PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV,
DECORATED WITH THE ORDER OF THE OCTOBER REVOLUTION AND THE ORDER OF THE RED BANNER OF LABOR

Vol. 302　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1976

STUDIES ON THE AMALGAM-POLAROGRAPHIC BEHAVIOR OF MANGANESE AND ITS DETERMINATION IN NITRIC ACID AND A SERIES OF SALTS

I. P. MAMONTOVA, A. A. KAPLIN

(Presented by the Scientific-Methodological Seminar of the Department of Physical and Colloid Chemistry)

In a number of works [1—3], the electrochemical behavior of manganese has been studied. By the method of amalgam polarography with accumulation, the solubility of manganese in mercury was determined to be \((1.2 \pm 0.4)\cdot 10^{-3}\) wt.% at \(25^\circ\)C. These data agree well with results obtained by other methods. In work [1], the nature of manganese amalgams was studied by the method of amalgam polarography with accumulation on a stationary mercury drop. The authors subjected manganese amalgam to anodic oxidation, in some cases immediately after preparation, in others after keeping the amalgam in contact with the solution. It was found that, upon aging of manganese amalgam in contact with the solution, the peak of anodic oxidation of manganese at \(-1.25\) V disappears and new peaks appear at more positive potentials. In the authors’ opinion, this phenomenon is explained by the formation of a compound of Mn with Hg. However, the authors of [5] do not confirm this assumption; they showed that in a pure solution after aging of the amalgam only the anodic oxidation peak of Mn at \(\varphi=-1.3\) V (S.C.E.) is observed, while the appearance of peaks at more positive potentials is due not to the formation of an intermetallic compound of Mn with Hg, but to cementation by manganese of impurities of more electropositive metals (for example, Zn, Pb) from the solution. Reduction of manganese to the metal can be carried out in alkaline solutions with an alkali-metal chloride \((\varepsilon_{1/2}=-1.45\) to \(-1.60\) V), as well as in the presence of certain ligands \((\varepsilon_{1-2}=-1.6\) to \(-1.7\) V). In acidic solutions, manganese reduction is hindered by hydrogen evolution. In acetate-buffer solutions, Mn is not reduced. In work [6], the possibility was studied of using stationary and rotating Pt electrodes for preliminary accumulation of Mn in the form of \(\mathrm{MnO_2}\) against the background of borate and ammonia buffer solutions. Upon dissolution of \(\mathrm{MnO_2}\), a distinct peak is observed on the polarogram; its potential depends on the pH of the supporting electrolyte, and its height is proportional to the Mn concentration in solution in the range \(10^{-6}\)—\(10^{-5}\) M \((\varphi_{\mathrm{e}}=-1.8\) V at pH 7). The determination is not interfered with by alkali metals, Zn, Co, Cd, Cu; Fe, Al, Pb interfere. The study of manganese at stationary dropping and film mercury electrodes is difficult because of the low solubility of manganese in mercury and the ready oxidizability of manganese amalgam. In work [7], conditions were found for the determination of submicrogram amounts of Mn in the presence of \(10^{-6}\)% impurities of Cu, Pb, Cd, Zn, and Ni, in the presence of \(\mathrm{Fe}^{+2}\) ions not exceeding the manganese concentration by more than 50 times. Work on the study of amalg-

...there is comparatively little on the amalgam-polarographic behavior of Mn and its determination in especially pure substances. We have begun systematic studies in this direction. Table 1 gives the characteristics of the anodic peaks of manganese on a number of supporting electrolytes. The working electrode was a mercury film electrode; the reference electrode was a saturated calomel electrode. Aqueous solutions of \(\mathrm{MnCl_2}\) or \(\mathrm{MnSO_4}\) of various concentrations were used as standard manganese solutions. Dependences were obtained of the height of the anodic Mn peak on the potential and electrolysis time. The dependence of the height of the anodic Mn peak on its concentration in solution is a straight line passing through the origin, which makes it possible to use this method for analytical purposes. The scatter of points beginning at a concentration of \(2.5 \cdot 10^{-4}\,M\) is probably connected with attainment of the solubility limit of manganese in mercury.

