Scientific paper
Ion-Association Complexes of Gallium(III) -4-(2-Pyridylazo)-resorcinol Anionic Chelates and Dicationic Tetrazolium Reagents
Kirila T. Stojnova, Kiril B. Gavazov* and Vanya D. Lekova
Department of General and Inorganic Chemistry, University of Plovdiv "Paisii Hilendatski", 24 Tsar Assen St., 4000 Plovdiv, Bulgaria
* Corresponding author: E-mail: kgavazov@abv.bg
Received: 21-02-2013
Abstract
The formation and liquid-liquid extraction of ion-association complexes between gallium(III)-4-(2-pyridylazo)resorci-nol (PAR) anionic chelates and cations of four ditetrazolium chlorides (DT2+) were studied: Neotetrazolium chloride (NTC), Blue Tetrazolium chloride (BTC), Nitro Blue Tetrazolium chloride (NBT) and Tetranitro Blue Tetrazolium chloride (TNBT). The optimum extraction-spectrophotometric conditions, composition of the extracted species {1:2:1 and/or 1:2:2 (Ga:PAR:DT)}, some equilibrium constants {constants of association (fi), constants of distribution (KD) and constants of extraction (Kex)} and analytical characteristics {molar absorptivity (£), Sandell's sensitivity, intervals of adherence to Beer's law, etc.} were found. Relationships involving the number of nitro groups in DT2+ (Nnitro; DT2+ = BT2+, NBT2+ and TNBT2+) or molecular mass of DT2+ (MM; DT2+ = NT2+ and BT2+) were discussed: Log fi = /(Nmtt0), Log Kd = /(Nmtt0), Log K„ = /(NmtJ, Log £ = /(NmtJ and Log fi = /(Log MM).
Keywords: Gallium-PAR chelate, bis-tetrazolium salt, nitro group, linear relationship, ion-associate, solvent extraction
1. Introduction
Ion-association reagents are usually bulky compounds capable of forming cations or anions with a charge of unity, which is distributed over the whole ion.1 However, some organic salts can form analytically important cations with a charge of two.2-11 Such salts, and the reactions in which they participate, are generally less studied, despite the fact that they may offer some advantages in comparison to typically used monocationic ion-associa-
5 12
tion reagents.5 12
In previous papers we described the complex formation and liquid-liquid extraction from water to chloroform in systems containing gallium(III), 4-(2-pyridylazo)-re-sorcinol (PAR) and monotetrazolium salt {2,3,5-trip-henyl-2H-tetrazolium chloride,13 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide,14 3-(2-naphtyl)-2,5-diphenyl-2H-tetrazolium chloride,15 and 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazo-lium chloride15}. Here, we focused our attention on four similar systems involving commercially available ditetra-zolium chlorides (DTCs), namely: Neotetrazolium chloride (NTC), Blue Tetrazolium chloride (BTC), Nitro Blue
Tetrazolium chloride (NBT) and Tetranitro Blue Tetrazolium chloride (TNBT). General formula and nomenclature names of the mentioned DTCs are shown in Table 1.
