluted to the mark with ethyl acetate.The absorbance of the organic phase was measured against the reagent blank at 381 nm.The total concentration of the cerium ion [Ce 4+ ] species remaining in the aqueous phase [Ce 4+ ] (aq) was measured by ICP OES.
The concentration of the metal ion extracted into the organic phase [Ce 4+ ] (org) as complex was estimated by [Ce 4+ ] (org) = [Ce 4+ ] (aq, initial) -[Ce 4+ ] (aq final) , where [Ce 4+ ] (aq, initial) and [Ce 4+ ] (aq, final) are the initial and final concentration of the metal ion in the aqueous phase respectively.The percent extraction (%E), was calculated by The extracted Ce(IV)-C4RAHA complex in ethyl acetate after appropriate dilution was also determined by ICP OES.

Transport studies of cerium(IV)
Transport of Ce(IV) through the liquid membrane in a three phase system was carried out in a glass assembly as shown in Figure 2. The reaction cell was 6.6 cm in inner diameter and 9.0 cm in height with a total capacity of 305 mL.U-tube (2.0 cm diameter, 20 cm length) was fused from the base of the cell.The height of the tube inside the cell was 4.0 cm from the basal plane.The whole assembly was kept in a thermostat maintained at 30 °C.The transport experiments were performed with 50.0 mL of 7.14 × 10 -5 mol L -1 Ce(IV) solution at pH 9.0 as source phase and 50.0 mL of 0.1 mol L -1 HCl as receiving phase.The liquid membrane consisted of 75 mL of 7.14 × 10 -5 mol L -1 C4RAHA in ethyl acetate.A synchronous motor (200 rpm) provided constant reproducible stirring from the top.The amount of Ce(IV) transported from the source phase to the receiving phase was measured by ICP OES.The transport data's are the average of 5 runs with an experimental error of less than 2%.

Complexation studies and statistical significance
The synthesized Calix [4]resorcinarene-N-fenilacetohydroxamic acid was used for the extraction and spectrophotometric determination of Ce(IV).The complexation of Ce(IV) with C4RAHA was studied under optimum conditions of pH, solvent, and reagent (C4RAHA) concentration.Maximum extraction of Ce(IV) was reached in the pH range of 8.0-9.0,however, above and below this range, the extraction of the complex was incomplete.It was also found that 5 min of shaking is sufficient for quantitative extraction of Ce(IV).The extraction was not affected by further shaking, indicating that the equilibrium state had been attained.The effect of solvent on the extraction of Ce(IV)-C4RAHA complex was restricted to ethyl acetate, chloroform, toluene and benzene due its limited solubility in other organic solvents and was found that quantitative extraction is possible in ethyl acetate with molar absorptivity 1.06 × 10 4 L mol -1 cm -1 .The spectra of the complex of Ce(IV) showed a bathochromic shift of 58 nm from that of the reagent C4RAHA in ethyl acetate as shown in Figure 3.
Under the optimum conditions, quantitative extraction was possible with 3.0 mL, 7.14 × 10 -5 mol L -1 C4RAHA  in ethyl acetate.The molar absorptivity measured at the wavelength of maximum absorption (λ max 381 nm) for the extracted complexes was 1.06 × 10 4 L mol -1 cm -1 .The system obeys Beer's law over the range of 0.4-15.0mg mL -1 and the regression analysis equation obtained using reference solutions can be represented as A = 0.0765 × Conc.-0.000057 with the a linear correlation coefficient of r = 0.9988.The Sandell's sensitivity calculated for the proposed method is 0.013 μg cm -2 .The precision of the proposed method was investigated based on the optimum condition of Ce(IV) extraction and expressed as relative standard deviation of 1.8%.The limit of detection (LOD, 2.3 μg L -1 ) was defined as that analyte concentration giving a signal equal to three times the standard deviation of blank signal. 39The limit of quantification (LOQ , 7.6 μg L -1 ) was then set as ten times the standard deviation of the blank signal.

Stoichiometry of the complex
The stoichiometric ratio of the complex was determined by the modified Job's method of continuous variation.The absorbance of complex A c of a series of solution containing cerium ion (Ce 4+ ) a and C4RAHA b at a constant total concentration a+b, but different concentration ratios were measured at 381 nm.The absorbance of b was subtracted from the observed absorbance A c to obtain the true absorbance ΔA.

ΔA = A c -A b
ΔA values were plotted against the molar ratio of Ce(IV) ion a / (a+b).The maximum value for ΔA was clearly obtained at a / (a+b) = 0.78, indicating 4 : 1 stoichiometric ratio of Ce 4+ and C4RAHA.The accuracy of this result is possible if only a single complex is formed.To verify this, measurements were taken at a different selected wavelength, which gave same value for a / (a+b) ratio.
To get more information about the nature of the extracted complex, the extract was evaporated to dryness and subjected to elemental analysis.Further, a known mass of the dry complex was digested with perchloric and nitric acid mixture.It was centrifuged and cerium content was determined by ICP OES.The results obtained are in conformity with the complex formulae.The experimental results obtained for elemental analysis of the complex are: carbon 47.44%, nitrogen 4.57%, hydrogen 3.58% and cerium 23.01% and are comparable with the theoretical values: carbon 47.50%, nitrogen 4.62%, hydrogen 3.66% and cerium 23.11%.

