Investigation on the Synergistic Complexation of Ni ( II ) with 1 , 10-Phenanthroline and Dithizone at Hexane-Water Interface Using Centrifugal Liquid Membrane-Spectrophotometry

Complex formation of Ni(II) and 1,10-phenanthroline (C12H8N2/Phen) with the addition of dithizone (C13H12N4S/HDz) at the hexane-water interface has been studied by direct measurement spectrophotometry using the centrifugal liquid membrane (CLM) method. Ni(II) ion with Phen formed a cationic complex of Ni(C12H8N2)2 or NiPhen2. That complex dissolved in the aqueous phase and had two UV absorption spectrum maxima wavelengths, λmax 270 and 292 nm. Observation of complex formation was performed variations of pH and ligand concentration. The pH caused protonation that affected the amount of the formed complex. With the variations of ligand concentrations, the greater was the concentration of ligands the greater was the formed complex. Based on the Batch method, the HDz ligand addition into the NiPhen2 cationic complex produced ion association complex of Ni(C13H11N4S)2(C12H8N2) or NiDz2Phen at λmax 403 nm, and was extracted in the organic phase. Measurement results using CLM method showed that NiDz2Phen complex was formed at hexane-water interface with λmax 523 nm. Comparison of Phen with HDz ligand concentrations affected the initial formation rate of NiDz2Phen complex. The greater concentration of Phen ligand increased the initial rate of formation for synergistic complex. The obtained data using CLM method indicated that the synergistic complex formation rate constant of NiDz2Phen at the interface, k was 0.30/s.


Introduction
Extraction behaviour of metal ions with ligand combinations has been studied in a liquid-liquid system [1].The system has been applied in several areas such as in the processing of hazardous metal ions [2] and in recovering precious metals from industrial products [3].During its development, it has been proven that the addition of certain elements can cause synergistic reactions resulting in improved performance and efficiency of the extraction [4][5][6][7][8][9][10].
Watarai et al. [11] reported that the interface plays an important role in the formation of complexes from liquid-liquid extraction.The interface region between the aqueous phase and the organic phase has a thickness of ± 1 nm [12].It has also been demonstrated that the complex formation of liquid-liquid extraction takes place through the interface [13][14][15][16][17][18].This observation is an important finding in the study of liquid-liquid extraction.To date, many researchers have investigated the interface in their studies [19][20][21][22].This proves the importance of the interface to study the reaction mechanism in the liquid-liquid extraction system.
The rate of synergistic extraction has been studied to observe the formation of Ni(II) complex with dithizone and 1,10-Phenanthroline [23] as well as the formation of Ni(II) complex with dithizone and 4.7-diphenil-1,10-Phenanthroline [24].Both of these researches study the effects of phenanthroline on the rate of extraction of Ni(II) with dithizone in chloroform using the high speed stirring (HSS) method.The results showed that the synergistic extraction system can be considered as a combination of the chelate extraction system and ion association.
Starting from studies using indirect measurements, this research was conducted using direct measurements to study the kinetics and mechanism of Ni(II) complex formation with 1,10-Phenanthroline and dithizone at the hexane-water interface.Data was collected from a combination sequence of added ligands and the parameters that influence it.Our initial research [25] studied the formation of addition complex at the interface where 1.10 Phenanthroline ligand was added to Ni(II)-dithizone complex in a hexane-water system.The results showed a faster formation rate of complex.This research studies the formation of a synergistic complex at the hexane-water interface through the addition of dithizone to the Ni(II)-Phenanthroline complex.The rate of interfacial complexation obtained was then compared with the rate of complex formation obtained from a previous study [25].This study used direct measurements from the centrifugal liquid membrane (CLM)-spectrophotometry to observe formation of complexes and seek formation rate constants of complexes at the hexane-water interface.The study also investigated variations of ligand concentrations and pH that affect the rate of complex formation.

Experiment
Chemicals.Determining the molar ratio of Ni(II):Phen.5 ml of solution containing Ni(II) ion 1.0 x 10 -5 M (pH 3.88, ionic strength (NaCl) 0.1 M) was mixed with 5 ml Phen ligand in a sealed test tube (diameter 1.5 cm and height 15 cm), both in the aqueous phase.The mixture was shaken 15 minutes until it reached an equilibrium reaction.Measurements were taken with a UV-Vis spectrophotometer at wavelength, λ 190-800 nm (Hitachi, double beam with tungsten and deuterium lamp light sources).Ligand concentrations varied at 1.0 x 10 -6 -4.0 x 10 -5 M.
Batch methods.This method is used to study the behavior of complex formation of Ni(II) with Phen in the aqueous phase, and the formation of a synergistic complex (Ni:Phen) with HDz in a hexane-water system.The experiments were conducted with variations in pH and ligand concentrations.

