Stock solution (1 mg/mL) of Mo(VI) was prepared by dissolving 1.8402 g (NH₄)₆Mo₇O₂₄⸱∙4H₂O p. f. a. (Pure for analysis) in hot water. On cooling; solution was diluted with water in a measured flask to 1 L. The concentration of the solution was adjusted gravimetrically [36] . Working solution with concentration of 0.1 mg/ml was prepared by dilution of stock with deionised water. Solutions of DP and AP in chloroform (0.01 M) were used. Their purity was verified by paper chromatography and melting point determination. AP was used in a recrystallization form. The extractant was purified chloroform. The ionic force of solutions was supported a constant (µ = 0.1) by introduction of the calculated quantity of KCl. To create the optimal acidity, 1 M solutions of KOH and HCl were used.

A buffer solution of pH 5.0 was prepared by mixing proper proportions of acetic acid (0.2 M) and sodium acetate solutions (0.2 M). The stock solution of various metal ions and anions were prepared by dissolving the appropriate metal salts in distilled water or with suitable dilute acids and making up to a known volume.

The adsorbance of the extracts was measured using a Shimadzu UV1240 spectrophotometer and KFK-2 photocolorimeter (USSR). Glass cells with an optical path of 5 or 10 mm were used. The pH of the aqueous phase was measured using an l −120.2 potentiometer with a glass electrode. The ash furnace was used to dissolve the samples. The thermolysis process of the compounds was investigated using the compounds was investigated using the cyvatography system “Shimadzu TGA-50H”. IR spectra were recorded on a spectrophotometer “Bruker” (Germany). ¹H-NMR spectra were recorded on “Bruker” Fourier Transform (300.18 MHz) in deuterated benzene (C₆D₆).

All specified devices passed state check. All measurements were carried out at 20˚C ± 1˚C.

To an aliquot containing 50 µg of molybdenum, add 1.8 ml 0.01 M DP, and a 1.3 ml 0.01 M AP 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. After the complete separation of the phases, the organic phase was separated from the aqueous phase in 10 minutes and the absorbance of the extracts was measured on KFK-2 photocolorimeter using 540 nm (l = 0.5 cm) and at room temperature.

A 0.5 - 1.0 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 a graphite cup in a mixture of 16 ml of HF (conc.), 5 ml of HNO₃ (conc.), and 15 ml of HCl (conc.) at 50˚C - 60˚C to remove excess hydrogen fluoride. A further 8 ml portion of HNO₃ (conc.) 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. Molybdenum was determined in aliquots of the solution using the procedure proposed by us.

The water samples were filtered through Whatman No. 40 filter paper then 100 ml of each filtered water sample was accurately transferred into a 250 ml round bottom flask, and 10 ml of a mixture consisting of HNO₃ and H₂O₂ (1:9, v/v) were added. These samples were digested by heating under reflux for 1.5 h. The cooled samples were transferred into 100 ml volumetric flask and made up to the mark. With deionized distilled water, mixed well, then subsequently analysed by the proposed spectrophotometric methods.

A wet ash method was employed in the preparation of the sample solution. 0.5 g of the sample was dissolved in a 1:1 mixture of nitric acid and perchloric acid. The solution was evaporated to dryness, and the residue was ashed at 300˚C. The ash was dissolved in 2 ml of 1 M sulphuric acid and made up to the volume in a 25 ml standard flask with distilled water.

It is known that dithiophenols have reducing properties in acidic medium. To elucidate the oxidation state of Molybdenum in MLC, we conducted two series of experiments. In the first series we had used Mo (VI), while in the second series we had used Mo(V) obtained by addition of a supplementary reducing agent (SnCI₂ or KI). It was found that the spectrophotometric characteristics of the MLC of Mo (VI) and Mo(V) were identical i.e. in the interaction with DP, Mo(VI) was reduced to Mo(V).

This fact was also confirmed by EPR spectrometry [41] . Molybdenum(VI) does not have unpaired electrons and is diamagnetic, while molybdenum(V) has one d electron and exhibits electron paramagnetic absorption. According to the value of the nuclear spin j = 7/2, the ESR spectra of molybdenum (VI) consisted of eight lines with the hyperfine structure associated with the interaction of the magnetic moment of an unpaired electron with the magnetic moment of the ⁹⁵Мо and ⁹⁷Мо nucleus. Hyperfine structure consisting of 6 lines (2J + 1 = 2 × 5 / 2 + 1 = 6) was observed in the ESR spectra of chloroform extracts of MLC from aqueous solutions of Mo(VI) and Mo(V) salts.

