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541.49: 543.4:542.61:546:882 LIQUID-LIQUID EXTRACTION OF THE CHROMOGENIC SYSTEMS CONTAINING NIOBIUM(V), 2-HYDROXYTHIOPHENOL AND HYDROPHOBIC AMINES
A.M.Maharramov*, N.A.Verdizade, A.Z.Zalov
*Baku state university Azerbaijan State Pedagogical University
Received 28.09.2015
The complex formation and liquid-liquid extraction in the niobium(V)-hydroxythiophenol-hydrophobic amine-water-organic solvent system was studied. The hydrophobic amines were aniline, amino-pyridine, dipyridyl. The optimum conditions for the extraction of mixed ligand complexes (organic solvent, extraction time, acidity of the aqueous phase, concentration of reagents), some key constants {association constant (P), extraction constant (Kex), equlibrium constant (Keq), extent polymerization (y)} and analytical characteristics were determined.
Keywords: niobium, spectrophotometry, differentligand complexes, 2-hydroxythiophenol, hydrophobic amines.
Introduction
Niobium is widely used for the fabrication of steels and superalloys, as well as special materials for microelectronics and optics, superconductors, refractory materials and catalysts [1]. As an important microalloying element in steels niobium can significantly affect the properties of the sample, such as the intensity at high temperature, the ability of tarnish resistance and temperate brittleness.
Spectrophotometric method remains one of the most popular among widely used methods in analytical practice. The simplicity of the experiment, which does not require expensive equipment, flexibility and sufficient accuracy of determination, makes the method cost-effective.
An extractive spectrophotometric method is developed for the trace determination of niobium in acidic medium. A yellow (1:3) complex of niobium(V) is formed with 3-hydroxy-2-(4'-methoxyphenyl)-4-oxo-4H-1-benzopyran in perchloric acid medium [2].
The formation and solvent extraction of new ion-association complexes between anionic chelates of niobium(V) with nitroderivatives of catechol (NDC) - 3,5-dinitrocatechol (3,5-DNC) and 4-nitrocatechol (4-NC), tetrazolium cations {2,3,5-triphenyl-2H-tetrazolium (TT) and 3,3'-[3,3'-dimetoxy(1-1'biphenyl)-4,4'-diyl]-bis-[2,5-diphenyl-2H-tetrazolium] (BT)} were studied [3].
Very sensitive technique based on the formation of mixed complexes of niobium with o-nitro-fenilfluoron and diantipirilmetan [4]. Still higher sensitivity achieved using salicyl fluorone and surfactants [5, 6]. Other organic reagents for determination of niobium - 8-hydroxyquinoline-5-sulfonic acid [7], 5,7-dichloro-8-hydroxyqui-noline [8], 5-chloro-7-iodo-8-hydroxyquinoline [9], lumogallion [10] and thioglycolic acid [11] was investigated.
Hydroxythiophenolate complexes of niobium are insoluble in chloroform, while mixed-ligand complexes with hydrophobic amines and aminophenols easily dissolve in various organic solvents [12, 13].
In this paper, we study the formation and liquid-liquid extraction (LLE) of ternary ion-association complexes of niobium(V) with 2-hydroxythiophenol (HTP, H2L, L) and hydrophobic amines (HAs) as the first step of using these complexes for LLE-spectrophotometric determination of Nb(V). The HAs were aniline (An), 2-aminopyridine (AmPy), dipyridyl (Dipy).
Experimental
Reagents and Instruments. A stock Nb(V) solution (1 pg/ml) was prepared by dissolving 0.1430 g of Nb2O5 in 4 g K2S2O7 as described in [14]. The melt was dissolved in a hot 5% solution of tartaric acid, cooled and diluted with a solution of tartaric acid to 100 ml in a
volumetric flask. Working solutions were prepared by appropriate dilution of standard solution with 2% solution of tartaric acid. The concentration was of the niobium solution was adjusted gravimetrically [14].
HTP were synthesized according to the procedure [15]; their purity was verified by paper chromatography and melting point determination. Solutions of HTP and HA in chloroform (0.01 M) were used. To create the optimal acidity, 1M solutions of NaOH and HCl were used. The extractant was purified chloroform.
The absorbance of the extracts was measured using a KFK-2 photocolorimeter and an SF-26 spectrophotometer. The equilibrium value of the pH of aqueous phase was measured using a I-120. 2 potentiometer with an glass electrode.
