Synthesis of Different Ligand Complex Mo (VI) with Stillbazo and Cetylpyridinium Chloride ()
1. Introduction
Polyfunctional azopyrocatechols are typical and valuable reagents for the spectrophotometric determination of metal ions [1] [2]. Their analytical capabilities can be significantly expanded if complexation reactions with metal ions are carried out in the presence of a Superficially Active Substance (SAS) [3] [4].
When using SAS, the sensitivity and contrast of the reaction significantly increase, bathochromic and hyperchromic effects are observed, and the pH of complex formation decreases [3].
In the presence of cationic SAS (cSAS), the group of easily hydrolyzed elements W (VI), Zr (IV), Ti (IV), Mo (VI) is of greatest practical interest [5] [6].
The aim of this work is to study the complexation of Mo (VI) with stillbazo in the presence of a cationic surfactant (CPCl) and use the data obtained for the development of a selective and sensitive spectrophotometric method for the determination of molybdenum (VI).
2. Experimental Part
2.1. Equipment
The absorption spectra of the complex and reagents were recorded on an SFV-1800 and KFK-2 spectrophotometer. The pH value of the colored solutions was measured using a BENCHTOP METERS pH meter.
2.2. Reagents and Solutions
A standard solution of molybdenum (VI) with a concentration of 10−4 M was prepared from ammonium molybdate. The titer of the solution was determined as described in [7]. Used 5 × 10−3 M aqueous solution of stillbazo, a 10−3 M solution of CPCl was prepared by dissolving a sample of analytical grade reagent in distilled water, followed by heating.
To create a certain acidity of the medium, we used acetate-ammonia solutions, pH 3 - 7, prepared from 2M CH3COOH and NH4OH. Buffer solutions with pH 1 - 2 were prepared from the standard titrimetric substance of HCl.
2.3. Methodology
Mo (VI) solutions of various concentrations (0.1 - 17 μg/ml) were placed in volumetric flasks with a capacity of 25 ml. 1.2 ml (2.4 × 10−4 M) of stillbazo solution and 1.6 ml (6.4 × 10−5 M) of CPCl solution were added, the volume was brought to the mark with a buffer solution of a standard titrimetric substance HCl with pH 1 - 2. A blank solution was prepared in a similar way. The optical density of the prepared solutions was measured on a KFK-2 (l = 1 cm) relative to a blank solution.
To construct a calibration graph, 1.6 × 10−4 M stillbazo, 4 × 10−5 M (CPCl), varying concentrations of Mo (VI) were placed in flasks with a capacity of 25 ml, and the volume was brought to the mark with a buffer solution of pH 1.1. The optical density of the prepared solutions was measured by KFK-2 at 640 nm in a cuvette with l = 1 relative to the blank experiment.
3. Results and Its Discussion
When mixing solutions of molybdenum (VI) and a solution of reagents, a mixed-ligand complex compound colored in a violet-blue color is formed. The absorption spectra of the reagent with molybdenum, as well as in the presence of CPCl, were recorded in a wide range of wavelengths and at various pH values of the medium.
It was found that the maximum absorption of the reagent in a weakly acidic medium with pH 5.6 is observed at a wavelength of 340 nm, and its single-ligand complex with molybdenum—at 409 nm. Optimum complication conditions are observed at pH 2.6. For the complete binding of Mo (VI) ions into the complex, 0.6 ml of 1.2 × 10−4 M reagent solution is required.
It was found that in the presence of CPCl, a mixed-ligand complex Mo (VI)-tilbazo-CPCl is formed. The study of the absorption spectra of the mixed-ligand complex depending on the pH of the medium showed that during its formation the maximum of light absorption shifts to an acidic medium and a longer wavelength region of the spectrum. The maximum yield of the Mo (VI)-tilbazo-CPCl complex is observed at pH 1.1 and λ = 640 nm. For complete binding of CPCl, 0.8 ml of a 3.2 × 10−5 M CPCl solution is required (Figure 1).
The study of the dependence of the formation and stability of the solution of the complexes on time and temperature showed that the homogeneous ligand and mixed-ligand complexes are formed immediately after mixing the solutions of the components and the resulting complexes are stable during the working day and when heated to 70˚C.
In the presence of surfactants, a significant hyperchromic effect and a bathochromic shift are observed in the spectra of the complexes, and the values of optical densities are the maximum, as a rule, at surfactant concentrations close to CMC [8].