2. Experimental Procedure
2. 1. Reagents and Apparatus
A stock gallium(III) solution was prepared by dissolving 0.1312 g of Ga2O3 (Koch-Light Laboratories Ltd., 99,99%) in a hot concentrated HCl solution (20 mL). After cooling, the obtained clear solution was collected into a 100-mL calibrated flask and diluted to the mark with 6.5 mol L1 solution of HCl. Fresh working solutions (50 mL; 1.4 ■ 10-4 mol L1 Ga(III)) with pH ca. 1.1 were prepared every day by mixing 0.5 mL of the stock solution, 0.3 mL of 6.5 mol L1 solution of HCl and distilled water. Aqueous 2 ■ 10-3 mol L1 solutions of the reagents were used: PAR, NTC, BTC and TNBT (all from Sigma-Aldrich Chemie GmbH) and NBT (from Merck KGaA). The organic solvent, chloroform, was distilled before use. The acidity of the aqueous medium was set by the addition of buffer solution, prepared by mixing 2.0 mol L1 aqueous solutions of
Table 1. Ditetrazolium chlorides (DTCs) used in the present study
Substituents
R1 R2 R3
Name
Abbreviation
H H H H H OCH
NO2 H och3
no2 no2 och3
3,3'-(4,4'-biphenylene)bis(2,5-diphenyl-2H-tetrazolium chloride)	NTC
(Neotetrazolium chloride) 3,3'-(3,3'-dimethoxy-4,4'-biphenylene)bis(2,5-diphenyl-2H-tetrazolium chloride)	BTC
(Blue Tetrazolium chloride) 3,3'-(3,3'-dimethoxy-4,4'-biphenylene)bis[2-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride] NBT
(Nitro Blue Tetrazolium chloride) 3,3'-(3,3'-dimethoxy-4,4'-biphenylene)bis[2,5-di(4-nitrophenyl)-2H-tetrazolium chloride] TNBT (Tetranitro Blue Tetrazolium chloride)
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 Operating Conditions
Aliquots of Ga(III) solution, PAR solution (up to 1 mL), DTC solution (up to 1.3 mL) and buffer solution (5 mL; pH ranging from 4 to 9.4) were introduced into se-paratory 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 3-300 sec. Portions of the organic extracts were transferred through paper filters into cells. The absorban-ces were recorded against respective blank samples.
2. 3. Procedure for Determination of the Distribution Constants
The distribution constants KD were found from the ratio Kd = A1/(A3-A1) where A1 is the light absorbance obtained after a single extraction (at the optimum operating conditions - see Table 2) and A3 is the absorbance obtained after a triple extraction under the same condi-
3. Results and Discussion 3. 1. Absorption Spectra
Spectra of the ternary Ga(III)-PAR-DTC complexes extracted in chloroform at the optimum conditions are shown in Fig. 1. Their chief absorption maxima are situa-
tions.1
The final volume of the measured solutions in
both cases was 25 mL.
Fig. 1. Absorption spectra of the ternary complexes (curves 1-4; CGa(III) = 7.2 • 10-6 mol L-1) and blank samples (curves 1'-4') in chloroform at the optimum extraction conditions. (1,1') CPAR = 1.4 • 10-4 mol L-1, CNTC = 1.8 • 10-4, pH = 7.5; (2,2') CPAR= 1.0 • 10-4 mol L-1, CBTC = 1.2 • 10-4, pH = 5.7; (3,3') CPAR= CNBT = 1.6 • 10-4 mol L-1, pH = 6.0; (4,4') CPAR= 1.4 • 10-4 mol L-1, CTNBT = 2.0 • 10-4, pH = 5.0.
Table 2. Optimum extraction-spectrophotometric conditions
Extraction systems	Extaction time, min	PH	CPAR, mol L-1	CDtc, mol L 1	nm
Ga(III)-PAR-NTC	2	6.5-8.0	1.4 x 10-4	1.8 x 10-4	510
Ga(III)-PAR-BTC	2a, b	5.5-6.0a, 7.0-8.0b	1.0 x 10-4 a, b	1.2 x 10-4 a, b	510 a, b
Ga(III)-PAR-NBT	2	5.0-7.0	1.6 x 10-4	1.6 x 10-4	511
Ga(III)-PAR-TNBT	2	4.9-5.1	1.4 x 10-4	2.0 x 10-4	513
a for the complex with Ga-PAR-BTC ratio of 1:2:1 b for the complex with Ga-PAR-BTC ratio of 1:2:2
ted in the interval 510-513 nm (see Table 2). BTC and NTC - salts which do not contain NO2 groups - form the most intensely absorbing complexes (Fig. 1, curves 1 and
2 ¿max = 510 nm, e510 =
8.5 ■ 104 L mol-1 cm-1).