General Procedure
General Procedure for the Determination of Niobium(V). Portions of stock solutions of Niobium(V) varying from 0.1 to 1.0 ml with a 0.1/ml step, a 2.0-2.5 ml portion of a 0.01 M solution of HTP and solution of HA were placed in to calibrated test tubes with ground-glass stoppers (the volume of the organic phase was 5 ml). The required value of pH was adjusted by adding 0.1 M HCl. The volume of the aqueous phase was increased to 20 ml using distilled water. The tube was closed and shaken for a fixed time (15 min). After separation of the layers, a portion of the organic extract was transferred into a cell (/=0.5 sm) and the ab-sorbance was read against simultaneously prepared blank sample (KFK-2; X=440 nm).
Determination of Niobium(V) in Steel. 0.2 g [CBT-1 (0.017 % Nb); CBT-3 (0.08 % Nb); CBT-6 (0.11 % Nb)] was dissolved in 20 ml of H2SO4 (1:1) and was oxidized with a few drops of concentrated nitric acid and evaporated twice to vapor SO3. The precipitated salt was dissolved in 20 ml of 15% tartaric acid under heating, the solution was cooled, adjusted with water to 100 ml in a volumetric flask, stirred and filtered. An aliquot of 5 ml was put into a separatory funnel, was added 1 ml of 10% hy-droxylamine solution, 1 ml of 3% ascorbic acid,
and pH was adjusted to ca 4.0-4.5 with NaOH and was determined niobium using the proposed procedures.
Results and Discussion
Charge of the complexes and choice of
organic solvent. The binary complexes, Nb(V)-HTP cannot be extracted in chloroform. Experiments with KU-2 and AV-17 ion-exchangers showed that these yellow species are charged negatively. Electroneutral different liqand complexes (DLC) can be formed in the presence of HAs.
The following organic solvents were tested for the extraction of these complexes: chloroform, 1,2-dichloroethane, carbon tetrachloride, benzene, toluene, xylene, isobutanol, and isopentanol. Chloroform was found to be the most effective. The concentration of niobium in the organic phase was determined with bro-mopyrogallol red [16] by photometric measurements after back extraction, while in the aqueous phase it was determined by the difference. The basicity of HA hardly influences the recovery of niobium. At the optimum conditions this solvent provides degrees of extraction R= 98.198.3% (Table 1).
Effect of pH. The effect of pH on the intensity of the color reaction is shown in the Figure 1. The optimum pH for complex formation and extraction is 4.0-4.5. Extraction of Nb(V) enhanced with the increase in the acidity of the initial solution; the further increase in acidity lead to the gradual decrease of recovery, which was obviously associated with a decrease in the concentration of the ionized form of HTP. At pH > 6, the complexes were hardly extracted, obviously because of the decrease in the degree of HA protonation (Table 1).
Reagents concentrations, effect of time, absorption maximum and molar absorptivi-ties. The optimum conditions are listed in Table 1. Complete extraction is achieved at HTP and HA concentrations not lower than (1.3-1.5^10-3 mol/l and (5.2-9.4)-10-4 mol/l, respectively. The absorbance of the extracts is stable for at least 48 hours. The optimum shaking time is 510 min.
Table 1. Optical characteristics, precision and accuracy of the spectrophotometry determination of Nb(V) with HTP and HA
Parameter An Dipy AmPy
Color yellow yellow yellow
The pH range of education and extraction 2.8-6.0 2.0-6.0 2.9-5.1
The pH range of maximum extraction 4.2-4.5 4.0-4.5 4.2-4.3
Concentration of DTP, mol/l 1.3 10-3 1.4 10-3 1.5 10-3
Concentration of HA, mol/l 5.2^10-4 7.3-10-4 9.4^0-4
Organic solvent CHCl3 CHCI3 CHCls
Extraction time, min 5 8 10
^ma« A™ 435 458 445
Molar absorptivity 3.3 2.5 2.1
Sandell's sensitivity, ^g/cm2 0.028 0.037 0.044
R, % 98.5 98.6 98.3
Distribution constant, KD 263 282 231
The equation of calibration curves 0.075+0.057x 0.0067+0.063x 0.073+0.062x
lgKec, 2.78 8.5 8.93
lgKex 8.61 14.53 14.84
Stability constant, ß 8.89 8.79 8.83
Beer's law range, ^g/ml 2.0-95 1.8-90 1.5-115
Correlation coefficient 0.9451 0.9538 0.9279
Limit of detection (LOD), ng/ml 11 16 17
Limit of quantification (LOQ), ng/ml 38 49 52
A 0.5
0.4 -
0.3 -
0.2
0.1
2
3
4
5
6
pH
Figure 1. Absorbance of differentligand complexes as a function of the pH of the aqueous phase: 1 - An, 2 - Dipy, 3 - AmPy; CNb= 2.15^10-5, CHTP=(1.3-L5yi0-3, Cha=(5.2-9.4V10-4 mol/l, /=0.5 sm.
