I. B. Kogan and S. L. Makhover

Polarographic Determination of Manganese in Air

From the Kharkov Regional Sanitary and Epidemiological Station

Manganese and its salts occur in the air of a number of machine-building and metalworking facilities, among others. We have developed a polarographic method for determining manganese in the air of industrial premises.

A well-defined manganese wave is obtained in a 0.2 N potassium thiocyanate supporting solution in the presence of 0.25% gelatin. The half-wave potential is 1.55 V relative to the saturated calomel electrode; the diffusion current is well defined.

The work was carried out using a Gorky polarograph (model M-7). The galvanometer sensitivity was \(1.53\cdot10^{-9}\) A mm/m.

Determinations were carried out at \(S=\frac{1}{25}\).

Distilled mercury placed at the bottom of the electrolytic cell was used as the anode.

The figure shows a calibration curve for \(\mathrm{MnO_2}\), expressed as \(\mathrm{Mn}^{2+}\), at a galvanometer sensitivity of \(S=\frac{1}{25}\). Polarographic recording was started at 1.2 V.

Figure: Calibration curve for \(\mathrm{Mn}^{2+}\) in a potassium thiocyanate supporting solution. Vertical axis: wave height, mm. Horizontal axis: Mn concentration, \(\gamma\).

A weighed portion of chemically pure manganese dioxide is dissolved in HCl (specific gravity 1.17), and a measured quantity of the solution is evaporated to dryness. After the residue has been dissolved in several milliliters of water, the resulting solution is again evaporated to dryness. This operation is repeated until the excess acid has been completely removed, since the solution must be neutral. The residue is dissolved in 1 ml of water, 2 ml of 0.2 N KCNS solution and gelatin are added, and the manganese polarogram is recorded.

Table 1 presents the results of the polarographic determination of \(\mathrm{Mn}^{2+}\).

As Table 1 shows, Mn is determined quite satisfactorily.

To collect manganese from the air, we used absorbent cotton in a dust-sampling tube. The sample collected on the cotton was burned in a crucible over a small burner flame and then ignited at \(600^\circ\) in a muffle furnace.

The ash was dissolved in 1 ml of concentrated hydrochloric acid and evaporated to dryness on a water bath; evaporation was continued until the reaction to litmus was neutral.

Table 1

Table 2

The clear solution was transferred from the crucible to the electrolytic cell. Then 2 ml of 0.2 N potassium thiocyanate solution was added in small portions to the same crucible, and the solution was transferred from the crucible to the electrolytic cell. A drop of 0.15% gelatin solution was added, and polarography was performed.

Table 2 presents the results of the determination of $\mathrm{Mn^{2+}}$ after ignition of the cotton wool in the crucible.

The data in Table 2 clearly demonstrate that $\mathrm{Mn^{2+}}$ can be determined in air.

Blank experiments conducted under the analytical conditions showed no polarographic waves at the manganese reduction potential.

To test the polarographic determination of manganese in practice, manganese dioxide aerosol was determined under industrial conditions during the welding of frames with manganese electrodes. The analyses were performed by chemical and polarographic methods (Table 3).

Table 3

As Table 3 shows, the polarographic and chemical analyses give entirely satisfactory results.

The sensitivity of the method for the quantitative determination of manganese in a potassium thiocyanate supporting electrolyte is 2–5 $\gamma$ in the sample analyzed, with an accuracy of 5–10%.