PHOTOMETRIC DETERMINATION OF ALUMINUM WITH METHYLTHYMOL BLUE IN HIGH-PURITY IRON

V. B. Sokolovich, V. A. Malevannyi, Yu. L. Lel’chuk*

(Submitted by the scientific seminar of the CTF)

Methylthymol blue—tetrasodium salt of 3,3¹-bisdi(carboxymethyl)aminomethylthymolsulfophthalein, \(C_{37}H_{40}O_{13}N_2SNa_4\), mol. wt. 844.76—forms complex compounds with the cations of many metals at different pH values [1]. With aluminum at pH 3.15–3.53, methylthymol blue forms a colored complex of composition (1 : 1) [2] [3]. The interaction proceeds very slowly under room-temperature conditions and is accelerated many times by heating [1]. Methylthymol blue is one of the most contrast photometric reagents for aluminum, is characterized by low sensitivity to fluctuations in the pH of the solution, and makes it possible to determine aluminum in the presence of traces of iron. Its specific sensitivity to iron(III) is 1.9 and to iron(II) is 2.9 [4]. We used methylthymol blue in the photometric determination of traces of aluminum in high-purity iron.

The proposed procedure is based on decomposition of the weighed sample with hydrochloric acid under mild heating, oxidation of iron to the trivalent state, removal of it by extraction with diethyl ether, formation of the aluminum complex with methylthymol blue, and photometry of the complex on an FEK-N-57 with a green light filter having a transmission maximum of 536 mµ. The duration of the analysis is 3–4 hours. The sensitivity of the method is \(1 \cdot 10^{-4}\%\). The relative error of determination is \(\pm 5\text{–}9\%\).

In the work, triply distilled water was used, as well as isopiestic hydrochloric acid obtained by isothermal distillation of concentrated hydrochloric acid. The remaining reagents were of chemically pure or analytically pure grade. An aqueous 0.1% solution of methylthymol blue was prepared from an imported reagent from the firm “Reanal.” The initial standard aluminum solution containing 1000 µg in 1 ml was prepared from high-purity aluminum sulfate (class A₂, VTU 3-334-60). The titer of the standard solution was checked by the gravimetric method.

Construction of the Calibration Curve

From the initial standard aluminum solution containing 1000 µg in 1 ml, immediately before use a solution is prepared containing 1 µg of aluminum in 1 ml. Into a series of 50-ml volumetric flasks, 5 ml portions of acetate buffer solution with pH 3.3 are introduced,

* With the participation of student G. E. Gombotsova.

2 ml of a freshly prepared 5% solution of ascorbic acid, the required volumes of freshly prepared standard aluminum solution containing 1 μg in 1 ml, corresponding to 0, 1, 2, 3, etc., up to 10 μg of aluminum, and 1 ml of a 0.1% aqueous solution of methylthymol blue. Everything is shaken, heated, and boiled for 3 minutes to form the aluminum complex with methylthymol blue. After rapid cooling, the contents of each flask are brought to the mark with acetate buffer and mixed. After 10 minutes, the optical densities of the solutions are measured on an FEK-N-57 with green light filter No. 5, with a transmission maximum of 536 mμ, in cuvettes with a 50 mm layer thickness. The reference solution is tridistillate. From the optical density of each solution, the optical density of the solution with zero aluminum content is subtracted, and a calibration curve is constructed. Since the optical properties of the methylthymol blue solution change during storage, the calibration curve has to be renewed frequently. More accurate results are obtained when the calibration curve is constructed simultaneously with the determination of aluminum in the sample. The determination can also be completed without a calibration curve according to the procedure given below.

