Luminescent Determination of Trace Aluminum in High-Purity Tin
Abstract Generated abstract
A luminescent method is developed for determining trace aluminum in high-purity tin, addressing the limited sensitivity, reproducibility, and efficiency of existing chemical and spectral procedures. The procedure uses salicylal-ortho-aminophenol to form a fluorescent aluminum complex after decomposition of a 1 g tin sample with hydrochloric acid, hydrobromic acid, and bromine, followed by near-complete removal of tin through repeated hydrochloric acid evaporations and measurement at 520 mμ. The study evaluates interferences from tin, copper, iron, pH, complex aging time, buffer purification, and calibration versus standard additions. The resulting method reaches a sensitivity of 1 x 10^-6 to 5 x 10^-7 percent aluminum, with a reported relative error not exceeding 20 to 25 percent.
Full Text
Yu. L. Lel’chuk, V. B. Sokolovich, E. A. Kurysheva
LUMINESCENT DETERMINATION OF TRACE ALUMINUM IN HIGH-PURITY TIN
(Presented by the scientific seminar of the Chemical-Technological Faculty)
Existing chemical methods for determining trace aluminum in high-purity tin are lengthy and labor-intensive, have low sensitivity and poor reproducibility, and require a large weighed portion of the tin being analyzed. The spectral method now in use has a sensitivity of \(3 \cdot 10^{-4}\%\). The error of determination at the sensitivity threshold of the method is large.
In search of a more sensitive and more accurate method, we studied the available literature data on the luminescent determination of aluminum with salicylal-ortho-aminophenol, experimentally demonstrated the possibility of using this reagent for determining trace aluminum in high-purity tin, and established all details of the method for aluminum contents in the range \(4 \cdot 10^{-5}\)—\(5 \cdot 10^{-7}\%\).
Salicylal-ortho-aminophenol was proposed in 1953 by Holzbecher [1] for the detection and semiquantitative determination of trace aluminum. The conditions for quantitative luminescent determination of trace aluminum with this reagent were studied in detail by Bozhevol’nov [2]; quantitative methods for determining trace aluminum in sodium acetate, in hydrogen peroxide, and in hydrochloric, sulfuric, nitric, and hydrofluoric acids were developed by Bozhevol’nov and Yanishevskaya [3], in lead salts by Klimov, Didkovskaya, and Kozachenko [4], and in red phosphorus by Yanisheva, Sazonova, and Kapilovich [5].
According to Bozhevol’nov [3], apart from aluminum, no other cations, at contents up to 25 µg in 5 ml of solution with salicylal-ortho-aminophenol, produce fluorescence. Copper and iron, in amounts greater than 0.1 µg in 5 ml, quench the fluorescence of the aluminum complex with salicylal-ortho-aminophenol.
Using pure salts and mixtures of salts, we studied the effect of tin and its tartaric-acid complex on the course of the luminescent determination of trace aluminum. The experiments showed that tin must be completely removed. Traces of it cause an increase in the fluorescence of the aluminum complex, while appreciable amounts lead to precipitation of hydrolysis products of its salts, making it impossible to measure the luminescence intensity of the aluminum complex. Attempts to eliminate the interfering effect of tin by binding it into a tartaric-acid complex did not give positive results.
Of the several variants tested by us, the best proved to be the method of removing tin in the form of tin chloride during decomposition of the weighed sample of the tin to be analyzed with a mixture of hydrochloric acid, hydrobromic acid, and bromine, purified from traces of metals. Threefold evaporation of the solution to dryness after each addition of hydrochloric acid makes it possible to remove tin from the solution being analyzed practically completely.