3. 2. Effect of pH
The effect of pH on the extraction is represented in Fig. 2. The differences in the pH intervals for maximum extraction are an indication of a different ability of DTCs (NTC, BTC, NBT and TNBT) to facilitate deprotonation of 1-hydrohyl group in the complexed PAR during the formation of ternary species; another explanation can be their ability to stabilize complex anions with different composition and charge.17
A narrowest pH interval for maximum extraction is recorded for the system containing TNBT (curve 4). A similar behaviour of this DTC (in comparison to other DTCs) has been reported for the extaction systems containing V(V)-PAR,17 V(IV)-4-(2-thiazolylazo)resorcinol17 and In(III)-PAR.16 The complicated course of curve 2 (DTC = BTC) will be discussed in Sec. 3.5.
Fig. 2. Absorbance of Ga(III)-PAR-DTC complexes in chloroform vs. pH of the aqueous phase plots. CGa(III) = 7.2 • 10-6 mol L-1; CPAR = CDTC = 2.0 • 10-4 mol L-1. (1) DTC = NTC, X = 510 nm; (2) DTC = BTC, X = 510 nm; (3) DTC = NBT, X = 511 nm; (4) DTC = TNBT, X = 513 nm.
3. 3. Effect of Shaking Time
The extraction equilibrium for Ga(III)-PAR-DTC systems is reached for about 60-90 seconds (Fig. 3). In all cases, longer shaking time does not affect the absorbance. In order to avoid accidental errors, caused by a combination of short shaking times and different shaking rates, we extracted in our further experiments for 2 min.
3. 4. Effect of Reagents Concentration
The effect of PAR and DTC concentrations on the absorbance of the extracted species is shown in Fig. 4 and Fig. 5, respectively. One can conclude that the saturation with PAR and DTC is reached most easily in the system in
Fig. 3. Effect of shaking time on the extraction. CG mol L-1; CPAR= CDTC = 2.0 • 10-4 mol L-1.
: 7.2 ■ 10-6
which DTC = BTC (Fig. 4, curve 2 and Fig. 5, curve 2). The optimum reagent concentrations deduced from the mentioned figures are shown in Table 2.
0.600
0.400
0.200
o.ooo
	♦ * m i
	» * i
r **	
	
/	♦(1 ) G a-PAR-NT C
if	■(2) Ga-PAR-BTC
ÏÏ	•(3) Ga-PAR-NBT
/ .	(4) Ga-PAR-TNBT
o.o
0.3
0.6
0.9 Cpar 1
1.2 (mol L-
1.5
1.8
Fig. 4. Absorbance of the extracted ternary Ga(III)-PAR-DTC complexes vs. concentration of the PAR plots. CGa(In) = 7.2 • 10-6 mol L-1, CDTC = 2.0 • 10-4 mol L-1. (1) pH = 7.5, X = 5510 nm; (2) pH = 5.7, X = 510 nm; (3) pH = 6.0, X= 511 nm; (4) pH = 5.0, X = 513 nm.
Fig. 5. Absorbance of the extracted ternary Ga(III)-PAR-DTC complexes vs. concentration of the DTC plots. CGa(IE) = 7.2 • 10-6 mol L-1. (1) CPAR= 1.4 • 10-4 mol L-1, pH = 7.5, X = 510 nm; (2) CPAR = 1.0 • 10-4 mol L-1,pH = 5.7, X = 510 nm; (2') CPAR= 1.0 • 10-4 mol L-1, pH = 7.5, X = 510 nm; (3) CPAR = 1.6 • 10-4 mol L-1,pH = 6.0, X = 511 nm; (4) CPAR= 1.4 • 10-4 mol L-1,pH = 5.0, X= 513 nm.
