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7ФУНДАМЕНТАЛЬНЫЕ И ПРИКЛАДНЫЕ ИССЛЕДОВАНИЯ
Естественно-математические науки
UOT 543.552.054.78.77
1 12
Ali Z. Zalov , Nazani A. Novruzova , Sultan G. Aliyev
Azerbaijan State Pedagogical University, Baku, Azerbaijan
2Azerbaijan State University of Oil and Industrial, Baku, Azebaijan
Corresponding author: Ali Z. Zalov, zalov1966@mail.ru
EXTRACTION-SPECTROPHOTOMETRIC STUDY ON THE CHROMIUM (VI)- 2-HYDROXY-5-NITROTHIOPHENOL-HYDROPHOBIC AMINES-WATER-CHLOROFORM SYSTEM
Annotation. The present work is devoted to the study of the conditions for the interaction of chromium (VI) with 2-hydroxy-5-nitrothiophenol. In the presence of hydrophobic amines, these compounds are extracted into the organic phase in the form of a mixed-ligand complexes. It was found that the spectrophotometric characteristics of mixed-ligand complexes Cr (VI) and Cr (III) are identical, that is, in interaction with 2-hydroxy-5-nitrothiophenol, Cr (VI) were reduced to Cr (III). The proposed method under the already established optimum conditions was applied for the determination of Cr (III) in alloys,in sewage water and Bottom sediments and in soils.
Keywords: chromium, 2-hydroxy-5-nitrothiophenol, mixed-ligand complexes, extraction -photometric method
Али З. Залов1, Назани А. Новрузова2, Султан Г. Алиев3
1 Азербайджанский государственный педагогический университет, Баку,
Азербайджан
Азербайджанский государственный университет нефти и промышленности, Баку,
Азербайджан
Автор, ответственный за переписку: Али З. Залов, zalov1966@mail.ru
ЭКСТРАКЦИОННО-СПЕКТРОФОТОМЕТРИЧЕСКОЕ ИССЛЕДОВАНИЕ В СИСТЕМЕ ХРОМ (VI)- 2-ГИДРОКСИ-5-НИТРОТИОФЕНОЛ-ГИДРОФОБНЫЕ АМИНЫ-ВОДА-ХЛОРОФОРМ
Аннотация. Настоящая работа посвящена изучению условий взаимодействия хрома (VI) с 2-гидрокси-5-нитротиофенолом. В присутствии гидрофобных аминов эти соединения экстрагируются в органическую фазу в виде разнолигандных комплексов. Установлено, что спектрофотометрические характеристики разнолигандных комплексов Cr (VI) и Cr (III) идентичны, т. е. при взаимодействии с 2-гидрокси-5-нитротиофенолом Cr (VI) восстанавливаются до Cr (III). Предложенный метод при уже установленных оптимальных условиях был применен для определения Cr (III) в сплавах, в сточных водах и донных отложениях, а также в почвах.
Ключевые слова: хром, 2-гидрокси-5-нитротиофенол, разнолигандные комплексы, экстракционно-фотометрический метод
Introduction
Chromium is one of the toxic metals. Chromium (VI) compounds, which are among the most dangerous pollutants of natural objects, have the most dangerous carcinogenic effect [1]. In this regard, in modern practical chemical analysis, it becomes necessary to determine this pollutant
element [2]. Drinking, natural and technical waters require special control, the chromium content of which is strictly regulated. To assess the content of toxic components in various environmental objects, methods based on the determination of elements with organic reagents are promising and are being actively developed.
The interaction of chromium(VI) with 1,5-diphenylcarbazide immobilized in a transparent polymethacrylate matrix was studied. The optimal conditions for the interaction of a metal with a reagent in the solid phase were found [3].
A method has been developed for photometric determination in solutions of Cr(III) and Fe(III) with simultaneous presence. The method is based on the difference between the electronic spectra of Cr(III) and Fe(III) complexes with Na2EDTA. The optimal conditions for carrying out the reaction and the metrological characteristics of the method were determined. The iron cation complex is formed at the moment of mixing the solutions. Complexation of the Cr(III) cation occurs slowly at room temperature. The results of the reaction are well repeated, the coefficient of variation does not exceed 1.4% [4].
Mixed-ligand complexes (MLCs) of chromium (VI) with o-hydroxythiophenols (HTPD) and
its derivatives [2-hydroxy-5-chlorothiophenol, 2-hydroxy-5-bromothiophenol and 2-hydroxy-5-
iodothiophenol in the presence of hydrophobic amines studied by spectrophotometric method. It has
been established that mixed-ligand complexes are formed in a slightly acidic medium (pH 3.0-5.0).