Hence, in the complex formation with DTBP, molybdenum (VI) is reduced to molybdenum (V) by the reagent itself. The results of the studies are presented in Figure 1. The Mo spectra are anisotropic, and are similar to the V(IV) spectrum.

As DP is insoluble in water, an organic solvent was used for the system. Of the various solvent (chloroform, 1,2-dichloroethane, tetrachloromethane, dichloromethane,

benzene, chlorobenzene, toluene, xylol, isobutanol, isoamyl alcohol, cyclohexane, ethyl acetate, 1-butanol, isoamyl acetate and their mixes) studied, chloroform, 1,2-dichloroethane and carbon tetrachloride was found to be the best solvent for the system (Figure 2). No absorbance was observed in the organic phase with the exception of 1-butanol. In a 50% ethanol medium however, the maximum ab­sorbance was used in the determination procedure. After a single extraction with chloroform, 98.4% - 98.7% of molybdenum was extracted as an ion associate (in a case the dichloroethane and carbonetrachloride was removed 96.5% - 97.8% of molybdenum). Further researches were conducted with chloroform. The concentration of molybdenum in the organic phase was determined using 8-mercaptoquinoline [3] by photometric measurements after reextraction, and in the aqueous phase by difference.

To study the effect of pH on maximum color development and also on quantitative extraction of the colored complex into chloroform, the color reaction and the extraction were carried out at different pH values. Extraction of the complex into chloroform is quantitative and the complex shows maximum absorbance in the pH range 4.1 - 5.9 (Figure 3). The complete extraction of the complex into chloroform is confirmed by the fact that the aqueous phase does not show any absorbance. However, outside this pH range, it is observed that either the extraction of the complex into chloroform is incomplete or the solution does not attain the maximum color. For all subsequent studies, therefore, the pH is maintained at an optimum level of 5.0. Thus further extraction and determination carried

out at pH 5.0. The presence of one maximum optical density within the indicated pH limits confirms the assumption that a single complex compound is formed. The analyzed solution should not contain oxidizing agents (for example, HNO₃) due to the high tendency of DP to oxidation. At higher pH values, the complexation process is complicated by hydrolysis.

Under the experimental conditions, the absorption spectrum of the Mo(V)-DTMP-AP complex was recorded against the blank solution. The absorption maxima (λ_max) of the ternary Mo(V)-DP-AP complexes lie in the range of 516 - 534 nm (Figure 4, Table 3). All colour reactions were very contrast since the initial solutions are colourless (λ_max (dithiolphenols) = 274 - 280 nm). Thus, bathochromic shift makes 242 - 260 nm. Close values of λ_max of light absorption allow to draw a conclusion that the formed complexes were ionic associates. Contrast of reactions was high i.e. initial reagents are colourless while complexes are intensively painted. Molar coefficients of absorption make (4.16 - 5.34) × 10⁴

dm³ × mol⁻¹ × cm⁻¹. The color of both the mixed ligand complexes and the homogeneous ligand complex HLC is due to the same electronic transitions—the transfer of electrons from the dithiolphenolate ion to the metal ion, and these complexes are charge-transfer complexes.

The reagents have a minimum absorbance at the maximum absorbance of the complex, and hence do not interfere with the determination of molybdenum. All the spectral measurements of the complex were carried out at 540 nm.

Studies of the effect of reagent concentration showed that an excess of reagent 2 - 3 times was optimal for the formation of complexes. The presence of a small excess of the reagent solution does not interfere with the color reaction. Temperature measurement in the range of 10˚C - 70˚C does not affect the color of the complexes. It was found that the optimal concentration of dithiophenols and AP for the formation of the complex is 7.2 × 10⁻⁴ and 5.2 × 10⁻⁴ M, respectively.

The stoichiometric reaction coefficients of Mo(V) with DP and AP were determined by the equilibrium shift method and confirmed by straight-line method of Asmus and relative yield method [42] . Both methods showed that the ratio of the reacting components of Mo:DP:AP = 1:1:2 (Figure 5).