The absorption spectrum of the complex indicates that the maximum lies in the range 435-458 nm where the reagent blank has minimal absorbance (Figure 2). All colour reactions are very contrast since the initial solutions are colourless (Vax HTP=278 nm; A Vax=157-180 nm). The Komar-Tolmachev method [17] also allows calculating the true molar absorptivity of the complex: s=(2.1-3.3)x104.
A 1.2
1.0 .
0.8 0.6 0.4
30
60
90 120
Nb(V), ^g/ml
Figure 2. Analytical determination of Nb(V): 1 - Nb(V)-L-An, 2 - Nb(V)-L-Dipy, 3 - Nb(V)-L-AmPy; CHTP=(1.3-1.5)^10-3, CHA=(5.2-9.4)^10-4 mol/l; pH 4.0-4.5; 1 440 nm; /=1.0 sm.
Stoichiometry of the ternary complexes. The molar ratios of the components of the ternary complexes were established by the equilibrium shift method [17] and the method of As-mus [17]. The results show a complex composition of 1:2:1 [Nb(V):L:HA], in the case of aniline 1:2:2. Using the Nazarenko's method [18],
we found that the niobium complexation form is Nb(OH)2+ . Hence, the complexes can be regarded as ion associates between doubly charged an ionic chelates [Nb(OH)3L2]2- and two protonated HA species: (AH+)2[Nb(OH)3L2]. Structure ex-tractable complexes can be represented as:
OH
'O'
rT<VS \ I *
I Nb
HO OH
+ ■
NH2
Nr^
The mechanism of complexation. The
disappearance of the pronounced absorption bands in the 3200-3600 cm 1 with a maximum at 3460 cm-1 observed in the spectrum of HTP, says that the OH-group is involved in the formation of the complex. The observed decrease in the intensity, absorption bands in the area 2580 cm-1 shows that SH-groups involved in the formation of coordination bond in the ionized state. Detection of the absorption bands at 2310 cm-1 indicates the presence of a protonated aminophenole [19, 20].
The colour of the anionic chelate intensifies when the compound is extracted to clo-roform as an ion-associate, [Nb(OH)3L2]2-with a amina (AH+) counter cation. The suggested equations of complex formation and extraction, based on the mentioned molar ratios and data for the niobium state in pH 3-4:
Nb(OH) +2H2L-
>[Nb(OH)3L2]2-+4H
(1)
[Nb(OH)3L2]2"+2AH+^[Nb(OH)3L2](AH)2, (2) [Nb(OH)3L2f+AH2+^[Nb(OH)3L2](AH2), (3) {[Nb(OH)3L2](AH)2 }aq^{[Nb(OH)3L2](AH)2}orq. (4)
Equation 2 presents the association of [Nb(OH)3L2]2- with the cations of the type AH+ (An) and equation 3 - with those of the type AH+ (AmPy and Dipy).
Constants of stability, equilibrium extraction and extent polymerization. For evaluation of the stability of the complex we used the method of crossed lines [17]. The experiments were performed with constant Nb(V) and HA concentrations and two different HTP concentrations. The calculated value of the two-phase stability constant was logP = 8.79- 8.89.
The constants of equlibrium were calculated according to the equation:
lgKeq = lg D - 2lg [HA+] (5)
and in case with AmPy and Dipy (equation 3) lgtfeq = lgD - lg [H2A2 ] . (6)
The extraction constants were calculated according to equation:
lg^ex=lgD-2lg[L]-2lg[HA+], (7) lgKex= lgD-2lg [L]-lg[H2A2+], (8) The peaks of lgKeq u lgKex, calculated by formulas (5)-(8) for the complexes Nb(V)-L-HA shown in Table 2 below.
