EXTRACTION–PHOTOMETRIC METHOD FOR THE DETERMINATION OF TRACE MANGANESE IN HIGH-PURITY IRON

R. D. Glukhovskaya, Yu. L. Lel’chuk, E. D. Lukker

(Presented by the Scientific-Methodological Seminar of the Chemical-Technological Faculty)

For the determination of manganese in various objects, methods based on the oxidation of manganese to the permanganate ion and its photometric measurement by one means or another are widely used. The principal disadvantages of these methods are the need to remove chloride ions and the danger of formation of compounds of tetravalent manganese [1]. In addition, these methods have low sensitivity.

Recently, for the photometric determination of microquantities of manganese in especially pure substances, the formaldoxime method has gained recognition [6]. It is based on the formation, in an alkaline medium, of a water-soluble complex compound of manganese with formaldoxime, colored reddish brown. The sensitivity of the determination of manganese with this reagent exceeds the permanganate method by an order of magnitude and is \(5 \cdot 10^{-3}\ \mu\text{g}/\text{cm}^2\) at an optical density of 0.001 and a wavelength of 455 mµ [2].

The determination of manganese with formaldoxime is not interfered with by Ag, Pb, Cd, As, Sb, Sn, Pt, Au, W, Mo, Al, and Zn [5], nor by large amounts of chloride, sulfide, carbonate, nitrate, and phosphate salts of alkali and alkaline-earth metals [3, 4].

The determination is interfered with by copper, nickel, cobalt, chromium, and vanadium. If the concentrations of these elements are appreciably greater than the concentration of manganese, they must be removed. Interferences caused by the presence of small amounts of iron, vanadium, and others can be eliminated by heating the solution to \(65^\circ\text{C}\) [3, 4], or by binding these elements with complexon-III in the presence of reducing agents [7—9], or with potassium cyanide in the presence of tartrate [9].

This method has been used for the determination of small amounts of manganese in feed and food products [11, 12], in silicate rocks and sediments [8], and in soil extracts and plant materials [9].

In the present work a procedure is described for the determination of traces of manganese with formaldoxime in high-purity iron. The bulk of the iron is removed by extraction with diethyl ether; the remaining iron is separated as the hydroxide with ammonia, and interferences caused by traces of iron are eliminated by heating the solution after the addition of all reagents for 20 min at a temperature of \(65^\circ\text{C}\).

In developing the method, we established the following:

1. The concentration of the solution with respect to ammonia must be not less than 1 N. Further addition of ammonia has practically no effect on the optical density of the solutions.

1. The manganese complex with formaldoxime forms immediately, but the optical density of the solution stabilizes only 10 min after addition of the reagents and does not change for several days.

2. To destroy the complexes of traces of iron and other interfering elements with formaldoxime, heating the solution for 15–20 min at 60°C is sufficient. Under these conditions, however, the color of the manganese formaldoxime complex remains unchanged.

Experimental Part

Required reagents and solutions

1. Water of special purity. Obtained by triple distillation from a quartz distillation apparatus with ground joints.

2. Isopiestic hydrochloric acid. Obtained by saturating tridistillate with hydrogen chloride.

3. Especially pure hydrochloric acid. Obtained by passing isopiestic (8–10 N) hydrochloric acid through an AN-2F anion exchanger in the chloride form.

4. Standard manganese solution with a titer of 1 μg/ml. Prepared by diluting a more concentrated solution made from the dry preparation MnSO₄ · 5H₂O.

5. Aqueous formaldoxime solution. 4 g of hydroxylamine hydrochloride are dissolved in a small amount of water, 5 ml of 40% formalin solution is added, and the volume of the solution is brought to 100 ml with water [6].

Analytical procedure

The sample weight of the substance being analyzed is taken depending on the expected manganese content. At a manganese content of \(10^{-3}\%\) and higher, a 0.1 g sample is sufficient; at a content of \(10^{-4}\) and lower, 1 g and more is used.

