CC BY
70
https://doi.org/10.29013/AJT-20-3.4-55-58
Yakhshieva Zukhra Ziyatovna, Doctor of Chemistry, Professor Kalonov Rustam Mamadierovich, Teacher, Djizakh State Pedagogical Institute named after A. Qodiriy, Uzbekistan E-mail: [email protected] Khudaiberdieva Umida Ermatovna, chemistry teacher of general education school No. 12 in Pakhtakor City, Jizzakh Region, Uzbekistan
Muyassarova Kumush Ikrom, Kizi, third-year student of Djizakh State Pedagogical Institute, Uzbekistan
ASPECTS OF APPLICABILITY OF DIETHYLDITHIOCARBAMATE
SALTS IN ANTIMONY TITRATION
Abstract. The paper shows aspects of applicability of carbamates and optimizes the conditions for amperometric titration of antimony with a solution of lead diethyldithiocarbamate with two platinum indicator electrodes in media having different acid-base properties relative to the current of anodic oxidation of reagent and cathodic reduction of titrated metal ions. During experiment, the metrological characteristics (reproducibility, correctness, band extension of determined contents, sensitivity, selectivity) were improved, the fields of use of the antimony titration method were expanded with a view to their subsequent application when controlling the composition of natural objects and industrial materials.
Keywords: amperometric titration, electrochemistry, selectivity, reproducibility, reagent, selectivity, expressivity.
Introduction. Increased requirements for analytical composition control of poorly water-soluble organic objects at all stages of technological processes dictate increased requirements for accuracy, expressivity, selectivity and expansion of the range of determined contents of analysis methods developed or implemented in production. The most accurate ways to indicate the endpoint of titration include electrochemical methods, in particular, amperometric methods with one or two polarized indicator electrodes.
Theoretical analysis. Lead diethyldithiocarbamate (Pb(DTC)2) forms together with many cations
very sound and almost non-water-soluble complex compounds that can be extracted by many organic solvents (benzene, butanone, carbon tetrachloride, chloroform, etc.).
Trial experiments have shown that when titrating the antimony cation in protolytic media with two platinum indicator electrodes, clear curves are registered that allow to accurately find the position of equivalence point using a conventional graphical technique. Determination of a titration endpoint (TEP), regardless of the nature of solvent used, corresponds to the formation of complexes
in a molar ratio Me: ligand -1:3 for trivalent cation ions.
Experimental procedure. Amperometric titration with two platinum indicator electrodes, as follows from the current-voltage characteristic of Pb(DTC)2, should be performed at voltages below 0.5 V on the acetate background, 0,4 V on the nitrate background and 0,5 V on the perchlo-rate background, so that the processes of anodic titrant oxidation and cathodic oxygen reduction could simultaneously occur. If the voltage is too high and Pb(DTC)2 is used as a titrant, the current can also occur through anodic oxidation of lead ions formed during the reaction and cathodic oxygen reduction.
In order to optimize the titration conditions with Pb(DTC)2 solutions, the effect of external voltage magnitude applied to the indicator electrodes, the nature and concentration of background electrolyte, additives of an inert solvent, different foreign cations, interfering anions, a number of complexing compounds and other factors have been studied.
Results and discussion.
External voltage effect. In accordance with current-voltage characteristics of Pb(DTC)2, with antimony ions participating in complexation reaction, as well as dissolved oxygen, amperometric titration of metal ions with two platinum indicator electrodes on various background electrolytes should be performed at a voltage not lower than 0,30 V so that after the equivalence point, the processes of anodic free reagent oxidation and cathodic oxygen reduction could simultaneously occur. Dimethylformamide dissolved oxygen starts to recover with a noticeable rate only at potentials of 0.35 V. The voltage was changed in the range of 0.3-1.1 V stepwise of 0.2 V.
Taking into account this circumstance, as well as indicated values of the half-wave potentials (E ) of Pb(DTC)2 and the oxidation starting point of acetic acid, n-propanol, dimethylformamide or di-methylsulfoxydum it can be concluded that the amperometric titration of non-reducible cations upon
platinum cathode should be controlled at a voltage of at least 0.8 V [1].