Calculation of extent of polymerization of complexes was carried out on equation [21]. The made calculations showed that DLC in an organic phase won't be polymerized and are in a monomeric form (y=1.00-1.05).
Influence of interfering ions. To evaluate the complex applicability for photometric determination of niobium, we examined the influence of foreign ions and reagents. The results showed that great excesses of alkali, alkali earth, and rare earth elements, as well as NO T,
ClO ;
so
and CH3COO do not interfere
determination of niobium with HTP and HA. Interference of most cations masked by the addition of complexone III. Tartrate masks the milligram quantities of Ta, Ti, W and Mo. Zr fluorides should mask, and copper-thiourea. The results are summarized in Table 2.
Effect of Niobium(V) concentration. The adherence to Beer's law was studied by measuring the absorbance value of the series of solutions containing different concentrations of the metal ion. A linear calibration graph drawn between absorbance and the metal ion concentration indicates that Nb(V) may be determined in the range 1.5-115 p,g/ml. The pertaining calibration graph is shown in the Figure 2.
In conclusion the analytical parameters pertaining to the proposed method are given in Table 1.
The proposed method compares favourably with the existing ones (Table 3) and offers the advantages of better simplicity, rapidity, sensitivity and selectivity [3, 16, 22].
Table 3. Comparative characteristics of the prosedures for determining niobium
Table 2. Influence of interfering ions on the determination of niobium(V) as DLC with HTP and An (30.0 ^g Nb added)
Ion Molar excess of the ion Masking agent Found Nb, ^g RSD, %
Co(II) 50 30.3 2
Ni(II) 50 29.4 1
Fe(II) 50 29.9 4
Fe(III) 40 SnCl2 30.1 3
Cd(II) 180 30.2 2
Al(III) 150 29.9 2
Mo(VI) 30 Tartaric acid 29.8 3
Zr(IV) 50 29.9 5
Cu(II) 50 SC(NH2)2 29.6 2
Hg(II) 50 30.3 1
Ti(IV) 50 Ascorbic acid 29.8 3
V(IV) 50 30.0 2
V(V) 50 EDTA 30.2 4
W(VI) 50 Tartaric acid 30.4 5
Cr(III) 140 29.8 5
Ta(V) 50 Tartaric acid 30.4 1
uo2+ 50 CH3COO- 30.0 3
Cr(VI) 50 SC(NH2)2 30.2 4
Mn(VII) 50 SC(NH2)2 30.4 2
Mn(II) 50 29.9 4
Ascorbic acid 100 30.3 2
Tartaric acid 100 30.4 5
Oxalate 5 30.8 4
Fluoride 1.5 29.6 4
Phosphoric acid 30 30.4 3
Thiourea 20 31.2 6
Reagent(s) pH (Cm) Solvent X, nm e-10-4 Composition Nb:Rj:R2 Linear range, tig/ml Ref.
Catechol + TTC CHCls 1.9 1:2:1 [221
Pyrocatechol violet 1.6 - 2.0 2.2 - 2.3 600 0.93 1:2 0.2-6.0 [16]
Tiosianat 2.5M HCl Water + asetone 385 3.5 1:5 0.8-9.0 [1,161
4-NC + TV 0.7 M H2SO4 C2H4O2 440 1.59 1:2:1 0.41-4.54 [3]
Pyrocatechol + TV 0.4 - 1 M H2SO4 CHCls 390 1.60 1:2:2 1.25-11.78 [3,16]
Bromopyrogallol red 5.8-6.6 560 4.75 1:2 [16]
3,5-DNC + TT 0.15-0.28 M H2SO4 CHCls 400 2.48 1:2:2 0.51-2.3 [3]
4-NC+ BT 0.08-0.15 M H2SO4 CHCls 415 4.6 1:2:1 0.42-1.6 [3]
HTP+An 4.2-4.5 CHCls 435 3.3 1:2:2 0.4-19
HTP+Dipy 4.0-4.5 CHCls 458 2.5 1:2:1 0.36-18
HTP+AmPy 4.2-4.3 CHCls 445 2.1 1:2:1 0.3-23
Note: TTC - 2,3,5-triphenyl-2-H-tetrazolium chloride; TV - tetrazolium violet
Analytical applications. The proposed method under the already established optimum conditions was applied for the determination of Nb(V) in steels of different brands [CBT-1 (0.012% Nb), CBT-6 (0.29% Nb), CO № 158a
(1.21% Nb), CO № 231(0.37% Nb)]. The results presented in Table 4 indicate the successful applicability of the proposed method to real sample analysis.
