AMPEROMETRIC TITRATION OF MERCURY (II) WITH MPCMDEDTC SOLUTION IN DIMETHYL SULFOXIDE

. The article studies the voltammetric behavior of 4-methoxyphenyl carboxymethyl diethyldithiocarbamate (MPCMDEDTC) in dimethyl sulfoxide (DMSOn the presence of background electrolytes with different acid-base properties in the anodic polarization region of a platinum microdisk electrode. MPCMDEDTC is oxidizied in 0.125 M LiClO 4 solution forming one clearly pronounced anode wave with E1 /2 = 0.85 V, while the wave height varies in proportion to the concentration of the depolarizer. A procedure has been developed of amperometric titration of mercury (II) with a standard MPCMDEDTC solution in DMSO using two indicator electrodes.


Relevance of the work
Electroanalytical methods, amperometric titration in particular, can be used to determine different elements in broad concentration range and, with an appropriate selection of analysis conditions, they also become highly selective.
Since mercury, which is often found in natural and technological environments in quantities exceeding the permissible exposure limit (PEL), is toxic, monitoring and determination of its content is a relevant issue throughout the world.
This work is devoted to studying the possibility of amperometric titration of mercury (II) with two indicator electrodes with a MPCMDEDTC solution in a medium widely used in electrochemical studies of an aprotic dipolar solvent, dimethyl sulfoxide (DMSO), which exhibits weak basic properties.
The initial 0.002 М mercury (II) nitrate solution was obtained by dissolving a portion of Hg(NO 3 ) 2 . H 2 O (chemically pure) в ДМСО in DMSO and standardized by an aqueous solution of potassium iodide amperometrically [5]. Solutions of lower concentrations were obtained by diluting the original immediately before use. A standard 0.005 M solution of MPCMDEDTC was prepared by accurately weighing the reagent. Stock solutions of background electrolytes (10% LiClO 4 ; 7.5% CH 3 COONa and CH 3 COOH) were prepared by dissolving the corresponding salts in DMSO.
Amperometric titration with two indicator rotating platinum electrodes was carried out according to the setup described in [6]. Titration was carried out with a piston microburette [7] with an accuracy of 0.0005 ml. the final volume of the solution prepared for titration was 10.0 ml.
To obtain the current-voltage curves of MPCMD-EDTC, a complex of mercury (II) with MPCMD-EDTC nd free ions of mercury (II) for the anodic and cathodic polarization of a platinum microdisk electrode (l = 1.0 mm), we used a three-electrode cell and a PPT-1 polarograph, the reference electrode was SCE.
Results and discussion. It can be seen from the obtained experimental data that the current-voltage curves of mercury (II) ions in DMSO are restored at E 1/2 =0.12 V. he cathode wave of mercury (II) chelating agent is shifted by approximately 0 with respect to the wave of free mercury (II) ions. 2 V towards more negative potentials. MPCMDEDTC anode wave is not clearly expressed due to the partial application of the oxidation current of the solvent itself, which begins to flow at a noticeable rate even at 0.8 V. The mercury (II) chelating agent also oxidizes at the platinum anode at slightly higher potential values (E 1/2 = 0.92 V), and therefore its wave is expressed even less clearly than the wave of free MPCMDEDTC.
Taking into account the voltammetric behavior of all components of the reaction of formation of mercury (II) chelating agent, it can be concluded that amperometric titration of mercury (II) should be carried out at a half-wave potential in the range of E 1/2 = =0.80-0.90 В. Under such conditions, to the point of equivalence, the indicator current will be due to the simultaneous reduction of titrated mercury (II) ions at the cathode and the oxidation of the resulting mercury (II) chelating agent at the anode. Since in this case the concentration of mercury (II) will continuously fall from its initial to almost zero value, and the concentration of mercury (II) chelating agent, on the contrary, will increase, the indicator current will first increase to a certain maximum value, reached at the time of titration of approximately half of all mercury (II), and then fall in this case, a continuously increasing current is observed, due to the reduction of mercury (II) chelating agent at the cathode and the oxidation of free MPCMDEDTC on the anode.
When mercury (II) is titrated against a background of 0.125 М LiClO 4 a fairly clear titration curve is obtained with a smooth curve in the vicinity of the titration end point, which indicates a slightly lower strength of mercury (II) chelating agent in DMSO compared with its strength in water. The titration endpoint, determined by the generally accepted graphic method, corresponds to the formation of a normal complex of mercury (II) with MPCMDEDTC with a ratio of mercury (II): MPC-MDEDTC = 1/1. Unlike solutions containing lithium perchlorate as a background, solutions of mercury (II) nitrate against the background of sodium acetate and acetic acid are titrated much worse: the titration curve is less clear and the equilibrium time sharply increases after each titrant addition. This is explained by the ability of mercury (II) to form very strong complexes with acetate ions.
In order to assess the reproducibility and correctness of the developed method, various amounts of mercury (II) were titrated against a background of 0.1 M lithium perchlorate with a 3-fold repetition of each determination. The results of titrations of mercury (II) with the standard MPCMDEDTC solution showed that the difference between the taken and found amounts of mercury (II) in all cases does not go beyond the confidence interval, which indicates the absence of systematic errors.
Amperometric titration of mercury (II) with MFKMDEDTK solution can be used in the analysis of certain paints, pharmaceuticals and perfumes containing mercury that are poorly soluble in water.