Using the Nazarenko’s method [43], we found that the molybdenum complexation form is MoO(OH)²⁺ [44] . Hence, the complexes can be regarded as ion associates between doubly charged anionic chelates [Mo(OH)(DP)₂]²⁻ and two protonated AP species: (APH⁺)₂[MoO(OH)(DP)₂].

In the IR spectra of the Mo-DTMP-An complex, in addition to a narrow band in the region of 950 - 930 cm⁻¹, corresponding to the M = O bond, a somewhat intense absorption band appears at 1090 cm⁻¹, which indicates the presence of the M − OH group (Figure 6). The disappearance of the pronounced absorption

bands in the 3620 - 3250 cm⁻¹ with a maximum at 3475 sm⁻¹ observed in the spectrum of DTMP, says that the -OH group is involved in the formation of the complex. The observed decrease in the intensity, absorption bands in the area 2570 sm⁻¹ shows that one of the -SH groups involved in the formation of coordination bond in the ionized state. Detection of the absorption bands at 2380 cm⁻¹ indicates the presence of a coordinated aminophenole [39] [40] .

Thermogravimetric study of the complex Mo-DTMP-AP₁ shown that thermal decomposition of the complex takes place in three stages: at 50˚С - 80˚С water evaporates (weight loss-4.95%), at 420˚С - 490˚С-decomposed AP (weight loss—30.56%), and at 490˚С - 530˚С-DTMP (weight loss—46.75%). The final product of the termolysis of the complex is MoO₃.

Additional experiments by the Akhmedly’s method [45] showed that the complex exists in monomeric form in the organic phase (the obtained coefficient of polymerization γ was equal to 1.05).

Given the molar ratio of components in the complexes, the complexing form of central ions, monomeric complexes in the organic phase, IR spectroscopic data, thermogravimetric studies and chemical analysis, it can be assumed that the Mo(V) with DTMP and AP forms mixed ligand complexes (Figure 7).

To assess the expediency of the proposed method, the effect of several diverse ions was examined under already established optimum conditions. The interference of various ions was determined by measuring the absorbance of solutions containing a constant amount of Mo(VI) and varying amounts of diverse ions. The tolerance limit was considered to be the amount that caused a ±1% deviation in the absorbance value. The tolerance limits of various diverse ions are given in Table 4.

Ba(II), Mg(II), Ca(II), Sr(II), Cr(III), Zn(II), Cd(II), Hg(II), Al(III) and Bi(III)

did not affect the absorbance of Mo(V)-DTMP-AP complex when present in 10 mL of the aqueous phase and were found to cause < 1% error. However, Hg(II), Pb(II), U(VI), Mn(II), V(IV), Fe(II), Fe(III), Cr(VI), Pd(II), Pt(IV), V(V), Zr(IV), Ti(IV), Ta(V), Co(II), Cu(II), Nb(V), Ni(II) and W(VI) required masking agents, as mentioned under the procedure.

The interference of Cu(II) can be eliminated by using 1.0 mL 0.2% EDTA as a masking agent. Fe(II) and Fe(III) are masked with 1.0 mL of 3% sodium fluoride. The interference of Zn(II), Cd(II) can be eliminated by using 1.0 mL of 0.5% thiosulphate solution. Anions like fluoride, thiocyanate, thiosulfate, and thiourea do not interfere when present up to 3.000 µg, with the determination in the method. Chloride, citrate, and borate are tolerated up to 1.500 µg. The interfering influence of Nb(V), Ta(V), Mn(II) and Hg(II) removed by controlling pH.

A series of solutions containing different amounts of the metal ion were prepared as per the general experimental procedure. The absorbance of the solutions was measured at 500 nm. A calibration graph drawn between absorbance and the metal ion concentration indicates that Mo(V) can be determined in the concentration range 0.3 to 22.0 µg mL⁻¹ (Figure 8).

Detection limit estimated from the standard deviation (SD) of the blank and calibration sensitivity (slope of calibration line), (LOD = 3 SD/sensitivity amounted to 9 - 10 ng∙mL⁻¹ of Mo(V) (Table 5). Limit of quantication (LOQ) is 28 - 34 ng∙mL⁻¹ of Mo(V), and the LOD/LOQ ratio is 3.1 - 3.4.