In the course of the investigation we tested the influence of the impurities ordinarily encountered in high-purity tin on the course of the luminescent determination of aluminum with salicylal-ortho-aminophenol, the efficiency of purification of the acetate buffer mixture from traces of aluminum on EDE-10 anion exchanger with the aid of lumogallion according to the procedure described by Bozhevolnov, the influence of the pH of the solution and of the aging time of the complex on the intensity of its luminescence, established the necessity of strict regulation of the sensitivity of the luminescence apparatus before each series of measurements, studied the conditions for determining aluminum by the calibration-curve method and by the method of additions, and a number of other conditions ensuring highly sensitive determination of traces of aluminum by this method.
Fig. 1. Diagram of the luminescence apparatus: 1) circuit for switching on the PRK-2 lamp; 2) PRK-2 lamp with water cooling; 3) UFS-3 light filter; 4) cuvette with the solution being analyzed; 5) achromatic condenser with \(F = 94\) mm; 6) shaped diaphragm; 7) UM-2 monochromator; 8) quenching resistance; 9) FEU-19M photomultiplier; 10) M4-type kilovoltmeter; 11) load of the FEU-19M; 12) M 198/2 millivolt–microammeter; 13) battery set up to 1600 V; 14) tumbler-type switch; 15) signal lamp for switching on the FEU-19M; 16) illumination lamp.
As a result of the investigation, a procedure was developed for the luminescent determination of traces of aluminum in high-purity tin, with a sensitivity of \(1 \cdot 10^{-6}\)—\(5 \cdot 10^{-7}\%\) for a gram weighed sample of the tin being analyzed.
The method is based on decomposition of the weighed sample of tin to be analyzed with a mixture of hydrochloric acid, hydrobromic acid, and bromine; removal of tin by threefold evaporation of the solution with hydrochloric acid at a temperature of 85–90°C; dissolution of the dry residue in the smallest possible amount of hydrochloric acid; dilution of the solution to a definite volume with an acetate buffer mixture purified from traces of aluminum with lumogallion on the anion exchanger EDE-10; addition to an aliquot portion of the solution of a 0.1% acetone solution of salicylal-ortho-aminophenol; and determination of the aluminum content on a luminescence apparatus from the intensity of the luminescence of the complex formed at 520 mµ.
Our luminescence apparatus consists of a PRK-2 mercury-quartz lamp, a UFS-3 light filter, a UM-2 universal monochromator, an FEU-19M photomultiplier, and an M 198/2 millivolt-microammeter. The circuit diagram of the instrument is given in Fig. 1.
When the content of copper and iron in high-purity tin is less than \(5 \cdot 10^{-4}\%\), the luminescent completion of the determination may be carried out either by means of a calibration curve or by the method of additions; at higher contents, only by the method of additions.
Hydrochloric acid, hydrobromic acid, and bromine are purified from traces of metals by three- to fourfold distillation in a quartz distillation apparatus.
To purify the acetate buffer mixture from traces of aluminum, for each 100 ml of mixture 5 ml of a 0.1% aqueous solution of lumogallion is added. The solution is allowed to stand for 1 hour and is then passed through a chromatographic column filled with the anion exchanger EDE-10 at a rate of 130 ml/cm² per hour.
Decomposition of the weighed sample and removal of tin take 3.5–4 hours; the luminescent completion takes 1.5–2 hours. The error of determination does not exceed 20–25% relative.
Table 1
Check of the accuracy of the luminescent determination of aluminum by the method of additions
| Al taken, µg | Al found, µg | Absolute error, µg | Relative error, % |
|---|---|---|---|
| 0.300 | 0.324 | +0.024 | +8.0 |
| 0.300 | 0.288 | −0.012 | −4.0 |
| 0.300 | 0.273 | −0.027 | −9.0 |
| 0.200 | 0.229 | +0.029 | +14.5 |
| 0.200 | 0.222 | +0.022 | +11.0 |
| 0.200 | 0.213 | +0.013 | +6.5 |
| 0.200 | 0.191 | −0.009 | −4.5 |
| 0.200 | 0.201 | +0.001 | +0.5 |
| 0.200 | 0.180 | −0.020 | −10.0 |
| 0.200 | 0.232 | +0.032 | +16.0 |
| 0.020 | 0.017 | −0.003 | −15.0 |
| 0.020 | 0.022 | +0.002 | +10.0 |
Table 1 gives part of the data obtained by us in checking the accuracy of the method on a standard solution of an aluminum salt, carried out-
through all stages of treatment of the weighed portion of high-purity tin; in Table 2 are the results of parallel determinations of traces of aluminum in two samples of high-purity tin.