It has been established that the optimal concentration of HTPD and Am for complexation is 1.0*103 3
M and 1.12*10- M, respectively. The maximum absorption of light is observed at 475-488 nm. The calculated molar absorption coefficients (smax) belong to the interval (2,92 - 3,28)*104.
A linear calibration plot plotted between absorbance and metal ion concentration shows that Cr(III) can be determined in the range of 0,5-16 ng/ml. Extraction-photometric methods for the determination of chromium have been developed. The proposed method is successfully used to determine the amount of chromium in alloys and soil [5].
This work is devoted to the study of the conditions for the interaction of chromium (VI) with 2-hydroxy-5-nitrothiophenol (HNTP). In the presence of hydrophobic amines (Am), these compounds are extracted into the organic phase as MLCs. Of the hydrophobic amines (Am), N,N-dimethylaminomethyl-4-methylphenol (Am1) and N,N-dimethylaminomethyl-4-ethylphenol (Am2) were used.
Experimental
Reagents and apparatus. A solution of Chromium (VI) (1 pg/ml) was made by dissolving (0.1935 g) of K2Cr2O7 (pure for analysis) in 1000 ml of water. Concentration of solution of chromium was established gravimetric [6]. Working solution with concentration of 0,1 mg/ml was prepared by dilution of stock with deionized water.
HNTP were synthesized according to the procedure [7]. HNTP their purity was verified by paper chromatography and melting point determination. Solutions of HNTP and Am in chloroform (0.01M) were used. As an ekstragent the cleared chloroform was applied.
The ionic force of solutions was supported a constant (^ = 0.1) introduction of the calculated quantity of KCl. To create the optimal acidity, 1M solutions of KOH and HCl were used.
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 a glass electrode. All measurements was carried out at 20±5o C.
Studies on the oxidation state of chromium (VI). It is known that o-hydroxythiophenols have reducing properties in an acidic environment [5, 8-12]. To determine the oxidation state chromium in MLC, we conducted two series experiments. In the first series, we used Cr (VI), and in the second series we used Cr (III), obtained by adding an additional reducing agent (SnCl2 or KI). Was found that the spectrophotometric characteristics of MLC Cr (VI) and Cr (III) were identical, that is, in interaction with HNTP, Cr (VI) were reduced to Cr (III).
General procedure for the determination of chromium (III). Portions of stock solutions of chromium (III) varying from 0.1 to 1.0 ml with a 0.1ml step, a 2.2 ml portion of a 0.01 M
solution of HNTP, and a 2.5 ml portion of a 0.01M solution of Am were placed in to calibrated test tubes with ground-glass stoppers (the volume of the organicphase was 5 ml). The required value of pH was adjusted by adding 1M HCl. The volume of the aqueous phase was increased to 20 ml using distilled water. In 15 minnute after the complete separation of the phases, the organic phase was separated from the aqueous phase and the absorbance of the extracts was measured on KFK-2 at room temperature and 440 nm (l=0.5cm).
Charge of complexes. Experiments on electromigration in a U-tube and sorption on EDE-10P, anion exchangers demonstrated anionic character of single use, ligands, at studying the electromigration of complexes, it was found that the complexes of chromium (VI) red binary o-hydroxythiophenolate are transferred to the cathode. When the charge sign of complexes with one ligand was determined by ion chromatography, the anion exchange EDE-10P completely absorbed the colored component of the solution. When hydrophobic amines (Am) were introduced into the system, extraction of these compounds in the organic phase as a complex of MLC was observed. Among the hydrophobic amines, N,N-dimethylaminomethyl -4-methylphenol (Ami) and N, N-dimethylaminomethyl-4-ethylphenol (Am2) were used. On the basis of the data obtained, new selective and highly sensitive prospectuses were developed for extraction-spectrophotometric determination of a small amount of chromium in alloys of different varieties, soils and water.
Results and discussion
The choice of the extractant. For the extraction of complexes we used CH3Cl, C2H4Cl2, CCl4, C6H6, C6H5Cl, C6H5-CH3, xylol, z'so-butanol, z'so-pentanol, ethyl oxide, «-butanol and their mixes. Extractibility of complexes was estimated in coefficient of distribution and extent of extraction. CH3Cl, C2H4Cl2 and CCl4 appeared to be the best extractants (97,5-99,2 %). Fast division of layers and the maximum value of molar coefficient of absorption were received at extraction of complexes by chloroform. After a single extraction with CH3Cl 98.5% of chromium was extracted as an ion associate. Further researches were conducted with CH3Cl. The concentration of chromium in the organic phase was determined with diphenylcarbazide [13] by photometric measurements after back extraction, while in the aqueous phase it was determined by the difference. Thus basicity of amines has no noticeable impact on conditions and extraction of complexes.
