Flotation-Spectrophotometric Investigations on an Ion - SCIENTIFIC

Gavazov and Toncheva / Chemistry Journal (2013), Vol. 03, Issue 05, pp. 122-127

ISSN 2049-954X

Research Paper

Flotation-Spectrophotometric Investigations on an IonAssociation System Containing Zirconium (IV) Galya K. Toncheva and Kiril B. Gavazov* Dept. General and Inorganic Chemistry, Uni. of Plovdiv “Paissii Hilendarski”, 24 Tsaz Assen St., Plovdiv 4000, Bulgaria *E-Mail: [email protected]

Abstract A system containing zirconium (IV), 4-(2-pyridylazo) resorcinol (PAR), 2,3,5-triphenyl-2H-tetrazolium chloride (TTC), acetate buffer, water and organic solvents was studied. The optimum conditions for zirconium (IV) flotation and spectrophotometric determination as an ion-association complex were found: flotation solvent (chloroform), shaking time (2 min), pH (6.0-6.1, acetate buffer), concentration of the reagents (CPAR=2.8×10-4 mol L-1 and CTTC=2×10-4 mol L-1), sequence of their addition, dissolution solvent for the floated compound (dimethylsulphoxide) and its volume (4 mL). The molar ratio between Zr(IV), PAR and TTC in the floated compound was found to be 1:2:3. It could be regarded as an ion-associate between the triphenyltetrazolium cation (TT+) and an anionic chelate [Zr(OH)3–j(CH3COO)j(PAR)2]3– (most probably j=2 at the optimum conditions). The constant of association was calculated by two independent methods (Log β=12.8±0.2 and Log β=12.9±0.7). Some additional characteristics like conditional molar absorptivity, Sandell’s sensitivity, Beer’s law range, limit of detection, limit of quantification and relative standard deviation of the system for flotation-spectrophotometric determination of Zr(IV) were estimated. Beer’s law was obeyed for Zr(IV) concentrations up to 1.3 μg mL-1 at λmax=545 nm with a correlation coefficient of 0.9978. The conditional molar absorptivity was estimated to be ε545=1.2×105 L mol-1 cm-1. Keywords: Zirconium (IV), 4-(2-Pyridylazo) resorcinol, 2,3,5-Triphenyltetrazolium Chloride, Ion-Associate

1. Introduction 2,3,5-Triphenyl-2H-tetrazolium chloride (TTC) is a wellknown ion-association reagent (Gavazov et al, 2007) used for liquid-liquid extraction-spectrophotometric, potentiometric, chromatographic and gravimetric determination of various elements and species. In aqueous solutions, it forms hydrophobic 2,3,5-triphenyl-2H-tetrazolium cation (TT+), which readily associates with intensively coloured complex anions of the type [M(ADR)2]n–, where M is a metal ion {e.g. V(V), V(IV), Ga(III), In(III), Fe(III) and Co(III)} and ADR is an azoderivative of resorcinol {4-(2pyridylazo)resorcinol (PAR, H2L) or 4-(2-thiazolylazo) resorcinol}. The formed neutral ion-association complexes, (TT+)n[M(ADR)2], are chloroform-soluble. They can be easily extracted in this solvent and the obtained extracts can serve as a tool for spectrophotometric determination of the mentioned metal ions (Morgen & Dimova, 1984; Gav-

azov & Patronov, 2005; Genç et al, 2009; Gavazov & Racheva, 2011; Toncheva et al, 2011; Stojnova et al, 2012; Divarova et al, 2013 and Stefanova et al, 2013). It is known that Zr(IV) forms coloured species with PAR (Shtokalo, 1968; Nagarkar & Eshwar, 1974; Gupta & Munshi, 1976; Kragten & Parczewski, 1981; Kalyanaraman & Fukasawa, 1983; Kui et al, 1987 and Marczenko & Balcerzak, 2000). It can be represented with different formulae depending on the reaction conditions (pH, additional reagents used, etc.). Neutral e.g. [Zr(OH)3HL] (Shtokalo, 1968 and Toropova et al, 1975) and cationic species were detected e.g. [Zr(OH)2HL]+ or [Zr(OH)(HL)2]+ (Gupta & Munshi, 1976) at pH 1-4. In the presence of acetic acid the following mixed-ligand complexes were reported e.g. (at pH 1.5-3.2) [Zr(OH)(HL)(CH3COO)]+ and (at pH 4.5-10)