Experiments have shown that one gets the best-shaped curves and correct results of titration of metal ions when the indicator electrodes voltage is not lower than 0.9 V.
It was found that when the applied voltage is less than 0.4 V during amperometric titration with Pb(DTC)2 solution, the right ascending branch of a curve is too flat and quickly deviates from straightness, leading to a decrease in reproducibility of the curve shape and, consequently, to incorrect titration results.
The curve shape of amperometric antimony titration with Pb(DTC)2 solution is adequate to the curves obtained during its determination and to the equivalence point, cathodic antimony ions and anodic organic solvent oxidation will be reduced.
In order to get clearer curves of amperometric titration, the next experiments were performed with increased electrode voltage up to 0.9 V At this voltage, the right branch of the curve becomes steeper and its area of smooth curvature is significantly reduced.
Effect of background electrolyte nature and concentration. Since the acid-base properties and concentration of the background electrolyte have a very significant effect on the curve shape, conditions and results of titration of metal ions, the effect of its nature and concentration has been studied in detail at a voltage of 0.5-0.8 V (depending on the nature of titrated medium). The effect of three background electrolytes: potassium acetate, nitrate and lithium per-chlorate, that differ in their nature and exhibit basic, neutral and acidic properties, respectively, was studied.
Experiments have shown that the antimony titration with Pb(DTC)2 solution at background electrolyte concentration of 0.1-0.3 M proceeds relatively quickly, while the shape of titration curve, its precise-ness and position of TEP basically do not change when concentration of the background electrolyte varies. If its concentration is too high (more than 0.5 M), the shape of amperometric titration curve gets worse: the right branch quickly loses its steepness
and linearity, the area of smooth transition between the branches increases, and the position of equivalence point shifts towards overestimation. If the background concentration is too low because of the low electrical conductivity of the analyzed solution, the right branch of titration curve becomes curved and TEP, although slightly, is shifted in the direction of overestimation [2; 3].
Antimony titration with lithium nitrate is slightly faster than with potassium and ammonium acetate. The titrant demand at TEP changes in proportion to the amount of antimony added, but the equivalence point position does not exactly correspond to the stoichiometry of normal metal thiocarbaminate formation, as it occurs when titrating this cation on the acetate background. At the same time, one antimony atom accounts for two reagent molecules, that is, mixed-ligand complexes are formed. It should be emphasized that this ratio of1:3 components in the complex was seen with a solution of Pb(DTC)2 in glacial acetic acid.
Experiments have shown that amperometric titration of antimony with a solution of Pb(DTC)2 proceeds quickly enough and on an acetate background only, and it was impossible to get sufficiently reproducible and correct results with nitrate and lithium perchlorate. The titrant consumption at the equivalence point with potassium acetate is proportional to the amount of antimony taken. Changes in this background concentration in the range of 0.10.3 M has little impact on the curve shape and the results of antimony titration.
According to the observed data received during the titration of various metal ions, it can be concluded that the change in the concentration of the background electrolyte - potassium acetate or ammonium in the solution under s tudy (0,03-0,04 M), the shape and preciseness of amperometric titration curve remains largely unchanged, however, if the background concentration is too high (more than 0.4 M), the determination results are noticeably high. If their concentration is too low
(less than 0.2 M), the shape of titration curve is significantly deteriorated: its right branch becomes less steep and quickly loses its straightness, bending to the volume axis.
Thus, for amperometric titration of different metal ions (except for bismuth ions) with solutions of Pb(DTC)2, the best background electrolyte is considered to be potassium acetate with an optimal concentration in the range of 0.1-0.25 M [4].
Effect of the nature and concentration of inert solvent. In order to determine whether the proposed amperometric method with two indicator electrodes can be used for determining metal ions directly in extracts or in organic objects, the effect of carbon tetrachloride, chloroform, benzene and other solvents often and widely used as extraction agents on the conditions and results of titration with solutions of Pb(DTC)2 was studied. Titration was performed under optimal conditions, except that in the solution analyzed, one or another part ofprotolytic solvent was replaced with an inert solvent (from 10.0 to 50.0 vol.%).
