EXTRACTION–PHOTOMETRIC METHOD FOR THE DETERMINATION OF TRACE BORON IN HIGH-PURITY TIN

Yu. L. LELCHUK, V. A. IVASHINA

(Presented by the scientific-methodological seminar of the Chemical-Technological Faculty)

There is no description in the literature of methods for determining microquantities of boron in high-purity tin. In the search for a highly sensitive method, we investigated the possibility and conditions of applying, for this purpose, the most sensitive reagents used for the photometric determination of microquantities of boron in other objects. Of all the reagents studied and the various variants of completing the determination, methylene blue proved to be the best in an extraction-photometric finish with 1,2-dichloroethane as the extractant. As a result of the study, a procedure was developed for determining traces of boron in high-purity tin with a sensitivity of \(2 \cdot 10^{-5}\%\), with a weighed portion of the tin analyzed of 0.5 g.

Methylene blue, \(C_{16}H_{18}N_3SCl \cdot 3H_2O\), belongs to reversible oxidation–reduction indicators. Its normal oxidation–reduction potential at pH 7 is \(+0.011\), and at pH 0 is \(+0.53\) V. The reduced form is colorless; the oxidized form is blue [1].

For the determination of microquantities of boron [2–4], use is made of the ability of methylene blue, in an acidic medium at pH 1, to give with the tetrafluoroborate ion a blue-colored complex extractable with 1,2-dichloroethane. The composition of the complex and the conditions for determining from 0.2 to 25 μg of boron in steel were studied by Pastor and co-workers [3]. Karalova and Nemodruk [4] used this complex for the extraction-photometric determination of boron in uranium tetrafluoride with a sensitivity of \(8 \cdot 10^{-5}\%\).

According to Pastor [3], the composition of the tetrafluoroborate complex with methylene blue is 1 : 1; its structural formula is

\[ \left[ \begin{array}{cccccccc} \mathrm{CH_3} & \mathrm{H} & & & & \mathrm{H} & & \mathrm{CH_3} \\ | & & \mathrm{C} & & \mathrm{S} & & \mathrm{C} & | \\ \mathrm{N}{-}\mathrm{C} & \!\!/\!\! & \mathrm{C} & \!\!/\!\! & & \!\!\backslash\!\! & \mathrm{C} & {=}\mathrm{N} \\ | & | & \| & & | & & | & | \\ \mathrm{CH_2} & & \mathrm{C} & & \mathrm{C} & & & \mathrm{CH_3} \\ \mathrm{HC} & \!\!\backslash\!\! & & \!\!/\!\! & & \!\!\backslash\!\! & \mathrm{CH} & \\ & \mathrm{CH} & & \mathrm{N} & & \mathrm{CH} & & \end{array} \right]^+ \quad \begin{array}{c} \\[-1.5em] \\ \\ \\ \\ \mathrm{|}\\ \mathrm{BF_4} \end{array} \]

Our experiments showed that tin interferes with the determination of boron with methylene blue and must be completely removed from the solution being analyzed. In the procedure we propose, after decomposition of the sample with hydrochloric acid and hydrogen peroxide, tin is removed as tin tetrachloride by threefold evaporation at a temperature not exceeding \(90^\circ\mathrm{C}\). A higher temperature cannot be allowed, since the white needle-like precipitate formed under these conditions is not removed during subsequent evaporation with hydrochloric acid and makes determination of boron by this method impossible.

Oxidizing agents as well as reducing agents also interfere with the determination of boron by this method. Oxidizing agents convert methylene blue into its blue oxidized form, while reducing agents convert it into its colorless reduced form.

Dichloroethane extracts of the tetrafluoroborate-ion complex with methylene blue obey the Lambert—Beer law in the boron concentration range \(0\)—\(5\ \mu\mathrm{g}\) after centrifugation in a TsUM-1 electric centrifuge at 7000 rpm.