Effect of Phen concentration on Ni(II):Phen complexation.
The experiments were performed in the same way as the pH variation experiments, using the pH optimum obtained from these experiments.The Ni(II) concentration used 1.0x10 -3 M, ionic strength (NaCl) 0.1 M, the Phen concentration varied between 1.0 x 10 -7 -1.0 x 10 -5 M.

Synergistic complexation of Ni(II):
Phen with HDz. 5 mL from the aqueous phase containing Ni(II) ion 1.0 x 10 -3 M (pH optimum, ionic strength (NaCl) 0.1 M) was added the aqueous phase containing Phen 1.0 x 10 -5 M in a sealed test tube and then shaken 15 minutes to reach equilibrium reaction.Solution absorbance measurements were made with UV-Vis spectrophotometer at λ 190-350 nm.To this solution, 5 ml of the organic phase containing HDz 1.0 x 10 -5 M was then added, shaken for 1 h until a new equilibrium is reached, then left to settle until it separated.Each phase was measured with UV-Vis spectrophotometer at λ 300-800 nm.

Centrifugal liquid membrane (CLM) method synergistic complexation of Ni(II):Phen with HDz.
Observations with CLM were performed on blanks and samples in cylindrical glass cells (diameter 19 mm, height 32 mm and thickness 1 mm).Blanks are only performed at the aqueous phase without reagents, whereas the three solution samples are performed for Phen, Ni:Phen complex, and Ni:Phen-Dz complex.Measurements of the blanks were performed with 150 μL aquabidest using a micro-syringe injected into a cylindrical glass cell attached to UV-Vis spectrophotometer photodiode array (single beam Hewllett-Packard Agilent 8453 A).The cells were then rotated at a speed of 10,000 rpm using NSK (Nakanishi Electer Emax) rotator equipped with traffo 2PF1020 type.The absorption spectrum is measured directly at λ 300-800 nm.Measurements of Phen ligands were carried out by injecting 50 μL of Phen 1.3 x 10 -3 M, at the aqueous phase, into a cylindrical glass cell (at stop condition).The cylindrical glass cell was respun at a speed of 10,000 rpm.Absorption spectrum was measured again at λ 300-800 nm.Measurements of the Ni:Phen complex was conducted by injecting 50 μL of Ni(II) ions 5.0x10 -3 M (pH optimum, ionic strength (NaCl) 0.1 M), from the aqueous phase, into a cylindrical glass cell (spinning at 10,000 rpm).Absorption spectrum is measured again at λ 300-800 nm.Measurement of Ni:Dz-Phen complex was conducted after the Ni:Phen complex formation reaction had reached equilibrium.250 μL organic phase contain HDz 3.5 x 10 -5 M was injected into a cylindrical glass cell (at a fixed rotational speed of 10,000 rpm).Absorption spectrum of complex was measured at λ 300-800 nm.

Effect of Phen concentration on synergistic complexation.
Experiments were also conducted on blanks and samples.Blanks are only performed at the aqueous phase without reagents, whereas the three sample solutions are performed for Phen, Ni:Phen complex, and Ni:Phen-Dz complex.All experiments followed the same procedure as used in the previous experiments using concentrations of aqueous phase for Ni(II) 1.0 x 10 -3 M, ionic strength (NaCl) 0.1 M, pH optimum, and concentration of organic phase HDz 3.5 x 10 -5 M, but for measurements of the Phen ligand in the aqueous phase, the Phen concentration used was 1.0x10 - 5 M. The experiment was repeated for Phen concentrates of 1.9 x 10 -4 M and 2.6 x 10 -4 M.

Results and Discussion
Batch Method.Determining the molar ratio of Ni(II):Phen.The experiments aim to obtain a molar ratio between Phen ligands with Ni(II) ions to form a complex.The measurement results of UV-Vis absorption spectra show that Ni(II)-Phen complex has two maximum wavelengths; λ maks at 270 and 292 nm.To determine the molar ratio of Ni(II):Phen complex, it was analyzed for absorbance at λ maks 292 nm.