Table 2
Results of parallel determinations of aluminum in high-purity tin by the luminescent method
| Sample No. | Al found, % | Mean value, % | Deviation from the mean, % | Deviation from the mean, % |
|---|---|---|---|---|
| 22 | \(5.5 \cdot 10^{-5}\) | \(6.7 \cdot 10^{-5}\) | \(-1.2 \cdot 10^{-5}\) | −17.9 |
| 22 | \(5.2 \cdot 10^{-5}\) | \(6.7 \cdot 10^{-5}\) | \(-1.5 \cdot 10^{-5}\) | −22.4 |
| 22 | \(6.2 \cdot 10^{-5}\) | \(6.7 \cdot 10^{-5}\) | \(-0.5 \cdot 10^{-5}\) | −7.4 |
| 22 | \(7.2 \cdot 10^{-5}\) | \(6.7 \cdot 10^{-5}\) | \(+0.5 \cdot 10^{-5}\) | +7.4 |
| 22 | \(6.3 \cdot 10^{-5}\) | \(6.7 \cdot 10^{-5}\) | \(-0.4 \cdot 10^{-5}\) | −5.9 |
| 22 | \(11.0 \cdot 10^{-5}\) | \(6.7 \cdot 10^{-5}\) | \(+4.3 \cdot 10^{-5}\) | +64.2 |
| 28 | \(1.22 \cdot 10^{-4}\) | \(1.28 \cdot 10^{-4}\) | \(-0.6 \cdot 10^{-5}\) | −4.6 |
| 28 | \(1.16 \cdot 10^{-4}\) | \(1.28 \cdot 10^{-4}\) | \(-1.2 \cdot 10^{-5}\) | −9.3 |
| 28 | \(1.55 \cdot 10^{-4}\) | \(1.28 \cdot 10^{-4}\) | \(+2.7 \cdot 10^{-5}\) | +21.1 |
| 28 | \(1.35 \cdot 10^{-4}\) | \(1.28 \cdot 10^{-4}\) | \(+0.7 \cdot 10^{-5}\) | +5.4 |
| 28 | \(1.10 \cdot 10^{-4}\) | \(1.28 \cdot 10^{-4}\) | \(-1.8 \cdot 10^{-5}\) | −14.0 |
Procedure of Analysis
To the weighed portion of tin within the range of 1 g, in a wide and low quartz beaker of 50–60 ml capacity, covered with a watch glass and half immersed in cold water, add 2 ml of hydrochloric acid, 2.5 ml of hydrobromic acid, and carefully, dropwise, 1.5 ml of bromine. After the vigorous reaction ceases, the watch glass is removed and rinsed, and the beaker is transferred to a hot plate covered with Teflon for further decomposition of the weighed portion and removal of tin under an infrared lamp at a temperature of 85–90°C. The first evaporation is carried out to a slightly moist residue. To the residue add 1 ml of hydrochloric acid (1:1) and evaporate to dryness; add another 0.5 ml of hydrochloric acid and evaporate to dryness a third time.
When the aluminum content is within the range \(10^{-4}\)—\(10^{-5}\%\), the dry residue is moistened with 6 drops of hydrochloric acid (1:4), transferred with an acetate buffer mixture purified with lumogallion, pH 5.4, into a 25 ml volumetric flask, and brought to the mark with this solution. From the resulting solution take 2.5 ml and, in a volumetric flask, dilute to 25 ml with acetate buffer mixture of pH 5.8–6.0.
