Yu. L. Lelchuk, V. B. Sokolovich, O. A. Drelina

PHOTOCOLORIMETRIC METHOD FOR THE DETERMINATION OF TRACES OF ALUMINUM IN HIGH-PURITY TIN WITH ERIOCHROME CYANINE

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

From the literature, the works of a number of foreign investigators are known on the use of eriochrome cyanine for the colorimetric determination of aluminum in rocks, iron ores, cements, steel, and copper and zinc alloys, when its content ranges from 30 to 0.024 percent [1—6].

An aqueous solution of eriochrome cyanine has an orange-yellow color. With an increase in the pH of the solution, its color changes to blue-violet. With aluminum, in the presence of an ammonium acetate buffer, it forms a water-soluble red-violet complex. The determination of aluminum is interfered with by cations of trivalent iron.

Different authors recommend different compositions of the buffer mixture, different concentrations of the reagent, and different methods of preparing it. There is no consensus regarding the optimum pH of the solution, the required light filter, and the time for holding the solution before colorimetry. In the literature there are no data on the maximum sensitivity of the method, its accuracy and reproducibility in the determination of microquantities of aluminum, and insufficient data on the spectral characteristics of the reagent itself and of its complex with aluminum.

We have studied the conditions for using eriochrome cyanine for the photocolorimetric determination of traces of aluminum in high-purity tin, as well as the sensitivity, accuracy, and reproducibility of the method. As a result of the experiments carried out, we established that a 0.075% aqueous solution of eriochrome cyanine is sufficiently stable only when 5 g of sodium chloride, 5 g of sodium nitrate, and 0.4 ml of concentrated nitric acid are added to it for every 200 ml of reagent solution, as recommended by Hill [4] in one of the recent works on eriochrome cyanine. The maximum optical density of the complex is observed on an FEK-N-57 with a green light filter having a transmission maximum at 535 mµ. The optimum pH of the solution lies within 6.2—6.6 (Fig. 1). The greatest intensity of the color of the aluminum complex is reached 5—7 minutes after mixing the reagents and is retained for about 30 minutes, after which it gradually decreases.

After experimentally testing a number of possible variants for the chemical preparation of a sample of the high-purity tin being analyzed for the photocolorimetric determination of traces of aluminum in it with eriochrome cyanine, we ultimately arrived at a procedure that makes it possible to confidently

determine 0.1 μg and, less accurately, 0.05 μg of aluminum, which, for a gram sample of the tin being analyzed, amounts to \(1\cdot 10^{-5}\)—\(5\cdot 10^{-6}\) percent.

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

Cuvette 3 cm, comparison solution—bidistillate, light filter with maximum transmission at 535 mμ.

The method we have developed is based on decomposing the sample of the tin being analyzed with a mixture of hydrochloric acid, hydrobromic acid, and bromine; removing the tin by triple evaporation of the solution with hydrochloric acid at a temperature not exceeding 80–90°C; dissolving the dry residue in the smallest possible amount of hydrochloric acid; transferring the dissolved residue to a 25 ml volumetric flask with an ammonia–acetate buffer mixture; eliminating the interfering effect of trivalent iron by reducing it with ascorbic acid; adding a 0.075% solution of Eriochrome Cyanine; making the volume up to 25 ml; and photometering the aluminum complex formed after a seven-minute standing period on an FEK-N-57 photoelectric colorimeter-nephelometer with a green light filter having maximum transmission at 535 mμ.

Decomposition of the sample and removal of tin lasts 3.5–4 hours; the photometric completion 30–40 minutes. The error of determination does not exceed \(\pm 10\)—15% relative.

Hydrochloric acid, hydrobromic acid, and bromine are purified from traces of aluminum and other metals by three- to fourfold distillation in a quartz distillation apparatus. An ammonia–acetate buffer solution with pH 8.3 is prepared by mixing 460 ml of a two-normal solution of acetic acid with 540 ml of a two-normal solution of ammonia-

ca. The acetic acid solution is prepared from glacial acid of “khch” grade; the ammonia solution—from concentrated ammonia of “chda” grade. A 5% aqueous solution of ascorbic acid is prepared on the day of use by dissolving 5 g of “chda” grade ascorbic acid in 100 ml of water. The initial standard aluminum solution containing 1000 μg of aluminum in 1 ml is prepared from aluminum sulfate of special purity (class A₂, Krasnyi Khimik plant, VTU No. 3—334—60). The titer of the solution is established gravimetrically. Working standard solutions containing 1 μg and 0.1 μg of aluminum in 1 ml are prepared on the day of use by diluting the initial standard solution with water acidified with 0.5 ml of distilled hydrochloric acid per 1 liter. All solutions are prepared with twice-distilled water and stored in polyethylene vessels.

Construction of the calibration curve

Into a series of 25 ml volumetric flasks are introduced 8–10 ml each of ammonia–acetate buffer with pH 8.3, 6 drops of hydrochloric acid (1 : 4), 0.5 ml each of a 5% freshly prepared ascorbic acid solution, and the required volume of the working standard solution, calculated for a total aluminum content in the samples of 0.00; 0.05; 0.10; 0.30; 0.50;

Fig. 2. Calibration curve for the photocolorimetric determination of aluminum with Eriochrome Cyanine.
Cuvette—3 cm, solution—bidistillate, light filter with maximum transmission at 535 mμ.