The ratio of Phen and Ni(II) concentration ([Phen]/[Ni(II)]
) on absorbance at λ maks 292 nm is plotted on the graph (Figure 1).It can be seen that the increase in [Phen]/[Ni(II)] is accompanied by an increase in the absorbance of the complex.This shows that the more Ni(II)-Phen complex formed correlates with the greater concentrations of Phen used.However, when the ratio passed 2:1, the absorbance of the complex became constant.It shows that Ni(II) ion no longer reacts with Phen.Therefore, the use of Phen at higher concentrations had no effect on the amount of complex formed, so it was concluded that the molar ratio of Phen and Ni(II) is 2: (1) Formation of Cation Complex, NiPhen 2 2+ .In water solvent, Phen ligand has two ,λ maks that is 228 and 264 nm with molar absorptivity, ε = 3.72 x 10 4 and 2.59 x 10 4 M -1 cm -1 , respectively.Once added a solution of Ni(II) in a system containing Phen, it changes two λ maks as shown in Figure 2.
Figure 2 shows the λ maks complex at 270 nm, the λ maks shift of Phen ligands at 264 nm react with Ni(II).In addition to theλ maks , shift the complex has increased absorbance.Complexation reactions are also evidenced by the emergence of a new absorption at λ maks 292 nm.The emergence of this absorbance proves the formation of the cation NiPhen 2 2 + complex.Therefore, the analysis performed on NiPhen 2 2 + complex is performed at λ maks 292 nm.

Effect of pH on the complexation of NiPhen 2
2+ .The experiment was conducted to observe the effect of pH and to obtain the pH optimum for the formation of NiPhen 2 2+ complex.The results of observation illustrated in Figure 3, show that the formation of NiPhen 2 2+ complex reaches maximum absorbance that is pH optimum at pH 3.88.At pH> 3.88 it can be seen that the concentration of H + ions reduces or increases OH -ions which affect the amount of Ni 2+ ions that react with a decline in Phen, so that the formation of NiPhen 2 2+ complex is small, as shown by the decrease in absorbance values (Figure 3).This is because the Ni(II) begins to hydrolyze and precipitate as Ni(OH) 2 at pH> 6.50 for the concentration of [Ni(II)] = 1.0 x 10 -3 M. At pH <3.88 it can be seen that the concentration of H + ions increases, affecting the amount of NiPhen 2 2+ complex formed to become less.This is due to the number of H + ions protonating the N atoms of the Phen ligand.At low pH, the concentration of H + in the solution is high, so that more protonation occurs.Protonation of the N atom blocks the binding of Ni(II) on the ligand, so that less of the the complex is formed.From the pH optimum of complexation, the pH is then used to observe the synergistic complexation of Ni:Phen with HDz.

Effect of Phen concentration on the complexation of NiPhen 2
2+ .The experiment was to observe the effect of Phen concentration on the formation of cation NiPhen 2 2+ complex with conditions of ion concentration Ni(II)  [25], and from previous studies in solvent extraction systems the NiDz 2 Phen complex is extracted in the chloroform phase [23].
Observation of UV-Vis absorption spectra of the complex in the aqueous phase and the organic phase using the Batch method is shown in Figures 4 and 5. Figure 4 (solid line) shows the absorption spectra of NiPhen 2 2+ complex observed after the addition of HDZ with λ maks 270 and 292 nm in the aqueous phase.This spectrum is similar to the spectrum before the addition of HDz (dashed line) which is a NiPhen 2 2+ complex, but with much lower absorbance.Decrease in absorbance after the addition of HDz is because NiPhen 2 2+ reacts with HDz to form NiDz 2 Phen, so the presence of residue NiPhen 2 2 + does not react with HDz in the aqueous phase.Observations in the organic phase spectrum shows a new compound λ maks 403 nm (Figure 5, solid line).This spectrum differs from spectrum HDz (dashed line) that has λ maks 413 and 622 nm with ε dithizone in hexane phase 2.85 x 10 4 and 1.64 x 10 4 M -1 cm -1 , respectively.This spectrum was proposed as spectrum of ion association NiDz 2 Phen complex extracted in the organic phase.
CLM Method.This method uses a high rotation speed (10,000 rpm) to produce ultra-thin membranes from two phases system.Each phase has a thickness of 131 μm, with an interfacial area of the two phases (S i ) 19.11 cm 2 and the specific interfacial area (S i /V o ) 76.44 cm -1 .When measuring the absorption spectrum, the beam passes through the both phases of bulk and liquid-liquid interface in a cylindrical glass cell two times.Total absorption spectrum detected by UV-Vis spectrophotometer is at λ 300-800 nm [25].