The iron sample is dissolved in the cold or with gentle heating in 7.7 N HCl. Nitric acid (1:1) is added dropwise to the slightly heated solution until \(Fe^{2+}\) is completely oxidized to \(Fe^{3+}\). The end of the reaction is accompanied by vigorous foaming of the solution and a sharp change in color from dark green to dark brown.

After cooling, the solution is transferred to a separatory funnel; 20 ml of diethyl ether is added, rinsing the beaker at the same time; the funnel is stoppered and shaken for 3 min. During this time the stopper is opened from time to time to remove accumulated ether vapors. After phase separation, the lower aqueous layer is drained into the same beaker; to the ether layer in the separatory funnel, 1 ml of 7.7 N HCl is added, shaken for one minute, and the washing liquid is combined with the original solution. The ether layer is removed, and the extraction is repeated two more times.

The contents of the beaker are heated on a gently heated water bath until the odor of ether is completely removed. The solution is cooled, neutralized with concentrated ammonia solution until a faint odor appears, filtered through a white-ribbon filter into a 25 ml volumetric flask, and the filter is washed with a small amount of 1 N \(NH_4OH\). To the flask are added 3 ml of formaldoxime solution and 3.5 ml of concentrated \(NH_4OH\) solution; the mixture is stirred and immersed for 15–20 min in a water bath whose temperature is maintained at 60–65°C. The contents of the flask are cooled, another 5 drops of formaldoxime are added, the solution is diluted to the mark with water, mixed, and after 10–15 min the optical density of the solution is measured on an FEKN-57 in cuvettes with a layer thickness of 5 cm, using blue light filter 3 with a transmission maximum of 453 mμ.

At the same time a blank experiment is performed. The optical density of the blank sample is subtracted from the optical density of the solution being analyzed, and the manganese content in the iron portion is found from the calibration graph.

The percentage content of manganese in the iron sample being analyzed is calculated by the formula

\[ \%\,\mathrm{Mn}=\frac{B_{\mathrm{Mn}}}{H\cdot 10000}, \]

where \(B_{\mathrm{Mn}}\) is the Mn content in the portion, \(\mu g\);
\(H\) is the iron portion, g.

The results of determining traces of manganese in various iron samples are given in Table 1.

Table 1

Checking the accuracy of the analysis by the method of additions

REFERENCES

1. A. M. Dymov. Technical Analysis of Ores and Metals. Metallurgizdat, Moscow, 1948.

2. G. Charlot. Methods of Analytical Chemistry. Khimiya Publishing House, Moscow–Leningrad, 1966.

3. V. M. Peshkova, A. A. Ovsyannikova. “Zav. lab.,” 7, 800, 1937.

4. V. M. Peshkova, M. I. Gromova. Practical Guide to Spectrophotometry and Colorimetry. Moscow State University, Moscow, 1961.

5. Sandell. Colorimetric Methods for the Determination of Traces of Metals. Mir Publishing House, Moscow, 1967.

6. Methods for the Analysis of High-Purity Substances. Nauka, Moscow, 1965.

7. Kadzivara, Meisetsu, Goto Katsumi, Buleeki Kachaku, Japan Analyst, 13, 526, 1964; RZhKh, 10 G 66, 1965.

8. Abdulbach M. J. Analyst chimacta, 40, 526, 1968; RZhKh, 15 G 109, 1968.

9. Sapek Andrzej, Sapek Barbara, Roczn globoznawere, 18, 207, 1967; RZhKh, 18 G 173, 1968.

10. Cottlie B. A. Hecht F. Mikrochemie, 35, 337, 1950.

11. Goto Katsumi, Konatsu Tsugoshi, Jirukawa Toshiro. Analyst chim. acta., 27, 331, 1962; RZhKh, 14 G 165, 1963.

12. Henig Sen. A. Analyst., 91, 647, 1966; RZhKh, 7 G 185, 1967.