0.70; 1.00; 1.30; 1.50 μg. After mixing, 0.5 ml of a 0.075% solution of Eriochrome Cyanine is added to each, and ammonia–acetate buffer solution is added to the mark; the solutions are thoroughly mixed, and after 7 min the optical densities of the solutions are measured in 3 cm cuvettes on an FEK-N-57 photocolorimeter-nephelometer with a green light filter with maxi-

...with a maximum transmission at 535 mµ. Bidistillate serves as the comparison solution. The pH of the solutions before photometry is approximately 6.2. In our experiments we monitored it from time to time with a pH meter. Fig. 2 shows the calibration graph we obtained.

Preparation of tin samples for analysis

High-purity tin, during sampling, must be protected from contact with fingers by a thin polyethylene film. For ease of sampling and to accelerate dissolution of the weighed portion, the tin to be analyzed is converted into thin sheets. To do this, 5–10 g of tin are placed in a tall dry porcelain beaker, washed with chromic mixture, water, water purified with hydrochloric acid, and again with water, and the beaker is heated on an electric hotplate to slightly above the melting point of tin. The molten tin is poured from a height of 20 cm, drop by drop, onto a clean white glazed tile. If the tin is not overheated too much, thin sheets 0.1 to 0.2 mm thick and with an average weight of 1 g are obtained.

The selected tin sample on a watch glass is treated with two drops of hydrochloric acid, washed with bidistillate, dried with a drying lamp on a table covered with Teflon, in an organic-glass box, and weighed on an analytical balance. The weighed sample is pushed into a quartz beaker for decomposition, and the watch glass is weighed. From the difference in the weight of the watch glass with the tin and the empty glass, the weight of the tin sample is found.

Procedure of the analysis

To the weighed tin sample in a quartz beaker, covered with a watch glass and immersed halfway in cold water, add 2 ml of concentrated hydrochloric acid, 2.5 ml of hydrobromic acid, and, drop by drop carefully under the watch glass, 1.5–2 ml of bromine. After the vigorous reaction ceases, the watch glass is removed and washed, and the beaker is placed under an evaporation lamp on a table covered with Teflon for further decomposition of the weighed portion and removal of the tin. The solution is evaporated to a slightly moist residue, 1 ml of hydrochloric acid (1:1) is added, and it is evaporated to a dry residue at a temperature of 80–90°C; another 0.5 ml of hydrochloric acid is added and the solution is again evaporated to dryness.

The dry residue is dissolved in 6 drops of hydrochloric acid (1:4) and transferred with an ammonium acetate buffer mixture into a 25 ml volumetric flask. Add 0.5 ml of freshly prepared 5% aqueous ascorbic acid solution, mix, add 0.5 ml of 0.075% Eriochrome Cyanine solution, and ammonium acetate buffer solution to the mark. Mix thoroughly, and after 7 minutes measure the optical density of the solution on an FEK-N-57 photoelectric colorimeter-nephelometer in 3 cm cuvettes with a green light filter having a maximum transmission at 535 mµ. Bidistillate serves as the comparison solution. The pH of the solution is approximately 6.2.

A blank sample is carried through all stages of decomposition of the weighed portion and is photometered under analogous conditions. The optical density of the blank sample is subtracted from the optical density of the solution, and the aluminum content is determined from the calibration graph.

The accuracy and reproducibility of the method were checked by us on pure aluminum salts carried through all stages of tin treatment, on mixtures of salts, and on samples of high-purity tin. Some of the data obtained are given in Tables 1 and 2.

Table 1

Check of the accuracy of the method

Table 2

Results of parallel determinations of aluminum in high-purity tin

Conclusions

A highly sensitive photocolorimetric method has been developed for determining traces of aluminum with Eriochrome Cyanine. The sensitivity of the method is \(1 \cdot 10^{-5}\)—\(5 \cdot 10^{-6}\%\). The duration of the determination is 4–5 hours. The error does not exceed 10–15% relative.

Literature

1. Dozinel Chorles. Spectrophotometric determination of aluminum in copper alloys. Chim. analyt., 38, 244, 7, 1956.
2. Walraff Maria. Photometric determination of aluminum, iron, and titanium in cements. Zement—Kalk—Gips, 9, 186, 5, 1956.
3. Uno T. Hill. Direct photometric determination of aluminum in iron ores. Analytical Chemistry, 28, 1419, 9, 1956.
4. Uno T. Hill. Photometric determination of aluminum in steel. Analytical Chemistry, 31, 429, 3, 1959.
5. Minissi Glaudio Z., Rozado Epifinio. Determination of small quantities of aluminum in zinc. An. Assoc. quim. argent., 49, 22, 1, 1961.
6. Lu-Go-Tszyun. Direct photometric method for determining aluminum in rocks. Referativnyi zhurnal khimiya, 15, abstract 53123, 1959.