Synergistic complexation of Ni:Phen with HDz.
Results of the Batch method assume that NiDz 2 Phen is extracted in the organic phase.This method is observed in the aqueous phase and organic phase.Confirmation and further observations on the complex adsorbed at the hexane-water interface is conducted by directly measuring with CLM method.
The formation of NiDz 2 Phen complex begins with injecting the aqueous phase containing Phen into a cylindrical glass cell, followed by an aqueous phase containing Ni 2 + .After reaching equilibrium reaction, the organic phase containing HDz is injected.After injection, the visible appearance of the spectra at λ maks 523 nm increases absorbance exponentially with time (Figure 6).The spectra mentioned are spectra NiDz 2 Phen.The spectrum has different λ maks than the λ maks obtained from the Batch method (403 nm).This proves that formation of NiDz 2 Phen complex takes place at the hexane-water interface.
Figure 6 also shows spectra with λ maks 413 nm which decreases exponentially with time.The spectra mentioned shows the presence of HDz that diminishes as it reacts with NiPhen 2 2+ to form NiDz 2 Phen.It can be seen that two spectra peaks appear, indicating that HDz instantly expires when it reacts with NiPhen 2+ .This is due to the small concentration of HDz = 3.5x10 -5 M.This experiment observes changes in the visible region of the absorption spectra of NiDz 2 Phen.Spectra changes of Phen and cation NiPhen 2 2+ complex which absorbs in the UV region can not be observed because the CLM cylindrical glass cell used also absorbs UV light, thus distorting the measurement.
Research on synergistic NiDz 2 Phen complex formation through ion association reaction (NiPhen 2 2+ + HDz) is faster than previous studies [25] via addition reaction (NiDz 2 + Phen).This is evident in the increase of NiDz 2 Phen absorbance at λ maks 523 nm.In the addition reaction, an increase in NiDz 2 Phen absorbance is slow as the reaction equilibrium is reached after 1080 seconds.However, with the ion association reaction, the absorbance of the complex increases very rapidly and drastically.NiDz 2 Phen complex reactions can reach equilibrium after 120 seconds (Figure 7).Further verification is done by comparing the initial reaction rate (r o obs ) of both.
Calculation of r o obs performed using a second-order equation with least squares regression, where the increase in ion pair NiDz 2 Phen absorbance (523 nm) at the initial state is expressed exponentially with time (Figure 7).Using equation r o obs = (d [NiDz 2 Phen] i /dt), it obtained r o obs = 8.54 x 10 -5 M/s.Compared to previous studies [25], an addition reaction (NiDz 2 + Phen) resulted in r o obs = 1.11 x 10 -5 M/s.It can be seen that the combination of additional HDz or Phen ligands can affect the formation rate of NiDz 2 Phen complex.In this study, the order of adding Phen to Ni(II) ion formed cations NiPhen 2 2 + complex that speeds up the reaction of NiDz 2 Phen complex formation when treated with HDz.
Effect of Phen concentration on the synergistic complexation of NiDz 2 Phen.The experiment aims to observe the effect of Phen concentration on the formation of NiDz 2 Phen complex at the hexane-water interface.
Observations were made by spectrophotometry at λ maks 523 nm.The results of observations are shown in Figure 8.It appears that the greater concentration of Phen forms more NiDz 2 Phen complex, as can be seen from the higher absorbance.This is due to increased concentration of Phen that forms greater initial amounts of NiPhen 2 2+ complex.NiPhen 2 2+ complex subsequently undergo ion association with HDz to form NiDz 2 Phen that is adsorbed at the hexane-water interface.The extraction rate of NiDz 2 Phen complex is determined by the concentration of HDz and Phen [23].Formation of NiDz 2 Phen complex is an equilibrium reaction that occurs through the formation of intermediate complex Ni(C 13 H 11 N 4 S)(C 12 H 8 N 2 ) 2 + or NiDzPhen 2 + at the interface.If the HDz concentration is greater than Phen, the formation of intermediate NiDzPhen 2 + complex is slower, so NiDz 2 Phen in the organic phase is more dominant than NiDzPhen 2 + at the interface.This is evident from the low absorbance and small gain, visible from the narrow distance between the peaks of the spectra that almost coincide (Figure 9a).
If the HDz concentrate is smaller than Phen, then the formation of intermediate NiDzPhen 2 + complex is faster, so the NiDzPhen 2 + at the interface is more dominant than NiDz 2 Phen in the organic phase.This is evident from the high absorbance and large gain, visible from the wide distance between the spectral peaks (Figure 9b).Spectra at λ maks 413 nm show the amount of HDz during the reaction.With concentrations of HDz >> Phen the spectra decreased gradually (Figure 9a).At a concentration of HDz << Phen, the spectra decreased rapidly (Figure 9b).This is due to the small amount of HDz, so it instantly reacts with NiPhen 2 2 + .
Figure 10 shows the relationship of NiDz 2 Phen complex absorbance exponentially with time.It appears that the greater the concentration of Phen, the greater is the curve gradient.Based on this observation, the value of the formation rate constant (k) of ion association NiDz 2 Phen complex at the interface can be calculated.
Using a second-order equation with least squares regression, as shown in Figure 7 Determining the formation rate constant value of ion association NiDz 2 Phen complex at the interface, k is performed by plotting the r o obs value of the Phen concentration used.Based on this relationship, the resulting equation is a straight line y = 0.30x + 8.07x 10 -6 (Figure 11).k was obtained at 0.30/s.Values of k showed ease in the formation of synergistic complexes to be adsorbed at the liquid-liquid interface.This rate constant value was smaller than the value of k between 0.91-85.40/sthat were obtained from previous experiments on forming NiDz 2 Phen complex in a chloroform-water system using HSS spectrophotometry method [23].This value showed that the formation of complexes at the hexane-water interface (hexane, nonpolar) was slower than at the chloroform-water interface (chloroform, semipolar).The slow complex formation facilitates the observation of adsorbed complex at the interface.These results prove that the CLM spectrophotometry method can be applied to directly