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[Zr(L)2(CH3COO)2]2-. [Zr(L)2(CH3COO)2]2- can form a nbutanol-extractable ion-associate with diphenylguanid-ine: (DPG+)2[Zr(L)2(CH3COO)2] (Kui et al, 1987). Our preliminary investigations on the Zr(IV)-PAR system in the presence of acetate buffer confirmed the formation of coloured species over a wide pH interval. However, these species were insoluble in water-immiscible organic solvents (e.g. chloroform and n-butanol) even in the presence of TTC. Instead, during the shaking with chloroform they accumulated on the phase boundary (and on the wall of the separating funnel) giving an opportunity for separation and further examination of the obtained compound. In this paper, we share results obtained during our systematic flotation-spectrophotometric investigations on a system containing Zr(IV), PAR, TTC, acetate buffer and organic solvents.

2. Materials and Methods 2.1. Reagents and Apparatus Standard zirconium solution was prepared according to Marczenko & Balcerzak (2000) by dissolving 3.9 g of ZrOCl2.8H2O (Sigma Aldrich, Puriss. p.a.) in 2 mol L-1 HCl. The obtained clear solution was collected into a 1 L calibrated flask and diluted to the mark with 2 mol L-1 solution of HCl. The resulting solution was standardized gravimetrically by precipitation with ammonia and ignition to ZrO2. Working solutions {100 mL; 1.0×10-4 mol L-1 Zr(IV)} were prepared by mixing an aliquot of the stock solution, 4 mL solution of H2SO4 (1:1) and distilled water. Aqueous solutions of the reagents were used: PAR (Sigma Aldrich; 2×10-3 mol L-1) and TTC (Loba Feinchemie; 2×10-3 mol L-1). The organic solvents were chloroform (redistilled) and dimethylsulphoxide (DMSO; Lab-Scan). The acidity of the aqueous medium was set by the addition of buffer solution, prepared by mixing 2.0 mol L-1 aqueous solutions of CH3COOH and NH4OH. The resulting pH was checked by HI-83140 pH-meter (Italy). A Camspec M508 spectrophotometer (United Kingdom), equipped with 10 mm path-length cells, was employed for reading the absorbance. 2.2. Procedure for Establishing the Optimum Flotation Conditions Aliquots of Zr(IV) solution, PAR solution (up to 1.5 mL), TTC solution (up to 1.8 mL) and buffer solution (2 mL; pH ranging from 4.5 to 8.5) were introduced into 125-mL separatory funnels. The resulting solutions were diluted with distilled water to a total volume of 10 mL. Then 10 mL of chloroform were added and the funnels were shaken for a fixed time (10-300 sec). After separation of the

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phases, they were both carefully removed, leaving the precipitate accumulated on the wall of the funnel. 2.3. Dissolution of the Floated Compound and Spectrophotometric Measurements The separation funnel was closed with stopper after adding 4 mL of DMSO. After shaking to dissolve the adhered ionassociate, a portion of the obtained clear solution was transferred into a cell. Then, the absorbance was measured at 545 and 400 nm against a blank.

3. Results and Discussion 3.1. Choice of Reaction Medium and Flotation Solvent A possibility of the flotation of Zr(IV)-PAR-TTC ion associates from various media (acetate buffer, H2SO4, HCl) by several water-immiscible organic solvents (chloroform, 1,2-dichloroethane, n-butanol, n-pentanol, benzene, toluene, tetrachloromethane, 5-methyl-2-hexanone, cyclohexane) was investigated. The most promising was the system Zr(IV)-PAR-TTC-acetate-chloroform. It was the subject of the further studies. 3.2. Choice of Solvent for the Floated Compound Various organic solvents were examined to dissolve the adhered ion-associate from the wall of the separating funnel (Table 1). Only the polar aprotic solvent dimethylsulphoxide (DMSO) was found appropriate for further experiments. Table 1. Organic Solvents Examined to Dissolve the Adhered IonAssociation Complex

