V. B. Sokolovich, Yu. L. Lel’chuk, B. N. Besprozvannykh

PHOTOMETRIC DETERMINATION OF MICROAMOUNTS OF ALUMINUM WITH STILBAZO AND ALBERON

(Presented by the scientific seminar of the Faculty of Chemical Technology)

Stilbazo and alberon are known in the literature as the most sensitive photometric reagents for aluminum.

Stilbazo was proposed as a reagent for the colorimetric determination of aluminum in 1950 by Kuznetsov, Karanovich, and Drapkina [1]. According to these authors, with stilbazo it is possible to determine from 0.1 to 5.0 µg of aluminum in 5 ml. The determination is not hindered by alkali metals and many divalent metals. The influence of copper and trivalent iron is eliminated by ascorbic acid.

Agrinskaya, Petrashen’, Nazarenko, and others [2—5] determined 0.005% aluminum and more in various materials with stilbazo. The determination of aluminum with stilbazo is carried out photocolorimetrically [3,5—7], by colorimetric titration [1,7], and by a standard colorimetric scale [1,4].

Stilbazo consists of brown crystals and is used in the form of a 0.01—0.08% aqueous solution. Many authors consider pH 5.4 to be optimal for photocolorimetry [4,6]; Agrinskaya [2] recommends the pH interval 3.6—5.6, and Mukhina [7] recommends 5.2—5.4. The light filter is green [6].

The possibility of using alberon (dichlorosulfodimethyl-oxifuchsone dicarboxylic acid) for the photometric determination of aluminum was studied by Mustafin, Matveev, and Kashkovskaya [8,9]. According to these authors, the sensitivity of the reaction of aluminum with alberon is 0.01 µg in 1 ml. The color of the solution of the complex formed obeys the Lambert–Beer law within the range from 0.5 to 20 µg in 10 ml; the pH interval is 4.0—5.8; the maximum light absorption is at 530 mµ; the greatest optical density is reached after 15 minutes. The intensity of the color does not change over the course of two hours. Fe^..., Cu^.., Be^.., Ti^IV, V^IV, UO₂^.., which form intensely colored compounds with alberon at the same pH values, interfere with the determination. The interfering action of trivalent iron is eliminated by ascorbic acid, and the interfering action of copper by thiosulfate. Kashkovskaya and Mustafin [9] determined 0.022% and more aluminum in steels at pH 5.12. Shveitser and Rudenko [10] determined \(10^{-2}\)—\(10^{-4}\)% aluminum in tin with chromazurol S and an acetate buffer mixture at pH 5.37. Nakagawa Yoshinori [11] determined less than 15 µg of aluminum in 25 ml in magnesium with chromazurol S at pH \(6 \pm 0.2\) and 550 mµ.

We checked and refined the photometric conditions recommended in the literature for the indicated reagents using solutions of aluminum sulfate salt, and made attempts to use Stilbazo and Alberon for determining \(10^{-4}\)—\(10^{-5}\%\) aluminum in high-purity tin.

According to the data obtained, the sensitivity of aluminum determination with Stilbazo is \(0.1\ \mu\text{g}\) in \(25\ \text{ml}\); the sensitivity of determination with Alberon is \(0.4\ \mu\text{g}\) in \(10\ \text{ml}\).

A solution of the aluminum–Stilbazo complex has maximum optical density at pH 6.6 (Fig. 1). In the presence of an acetate

Fig. 1. Dependence of the optical density of Stilbazo and its complex with aluminum on the pH of the solution: 1—Stilbazo and its complex with aluminum; 2—Stilbazo; 3—Stilbazo complex with aluminum.

buffer mixture at pH 6.6, the maximum optical density of the solution being photometered is reached after 10 min and does not change over 45 min. The maximum light absorption is at \(508\ m\mu\). The optimal concentration of Stilbazo is \(0.5\ \text{ml}\) of a 0.05% aqueous solution per \(25\ \text{ml}\) of the solution being photometered. A study of the influence of foreign ions confirmed the literature data.

To construct calibration graphs, the following procedure was devised. Into several \(25\ \text{ml}\) volumetric flasks were introduced, in each case,

5–10 ml of acetate buffer mixture with pH 8.1, 0.2 ml of hydrochloric acid (1:4), the required volume of the standard aluminum solution, 0.2 ml of a 5% freshly prepared solution of ascorbic acid, 0.2 ml of 25% sodium acetate, and 0.5 ml of a 0.05% solution of Stilbazo. The contents of the flasks are diluted to the mark with acetate buffer mixture of pH 8.1 and mixed. The pH of the solution prepared for photometry is 6.6. After 10 minutes the solution is photocolorimetered on an FEK-N-57 in 50-mm cuvettes with green light filter No. 4, with maximum transmission at 508 mµ. The reference solution contains all reagents without aluminum. It was found that at an aluminum content below 0.1 µg in 25 ml, no linear relationship between optical density and concentration is observed. At an aluminum content from 0.1 µg to 1.0 µg in 25 ml, the Lambert–Beer law is obeyed, but reproducibility is poor. A similar effect is observed when calibration curves are constructed according to methods borrowed from the literature. Convergent results also could not be obtained in determining aluminum in high-purity tin after decomposition of the sample with practically complete removal of tin. By colorimetric titration and colorimetric-scale methods, reliable results in determining tenths of a microgram of aluminum likewise were not obtained.