Synergistic Complexation Reaction Mechanism.
From the results of experiments and observations made above a reaction mechanism for the formation of NiDz 2 Phen complex from ions Ni(II), Phen and HDz in the aqueous phase, the organic phase and the hexanewater interface, can be proposed, as shown in Figure 12.
Synergistic complex formation occurs in three stages.First, the NiPhen 2 2+ complex in the aqueous phase reacts with HDz in the hexane phase to form intermediate NiDzPhen 2 + complex at the hexane-water interface.Second, the intermediate complex undergoes disproportionation reactions to form neutral NiDz 2 Phen complex at the interface.Third, NiDz 2 Phen complex is desorped from the interface and extracted into the organic phase.It appears that the formation of ion association complex occurs gradually in three places, the aqueous phase, the interface, and the organic phase.

Conclusions
The formation of synergistic NiDz 2 Phen complex at the hexane-water interface has been successfully studied directly using UV-Vis spectrophotometry associated with CLM method.Based on the Batch method, the cation NiDzPhen 2 + complex has λ maks 270 and 292 nm.The complex dissolved in the aqueous phase with pH optimum 3.88.NiDz 2 Phen complex formed at λ maks 403 nm and extracted in the organic phase.Using the CLM method, NiDz 2 Phen complex formation was observed at λ maks 523 nm.Differences in λ maks obtained with the Batch method proves that formation of NiDz 2 Phen complex takes place at the hexane-water interface.The greater the concentration of Phen, the faster is the initial formation rate of NiDz 2 Phen complex.Based on the CLM method, the complexation rate constant value of ion association of NiDz 2 Phen at the interface, k is 0.30/s.
Ni(II):Phen complexation.5 ml of solution containing Ni(II) ion 1.0 x 10 -3 M (pH 2.06 measured by an electronic pH-meter (Ω Metro Ohm), ionic strength (NaCl) 0.1 M) was added a solution of Phen 1.0 x 10 -5 M in a sealed test tube and shaken 15 minutes to reach equilibrium reaction.Measurements were performed with a UV-Vis spectrophotometer at λ 190-350 nm.The same experiment was repeated with a solution of Ni(II) with pH 3.01, 3.88, 4.92, and 5.90.