3. 5. Composition of the Complexes, Suggested Formulae and Reaction Schemes
The saturation curves presented in Fig. 4 and Fig. 5 allowed us to determine the molar ratios PAR:Ga(III) and DTC:Ga(III) by the mobile equilibrium method18 and straight-line method of Asmus19 (Table 3). The results obtained by both methods show that the PAR-to-Ga(III) ratio in all cases is 2:1. However, the DTC-to-Ga(III) ratio appears to be different at the optimum extraction conditions when DTC = BTC (BTC:Ga = 1:1) or DTC * BTC (DTC:Ga = 2:1). It is known that under the working pH the predominant forms of Ga(III) and PAR are Ga(OH)4-20 and HL-.21,22 Having in mind the mentioned above molar ratios, the bulkiness and envelope properties of DTCs, their ability to form DT2+,7,12 and the literature describing the posible formation of ternary complexes with hy-drolyzed nature12,16,17,23 we can propose the following two different schemes for complex formation:
Table 3. Determination of the PAR-to-Ga(III) and DTC-to-Ga(III) molar ratios (n and m, respectively) under the optimum or non-optimum (*) extraction conditions by the method of Asmus19 from the experimental data given in Fig. 4 and Fig. 5, respectively.
Extraction	Correlation coefficient squared values (CC2)			
system	corresponding to molar ratios 1, 2 and 3			
	PAR:Ga(III)		DTC:Ga(III)	
Ga(III)-PAR-	0.9461 (n =	1)	0.9410 (m = 1)	
NTC-water-	0.9986 (n =	2)	0.9904 (m = 2)	
chloroform	0.9849 (n =	3)	0.9646 (m = 3)	
Ga(III)-PAR-	0.9525 (n =	1)	0.9993; 0.9148* (m =	1)
BTC-water-	0.9968 (n =	2)	0.9215; 0.9956* (m =	2)
chloroform	0.9571 (n =	3)	0.8381; 0.9731* (m =	3)
Ga(III)-PAR-	0.9607 (n =	1)	0.9538 (m = 1)	
NBT-water-	0.9968 (n =	2)	0.9992 (m = 2)	
chloroform	0.9734 (n =	3)	0.9756 (m = 3)	
Ga(III)-PAR-	0.9841 (n =	1)	0.9228 (m = 1)	
TNBT-water-	0.9945 (n =	2)	0.9969 (m = 2)	
chloroform	0.9575 (n =	3)	0.9838 (m = 3)	
Ga(OH)4- + 2HL- ^ [Ga(OH)L2]2 - + OH- + 2H2O
[Ga(OH)L2]2 - + BT2+ ^ (BT)[Ga(OH)L2]
(BT)[Ga(OH)LJaq ^ (BT)[Ga(OH)L2]org
(1.1) (1.2) (1.3)
Scheme 1. Complex formation and extraction in the system Ga(III)-PAR-BTC-water-chloroform at pH 5.7.
Ga(OH)4- + 2HL- ^ [Ga(OH)3L2]4 + H3O+
[Ga(OH)3L2]4 + 2DT2+; (DT)2[Ga(OH)3L2]
(DT^Ga^H)^
3L2]org
2	3 2 aq
(DT)2[Ga(OH)3L2]
(2.1) (2.2) (2.3)
Scheme 2. Complex formation and extraction in the systems Ga(III)-PAR-DTC-water-chloroform. When DTC = BTC, pH should be higher than optimum (ca. 7.5).
* CGa(III) = 7.2 • 10-6 mol L-1, CPAR = 1.0 • 10-4 mol L-1,pH = 7.5, A = 510 nm.
The formation of a different complex with BT (Scheme 1) could be explained in the frame of considerations given in Ref 17. However, the complicated course of the pH-curve of this reagent (Fig. 2, line 2) and the literature12,16 gave us reason to repeat the procedure for determination of the BTC-to-Ga(III) molar ratio at higher pH (pH 7.5). The results (Table 3,*) showed that the molar BTC-to-Ga ratio grows to 2:1. Hence, the dominant processes at pH close to 7.5 are the same as in the systems with the rest of DTCs at the optimum conditions (Scheme 2).