A study of the optical properties of the aluminum complex with Alberon showed that the absorption maximum corresponds to 536 mµ. The limits of optimum acidity are characterized by the range pH 5.0–5.5, with a maximum at 5.4 (Fig. 2). The maximum optical density is reached after 5 minutes and remains almost unchanged for 15 minutes. It was noted that when solutions prepared for photocolorimetry are allowed to stand, blue amorphous particles appear in them.

Fig. 2. Dependence of the optical density of the aluminum complex with Alberon on the pH of the solution.

An aqueous solution of Alberon is unstable during storage. Within 1–2 days after preparation, a film appears on the surface of the solution and a precipitate forms.

For the construction of calibration curves, the following procedure was drawn up. Into 50-ml conical flasks are introduced acetat-

buffer mixture with pH 5.4, calculated so that the total volume of the solution to be colorimetrically measured is 10 ml, the required volume of the standard aluminum solution, 0.2 ml of a freshly prepared 5% solution of ascorbic acid, and 0.3 ml of a 0.3% aqueous solution of Alberon. The contents of the flasks are shaken, allowed to stand for 5 minutes, and photocolorimetrically measured on an FEK-N-57 with green light filter No. 5 at 538 mµ in 20-mm cuvettes. The comparison solution may contain either only the reagents used, without aluminum, or the reagents with additions of 5–10 µg of aluminum. In the second case, tenths of a µg of aluminum are determined more accurately, since the optical density is measured on a more sensitive portion of the FEK-N-57 scale.

When calibration curves are constructed for aluminum amounts from 0.01 to 0.4 µg in 10 ml, no rectilinear dependence between optical density and concentration is observed. For aluminum amounts from 0.4 to 2.0 µg in 10 ml, the Lambert–Beer law is obeyed. Because of the instability of the solutions, the calibration curve should be constructed simultaneously with the determination of aluminum in the material under investigation.

Benzene, chloroform, carbon tetrachloride, butyl alcohol, isoamyl alcohol, butyl acetate, diethyl ether, and acetone, which we tested as extractants for the aluminum complex with Alberon, did not give positive results.

The accuracy of determining aluminum with Alberon was checked on solutions of aluminum sulfate salt. Some data are given in Table 1.

Table 1

Check of the accuracy of the photocolorimetric determination of aluminum with Alberon

Table 2

Results of determinations of aluminum with Alberon in a sample of high-purity tin

Some results obtained by us in determining aluminum in high-purity tin after decomposition of the sample with practically complete removal of tin [12] and dissolution of the dry residue in an acetate buffer mixture with pH 5.4 are given in Table 2.

Conclusions

1. A comparative study has been carried out of the photometric determination of traces of aluminum with stilbazo and alberon.

2. It has been established that, in the photocolorimetric determination of ten-thousandths of a percent of aluminum with the stilbazo reagent, reliable results are not obtained.

3. The reagent alberon can be used for the photocolorimetric determination of \(10^{-4}\)—\(10^{-5}\%\) aluminum in high-purity tin, provided that a calibration curve is constructed simultaneously. The error of determination is up to \(\pm 45\%\).

REFERENCES

1. V. I. Kuznetsov, G. G. Karanovich, and D. A. Drapkina. Colorimetric determination of aluminum with the new reagent stilbazo. Zavodskaya laboratoriya, XVI, 787, 7, 1950.

2. N. A. Agrinskaya, V. I. Petrashen. Colorimetric determination of aluminum in steel using the stilbazo reagent. Transactions of the Novocherkassk Polytechnic Institute, 31, 63, 1955.

3. N. A. Agrinskaya. An accelerated method for the determination of small amounts of aluminum in steels and cast irons. Zavodskaya laboratoriya, XXIII, 279, 3, 1957.

4. V. A. Nazarenko, E. A. Biryuk. On the determination of aluminum and fluorine with the stilbazo reagent. In: Modern Methods of Analysis in Metallurgy. Moscow, Metallurgizdat, 186, 1955.

5. N. A. Agrinskaya, V. I. Petrashen. Comparative study of colorimetric methods for the determination of aluminum with aluminon and stilbazo. Transactions of the Novocherkassk Polytechnic Institute, 72, 13, 1958 (1959).

6. B. B. Evzlina. Photometric determination of aluminum in slags from phosphorus furnaces. Scientific Institute for Fertilizers and Insectofungicides, 8, 87, 1958.

7. Z. S. Mukhina, E. I. Nikitina et al. Methods for the Analysis of Metals and Alloys. Moscow, Oborongiz, 139, 1959.

8. I. S. Mustafin, L. O. Matveev. Analytical application of phenolcarboxylic acids of the triphenylmethane series. Zavodskaya laboratoriya, XXIV, 259, 3, 1958.

9. E. A. Kashkovskaya. Determination of aluminum with the alberon reagent. Zavodskaya laboratoriya, XXIV, 1189, 10, 1958.

10. M. I. Shvaiger, E. I. Rudenko. Determination of small amounts of aluminum in tin. Zavodskaya laboratoriya, XXVI, 939, 8, 1960.

11. Nakatagava Yoshinori, Tanaka Takashi, Honda Shigeru. Spectrophotometric determination of aluminum with Chromazurol S. Referativnyi Zhurnal Khimiya, 20D45, 1962.

12. Yu. L. Lelchuk, V. B. Sokolovich, E. A. Kurysheva. Luminescent determination of traces of aluminum in high-purity tin. Izvestiya TPI, vol. 128, 1964.