Experiments have shown that carbon tetrachloride additives have least effect on the shape of curves and the results of amperometric titration of antimony. Even with its content of 40 vol.%, the titration results are still correct and reproducible. Hexane starts to overestimate the results of titration of metal ions at a content exceeding 35.0 vol.%, and chloroform and benzene - from 25.0 and 30.0 vol.%, respectively.
According to findings, when titrating Sb(lIl), Pd(ll), Hg(ll), Bi(lIl), Ag(l), Au(III) and other metals, the addition of increasing amounts of any of the studied non-aqueous solvents first markedly improves the shape of amperometric titration curve - its right branch becomes steeper, and the area of smooth curvature decreases, which indicates an increase in the effective stability constant (ESC) of the complex influenced by addition of inert solvent.
In addition, equilibration after each addition of the titrant is always significantly accelerated when its added to. Chloroform additives have a particularly favorable effect on the shape of titration curve.
However, starting from approximately 30-50 vol.% (depending on the solvent nature) of the added inert solvent, the steepness of the right branch of the titration curve continuously decreases as its content increases owing to decrease in the electrical conductivity of the solution under study. It should be noted that the sharpness of the amperometric titration curve slope after the equivalence point increases in all cases as the concentration of any of the studied inert solvent increases, indicating that the strength degree of the formed metal complex is increased [5-7].
Thus, based on our study, it can be concluded that the addition of small amounts of an inert solvent (not more than 30-40 vol.%) to protolytic solvents leads to a significant increase in the pre-
ciseness of the titration curve and, consequently, to an increase in the definition accuracy. The optimal content of an inert solvent that gives the best titration results depends on its nature. The most accurate value of this quantity is obtained for chloroform (45 vol.%), benzene (50 vol.%), and methyl ethyl ketone (40 vol.%).
It follows therefrom that it is best to perform amperometric titration of studied metal ions using benzene and chloroform extracts, since the best-shaped titration curves with less protolytic solvent are obtained at their presence.
Conclusion. Thus, the proposed method for titration of antimony ions with a solution of Pb(DTC)2 has quite satisfactory accuracy and speed of analysis.
References:
1. Геворгян А. М., Талипов Ш. Т., Хадеев В. А., Мухамеджанова Д. В. Вольт-амперометрическое поведение диэтилдитиокарбамината натрия на платиновом аноде в среде диметилформамида // Журн. аналит. химии. 1980.- Т. 35.- № 10.- С. 2026-2028.
2. Лурье Ю. Ю. Справочник по аналитической химии.- М.: Химия. 1979.- С. 230-236.
3. Фишер Р. А. Статистические методы для исследователей, занимающихся определением благородных элементов.- М.; Мир. 1988.- 287 с.
4. Геворгян А. М., Яхшиева З. З., Киреев Г. В. Комплексонометрическая амперометрия при определении благородных металлов в объектах окружающей среды. Тез. докл. Респ. научно-прак. конф. «Кимёнинг долзарб муаммолари» - Самарканд. 2009.- С. 65-66.
5. Геворгян А. М., Талипов Ш. Т., Хадеев В. А., Мирзоян И. И. Вольтамперные кривые окисления тио-налида на платиновом микроаноде в ледяной уксусной кислоте // Депонир. в ВИНИТИ № 4616-81 от 24.09.81.- С. 1-3.
6. Геворгян А. М., Яхшиева З. З., Жураева Л. К., Рахимбердиева Г. У Определение числа электронов, отдаваемых при электроокислении одной молекулы тиомочевины // Республиканская научно-практическая конференция с международным участием «Зеленая химия» - в интересах устойчивого развития.- Самарканд. 2012.
7. Yakhshieva Z. The conditions for amperometric titration of the Ag (I) ion with sulfur-containing reagents. // Universia Chemistry and Biology. Electronic scientific journal. 2016.- No. 4(22).
