Polarographic Determination of Microquantities of Titanium in Phosphor L-34
Abstract Generated abstract
This paper develops polarographic procedures for determining trace titanium in L-34 phosphor, an analytically difficult material because of limited solubility and matrix impurities. The authors evaluate oxalic acid in sulfuric acid as a supporting electrolyte and propose two separation approaches: extraction of the titanium oxine complex into chloroform followed by decomposition and polarography, and an accelerated hydroxide precipitation method with hydrazine treatment before polarographic measurement. Both methods use standard additions and achieve a reported sensitivity of 5·10⁻⁴ percent, with relative error not exceeding 10 to 15 percent. Analysis of plant samples gave titanium contents near 2.8·10⁻³ percent, with the accelerated method reducing the total analysis time to about 2.5 hours.
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POLAROGRAPHIC DETERMINATION OF MICROQUANTITIES OF TITANIUM IN L-34 PHOSPHOR
Yu. A. Karbainov, N. K. Troshchenko, S. N. Voyush
(Presented by the Scientific-Methodological Seminar of the Inorganic Departments of the Faculty of Chemical Technology)
At the request of the plant, it was necessary to develop a procedure for determining titanium in L-34 phosphor with a sensitivity of \(10^{-3}\)—\(10^{-4}\%\). This problem is rather labor-intensive, first, because the L-34 phosphor itself is a difficult object for analysis owing to its limited solubility in most widely used solvents. Second, a standard titanium solution is usually prepared from titanium dioxide, \(\mathrm{TiO_2}\), and here the operation of fusing \(\mathrm{TiO_2}\) with the corresponding reagent is unavoidable, which complicates and lengthens the analysis.
The literature describes a comparatively small series of electrolytes that can be used as supporting solutions in the polarographic determination of titanium in various objects. Thus, recently only pyrophosphoric acid solutions have found wide application in the determination of large amounts of titanium (up to 1%) [1–5]. True, other mixtures are also known, such as, for example, \(0.1\ M\) KCNS in \(0.25\ M\ \mathrm{H_2SO_4}\) [6], \(2M\ \mathrm{H_3PO_4}\) [7], \(2M\ \mathrm{CH_3COONa}+0.1\ M\) Trilon B [8], \(0.2\ M\) citric acid \(+ 0.1\ M\) KCNS [8], but they are used less often.
For the determination of microquantities of titanium, the most suitable supporting solutions proved to be solutions of oxalic acid in sulfuric acid at various concentrations of both components, for example, \(0.2\ M\ \mathrm{H_2C_2O_4}\) in \(0.5\ M\ \mathrm{H_2SO_4}\); \(0.1\ M\ \mathrm{H_2C_2O_4}\) in \(1\ M\ \mathrm{H_2SO_4}\) [9]. In our work, a mixture of \(0.2\ M\ \mathrm{H_2C_2O_4}\) in \(0.25\ M\ \mathrm{H_2SO_4}\) was used as the supporting solution. This mixture was used because it gave stable and well-reproducible results for titanium.
To separate titanium from the impurities accompanying it (Mn, Fe, Cu, etc.), two routes may be used: extraction and precipitation of titanium in the form of a sparingly soluble compound, for example, hydroxide.
In our investigations both of these possibilities were studied in detail, and, on the basis of the research carried out, two procedures are proposed for determining microquantities of titanium in L-34 phosphor.
- The procedure for the extraction-polarographic determination of microquantities of titanium in L-34 phosphor is based on separating titanium from the matrix by extracting it with chloroform in the form of a complex with oxine, followed by polarography after destruction of the complex in an oxalate supporting solution.
2.1. An accelerated procedure for determining traces of titanium in L-34 phosphor, based on preliminary separation of titanium from the matrix in the form of the hydroxide, dissolution of the precipitate obtained, and treatment of the solution with 20% hydrochloric hydrazine on heating (for reduction of \(\mathrm{Fe}^{\cdots}\) to \(\mathrm{Fe}^{\cdot\cdot}\)), followed by polarography in an oxalate supporting electrolyte.
Extraction–polarographic method for determining microquantities of titanium in L-34 phosphor
For analyses of L-34 phosphor samples by this method, the following reagents were used: hydrochloric acid, chemically pure grade (sp. gr. 1.184). Ortho-oxyquinoline, chemically pure grade, was additionally purified by twofold recrystallization from a boiling aqueous solution (bidistillate). After cooling, the oxyquinoline precipitated; it was filtered off and carefully dried; the chloroform used in the experiments was subjected to twofold fractional distillation over \(\mathrm{CaCO_3}\). Experience showed that the percentage extraction of titanium by chloroform depends very strongly on the purity of the latter, on the presence in it of by-products. Oxalic acid of chemically pure grade was used. Sulfuric acid, chemically pure grade, was not subjected to further purification.
