PHOTOCOLORIMETRIC DETERMINATION OF PHOSPHORUS IN HIGH-PURITY TIN AS PHOSPHOMOLYBDENUM BLUE

V. B. Sokolovich, Yu. L. Lel’chuk, O. A. Drelina

(Presented by the Scientific-Methodological Seminar of the Chemical-Technological Faculty)

Photometric determination of phosphorus is based mainly on colorimetry of yellow phosphomolybdic or phosphovanadomolybdic heteropoly acids, as well as of the blue products of their reduction, the so-called ceruleo acids and blues. The structure of these compounds has not been sufficiently elucidated. Some authors [1] indicate that ceruleo acids and blues are complex combinations of oxygen compounds of hexavalent molybdenum and molybdenum in lower oxidation states with phosphoric acid. Their composition is not constant, and their color is unstable. In the molecules of the blue, the ratio of \(M^{VI}\) to \(M^{V}\) is more constant than in the molecules of ceruleo acids, which accounts for the greater depth and durability of the blue color.

Solutions of the blue are characterized by an absorption maximum at 650–700 mµ; solutions of the “blue heteropoly compound” (ceruleo acid) at 820–830 mµ [2].

Greater sensitivity in the determination of phosphorus is achieved by colorimetry of the blue compounds, especially with the use of extraction.

Various and contradictory data are given in the literature concerning the conditions for obtaining phosphomolybdic heteropoly acid. An excess of molybdenum is recommended, with a molybdenum-to-phosphorus ratio from 16:1 to 250:1, and an acidity from 0.2 to 0.8 \(N\). It is stated that tin, arsenate, nitrate ions, large amounts of trivalent iron, and tungstate must be absent.

As reducing agents for heteropoly acids to blues, stannous chloride [4, 5], hydrazine sulfate, ascorbic acid, hydroquinone, and many others are recommended.

As extractants for heteropoly acids and blues, diethyl ether [6, 7], isoamyl alcohol, butanol, a mixture of butyl acetate with ethyl acetate, and others are most often used.

In preliminary investigations we established a method for decomposing a weighed sample of tin with practically complete removal of the tin while retaining the phosphorus in the solid residue; an extractant and a reducing agent were selected.

Diethyl ether was chosen as the extractant, and stannous chloride proved to be the best reducing agent for phosphomolybdic heteropoly acid to the blue in the ether medium.

It was established that the course of the reduction process, the optical density of the blue color formed, and the stability of the blue coloration are strongly influenced by the method of preparing the stannous chloride solution, since the acidity of the medium depends on it.

The most stable blue reduction product of phosphomolybdic heteropoly acid, with the highest optical-density value, was obtained when a freshly prepared 0.2% solution of stannous chloride in hydrochloric acid of isodensity concentration (1:1) was used.

The blue color developed almost instantaneously and remained without substantial change for about 24 hours.

It was also found that, under the conditions of the procedure we adopted, the content of pentavalent arsenic at an As:P ratio of 1:1 or less does not affect the result of the determination. At higher arsenic content, the density of the blue being colorimetrically measured increases. Silicon content, even at an Si:P ratio of 1000:1, does not change the optical density of the blue being colorimetrically measured.

Taking into account the literature data and the results of our preliminary investigations, a method was developed for determining traces of phosphorus in high-purity tin.

The method is based on decomposition of a weighed portion of tin with hydrochloric acid, bromine, and hydrobromic acid, with triple evaporation at a temperature of 80–90°C (which is accompanied by practically complete removal of tin and arsenic and conversion of silicon into the weakly active form of silicon dioxide), dissolution of the solid residue in a small volume of hydrochloric acid (1:3), formation, by heating with ammonium molybdate at pH 1.7–1.9, of phosphomolybdic heteropoly acid, which is extracted with diethyl ether and reduced in the extract by stannous chloride to molybdenum blue. The blue is colorimetrically measured on an FEK-N-57 photocolorimeter-nephelometer in 5 cm cuvettes with red light filter No. 8, having a transmission maximum of 656 mµ. The comparison solution may be either pure ether or ether containing all reagents and 5 µg of phosphorus. Decomposition of the sample and removal of tin lasts 2–2.5 hours; the photometric completion takes about 1 hour. The relative error of determination is about 10%. The sensitivity is 0.1 µg, or, calculated for a gram sample, \(1 \cdot 10^{-5}\%\).

Fig. 1. Calibration graph for the photocolorimetric determination of phosphorus in the form of phosphomolybdenum blue. Cuvette 5 cm, comparison solution—ether.

In carrying out the analyses, twice-distilled water and reagents of chemically pure and analytical grade were used; these were additionally purified by recrystallization or distillation [11]. The ether was purified from peroxides and distilled. Hydrochloric acid was of isodensity concentration. Standard phosphorus solutions were prepared from sodium hydrogen orthophosphate, in which the percentage phosphorus content had first been established gravimetrically [8]. The initial standard solution \(Na_2HPO_4\) ...

\(12\mathrm{H}_2\mathrm{O}\) contained 1000 μg of phosphorus in 1 ml. Its titer after preparation was checked gravimetrically.

Standard solutions containing 1 and 0.1 μg of phosphorus in 1 ml were prepared by diluting the initial standard solution immediately before use.

The reagent solutions were stored in polyethylene or quartz bottles.

The vessels used in carrying out the analysis were mainly quartz; before use they were washed with chromic acid mixture, steamed, and rinsed with distilled water and bidistilled water.

Construction of Calibration Curves

From the initial standard phosphorus solution, containing 1000 μg of phosphorus in 1 ml, a solution containing 1 μg in 1 ml is prepared. Dilution is performed with bidistilled water. The calibration curve is constructed for amounts from 0 μg to 3 μg of phosphorus.