Influence of the pH of the aqueous phase. The effect of pH on the formation of Cr(III)-HNTP-Am complex was studied, in order to find a suitable pH that can be adopted in the determination of Cr(III) (Table 1, Fig. 1). The absorbance was found to be maximum in the pH range 3,9-5,3. Hence further analytical investigations were carried out in media of pH 4,5. Extraction of Cr (III) enhanced with the increase in the acidity of the initial solution; the further increase in acidity lead to the gradual decrase of recovery, which was obviously associated with a decrease in the concentration of the ionized form of HNTP. Probably, it is present in the solution in the non-dissociated state. At pH > 8,6, the complexes were hardly extracted, obviously because of the decrease in the degree of Am protonation. The effect of pH on the intensity of the color reaction is shown in the Fig. 2. Existence of one maximum of absorbance in the specified limits pH confirms the assumption of formation of one complex connection. The nature of acids (HCl, H2SO4) almost does not influence a complex formation of chromium with HNTP and Am.
Fig.1. Absorbance of mixed-ligand complexes as a function of the pH of the aqueous phase.
A 0.6
2
3
4
5
6
7
9 pH
1- Cr(III)-HNTP-Am1; 2- Cr(III)-HNTP-Am2
Ccr=3.84x10'5M, Chntp =L6*10'3M, Cam=1.5^10~3M, KFK-2, X = 440nm, 1=0.5 cm.
8
Electronic absorption spectra. The absorption maxima (Vax) of the ternary Cr(III)-HNTP-Am complexes lie in the range of 465-470 nm (Table 1). All colour reactions were very contrast since the initial solutions are colourles (Xmax (HNTP) = 280 nm). Thus, batochromic shift makes 185-190 nm. Close values of maxima of light absorption allow to draw a conclusion that the formed complexes were ionic associates. Contrast of reactions was high: initial reagents - are colourless, and complexes - are intensively painted. Molar coefficients of absorption make (2.92-3.28)x104.
Optimal operating conditions. In a weakly acidic medium, HNTP reacts with chromium (III) to form a color anionic complex. In the presence of hydrophobic amines, a compound is formed which readily dissolves in chloroform. The maximum absorption of the extracted complex (in 5 ml of chloroform) is achieved when the pH of the aqueous phase is in the range of 4.0-5.3 and the reagent concentrations are (1.2 - 4.0)*10-3 mol/ml HNTP and (0.92 - 4.0)*10-3 mol/ml Am. Our
3 3
experiments are carried out at pH 4.5 in the presence of 1.6*10- mol/ml HNTP and 1.5*10-mol/ml at. The volume of the aqueous phase is 20 ml and contains 20 pg of Cr (III). Phase equilibrium is achieved after 90 s, but the extraction is carried out for 3 min in all experiments. Absorption of the workpiece remains constant for 15 minutes.
Stoichiometry of the Complexes and the Mechanism of Complexation. It was found using the Nazarenko method that Cr(III) in the complexes was present in the form of Cr3+. The number of protons replaced by chromium in one HNTP molecule appeared to be one [14]. Starik-Barbanel relative yield method, equilibrium shift method (Fig. 2), crossed lines method and Asmus' methods were employed to elucidate the composition of the complex [15]. The results show that the molar ratio of Cr: HNTP: Am is 1: 3: 3.
Fig. 2. Determination of the ratio of components by the equilibrium shift method for (a) Cr(III)-HNTP-Ami and(b) Cr(III)-HNTP-Am2.
1. Cr:HNTP; 2.Cr : Am.
Ions of chromium at interaction with three molecules of HNTP (HNTP = H2R) form three charged anionic complexes, which were extracted with three molecules of protonated Am. The composition of the extractable complexes can be represented by the formula [Cr(HR)3](AmH+)3. It is assumed that at a complex formation there are processes
Cr3+ + 3H2R ~[Cr(HR)3]3- + 3H+ [Cr(HR)3]3- + 3AmH+~ [Cr(HR)3](AmH+)3
The stability constant determined by crossed lines method. The sizes of equilibrium constant Ke calculated on a formula lg Кe = lg D - 2\g[4mN +] were presented in table 1.