From the dependence of the anodic-current magnitude on the concentration of Mn ions in solution, the solubility of Mn in mercury was calculated; it is \(2.0 \cdot 10^{-3}\) wt. % at \(23^\circ\mathrm{C}\). To calculate the diffusion coefficient of Mn in mercury from stripping currents, the procedure of Stromberg and Zakharova was used. The calculated value of the diffusion coefficient of Mn in mercury, \(1.3 \cdot 10^{-5}\ \mathrm{cm^2\,s^{-1}}\), as well as the value of the solubility of Mn in mercury, agrees with literature data.

Table 1

Anodic peaks of Mn on various supporting electrolytes. Experimental conditions: \(V\) of solution \(= 4\) ml; \(\varphi = -1.8\) V; \(\tau_\mathrm{e} = 5\) min; \(C_{\mathrm{Mn}} = 3.3 \cdot 10^{-7}\) g; \(S = 4 \cdot 10^{-8}\) a/mm

On the basis of the studies carried out, we propose procedures for the determination of Mn in especially pure \(\mathrm{HNO_3}\) and in a number of salts.

Determination of Mn in \(\mathrm{HNO_3}\)

Manganese in nitric acid is present as \(\mathrm{Mn(NO_3)_2 \cdot 6H_2O}\) with \(t_{\mathrm{boil}} = 129.4^\circ\mathrm{C}\); therefore, when evaporating a weighed portion of nitric acid, the temperature should not be raised above \(100^\circ\mathrm{C}\), in order to avoid losses of manganese. The weighed portion of \(\mathrm{HNO_3}\) should be evaporated to dryness, since hydrogen evolved at the electrode will mask the anodic peaks of manganese. The proposed procedure is based on evaporation of the nitric acid and subsequent polarographing of manganese against the background of a \(0.1\ N\) \(\mathrm{NaCl}\) solution. Manganese was not detected in the especially pure nitric acid available in the laboratory. The minimum determinable concentration of Mn, with a relative standard deviation of 0.33, will be \(1.14 \cdot 10^{-7}\%\) for a 5 g weighed portion. The dil...

the duration of the analysis is 2.5 hours. Together with Mn in nitric acid it is possible to determine Zn, Cd, Pb, and Cu.

Determination of Mn in lithium iodate

The method for determining Mn in $\mathrm{LiIO_3}$ is based on decomposing a weighed portion (0.1 g) in 3 ml of HCl without heating. The solution is then evaporated at 130°C, 4 ml of $\mathrm{H_2O}$ is added, and the polarographic determination of Mn is carried out on the resulting supporting electrolyte. Manganese was not found in $\mathrm{LiIO_3}$. The sensitivity of the method is $1.27 \cdot 10^{-5}\%$, with a standard deviation of 0.33. The duration of the analysis is 2 hours.

Determination of Mn in cesium arsenate and potassium phosphate

Both salts are readily soluble in water, but solutions of these salts cannot be used directly as supporting electrolytes for the polarographic determination of Mn, since the pH of the resulting medium is 5. As a result of our studies it was shown that the polarographic determination of Mn can be carried out only in an alkaline medium at $\mathrm{pH}=10$, owing to the interfering effect of hydrogen. The minimum determinable concentration of Mn in cesium arsenate and potassium phosphate is $1.26 \cdot 10^{-5}\%$ from a weighed portion of 0.05 g; the duration of the analysis is 1.5 hours. Mn was not detected in potassium phosphate; the results of the analysis of cesium arsenate are summarized in Table 2.

Table 2

Statistical processing of the analysis of $\mathrm{CsH_2AsO_4}$ for Mn content

REFERENCES

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