3. 6. Equilibrium Constants
The constants of association /3, characterizing Eq. 1.2 and Eq. 2.2, were calculated by the Holme-Langmyhr method,24 Harvey-Manning method25 and mobile equilibrium method.18 The constants of distribution KD, characterizing Eq. 1.3 and Eq. 2.3, were determined by comparison of the absorbance values obtained after single and triple extractions at the optimum conditions (as described in Sec. 2.3). Then, the constants of extraction (K ) and reco-
Table 4. Calculated values (P = 95%) of the extraction constants (Kex), distribution constants (KD), association constants (/) and recovery factors (R%)
Extraction system
Log P
Log Kd
Log K
» ex
R%
Ga(III)-PAR-NTC-H2O-chloroform Ga(III)-PAR-BTC-H2O-chloroform
Ga(III)-PAR-NBT-H2O-chloroform Ga(III)-PAR-TNBT-H2O-chloroform
8.9 ± 0.2a, 9.6 ± 0.6b, 9.6 ± 1.4c 5.3 ± 0.2a,d, 5.1 ± 0.2b,d,5.1 ± 0.2c,d 10.2 ± 0.5a,e, 11.0 ± 0.3b,e 9.7 ± 0.2a, 9.9 ± 0.3b, 9.8 ± 0.8c 9.2 ± 0.2a, 9.4 ± 0.2b, 9.0 ± 0.8c
0.79 ± 0.01 0.96 ± 0.01d 1.08 ± 0.01e 0.95 ± 0.01 0.56 ± 0.01
9.7	± 0.2,10.4 ± 0.6e
6.3 ± 0.2d,f, 6.1 ± 0.2d,g
11.3 ± 0.5e,f,12.1 ± 0.3e,g 10.7 ± 0.2f, 10.9 ± 0.2e
9.8	± 0.2f, 10.0 ± 0.2g
86.1 ± 0.1 90.0 ± 0.1d 92.4 ± 0.1e 89.9 ± 0.1 78.4 ± 0.1
a Calculated by the Holme-Langmyhr method24 b Calculated by the Harvey-Manning method25 c Calculated by the Mobile equilibrium method18 d at pH 5.7 e at pH 7.5 f Calculated by the equation Log Kex = Log / + Log KD, where / is the value obtained by the Holme-Langmyhr method g Calculated by the equation Log Kex = Log / + Log KD, where / is the value obtained by the Harvey-Manning method
very factors (R%) were determined by the formulae Kex = Kd p and R% = 100KD / (KD + 1), respectively. The results are presented in Table 4. All experiments were performed at room temperature of ~22 °C and the calculations were carried out at a probability of 95 %.
3. 7. Some Relationships Involving the Equilibrium Constants Obtained
Continuous investigations on tetrazolium ion-association complexes revealed that at least two factors noticeably influence the values of P: molecular mass of tetrazolium cation (MM) (I) and the presence of nitrophenyl substituent(s) in the tetrazolium ring (II).717 The studied in the present paper DTCs are suitable for testing both (I) and (II). BT2+, NBT2+ and TNBT2+ differ by the number of nitro groups (Nnitro): 0, 2 and 4, respectively. NT2+ and
Fig. 6. Logarithm of association constants of ion-association complexes containing DT2+ (TNBT2+, NBT2+ or BT2+) vs. number of nitro groups in DT2+ plots. (1) (DT2+)2[Ga(OH)3(PAR)2]; (1') (BT2+) [Ga(OH)(PAR)2]; (2) (DT^yMOH^PARy;16 (3) (DT2+)2 [VO(OH)2(TAR)2] {TAR = 4-(2-thiazolylazo)-resorcinol); DT2+ is NBT2+ and TNBT2+};17 (3') (BT2+)[VO(OH)2(TAR)];17 (4) (DT2+)2 [VO(OH)2(PAR)J;17 (5) (DT2+)3[VO2(PAR)2]2.26
BT2+ do not contain nitro groups; their MMs are 596.7 and 656.76, respectively.