Calculation of extent of polymerization of complexes was carried out on the equation [16]. The made calculations showed that MLC in an organic phase won't be polymerized and are in a monomeric form ( y=1,04-1,13).
Formed ion-association complex between anionic chelates of chromium (III) with HNTP and Am. The stability constant of Cr(III)-HNTP-Am complexes was calculated and found to be lgP = 8,13-10,34 at room temperature [17].
Influence of various ions. The effect of a diverse set of ions on the determination of chromium (III) with HNTP and Am is studied at pH 4,5 in the presence of 15 pg Cr(III) in 20 mL of an aqueous solution. The following ions (in «-fold excess) do not interfere: Cd (II) (435), Cu (II) (80), Ce (III) (125), Zn (II), Co (II) and HPO42- 1200, V (IV,V) and Ni (II) (130), Mo (VI) (10), Fe (II) (40), W (VI) (13), Mn (II) (30), Al (III), (120). The interference in the determination of chromium is caused by U (VI), Nb (V), Re (vII), Ti (IV), NO-3, SCN-, tartrate and urea. Using 1,5 mg F- and 2*10-3 mol/l sulfosalicylic acid as masking agents, let us assume a 125-fold excess of Al and a 10-fold excess of Nb (V). A solution containing 2*10-5 mol/ml a, a'-dipyridyl, will mask a 250-fold excess of Cu (II), and 10 mg of HPO42- will mask a 30-fold excess of Fe (III) and a 60-fold excess Mg (II). Elements of W, Nb and Si are separated as the corresponding insoluble acids during the dissolution of the sample.
In conclusion the analytical parameters pertaining to the proposed method are given in Table 1.
Table 1
Optical characteristics, precision and accuracy of the spectrophotometric determination of
Compound The pH range of maximum extraction R,% ^max (nm) e1 0-4 Y lgKe lgP Working range, Mg/ml
Cr- HNTP- Ami 4,1-5,3 97,5 470 2,92 1,04 5,40 8,13 0,5-16
Cr- HNTP- Am2 3,9-5,2 99,2 465 3,28 1,13 5,69 10,34 0,5-18
Effect of chromium (III) 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 Cr(III) may be determined in the range 0,5-18 pg/ml. Table 2 summarizes the calibration characteristics obtained with Cr (III)-HNTP-Ami and Cr(III)-HNTP-Am2.
Table 2
Analytical characteristics of some ternary complexes of Cr with HNTP and Am
Parameter Cr(III)-HNTP-Ami Cr(III)-HNTP-Am2
The equation of calibration curves y = 0,012 + 0,253x y = 0,016 + 0,257x
Correlation coefficient 0,9983 0,9989
Linear calibration range (^g/ml) 0,5-16 0,5-18
Limit of detection (ng/ml) 10 13
Limit of quantification (ng/ml) 33 32
Sandell's sensitivity (ng/ml) 1,9 2,0
Table 3 demonstrates the data which allow a comparison of the analytical parameters of the procedures for the determination of cromium with the earlier known procedures [13, 18-21].
Table 3
Comparative characteristics of the procedures for determining chromium
Reagent* pH ( solvent) X, nM sx 10-4 Beer's law range, p,g/ml [Ref.]
PAN 0,2-0,8 M HCl (CH3COCH3) 390400 1,28 0,3 - 2,0 [18]
5-Br-DMPAP 0,1-10 M HCl (CHCI3) 546 7,8 0,02 - 0,56 [19]
5-Br-PADAP 4,7 600 7,93 0,6 - 15,0 [20]
PAR 4,0-5,0 540 4,7 3,2-13,0 [21]
Cr- HNTP- Am1 4,1-5,3 (CHCI3) 470 2,92 0,5-16 Proposed method
Cr- HNTP- Am2 3,9-5,2 (CHCI3) 465 3,28 0,5-18
*Note: PAN - 1-(2-Pyridylazo)-2-naphthol; 5-Br-DMPAP - 2-(5-bromo-2-pyridylazo)-5-dimethylaminophenol); 5-Br-PADAP - 2-(5-brorno-2-pyridylazo)-5-diethylaminophenol; PAR - 4-(2-pyridylazo)resorcinol; HNTP - 2-hydroxy-5-nitrothiophenol; Am1 - N, N-dimethylaminomethyl -4-methylphenol; Am2 - N, N-dimethylaminomethyl -4-ethylphenol.