The same operations are repeated with a blank sample, to which all the same reagents are added, in the same amounts and in the same sequence.
When the aluminum content is within the range \(10^{-6}\)—\(10^{-7}\%\), its determination is carried out from the entire weighed portion of tin. In this case, after moistening the dry residue with 6 drops of hydrochloric acid (1:4), add 9.7 ml
acetate buffer mixture so that the total volume of the solution is 10 ml.
At the end of the determination by the method of additions, 4 thoroughly washed conical flasks are charged with 4.7 ml of acetate buffer mixture with pH 5.8–6.0. Into the first flask are added 5 ml of the blank sample; into the second, 5 ml of the tin sample solution; into the third, 5 ml of the tin solution and an addition of a known amount of aluminum; into the fourth, 5 ml of the tin sample solution and another addition of a known amount of aluminum. Then to each of these flasks is added 0.3 ml of a 0.1% acetone solution of salicylal-ortho-aminophenol; the flasks are closed with stoppers protected by a thin polyethylene film, mixed thoroughly, and after 50 minutes the luminescence intensity of the aluminum complex is measured on the luminescence apparatus at 520 mμ.
The aluminum content in the aliquot is determined from the formula
\[ B_{\mathrm{Al}}=\frac{D_{\mathrm{Al}}\,(I_H-I_0)}{I_{H+D}-I_H}, \]
where
\(B_{\mathrm{Al}}\) is the weight of aluminum in the aliquot portion of the solution (5 ml), in μg;
\(D_{\mathrm{Al}}\) is the weight of added aluminum, in μg;
\(I_H\) is the reading of the millivolt microammeter, in mA, when measuring the luminescence intensity of the analyzed sample;
\(I_0\) is the reading of the millivolt microammeter, in mA, when measuring the luminescence intensity of the blank sample;
\(I_{H+D}\) is the reading of the millivolt microammeter, in mA, when measuring the luminescence intensity of the analyzed sample with an addition of a known amount of aluminum.
The weight of aluminum is calculated from the addition of a known amount of aluminum to the third flask and the luminescence intensity of this solution; the calculation is then repeated from the addition of aluminum to the fourth flask and the luminescence intensity of the corresponding solution. To determine the percentage of aluminum in the analyzed tin, the arithmetic mean of these two values is taken.
The percentage of aluminum in the analyzed tin is calculated from the formula:
\[ \%\,Al=\frac{B_{\mathrm{Al}}\cdot V_H\cdot 10^{-4}}{V_a\cdot H}, \]
where
\(B_{\mathrm{Al}}\) is the weight of aluminum in μg in the aliquot portion of the solution;
\(V_H\) is the volume in which the entire weighed portion of tin is dissolved (250 ml);
\(V_a\) is the volume of the aliquot (5 ml);
\(H\) is the weight of the tin sample, in grams.
When the content of copper and iron in the analyzed tin is less than \(5\cdot10^{-4}\%\), the determination of aluminum may be carried out by means of a calibration curve. In this case, into a thoroughly washed conical flask are introduced 4.7 ml of acetate buffer mixture purified with lumogallion, pH 5.8–6.0; 5 ml of the analyzed sample; and 0.3 ml of a 0.1% acetone solution of salicylal-ortho-aminophenol. The solution is mixed thoroughly, and after 50 minutes the luminescence intensity of the solution is measured.
Analogous operations are carried out with the blank sample. The instrument reading for the blank sample is subtracted from the instrument reading for the tin sample, and from these data, using the calibration curve obtained by the same procedure for a standard aluminum solution, the true content in the aliquot portion of the sample solution is established.
Conclusions
A procedure has been developed for the luminescent determination of traces of aluminum in high-purity tin with salicylal-ortho-aminophenol. The sensitivity of the method is \(1 \cdot 10^{-6}\)—\(5 \cdot 10^{-7}\%\). The error of determination does not exceed 20–25% relative.
References
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