Table 4. Determination of niobium in steel (n=5, P=0.95)
Steel Method Procedure X, % RSD,%
СВТ-1 Standard Thiocyanogen 0.0129+0.004 2.5
Catechol + TTC 0.0134+0.001 2.8
Pyrocatechol violet 0.0119+0.002 1.6
Proposed HTP+An 0.0123+0.009 2.7
СВТ-6 Standard Catechol + TTC 0.2841+0.042 2.3
Pyrocatechol violet 0.3125+0.056 2.2
Proposed HTP+Dipy 0.2971+0.013 2.3
CO № 158a Standard Catechol + TTC 1.2493+0.030 2.8
Proposed HTP+An 1.2076+0.027 2.7
HTP+AmPy 1.2181+0.029 2.1
CO № 231 Standard Thiocyanogen 0.3840+0.024 1.8
HTP+An 0.3852+0.039 1.9
HTP+AmPy 0.3695+0.042 2.5
References
1. Гибало И.М., Виноградов А.П. Аналитическая химия ниобия и тантала. М.: Наука, 1967. 352 с.
2. Agnihotri N., Agnihotri R. Extractive Spectrophotometry Determination of Niobium(V) Using 3-Hydroxy-2-(4'-Methoxyphenyl)-4-Oxo-4H-l-Ben-zopyran as a Complexing Agent // The Open Analytical Chemistry J. 2012. V. 8. No 6. P. 39-44.
3. Lekova V., Racheva P., Stojnova K., Dimitrov A., Gavazov K. Ternary complexes of niobium(V) with nitroderivatives of catechol and tetrazolium salts. extraction-spectro-photometric investigations // Chemija. 2010. No 1. P. 106-111.
4. Ганаго Л.И., Бухтеева Л.Н. Исследование раз-нолигандных комплексов ниобия с о-нитрофе-нилфлуореном и диантиприлметаном // Журн. аналит. химии. 1979. Т. 34. С. 2186-2191.
5. Чернышева М.А., Голик Н.Н., Малютина Т.М., Антонович В.П., Винарова Л.И. Реакции между ниобием и салицилифлуореном в присутствии поверхностно-активных веществ и маскирующих лигандов // Журн. аналит. химии. 1987. Т. 42. № 12. С. 1963-1968.
6. Wang Z.Q., Li J.J., Shen H.X. Criteria for selecting analytical wavelengths for multicomponent mixtures by the CPA matrix method and simultaneous spectrophotometric determination of niobium and tantalum // Anal. Chim. Acta. 1988. V. 212. Р.145-153.
7. Alonso J.I.G., Garcia M.E.D., Medel A.S. The surfactant-sensitized analytical reaction of niobium with 8-hydroxyquinoline-5-sulphonic acid // Talan-ta. 1984. V. 31. No 5. P. 361- 366.
8. Medel A.S., Diaz Garcia M.E. Extractive Spectro-photometric Determination of Niobium in Pyro-chlore-Bearing Rocks with 5,7-Dichloroquinolin-8-ol // Analyst. 1981. V. 106. No 10. P. 1268-1274.
9. Sharma Y. A Sensitive Spectrophotometry Determination of Niobium(V) with the Reagent 5-Chloro-8-hydroxy-7-iodoquinoline // Microchim. Acta. 1982. V. 78. No 3. P. 297-303.
10. Егорова Л.А., Гудзенко Л.В., Тимченко А.К. Спектрофотометрическое определение активирующих добавок в фторида лития монокристаллов // Заводская лаборатория. 1993. Т. 59. № 1. С. 13-15.
11. Dutta R.K., Baneijee S. Spectrophotometry determination of niobium and its application to niobium-stabilized stainless steel // Talanta. 1974. V. 21. No 10. P. 1091-1094.
12. Kuliyev K.A., Verdizadeh N.A. Spectroscopic investigation of the complex formation of niobium using 2,6-dithiophenol and aminophenols // American Journal of Analytical Chemistry. 2015. V. 6. P. 746-756.