Procedure

A 0.5 g sample of iron is placed in a quartz beaker, treated with 10 ml of ultrapure hydrochloric acid \(\sim 8\text{ N}\), and gently heated for 10–15 minutes until complete decomposition. To oxidize the iron to the trivalent state, 15 drops of nitric acid (1 : 1) are added to the hydrochloric-acid solution. Nitrogen oxides are removed by heating. The solution is cooled and transferred to a 200 ml separatory funnel; the beaker is rinsed with 5 ml of \(\sim 8\text{ N}\) ultrapure hydrochloric acid. From the 6 N hydrochloric-acid solution, the iron is extracted with diethyl ether in the form of chloride complexes. The extraction is repeated three times with 20 ml portions of ether, shaking for three minutes. After extraction is complete, the aqueous layer is drained into a beaker, traces of ether are removed by gentle heating, the solution is cooled, neutralized with ammonium hydroxide to pH \(\sim 3.0\), transferred to a 50 ml volumetric flask, and diluted to the mark with acetate buffer.

The analysis is then continued with an aliquot portion of the resulting solution. The volume of the aliquot depends on the aluminum content. At low aluminum content, the entire solution obtained after decomposition of the sample may be subjected to the subsequent analytical operations.

When the aluminum content is on the order of \(10^{-3}\%\), a 10 ml aliquot is placed in a 50 ml volumetric flask, 2 ml of freshly prepared 5% ascorbic acid solution is added to reduce traces of iron, and 1 ml of a 0.1% solution of methylthymol blue is added. Everything is heated and boiled for 3 minutes to form the aluminum complex with methylthymol blue, then rapidly cooled, brought to the mark with acetate buffer, and mixed. A blank determination is carried out through all stages of the analysis (including extraction), and, if the determination is completed without a calibration curve, an exact portion of the standard aluminum solution is also carried through. After 10 minutes, the optical densities of the complex solutions are measured on an FEK-N-57 with green light filter No. 5, with a transmission maximum at 536 mμ, in cuvettes with a 50 mm layer thickness. The reference solution is tridistillate.

When completing the determination without using a calibration curve, the aluminum content in weight percent is calculated by the formula

\[ \%\,\mathrm{Al} = \frac{C_{\mathrm{Al}}(D_x-D_o)\cdot 10\cdot 10^2}{(D_{\mathrm{st}}-D_o)\cdot 10^6} = \frac{C_{\mathrm{Al}}(D_x-D_o)}{(D_{\mathrm{st}}-D_o)}\cdot 10^{-3}, \]

where \(C_{\mathrm{Al}}\) is the aluminum content in the standard solution;
\(D_{\mathrm{st}}\) is the optical density of the aluminum complex with methylthymol blue in the standard solution;
\(D_x\) is the optical density of the complex in the solution being analyzed;
\(D_0\) is the optical density of the complex in the blank test;
10 is the conversion coefficient to a gram sample.

The results of the statistical treatment of the experimental data obtained are presented in Table 1.

Table 1

Results of the photometric determination of aluminum with methylthymol blue in high-purity iron

Conclusions

A procedure has been developed for the photometric determination of aluminum with methylthymol blue in high-purity iron. The sensitivity is \(1\cdot 10^{-4}\%\). The accuracy of the determination is \(\pm 5\)—\(9\%\).

References

1. T. V. Mal’kova, N. D. Medvedeva, K. B. Yatsimirskii. Complex compounds of aluminum with methylthymol blue. Zh. neorg. khim., IX, 10, 2347, (1964).

2. N. D. Lukomskaya, T. V. Mal’kova, K. B. Yatsimirskii. Spectrophotometric method for the determination of aluminum and gallium with methylthymol blue. Izv. vyssh. uch. zaved. SSSR, “Khimiya i khim. tekhnologiya”, X, 9, 995, (1967).

3. V. A. Malevannyi, Yu. L. Lel’chuk. On the question of the photometric determination of aluminum with methylthymol blue in technical titanium dioxide. Izv. TPI, vol. 233, Tomsk, 1974.

4. V. A. Malevannyi, Yu. L. Lel’chuk. On the quantitative assessment of the interfering action of foreign cations in the spectrophotometric method of analysis. Zh. analit. khim., XXIII, 10, 1558, (1968).