Organic Solvent Diethyl ether n-Butanol Ethanol Methanol Ethyl acetate Acetone Dimethyl Sulphoxide

Dipole Moment

Dielectric Solubility of Constant the Complex

1.15 D 1.63 D 1.69 D 1.70 D 1.78 D 2.88 D

4.3 18 24.55 33 6.02 21

Partial

3.96 D

46.68

Excellent

3.3. Absorption Spectrum Spectra of the complex and blank in DMSO are shown in Figure 1. The complex is characterized by two maxima: at 545 nm and 400 nm. The first one is better for spectrophotometric determination of Zr(IV); it is more intensive and the blank at 545 nm is lower.


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Figure 1. Absorption Spectra of the Ternary Zr(IV)-PAR-TTCAcetate Complex and The Blank (PAR-TTC-acetate) in DMSO. CZr(IV) = 5.0 × 10–6 mol L–1, CPAR = 2.8 × 10–4 mol L–1, CTTC = 2.0 × 10–4 mol L–1, pH = 6.1 (2 mol L–1 Acetate Buffer), VDMSO= 4 mL, l = 1 cm

3.4. Order of Reagents Addition The order of reagents addition is of importance for colour development. A change of the order suggested in Sec. 2.2 (Zr, PAR, TTC and acetate buffer) e.g. introducing the buffer between Zr(IV) and PAR, may bring about a sharp decrease in the absorbance values due to irreversible hydrolysis of zirconium (Kalyanaraman & Fukasawa, 1983).

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Figure 2. Absorbance of ion-associate complex vs. pH of the Aqueous Phase Plots. CZr(IV) = 7.0 × 10–6 mol L–1, CPAR = 2.8 × 10–4 mol L–1, CTTC = 2.0 × 10–4 mol L–1, pH = 6.1 (2 mol L–1 acetate buffer), VDMSO= 4 mL, l = 1 cm

The effect of PAR and TTC concentrations on the absorbance is shown in Figure 3. One can conclude that the saturation with TTC is reached most easily (curve 1). The optimum reagent concentrations deduced from Figure 3 are shown in Table 2.

3.5. Buffer Solution and Effect of pH Buffer solutions prepared from acetic acid and ammonium hydroxide with concentration of 2 mol L-1 were applied to control pH. One reason for using buffered system was the kinetic phenomenon described by Kragten & Parczewski (1981)-if unbuffered Zr(IV)-PAR solution with pH 1 is neutralised rapidly to pH 3-3.5, the solution will remain at this pH for about 10 sec, then the pH jumps to ∼8. Another reason was the enhanced molar absorptivity of the complexes formed in the presence of acetate (Kui et al, 1987). The use of 1.5-2.5 mL of the buffer solution per 10 mL (final aqueous solution) was found to give a constant absorbance. The effect of pH on the absorbance is represented in Figure 2. It could be seen that the coloured compound is formed in a great extent at pH values between 5.9 and 6.2. All further experiments were performed with 2 mL buffer solutions with pH 6.0-6.1.

Figure 3. Absorbance of the Ion-Associate Complex vs. Concentration of the Reagent Plots 1: TTC. CZr(IV) = 5.0 × 10–6 mol L–1, CPAR = 2.8 × 10–4 mol L–1, pH = 6.1 (2 mol L–1 acetate buffer), VDMSO= 4 mL, l = 1 cm, λ=400 nm*; 2: PAR. CZr(IV) = 7.0 × 10–6 mol L–1, CTTC = 2.4 × 10–4 mol L–1, pH = 6.1 (2 mol L–1 acetate buffer), VDMSO= 4 mL, l = 1 cm, λ=540 nm *The results obtained at λ=400 nm during our optimization experiments with low TTC concentrations were more stable and reproducible than these at 545 nm