Literature2,12,16,17,23,26 and the results described in the previous section (3.6) were used in the construction of Fig. 6 and Table 5. It could be seen (Fig. 6; Table 5, row 1) that a well-defined negative linear relationship (slope of -0.275; squared correlation coefficient of 0.9973) exists between Log P and Nnitro for the series (TNBT2+)2[Ga (OH)3(PAR)2], (NBT2+)2[Ga(OH)3(PAR)2] and (BT2+)2 [Ga(OH)3(PAR)2]. Similar is the situation with the dependences Log Kd = /(Nnitro), Log K^ = /(NùJ and Log e = /(Nnitro) (Table 5, rows 2, 3 and 4). The slopes in row 5 (Table 5) are positive; this is a confirmation that heavier DTs2+ form more stable ion-associates.7
3. 8. Beer's Law and Analytical Characteristics
The adherence to Beer's law for each Ga(III)-PAR-DTC-water-chloroform system was examined under the optimum extraction-spectrophotometric conditions (Table 2). Calculated molar absorptivities (e) are listed in Table 6, along with some other important analytical characteristics. One can conclude that BTC-PAR and NTC-PAR ensure highest sensitivity of determination. In this criterion, they are better than the reagents used in similar systems: Ga(III)-1-(2-pyridylazo)-2-naphthol-wa-ter-chloroform (e = 2.7 ■ 104 L mol1 cm-1),22 Ga(III)-Eriochrom Black T-water- chloroform- n-butanol- ca-pronic acid (e = 3.4 ■ 104 L mol1 cm-1),27 Ga(III)-4-(2-thiazolylazo)-resorcinol-2,3,5-triphenyl-2H-tetrazolium chloride (e = 4.6 ■ 104 L mol1 cm1),13 Ga(III)-Pyroca-techol Violet-tridodecylethylammonium bromide-wa-ter-xylene (e = 8.0 ■ 104 L mol1 cm1),28 Ga(III)-PAR-tetraphenylarsonium chloride-water-1,2-dichloro-benzene (e = 8.2 ■ 104 L mol1 cm-1)29 and Ga(III)-PAR-2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetra-zolium chloride-water-chloroform (e = 8.2 ■ 104 L mol1 cm-1).15
Table 5. Straight-line equations (y = ax + b) and squared correlation coefficients (CC2) for series of ion-associates containing DT2+ and PAR. DT2+ = BT2+, NBT2+ and TNBT2+ (Rows No. 1, 2, 3 and 4); DT2+ = NT2+ and BT2+ (Row No. 5)
No.	System	In(III)-PAR-DTC-	V(V)-PAR-DTC-	V(IV)-PAR-DTC-	Ga(III)-PAR-DTC
	Relationship	H2O-CHCl3	H2O-CHCl3	H2O-CHCl3	H2O-CHCl3
		[Ref. 16, 23]	[Ref. 2, 26]	[Ref. 12, 17]	[This work]
1	Log P= f(Nmto)	y = -0.25x + 9.70	y = -0.425x + 8.42	y = -0.700x + 11.43	y = -0.275x + 10.22
		CC2 = 0.8929	CC2 = 0.9988	CC2 = 0.9017	CC2 = 0.9973
2	Log Kd = f(Nmtt0)	y = -0.40x + 1.24	y = -0.05x + 1.37		y = -0.13x + 1.12
		CC2 = 0.8928	CC2 = 0.9932	-	CC2 = 0.9231
3	LOg Kex = f(Nnitro)	y = -0.65x + 10.93	y = -0.50x + 9.80		y = -0.40x + 11.37
		CC2 = 0.9980	CC2 = 1.000	-	CC2 = 0.9796
4	Log £= f(Nnitro)	y = -0.152x + 4.94	y = -0.08x + 4.63	y = -0.039x + 4.557	y = -0.033x + 4.93
		CC2 = 0.8362	CC2 = 0.9498	CC2 = 0.5773	CC2 = 0.9866
