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 (
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 [
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 [
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 [
Me(H2O)n + L → MeL(OH)n-1 + H2O.
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 MoO 2 2 + 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 [
According to [
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 [
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 Steels1 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) (
| Object | Mo content according to the passport, % | Found Mo, % | Sr |
|---|---|---|---|
| Steel 86 е | 0.21 | 0.22 ± 0.05 | 0.049 |
| Steel 316 | 2.77 | 2.75 ± 0.02 | 0.013 |