3.6. Effect of Shaking Time Further optimization of the flotation process was performed by testing different shaking times (from 10 to 300 seconds). It was found that the absorbance is maximum and constant for the interval between 60 and 300 seconds. Hence, we shook for about 120 seconds in our further experiments. 3.7. Effect of Reagents Concentrations

3.8. Composition of the Complex, Suggested Formula and Complex Formation Scheme The molar PAR-to-Zr(IV) and TTC-to-Zr(IV) ratios were determined at 545 and 400 nm by several methods i.e. the mobile equilibrium method (Figure 4) (Zhiming et al, 1997), the straight-line method of Asmus (1960), and the


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method of Bent & French (1941). The PAR-to-Zr(IV) ratio was 1:1 or 2:1 (by methods mentioned above) at low and high (optimum) PAR concentrations, respectively. Under the optimum PAR concentration the TTC-to-Zr(IV) ratio was 3:1 (mobile equilibrium method, method of Asmus). Having in mind these findings, the initial acidic media ensuring the existence of Zr4+ (Kragten & Parczewski, 1981 and Noren, 1973) and protonated PAR species (Gupta & Munshi, 1976 and Marić & Široki, 1996), and the ionassociation properties of TT+ (Gavazov et al, 2007) one can suggest the following scheme of complex formation under the optimum reagents concentrations:

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ic system (Equation 2), most pro-bably can be represented under the optimum pH with a for-mula in which j=2: (TT+)3[Zr(OH)(CH3COO)2L2].

Table 2. Optimum Conditions and Analytical Characteristics*

Optimum Conditions

Analytical Characteristics

Conditional molar Absorption maximua: 545 absorptivity: ε(I) = 1.2 × 105 nm (I); 400 nm (II) L mol-1 cm-1; ε(II) =9.5 × 104 L mol-1 cm-1 Beer’s law range: pH: 6.0-6.1 (acetate buffer) up to 1.3 µg mL-1 Limit of detection: 48 ng mL-1 CPAR: 2.8 × 10-4 mol L-1 (I); 88 ng mL-1 (II) Limit of quantification: 160 CTTC: 2.0 × 10-4 mol L1 ng mL-1 (I); 290 ng mL-1 (II) Sandell’s sensitivity: 0.77 ng Shaking time: 2 min cm-2 (I); 0.96 ng cm-2 (II) Relative standard deviation (5 replicate measurements of 0.3 Volume of DMSO: 4.0 mL µg mL-1 Zr(IV)): 2.9% (I); 3.3 (II)

Figure 4. Determination of the TTC-to-Zr(IV) (Straight Line 1) and PAR-to-Zr(IV) (Straight Line 2) Molar Ratios by the Mobile Equilibrium Method 1: TTC. CZr(IV) = 5.0 × 10–6 mol L–1, CPAR = 2.8 × 10–4 mol L–1, pH = 6.1 (2 mol L–1 acetate buffer), VDMSO= 4 mL 2: PAR. CZr(IV) = 7.0 × 10–6 mol L–1, CTTC = 2.4 × 10–4 mol L–1, pH = 6.1 (2 mol L–1 acetate buffer), VDMSO= 4 mL

3.9. Association Constant

* All Experiments were performed at room temperature of ~22 °C and the calculations carried out at a probability of 95%

The association constant (β), characterizing Equation 2, was calculated by two independent methods: Holme-Langmyhr method (1966) and mobile equilibrium method (Zhiming et al, 1997) (Figure 4, straight line 1). Statistically indistinguishable values were obtained: Log β = 12.8±0.2 (Holme-Langmyhr, 1966, method) and 12.9±0.7 (mobile equilibrium method). This is an indication of the correctness of performed experiments and suggested complex formation scheme.

i) Formation of an anionic chelate with a charge of 3:

3.10. Beer’s Law and Analytical Characteristics

Zr4++2H3L+ +jCH3COO–+(3-j)H2O ⇔ [Zr(OH)3–j(CH3COO) jL2]3– + (9-j)H+.....1

ii) Ion-associate formation [Zr(OH)3–j(CH3COO) jL2]3– + 3TT+ ⇔ (TT+)3[Zr(OH)3–j(CH3COO) jL2]……...2