The mechanism and rate of conversion of boric acid into the tetrafluoroborate ion were studied by Babko and Marchenko [5], and the effect of HF concentration on the rate of the process by Pastor and co-workers [3]. According to Babko, the process proceeds in two stages. The first stage, with formation of hydroxyfluoroboric acid, proceeds practically instantaneously.

\[ \mathrm{I.}\quad \mathrm{H_3BO_3 + HF \rightleftarrows HBF_3OH + 2H_2O} \]

The formation of tetrafluoroboric acid in the second stage of the process is catalytically accelerated by hydrogen ions.

\[ \mathrm{II.}\quad \mathrm{HBF_3OH + HF \rightleftarrows H[BF_4] + H_2O} \]

The shift in the equilibrium of this process depends on the acidity of the solution and the HF concentration. The greater the acidity of the solution and the concentration of HF, the faster and more completely boron is converted into \([\mathrm{BF_4}]^-\).

Our experiments showed that, when 3 ml of 5 N sulfuric acid, 2 ml of water, and 5 ml of a 5% hydrofluoric acid solution are added to the dry residue after removal from the solution being analyzed, one hour is sufficient for practically complete conversion of boron into the tetrafluoroborate ion at room temperature.

Pastor and co-workers [3] photometered dichloroethane extracts of the complex using a light filter with maximum transmission at 660 m\(\mu\), and Karalova and Nemodruk [4] using a light filter with maximum transmission at 656 m\(\mu\). In our experiments, more accurate and reproducible data were obtained with a light filter having maximum transmission at 610 m\(\mu\).

The procedure for determining microimpurities of boron in high-purity tin consists in decomposing a weighed portion of the tin being analyzed with hydrochloric acid and hydrogen peroxide, removing the tin by threefold evaporation with HCl, converting boron into \([\mathrm{BF_4}]\) and then into the colored complex with methylene blue, extracting the complex with dichloroethane, and photometering the extract on a FEKN-57 with red light filter No. 7, with maximum transmission at 610 m\(\mu\).

The weighed portion of tin is 0.5 g. The duration of the analysis is 4 hours. The sensitivity of the method is \(2 \cdot 10^{-5}\%\). The error of determination is \(\pm 0.1\ \mu\mathrm{g}\).

By increasing the weighed portion of the tin being analyzed, the sensitivity of the method can be correspondingly increased to \(1 \cdot 10^{-5}\%\) with a gram-sized sample and to \(1 \cdot 10^{-6}\%\) with a ten-gram sample.

The water used is twice distilled in a quartz apparatus; the hydrochloric acid is isopiestic; the hydrogen peroxide is a 30% solution.

chemically pure, dichloroethane twice distilled, methylene blue, 0.002 M solution in bidistillate, sulfuric acid, chemically pure, hydrofluoric acid, analytical grade, distilled in the presence of mannitol, mannitol, analytical grade, 1% solution. Decomposition of the weighed sample and removal of tin are carried out in an organic-glass cabinet under a drying electric lamp.

High-purity tin, when samples are taken, must be protected from contact with the fingers by a thin polyethylene film. For convenience in sampling and to accelerate decomposition of the weighed portion of the tin being analyzed, it is converted into thin sheets; the preparation of such sheets has been described by us previously.

Procedure of the analysis

To a weighed portion of tin within the range of 0.5 g, in a quartz cup or in a glass cup that contains no boron, add 5 ml of azeotropic hydrochloric acid and, dropwise with stirring, 25 drops of 30% hydrogen peroxide. After dissolution of the weighed portion, add 1 ml of 1% mannitol solution, and evaporate the solution to dryness in an organic-glass cabinet under a drying lamp at a temperature not exceeding 90°C. To the dry residue add 1 ml of hydrochloric acid and evaporate to dryness; add another 1 ml of this acid and evaporate to dryness a third time. The residue is then treated with 3 ml of 5 N sulfuric acid; the solution is transferred to a polyethylene cylinder; the remains of the solution in the cup are rinsed with 2 ml of bidistillate; 5 ml of 5% hydrofluoric acid solution are added; and the mixture is left to stand at room temperature for 1 hour.