5	Log p= f(Log MM)	y = 24.0x - 57.9	y = 28.8x - 72.8	y = 58.6x - 153.3	y = 31.2x - 77.7
Table 6. Characteristics concerning the application of the ion-association complexes for extractive-spectrophotometric determination of gallium
Extraction system	Ga(III)-PAR-NTC- Ga(III)-PAR-BTC- Ga(III)-PAR-NBT- Ga(III)-PAR-TNBT-
Analytical characteristics	H2O-CHCl3	H2O-CHCl3	H2O-CHCl3	H2O-CHCl3
Apparent Molar absorptivity (e), L mol-1 cm-1	8.5 • 104	8.5 • 104	7.1 • 104	6.2 • 104
Sandell's sensitivity (SS), ng cm-2	0.82	0.82	0.99	1.13
Adherence to Beer's law, |g mL-1	up to 1.4	up to 0.9	up to 1.4	up to 1.4
Squared correlation coefficient (CC2)	0.9990	0.9997	0.9957	0.9999
Limit of detection (LOD), |g mL-1	0.047	0.02	0.10	0.016
Limit of quantification (LOQ), |g mL-1	0.16	0.07	0.33	0.052
Absorbance of the blank at Amax	0.112 ± 0.005	0.089 ± 0.002	0.132 ± 0.003	0.93 ± 0.003
(in parenthesis)	(510 nm)	(510 nm)	(511 nm)	(5131 nm)
4. Conclusions
1.	Ga(III) readily forms chloroform-extractable species with PAR and DTCs.
2.	All investigated DTCs can form complexes with a composition of 1:2:2 (Ga:PAR:DTC) and a suggested general formula (DT2+)2[Ga(OH)3(PAR)2]. Under the optimum extraction conditions BTC forms a simpler complex with a composition of 1:2:1 {suggested formula (BT2+)[Ga(OH)(PAR)2]}.
3.	With increase of the number of nitro groups (Nnitro) in the series BT2+, NBT2+ and TNBT2+ the stability, extrac-tability and molar absorptivity of the ionic associates (DT2+)2[Ga(OH)3(PAR)2] decrease. Well-defined negative linear relationships exists between Nnitro and Log ¡3 (CC = -0.9986), Nnltr0 and Log KD (CC = -0.9608), Nmtro and Log Kex (CC = -0.9897), and Nnitro and Log e (CC = -0.9933).
4.	BTC appears to be the best DTC for liquid-liquid extraction and spectrophotometric determination of Ga(III).
5. Acknowledgments
The authors would like to thank the Research Fund of the Plovdiv University for its long-time support.
6. References
1.	K. Toei, Anal. Sci. 1987, 3, 479-488.
2.	K. Gavazov, Z. Simeonova, A. Alexandrov, Anal. Lab. 1998,
7, 127-133.
3.	M. Kamburova, A. Alexandrov, Chem. Anal. (Warsaw) 1999,
44, 745-752.
4.	P. K. Martinelango, J. L. Anderson, P. K. Dasgupta, D. W.
Armstrong, R. S. Al-Horr, R. W. Slingsby, Anal. Chem.
2005,	77, 4829-4835.
5.	J. L. Anderson, D. W. Armstrong, G.-T. Wei, Anal. Chem.
2006,	78, 2892-2902.
6.	A. Dimitrov, V. Lekova, K. Gavazov, B. Boyanov, J. Anal.
Chem. 2007, 62, 122-125.
7.	K. B. Gavazov, A. N. Dimitrov, V. D. Lekova, Uspekhi Khim.
2007,	76, 187-198.
8.	P. K. Martinelango, P. K. Dasgupta, Anal. Chem. 2007, 79, 7198-7200.
9.	J. W. Remsburg, R. J. Soukup-Hein, J. A. Crank, Z. S. Breitbach, T. Payagala, D. W. Armstrong, J. Am. Soc. Mass Spec-trom. 2008, 19, 261-269.