Analytical applications The proposed method under the already established optimum conditions was applied for the determination of Cr (III) in alloys, in sewage water and Bottom sediments and in soils.
Determination of chromium (III) in alloys. A 0.4-0.7 g sample of alloys [Stainless Steel, (%)(Cr-11-13, C-0.1-0.4, Ni-10, Fe-77), Ferrochrome, (%) (Cr - 65, Fe - 35)] was carefully dissolved in 12 ml of H2SO4 (1:3). 2 ml of mix (1:3) conc. HCl and HNO3 was added and heated to release of nitrogen oxides. Filtered insoluble precipitate and a filtrate transferred into a 50 ml volumetric flask. After cooling solution was diluted with water to a tag. Select aliquot portions of the received solution, transfer to a in a separatory funnel, the required value of pH was adjusted by adding 0.1M HCl. 2,2 ml 0,01 M of HNTP and 2.5 mL 0.01M Am was added. The volume of an organic phase adjusted to 5 ml chloroform, and total amount - to 25 ml the distilled water. After 10
min of shaking, a portion of the organic extract was transferred through a filter paper into a cell and the absorbance was read at £=490 nm against chloroform. The Chromium content was found from a calibration graph. The results are summarized in Table 4.
Table 4
Determinal tion of chromium (III) content in alloys (n = 5, P = 0.95)
Method Sample composition Average of Cr(III) found (%) Sr
Cr- HNTP- Ami Stainless Steel 10,99±0,062 0,16
Cr- HNTP- Am2 Stainless Steel 10,99±0,062 0,19
Cr- HNTP- Am1 Ferrochrome 64,81 ± 0,045 0,12
Cr- HNTP- Am2 Ferrochrome 65,02 ± 0,026 0,14
Determination of chromium (III) in soils. The proposed procedures for the determination of Chromium were applied to its determination in light-chestnut soil from the Caspian zone. A 0,5 g weight was finely ground in an agate mortar and calcined in muffle furnace for 3 h. after cooling, the sample was treated and dissolved in an graphite cup in a mixture of 16 ml of conc. HF, 5mL of conc. HNO3, and 15 ml of conc. HCl at 50-60° C. to remove excess hydrogen fluoride, a 8 ml portion of conc. HNO3 was added triply to the solution that was each time evaporated to 5-6 ml. After that, the solution was transferred into a 100 mL volumetric flask and its volume was brought to the mark with distilled water. Chromium was determined in aliquot portions of the solution using the proposed procedures.
The proposed procedures for determining chromium in soils were verified by diphenylcarbazide method. The results of the analysis are listed in Table 5 indicate the successful applicability of the proposed method to real sample analysis.
Table 5
Correctness and reproducibility of determination of chromium in soil («=5, P=0.95)
Method X ", % xio-4 SD x10-4 S2 x +tp '5 — VÏÏ
Standard method
Diphenyl carbazide 2,89 0,156 0,6 (2,89±0,027)
Proposed method
HNTP-Am1 2,80 0,123 0,7 (2,80±0,013)
HNTP-Am2 2,84 0,139 0,5 (2,84±0,011)
Determination of chromium (III) in sewage water and bottom sediments, 1l taken for analysis of waste water is evaporated to obtain a precipitate, do not boil, The precipitate was dissolved in 5 ml of HNO3, was transferred to a 50 ml flask and diluted to the mark with water, Chromium was determined in aliquot portions of the solution using the proposed procedures,
The results of the analysis are shown in Table 6, which indicates the successful applicability of the proposed method to the actual analysis of the samples,
Table 6
Determination of chromium ( III) in sewage water and bottom sediment s («=5, P=0,95)
Analysis object Added, Vg Found, p.g Vg / kg Sr
Sewage water
Sample 1 3,0 3,47 0,47±0,045 0,13
Sample 2 7,0 7,73 0,73±0,013 0,18
Bottom sediments
Sample 1 3,0 4,60 1,60±0,028 0,29
Sample 2 5,0 6,25 2,25±0,033 0,30
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Information about the authors A.Z. Zalov - doctor of chemical sciences, chief of department of chemistry ASPU; N.A. Novruzova - Candidate of Chemical Sciences, Associate Professor of ASPU; S.G. Aliyev - Candidate of Chemical Sciences, Associate Professor of ASOIU.
Информация об авторах А.З. Залов - доктор химических наук, заведующий кафедрой химии АГПУ; Н.А. Новрузова - кандидат химических наук, доцент АГПУ; С.Г. Алиев - кандидат химических наук, доцент АГУНП.