After this, 2 ml of 0.002 M methylene-blue solution and 10 ml of 1,2-dichloroethane are added to the solution being analyzed; the polyethylene cylinder is tightly closed with a lid, thoroughly shaken for one minute, and, after phase separation, the dichloroethane extract is withdrawn with a dry pipette with a rubber bulb, transferred to a centrifuge tube, and centrifuged for 1 minute in a TsUM-1 electric centrifuge at 7000 rpm.

The centrifuged extract is photometered on an FEKN-57 photocolorimeter-nephelometer in a cuvette with a 3-mm layer thickness, using red light filter No. 7 with maximum transmission at 610 mμ; bidistillate serves as the reference solution.

The boron content in the tin being analyzed, in μg, is found from the calibration graph; the percentage content of boron is calculated by the formula

\[ \%B = \frac{B_{\mathrm{b}}}{10000H}, \]

where \(\%B\) is the percentage of boron in the tin being analyzed; \(B_{\mathrm{b}}\) is the weight of boron in the weighed portion of tin, in μg; \(H\) is the weighed portion of boron in g.

Construction of the calibration graph

From the initial standard boron solution containing 1000 μg of boron in 1 ml, working solutions containing 10 μg/ml and 1 μg/ml are prepared. Dilution is carried out with bidistillate.

Into 10 small quartz cups, or glass cups not containing boron, introduce the appropriate volumes of the standard working solutions with boron amounts of 0.0; 0.1; 0.3; 0.5; 0.7; 1.0; 2.0; 3.0; 4.0; 5.0 μg. To each cup add 5 ml of azeotropic hydrochloric acid, 10 drops of 30% hydrogen peroxide, and 1 ml of 1% mannitol solution; the solutions are evaporated to dryness under a drying electric lamp.

in an organic-glass cabinet at a temperature of 90–95°C. To each dry residue add 3 ml of 5 N sulfuric acid; transfer the solution to a polyethylene cylinder; rinse the cup with 2 ml of bidistillate; add 5 ml of a 5% solution of hydrofluoric acid and allow to stand at room temperature for 1 hour.

Table 1

Dependence of optical density on the amount of boron

Table 2

Checking the accuracy of the method on standard boron solutions passed through all stages of the analysis of high-purity tin

Table 3

Checking the accuracy and reproducibility of the method on a sample of high-purity tin

To the solution in the cylinder add 2 ml of a 0.002 M solution of methylene blue and 10 ml of dichloroethane; close the cylinder and mix thoroughly for 1 minute. After phase separation, using a dry pipette with a bulb, take the dichloroethane extract, transfer it to a centrifuge tube, and centrifuge for 1 minute on a TsUM-1 electric centrifuge at 7000 rpm.

The centrifuged extract is photometered on an FEKN-57 photocolorimeter in a cuvette with a layer thickness of 3 mm using a red light filter—

filter No. 7 with a transmission maximum at 610 mµ. Bidistillate serves as the reference solution.

Table 1 presents the data obtained by us for the boron concentration range of 0–5 µg.

The accuracy and reproducibility of the method were checked on standard boron solutions and on a high-purity tin sample with a boron content of \(3.70 \cdot 10^{-4}\%\) (Tables 2, 3).

Student G. N. Dembovskaya took part in carrying out the experimental part of the work.

Conclusions

A procedure has been developed for the extraction-photometric determination of microquantities of boron in high-purity tin from the optical density of dichloroethane extracts of the ion-association complex of the tetrafluoroborate ion with methylene blue.

The sensitivity of the method is \(2 \cdot 10^{-5}\%\) for a tin sample weight of 0.5 g. The duration of the analysis is 4 hours. The determination error is \(\pm 0.1\) µg.

References

1. Chemical Reagents and Preparations. Handbook edited by V. I. Kuznetsov. Goskhimizdat, Moscow–Leningrad, 1953.
2. L. Ducret. Anal. Chim. Acta, 17, 213, 1957.
3. L. Pasztor, I. D. Bode, Q. Fernando. Anal. Chem., 32, 277, 1960.
4. Z. K. Karalova, A. A. Nemodruk. Journal of Analytical Chemistry, 18, no. 5, 615, 1963.
5. A. K. Babko, I. V. Marchenko. Zavodskaya laboratoriya, 26, 11, 1202, 1960.