10.	X. Lin, A. R. Gerardi, Z. S. Breitbach, D. W. Armstrong, C. L. Colyer, Electrophoresis 2009, 30, 3918-3925.
11.	D. V. Kostova, M. A. Kamburova, J. Int. Sci. Publ. Ecol. Saf. 2010, 4, 162-169.
12.	F. Genf, K. B. Gavazov, M. Türkyilmaz, Cent. Eur. J. Chem. 2010, 8, 461-467.
13.	K. Stojnova, K. B. Gavazov, G. K. Toncheva, V. D. Lekova, A. N. Dimitrov, Cent. Eur. J. Chem. 2012, 10, 1262-1270.
14.	K. Stojnova, K. Gavazov, J. Mater. Sci. Eng. Sec. A. 2012, 2, 423-429.
15.	K. B. Gavazov, K. T. Stojnova, T. S. Stefanova, G. K. Toncheva, V. D. Lekova, A. N. Dimitrov, Chemija 2012, 23, 278-285.
16.	T. S. Stefanova, K. B. Gavazov, Cent. Eur. J. Chem. 2013, 11, 278-289.
17.	K. B. Gavazov, P. V. Racheva, V. D. Lekova, A. N. Dimitrov, M. Turkyilmaz, F. Genc, Croat. Chem. Acta 2012, 85, 53-58.
18.	Z. Zhiming, M. Dongsten, Y. Cunxiao, J. Rare Earths 1997, 15, 216-219.
19.	E. Asmus, Fresenius'J. Anal. Chem. 1960, 178, 104-116.
20.	S. A. Wood, I. M. Samson, Ore Geol. Rev. 2006, 28, 57-102.
21.	L. Maric, M. Široki, Anal. Chim. Acta 1996, 318, 345-355.
22.	F. I. Lobanov, G. K. Nurtaeva, E. E. Ergozhin, Ekstraktsiya kompleksov ionov metallov s piridinovymi oksiazosoedine-niyami, Nauka, Alma-Ata, 1983.
23.	T. S. Stefanova, G. K. Toncheva, K. B. Gavazov, Chem. J. 2012, 2, 146-152.
24.	A. Holme, F. J. Langmyhr, Anal. Chim. Acta 1966, 36, 383391.
25.	A. E. Harvey, D. L. Manning, J. Am. Chem. Soc. 1950, 72, 4488-4493.
26.	K. B. Gavazov, M. Türkyilmaz, Ö. Altun, Bulg. Chem. Commun. 2008, 40, 65-69.
27.	I. V. Pyatnitskii, L. L. Kolomiets, N. D. Sulima, Zh. Anal. Khim. 1985, 40, 115-119.
28.	Y. Shijo, T. Shimizu, K. Sakai, Bull. Chem. Soc. Jpn. 1983, 56, 105-107.
29.	M. Široki, M. J. Herak, Anal. Chim. Acta 1976, 87, 193-199.
Povzetek
Proučevana je bila tvorba in ekstrakcija ionskih asociatov med kompleksim Ga(III)-4-(2-pyridylazo)resorcinol (PAR) anionskim kelatom in kationi štirih ditetrazolijevih kloridov (DT2+) (neotetrazolijev klorid (NTC), modri tetrazolijev klorid (BTC), nitri modri tetrazolijev klorid (NBT) in tetranitro modri tetrazolijev klorid (TNBT)). Izvedena je bila optimizacija ekstrakcijsko-spektrofotometričnih pogojev, sestava ekstrahiranih zvrsti {1:2:1 in/ali 1:2:2 (Ga:PAR:DT)} ter določene nekatere ravnotežne konstante {konstante asociacije (P), konstante distribucije (KD) in konstante ekstrak-cije (Kex)} in analizne karakteristike {molska absorptivnost (e), Sandellova občutljivost, interval veljavnosti Beerovega zakona, in druge}. Obravnavan je tudi vpliv števila nitro v DT2+ (Nnitro;DT2+ = BT2+, NBT2+ in TNBT2+) ter molske mase DT2+ (MM; DT2+ = NT2+ in BT2+): Log p = f(Nmto), Log Kd = f(NmtI0), Log Kra = f(Nmto), Log e = f(NmJ in Log p = f(Log MM).