13. Залов А.З., Вердизаде Н.А., Джамалова Р.И. Экстракционно-фотометрическое определение ниобия (V) с 2-гидрокси-5-бромтиофенолом и гидрофобными аминами // Азерб. хим. журн. 2011. № 1. С. 97-102.
14. Коростелев П.П. Приготовление растворов для химико-аналитических работ. М.: Изд-во АН СССР. 1964. С. 401.
15. Кулиев А.М., Алиев Р., Мамедов Ф., Мовсум-заде М. Синтез аминометильных производных 2-гидрокси-5-трет-алкилтиофенолам и их расщепления тиолов // Журн. орг. химии. 1976. Т. 12. № 2. С. 426-431.
16. Марченко З., Бальцежак М.К. Методы спектро-фотометрии в УФ и видимой областях в неорганическом анализе. М.: Бином. Лаборатория знаний. 2007. 711 с.
17. Булатов М.М., Калинкин И.П. Практическое руководство по фотоколориметрическим и спек-трофотометрическим методам анализа. М.-Л.: Химия, 1986. 432 с.
18. Назаренко В.А., Бирюк Е.А. Изучение химии реакций ионов поливалентных элементов с органическими реагентами // Журн. аналит. химии. 1967. Т. 22. № 1. Р. 57-64.
19. Накамото К. ИК-спектры и спектры КР неорганических и координационных соединений. М.: Мир, 1991, 536 с.
20. Беллами Л. Инфракрасные спектры сложных молекул. М.: Мир, 1963. 590 с.
21. Ахмедли М.К., Клыгин А.Е., Иванова Л.И., Ба-широв Э.А. О химизме взаимодействия ионов галлия с некоторыми сульфофталинами // Журн. неорг. химии, 1974. Т. 19. № 8. Р. 2007-2012.
22. Racheva P., Lekova V., Stefanova T., Dimitrov A., Gavazov K. Complex formation and liquid-liquid extraction in the niobium(V)-4-nitrocatechol-thi-azolyl blue tetrazolium system // Plovdiv University "Paisii Hilendarski" - Bulgaria Scientific Papers. 2010. Т. 37. Book 5. P. 34-42.
NiOBiUM(V) 2-HiDROKSiTiOFENOL VO HiDROFOB AMiNlLORDON iBAROT XROMOGEN
SiSTEMiN EKSTRAKSiYASI
A.M.M3h3rramov, N.A.Verdizada, O.Z.Zalov
Niobiumun(V) 2-hidroksitiofenol va hidrofob aminlarla kompleksinin эшэ1э galma §araiti oyranilmi§dir. Hidrofob amin kimi anilin, aminopiridin va dipiridil istifada edilmi§dir. Muxtalifliqandli kompleksin amalagalma va ekstraksiyasinin optimal §araiti (uzvi reagent, ekstraksiya vaxti, su fazanin tur§ulugu, reagentlarin qatiligi) tapilmi§ va bir sira asas sabitlari [davamlilq sabiti (P), ekstraksiya sabiti (KJ, tarazliq sabiti) (Ktaraz), polimerla§ma daracasi (y)] hesablanmi§dir.
Agar sozlar: niobium, spektrofotometriya, muxtalifliqandh kompleksbr, 2-hidroksitiofenol, hidrofob aminlar.
ЭКСТРАКЦИЯ ХРОМОГЕННЫХ СИСТЕМ, СОДЕРЖАЩИХ НИОБИЙ(У), 2-ГИДРОКСИТИОФЕНОЛ И ГИДРОФОБНЫЕ АМИНЫ
А.М.Магеррамов, Н.А.Вердизаде, А.З.Залов
Изучены условия образования комплексов ниобияСУ) с 2-гидрокситиофенолом и гидрофобными аминами. В качестве гидрофобных аминов использованы анилин, аминопиридин, дипиридил. Найдены оптимальные условия образования и экстракции разнолигандного комплекса (органический растворитель, время экстракции, кислотность водной фазы, концентрация реагентов) и вычислены некоторые основные константы [константа устойчивости (Р), константа экстракции (KJ, константа равновесия (Kp^), степень полимеризации (у)].
Ключевые слова: ниобий, спектрофотометрия, разнолигандные комплексы, 2-гидрокситиофенол, гидрофобные амины.
