Spectrophotometric Determination of Gallium ( III ) with 4-( 2-Pyridylazo )-resorcinol and Nitron

The formation and liquid-liquid extraction of ion-association complex between gallium(III) − 4-(2-pyridylazo)-resorcinol (PAR) – 1,4-diphenyl-3-(phenylamino)-1H-1,2,4-triazole (nitron, Nt), water and chloroform were studied. The optimum conditions for gallium(III) extraction as an ion-association complex, (NtH)+[Ga3+(PAR)2]−, were found: pH, concentration of the reagents and shaking time. The following key constants were calculated: constant of extraction (logKex = 6.28 ± 0.07), constant of association (logβ = 4.98 ± 0.05), constant of distribution (logKD = 1.30 ± 0.02) and recovery factor (R / % = 95.17 ± 0.02). Beerʼs law is obeyed for Ga(III) concentration up to 0.8 μg cm−3 with apparent molar absorptivity of (10.3 ± 0.4)×104 dm−3 mol−1 cm−1 at λmax = 510 nm. Some additional characteristics, such as limit of detection (LOD = 0.072 μg cm−3), limit of quantification (LOQ = 0.24 μg cm−3) and Sandellʼs sensitivity (SS = 0.000675 ng cm−2) were estimated as well.


INTRODUCTION
Gallium is a post-transition metal of strategic importance for various priority areas of technology and science.It is known that it forms a colored anionic chelates with certain azo dyes {(4-(2-pyridylazo)resorcinol (PAR) and 4-(2-thiazolylazo)-resorcinol (TAR)}, which are associated with bulky organic cations to form a ternary ion associated complexes.−16 In our long term studies on ion associates a number of metals, such organic bases used are mono-and ditetrazolium salts.In this regard, studies have been conducted with the ion associates of Ga(III), which suffer the influence of the ligands, the influence of mono-and diterazolium bases, and of the various substituents on the molecule of the tetrazolium salt on the equilibria and the analytical characteristics of the test extraction systems.−19 In this work are presented research on the extraction system Ga(III)−PAR−Nt−H2O−CHCl3, which tetrazolium cation is replaced with triazolium.
Nt (Figure 1) is a low-cost and low-toxic analytical reagent that has been commercially available for more than a century; however, some novel aspects of its chemical nature have been recently disclosed.It should be said that extraction systems containing both PAR and Nt have been weakly studied.To the best of our knowledge, the only reported investigations in this field concern vanadium(V) 16 and cobalt(II) 20 extraction and spectrophotometric determination.

EXPERIMENTAL
The stock gallium(III) solution was prepared by heating for 20 min a known amount (0.1346 g) of Ga2O3 (Koch-Light Laboratories Ltd., 99,99 % in 37 % HCl (20 cm 3 ).After cooling, the obtained clear solution was collected into a 100-cm 3 calibrated flask and diluted to the mark with 6.5 mol dm −3 solution of HCl. 18Fresh working solutions (50 cm 3 ) were prepared every day by mixing an aliquot of the stock solution, 0.3 cm 3 of 6.5 mol dm −3 solution of HCl and distilled water.
Buffer solution, prepared by mixing 2.0 mol dm −3 aqueous solutions of CH3COOH and NH4OH.

Procedure for Establishing the Optimum Operating Conditions
Aliquots of Ga(III) solution, PAR solution (up to 1.2 cm 3 ), and buffer solution (5.0 cm 3 ; pH ranging 3.5 to 9.3) were introduced into 250-cm 3 separatory funnels.The resulting solutions were diluted with distilled water to a total volume of 10 cm 3 .Appropriate amounts of Nt solution and chloroform were added in a total volume of 10 cm 3 .Then the funnels were shaken for a fixed time (up to 5.0 min).A portion of the organic extract was filtered through a filter paper (to prevent the opportunity of water droplets transfer) into a cell and the absorbance read against a blank.