To construct the calibration curve, 20 ml of bidistilled water, 0.22 ml of hydrochloric acid (1:3), the required volume of the standard phosphorus solution, and 1.25 ml of a 10% aqueous solution of ammonium molybdate are introduced into a series of quartz beakers.

The contents of each beaker are heated until boiling begins, cooled, and transferred to a separatory funnel, to which 2.5 ml of azeotropic HCl and 8.5 ml of diethyl ether are added; with vigorous shaking of the funnel for 1.5–2 minutes, phosphomolybdic heteropoly acid is extracted.

After the layers have separated, the beaker is rinsed twice with small portions of the acid layer; the wash waters are poured back into the funnel. After repeated shaking and settling, the acid layer is drained off, and the ether layer is washed twice with hydrochloric acid (1:9) in 2.5-ml portions.

Fig. 2. Calibration curve for the photocolorimetric determination of phosphorus in the form of phosphomolybdenum blue. Cuvette 5 cm; the comparison solution contains all reagents and 5 μg of phosphorus.

The washed ether extract is run into a 25-ml volumetric flask; the funnel is washed several times with small portions of ether, which are added to the main extract.

To reduce phosphomolybdic heteropoly acid to the blue color, 0.2 ml of a freshly prepared 0.2% solution of stannous chloride in azeotropic hydrochloric acid (1:1) is added to the ether extract in each volumetric flask. The contents of the flask are brought to the mark with ether, shaken, and after 3–5 minutes colorimetrized on an FEKN-57 photoelectric colorimeter-nephelometer in 5-cm cuvettes with red light filter No. 8, having a transmission maximum at 656 mμ.

The comparison solution may be either pure diethyl ether or ether containing all the reagents and 5 μg of phosphorus.

When working by the second variant, the optical-density readings

are read on a more accurate portion of the scale, which helps increase the accuracy of the determination.

When constructing calibration graphs, the phosphorus content is plotted on the horizontal axis, and the optical densities of the blue solutions on the vertical axis.

Table 1 and graph 1 give the data we obtained for disubstituted sodium phosphate, chemically pure, when using diethyl ether as the comparison solution; Table 2 and graph 2, when using as the comparison solution ether containing all reagents and 5 μg of phosphorus.

Table 1

Dependence of optical density on phosphorus concentration.
Comparison solution—diethyl ether

Table 2

Dependence of relative optical density on phosphorus concentration.
Comparison solution—ether containing 5 μg of phosphorus and all reagents

Procedure for the Analysis of High-Purity Tin

Preparation of tin samples for analysis is carried out as we indicated previously [9].

To perform the analysis, 2 ml of concentrated hydrochloric acid, 1.5 ml of bromine [10], and 2.5 ml of bromine–hydrobromic acid are added to a tin sample weighing approximately 1 g in a quartz dish 5–6 cm in diameter and 2.5–3.0 cm high, covered with a watch glass and half-immersed in cold water. After the vigorous reaction ceases, the glass is removed, rinsed, and the beaker is placed under a lamp on a table covered with Teflon for further decomposition of the sample and removal of tin and arsenic.

At first, evaporation is carried out at 80–90°C to a slightly moist residue. Then the contents of the dish are moistened with 1 ml of hydrochloric acid (1:1) and 1 ml of hydrobromic acid and evaporated to dryness under the same conditions. After this, evaporation to dryness is repeated once more with 0.5 ml of hydrochloric acid (1:1).

To the dry residue are added 0.22 ml of hydrochloric acid (1:3), 20 ml of bidistillate, and 1.25 ml of a 10% aqueous solution of ammonium molybdate. The pH of the mixture is adjusted to 1.7–1.9.

The contents of the dish are heated to the onset of boiling, cooled, and transferred to a separatory funnel, to which 2.5 ml of 10 N hydrochloric acid and 8.5 ml of diethyl ether are added. The phosphomolybdenum formed upon heating with ammonium molybdate...

molybdenum heteropolyacid is extracted with ether by vigorously shaking the separatory funnel for 1.5–2 minutes. After complete separation of the layers, the cup is rinsed twice with small portions of the acid layer. The solution after rinsing is again poured into the separatory funnel. After repeated shaking and separation of the layers, the acid layer is discarded, and the ether layer is washed twice with hydrochloric acid (1:9) in 2.5-ml portions.

Table 3

Verification of the accuracy of determining phosphorus in the form of phosphomolybdenum blue in standard solutions

The washed ether extract is run into a 25-ml volumetric flask; the separatory funnel is rinsed twice with ether, which is poured into the same flask.

Table 4

Results of parallel determinations of phosphorus in a sample of high-purity tin

To the ether extract, 0.2 ml of a freshly prepared 2% solution of stannous chloride is added. The contents of the flask are brought to the mark with ether and shaken thoroughly. The phosphomolybdenum blue, which forms almost instantaneously, is colorimetrically measured after 3–5 minutes.

measurement is carried out on an FEK-N-57 in 5 cm cuvettes with red light filter No. 8, with a transmission maximum of 656 mµ. The comparison solution may be either pure diethyl ether or ether containing all reagents and 5 µg of phosphorus.

A blank experiment is carried through all stages of decomposition of the sample and is photometered under analogous conditions.

The accuracy of the developed procedure was checked by us using pure phosphorus salts carried through all stages of treatment of the tin sample, by the method of additions, and by parallel determinations of phosphorus in samples of high-purity tin. Some of the data obtained are given in Tables 3 and 4.

Conclusions

A procedure has been developed for determining traces of phosphorus in high-purity tin in the form of phosphomolybdenum blue, with a sensitivity of \(1 \cdot 10^{-5}\%\). The relative error is \(\pm 10\%\). The time required for a determination is 3–3.5 hours.

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