A SAS stabilizes a solution of molybdenum-stillbazo, prevents the precipitation of a poorly soluble compound, and acts as a protective colloid, which is usually used in gelatin [9].
It has now been established that the introduction of surfactants in low concentrations in aqueous solutions leads to the effect of water structuring. Hydrocarbon radicals of the introduced substances are located in the voids of water, thereby leading to an increase in the strength of hydrogen bonds in water, i.e., structuring water [10]. It is well known that water molecules are a competing ligand in reactions with organic reagents in aqueous solutions according to the following scheme:
Me(H2O)n + L → MeL(OH)n-1 + H2O.
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Figure 1. The adsorption spectra of the investigating compounds. 1) Mo (VI)-stillbazo; 2) Mo (VI)-stillbazo-CPCl. CMo(VI) = 1.7 × 10−5 M, CR = 2.4 × 10−4 M, CCP = 6.4 × 10−5 M.
Therefore, when hydrated molybdenum ions enter the field of structured water near the stillbazo-CP associate, the hydration shell of the molybdenum ion is loosened, as a result of which water is displaced by the ligand and easier coordination of two stillbazo ions becomes possible. A change in the structure of water leads to a change in the constant of water autoprotolysis. As a result, the hydrolysis of molybdenum is suppressed and the concentration of non-hydrolyzed
particles, capable of coordinating two stillbazo ions, increases. This is due to a sharp increase in the concentration of the complex with a large number of attached stillbazo particles in the presence of CPCl.
It was established by the method of isomolar series [11] that the ratio of reagents in the composition of the mixed-ligand and homogeneous-ligand complex Mo (VI)-stillbazo-CPCl = 1:2:4; Mo (VI)-stillbazo = 1:2.
According to [11], the 1:2 complex is formed by
ions. Taking into account the state of the reagent and molybdenum (VI), the resulting complexes can be schematically represented in the following form:
The stability constant of the mixed-ligand complex lgβk = 86 × 108 was determined by the method of curve intersection. With the formation of a mixed-ligand complex, the sensitivity of the reaction increases, i.e., the value of the molar light absorption coefficient increases, respectively, Mo (VI)-stillbazo, Mo (VI)-stillbazo-CPCl εк = 11835 and εк = 68985 [12].
Beer’s law is observed in the range 0.05 - 17.00 μg Mo (25 ml).
The influence of foreign ions and masking substances on the photometric determination of Mo (VI) has been studied. It was found that with the formation of a mixed-ligand complex, the selectivity and contrast of the reaction increase. The complexation is interfered with the ions W (VI), Fe (III), Ti (IV).
The developed technique was applied to determine Mo (VI) in industrial complex objects.
Determination of Molybdenum in Steels
1 g of a sample of steel is dissolved in 50 ml of HCl (1:1), adding a few milliliters of HNO3 (1:1) during dissolution. The solution is evaporated to 2 - 3 ml, transferred to a 100 ml volumetric flask, and the volume is adjusted to the mark with distilled water. An aliquot part (1 - 3 ml) is placed in a 25 ml volumetric flask, to which a buffer solution of pH 1.1 was previously added, then 5 ml of a 5% solution of ascorbic acid, 0.8 ml of 5 × 10−3 M stillbazo solution, 1 ml 1 × 10−3 M solution of CP, bring the volume to the mark with buffer solution (pH 1.1) and after 5 min. measure the optical density at 640 nm (l = 1 cm) (Table 1).
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Table 1. Results of the determination of molybdenum in steels (n = 3, p = 0.95).
4. Conclusion
The complexation of molybdenum (VI) with stillbazo was studied in the presence of a cationic surfactant (CPCl). It was found that in the presence of surfactants, the light absorption of the mixed-ligand molybdenum complex significantly increases, the bathochromic effect is observed in the absorption band spectrum, and complexation shifts to a more acidic region. The composition of Mo (VI)-stillbazo-CPCl = 1:2:4 complex was determined by various spectrophotometric methods. The stability constant (lgβk = 86 × 108) and the molar absorption coefficient (εк = 11835) of the complex were determined. It was found that Beer’s law is observed at a Mo (VI) concentration of 0.05 - 17 μg/ml. A method for the photometric determination of molybdenum in industrial facilities has been developed.