Kui et al (1987) showed that the molar Zr(IV)-toCH3COOH ratios in aqueous media can be 1:1 {[Zr(OH)(CH3COO)(HL)]+} or 1:2 {[Zr(CH3COO)2L2)]2-} at pH 3.5 and 7.5 respectively. The second complex is anionic; it contains PAR in de-protonated form (L2-) and can be extracted in organic medium in the presence of the cationic reagent diphenylguanidine. Comparing our results with those obtained by Kui et al (1987), we suggest that the ion-associate formed in our flotation-spectrophotometr-

Beer’s law, checked at optimum conditions (Table 2), was valid up to 1.3 μg mL-1 of Zr(IV). A straight-line with equation y=1.301x–0.0015 and a correlation coefficient of 0.9978 was obtained. The Limits of Detection (LOD) and quantification (LOQ) were calculated as 3 times and 10 times standard deviation of the intercept divided by the slope. Their values are shown in Table 2. The conditional molar absorptivity at λmax=545 nm was near 1.3 times higher than that obtained at λ=400 nm. The relative standard deviation for 5 replicate measurements of 0.3 µg mL-1 Zr(IV) (Table 2) was close to that reported by Pourreza et al (2010) for their flotation-spectrophotometric method for zirconium(IV) determination involving xylenol orange.

4. Conclusion


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Zirconium (IV) forms coloured species with PAR and TTC in acetate buffered medium, which can be used for flotation separation and spectrophotometric determination of zirconium. Optimum conditions for the formation of ion-association complex between TT+ and an anionic chelate with a charge of 3-{suggestible formula [Zr(OH)(CH3COO)2 (PAR)2]3-} were found. The constant of association was calculated. The following analytical characteristics were established: conditional molar absorptivity, Sandell’s sensitivity, Beer’s law range, limit of detection, limit of quantification, and relative standard deviation. The results show that Zr(IV) can be separated and subsequently determined by a simple, sensitive, reproducible and inexpensive procedure.

Gupta, G.P., and Munshi, K.N. (1976) Study of the complexing equilibria involved between zirconium and gallion and 4-(2-pyridylazo) resorcinol in aqueous solution. Microchem. J., 21(2), pp. 140-148.

Acknowledgments

Kragten, J., and Parczewski, A. (1981) Photometric complex-formation titration of submicromolar amounts of zirconium. Talanta, 28(3), pp. 149-155.

This work was supported by the Research Fund of the Plovdiv University (Grant No NI13-HF-006).

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(2013) Extraction-spectrophotometric characterization of a ternary complex of iron(III) with 4-(2-pyridylazo)resorcinol (PAR) and 2,3,5-triphenyl-2H-tetrazolium chloride (TTC). Sci. Res. Union Sci. Plovdiv., in press. Stojnova, K., Gavazov, K., Toncheva, G., Lekova, V., and Dimitrov, A. (2012) Extraction-spectrophotometric investigations on two ternary ion-association complexes of gallium(III). Cent. Eur. J. Chem., 10(4), pp. 1262-1270. Toncheva, G., Gavazov, K., Lekova, V., Stojnova, K., and Dimitrov, A. (2011) Ternary ion-association complexes between the indium(III)-4-(2-pyridylazo)resorcinol anionic chelate and some tetrazolium cations. Cent. Eur. J. Chem., 9(6), pp. 1143-1149. Toropova, V.F., Budnikov, G.K., Maysrenko, V.N., and Munin, E.N. (1975) Polarographic investigation of complex forming of zirconium (IV) and hafnium (IV) with 4-(2pyridylazo) resorcinol and 4-(2-pyridylazo)-2-naphtol. Zh. Neogr. Khim., 20(12), pp. 3269-3273. Zhiming, Z., Dongsten, M., and Cunxiao, Y. (1997) Mobile equilibrium method for determining composition and stability constant of coordination compounds of the form MmRn. J. Rare Earths, 15(3), pp. 216-219.


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