Procedure for Determination of the Distribution Constant
The distribution constant KD was found from the ratio KD = A1 / (A3 -A1), where A1 and A3 are the absorbances (measured against blanks) obtained after a single and triple extraction, respectively.The single extraction and the first stage of the triple extraction were performed under the optimum extraction-spectrophotometric conditions (Table 1).The organic layers were transferred into 25-cm 3 calibrated flasks and the flask for the single extraction was brought to volume with Nt solution.The second stage of the triple extraction was performed by adding another 10 cm 3 portion of the Nt solution to the aqueous phase, which remained after the first stage.After shaking, the organic layer was added to the one obtained after the first stage and the volume was brought to the mark with Nt solution.Before the spectrophotometric measurement, the calibrated flasks were shaken for homogenization.

Absorption Spectra
Spectra of the extracted ternary Ca(III)-PAR-Nt complex and the blank are shown in Figure 2. Maxima are recorded at 510 nm, where the blank absorbs insignificantly.It is shifted to 5−6 nm as compared to the maximum of the binary Ga-PAR chelate existing in aqueous solution with similar pH (λmaxGa(III0-PAR = 504-505 nm). 7,8

Effect of pH
The effect of pH for the extraction of Ga with PAR and Nt is represented in Figure 3.A buffer solution with a concentration of 2.0 mol dm −3 (prepared by mixing 2.0 mol dm −3 aqueous solutions of CH3COOH and NH4OH) was applied to control pH.All further experiments were carried out with 5.0 cm 3 buffer solution with pH 6.7−6.8.

Effect of Shaking Time
The extraction equilibrium is reached for a short shaking time (about 5 seconds) (Figure 4).To guarantee complete transfer of the complex into organic phase, the authors extracted in their experiments for 60 seconds.

Effect of Reagentʼs Concentration
The effect of PAR and Nt concentration on the absorbance are shown in Figure 5 and Figure 6, respectively.For up to 0.8 μg cm −3 of Ga, the use about 1.0 cm 3 2×10 −3 mol dm −3 PAR (13.9-fold excess) and 7.5 cm 3 3.5×10 −4 mol dm −3 Nt (2.4-fold excess) was found to be sufficient for a complete gallium extraction.

Composition of the Complex and Suggested Formula
The molar PAR-to-Ga(III) and Nt-to-Ga(III) ratios were determined by the mobile equilibrium method 21 (Figures 7 and 8), molar ratio method, 22 and the method of Asmus (Figures 9 and 10). 23The results showed that the ternary complex has a composition of 1 : 2 : 1 (Ga : PAR : Nt).We suggest the following formula of the extracted ternary species: (NtH) + [Ga 3+ (PAR)2] − .
In this formula, PAR is in deprotonated form (PAR 2− ), while Nt is in protonated form (NtH + ).

Equilibrium Constants and Recovery
The constant of association β were calculated by mobile equilibrium method. 21The constant of distribution KD, were determined in the above described procedure.The constant of extraction (Kex) and recovery factor (R / %) were determined by formulae Kex = KD×β and R / % = 100×KD / (KD + 1), respectively.The results are present in Table 2.

Beerʼs Law and Analytical Characteristics
The adherence to Beerʼs law for Ga(III)-PAR-Nt-waterchloroform system was examined under the optimum extraction-spectrophotometric conditions (Table 1).The linearity is observed up to 0.8 μg cm −3 of Ga.The molar absorptivity was calculated to be (10.3 ± 0.4)×10 4 dm 3 mol −1 cm −1 .This value could compete successfully with the ones obtained for similar PAR-containing complexes (Table 3).The limit of detection (LOD) and the limit of quantification (LOQ) were estimated.Sandellʼs sensitivity was calculated as well (Table 1).
The calculated equilibrium constants indicate that in the aqueous phase is formed, stable enough for this type of compounds, a ternary ion -associated complex, that is very well extracted into chloroform.These features, combined with analytical parameters give rise to claims that the analysis of gallium can be performed with high sensitivity and accuracy.The PAR and Nitron could complete successfully with many reagents for the spectrophotometric and liquid-liquid extraction for spectrophotometric determination of gallium(III). 7][18][19]24

Table 3 .
Spectral characteristics of some extracted in organic solvent Ga-PAR complexes