Extraction equilibrium
To evaluate the binding ability of C4RAHA (H 8 A) with Ce(IV), a liquid-liquid extraction in water-ethyl acetate system was carried out and the extraction constants were determined.
The β 4 and P Ce-H8A are the respective overall formation constant and distribution constant of [{Ce(OH) 2 } 4 A 8-] (o) complex in the two phases and the values of log β 4 .P Ce-H8A and log K ex at different temperatures are summarized in Table 1.

Effect of temperature and thermodynamic parameters
The extraction of Ce(IV) from aqueous solution was studied at different temperatures between 298 K and 318 K and the results show that the extraction equilibrium constant (k ex ) and two-phase stability constant (log β 4 .P Ce-H8A ) increases with increase in temperature (Table 1).The plot of (log β 4 .P Ce-H8A ) against 1/T yields a straight line equation 9167.4 × + 47.379 with slope(x) = -ΔH°/ 2.303 R. The mean enthalpy change accompany the complexation is found to be -177.52kJ mol -1 for cerium in the given range of temperature.The inspection of Figure 4 (ΔH°, -TΔS°, ΔG° → T) reveals that the complexation is driven by negative values of free energy and favorable enthalpy change.

Liquid membrane transport studies
The transport of Ce 4+ through the membrane containing 7.14 × 10 -5 mol L -1 , C4RAHA from source phase of [Ce 4+ ] = 7.14 × 10 -5 mol L -1 to the receiving phase of 0.1 mol L -1 HCl was carried out.As it can be seen in Figure 5, the concentration of Ce 4+ in the source phase decreases continuously and reached a lower concentration than the detection limit after 20 min (curve A), on the other hand concentration of Ce 4+ in the receiving phase increases (curve B).Therefore, it is clear that Ce 4+ moved from the source to receiving phase through the liquid membrane.The carrier in the membrane reacts with Ce +4 in the source phase at the interface and forms a complex [{Ce(OH) 2 } 4 A -8 ] (o) while releasing 8 moles of proton into the source phase.At the other interface of the membrane and the receiving phase the complex reacts with 8 moles of protons while releasing 4 moles of Ce +4 in the receiving phase.

Effect of diverse ions
Cerium was selectively separated and determined in presence of various metal ions at pH 9.0 and none of them affected the absorbance of cerium complex.The tolerance limit was set as the amount of foreign ions causing a change of ± 0.02 absorbance.However, a large concentration of vanadium(V) and molybdenum(VI) can be eliminated by stripping them with 0.075 mol L -1 H 2 SO 4 and 0.2 mol L -1 oxalic acid.Moderate amount of various metal ions associated with Ce(IV) were tolerated and also some common anions (Table 2).

Preconcentration of Ce(IV)
The concentration of Ce(IV) in natural water is too low for its direct determination.Therefore, preconcentration or enrichment step is necessary to bring the sample to the detectable limits of existing instrumental methods.The method was studied for the preconcentration of cerium in terms of its preconcentration factors.
The preconcentration study was carried out by extracting 20 μg of Ce(IV) in 1000 mL aqueous phase with 8.5 mL of 0.015% C4RAHA in ethyl acetate.To evaluate the efficiency of preconcentration, expressed as recovery, the concentration of cerium in organic phase and the aqueous phase was determined by ICP OES.
Quantitative determination was possible with recovery up to 98% with a preconcentration factor of 118.

Determination of Ce(IV) in standard samples
To check the applicability and reliability of the method, cerium(IV) was determined in monazite sand, standard geological samples (USGS) and sea water samples.Cerium was extracted from monazite sand after digesting 0.5 g sand with a mixture of concentrated HCl and HClO 4 (1:1 v/v) and diluting it to 250 mL with deionized water.An appropriate volume of aliquot of it was extracted at pH 9.0 with 10 mL of C4RAHA in ethyl acetate.Cerium was then stripped with 1-3 mL of 0.5 mol L -1 HNO 3 and the organic layer containing C4RAHA was recycled for recovery.The results for the analysis are summarized in Table 3.

Conclusions
The present method is selective for the determination of cerium(IV) in presence of large quantities of associated metal ions by Calix [4]resorcinarene-N-fenil-acetohydroxamic acid.The Calix [4]resorcinarene-N-fenilacetohydroxamic acid (C4RAHA) shows high affinity for cerium(IV) and binds in a 4:1 (metal:C4RAHA) ratio as evaluated by the Job's method.Transport studies for Ce(IV) are reported for the first time with liquid membrane in a three phase system.The preconcentration factor of 118 suggests the utility of C4RAHA in trace analysis.The results obtained from the analysis of monazite sand, standard geological and seawater samples proved the reliability of the proposed method for their application to various standard and natural samples.

Figure 4 .
Figure 4. Variation in thermodynamic parameters with temperature.

Figure 5 .
Figure 5. Transport profile of cerium(IV) through the liquid membrane containing C4RAHA at 30 o C.

Table 2 .
Effect of diverse ions on the extraction of Ce(IV)-C4RAHA complex a stripped with 0.2 mol L -1 oxalic acid , b stripped with 0.075 mol L -1 H 2 SO 4 .

Table 3 .
Determination of Ce(IV) in Monazite Sand, Geological samples (USGS) and Sea Water a Samples from Travancore, India, b average ± standard deviation (12 determinations).The values obtained by spectrophotometry and ICP OES are established at confidence level of 95%.