Course of the analysis
\(0.3\ \mathrm{g}\) of L-34 phosphor is dissolved in \(3\ \mathrm{ml}\) of concentrated HCl (\(d = 1.184\)). The undissolved residue is filtered off, ashed, after which a minimal amount of dry potassium pyrosulfate is added to it, and the residue is fused while being strongly ignited in the flame of a gas burner for 3–5 min. The contents of the crucible are cooled, dissolved in 1–2 ml of conc. HCl, and mixed with the initial solution being analyzed. After dissolution of the weighed sample, 5 ml of 0.02 M Trilon B solution is added to the solution, and the mixture is neutralized with concentrated NaOH to pH 5, after which the precipitate is filtered off, washed 2–3 times with bidistillate, and discarded. The filtrate is transferred to a 100 ml volumetric flask; with the aid of NaOH solution the pH of the mixture is brought to 8–9, after which it is made up to the mark with water. The contents are transferred to a separatory funnel of capacity \(\approx 200\ \mathrm{ml}\) and extracted with 10 ml of a 1% solution of oxyquinoline in chloroform, twice for 2 minutes each time. After completion of the extraction, the chloroform layer is separated, transferred to a platinum or quartz dish, and, in the presence of 0.1–0.15 g of oxalic acid, carefully evaporated to dryness; the residue is then slightly ashed (the oxalic acid is used here to prevent codistillation of titanium). To the residue after ashing, a minimal amount of dry \(\mathrm{Na_2CO_3}\) is added, and it is strongly ignited in the flame of a gas burner. It is cooled, dissolved in 5 ml of \(0.2\ \mathrm{M}\ \mathrm{H_2C_2O_4} + 0.25\ \mathrm{M}\ \mathrm{H_2SO_4}\), and polarographed in the potential interval \(-0.2\)–\(0.7\ \mathrm{V}\). Before analysis the supporting electrolyte is freed from oxygen by purging it with nitrogen for 10–15 min. The total duration of the analysis (two parallel samples and one blank sample) is 4–5 hours. The titanium content is determined by the method of additions of a standard solution. The sensitivity of the procedure is \(5 \cdot 10^{-4}\%\); the relative error is not more than 10–15%.
According to this procedure, analyses of plant samples of L-34 phosphor were carried out. The results of the analyses and their statistical treatment are given in the first row of Table 1; the conditions of polarography are also shown there.
Table 1
Results of statistical processing of polarographic analysis data for titanium content in L-34 phosphor
| No. | Measured quantity | Number of measurements, \(n\) | Arithmetic mean, % | Error of the arithmetic mean, \(S_x\) | Confidence level, \(P\) | Parameter, \(\tau_p\) | Accuracy of the mean, \(\varepsilon\) | Relative error (precision of the experiment), % |
|---|---|---|---|---|---|---|---|---|
| \(1^1\) | Titanium content, %, in L-34 phosphor | 16 | \(2.87 \cdot 10^{-3}\) | \(\pm 0.043\) | 0.05 | 2.131 | \(\pm 0.092\) | 3.2 |
| \(2^2\) | Titanium content, %, in L-34 phosphor | 23 | \(2.81 \cdot 10^{-3}\) | \(\pm 0.045\) | 0.05 | 2.074 | \(\pm 0.093\) | 3.3 |
\(^{1}\) Polarographic conditions: instrument sensitivity \(6 \cdot 10^{-8}\) A/mm; volume of the solution analyzed 5 ml; capillary characteristic \(m^{2/3}\tau^{1/6} = 1.74\ \text{mg}^{2/3}\ \text{s}^{-1/2}\).
\(^{2}\) Polarographic conditions: volume of the solution analyzed 10 ml; the remaining conditions are the same as above.
Accelerated Procedure for the Polarographic Determination of Microquantities of Titanium in L-34 Phosphor
In carrying out analyses of plant samples of L-34 phosphor by this method, the same reagents were used as those described in [10].
Course of the Analysis
A weighed portion of L-34 phosphor, 0.3 g, is dissolved in 3–5 ml of concentrated HCl (d = 1.184). The resulting solution is filtered off; the filter with its contents is ashed in a crucible, after which a minimum amount of dry potassium pyrosulfate is added to it, and the residue is fused while being strongly ignited in the flame of a gas burner for 2–3 min. The contents of the crucible are cooled and dissolved in 1–2 ml of concentrated HCl. The resulting solution is mixed with the initial analyte solution. The mixture is neutralized with concentrated NaOH, after which an additional 20 ml of 20% NaOH is introduced and the mixture is heated to 50°C. To the heated mixture, 15–20 ml of H₂O is added and the mixture is brought to 100°C. After 10–15 min, the precipitate of titanium hydroxide is filtered off; the contents of the filter are washed 1–2 times with hot water, and then several times with cold water, until the wash waters are neutral. The washed precipitate is dissolved in 10 ml of 0.2 M H₂C₂O₄ + 0.25 M H₂SO₄. To the resulting solution, 4–5 drops of 20% hydrochloric-acid hydrazine are added, and it is heated to 100°C, cooled, transferred to an electrolyzer, and analyzed. The titanium content is determined by the method of standard additions. The total duration of the analysis (two parallel samples and one blank sample) is no more than 2.5 hours. The sensitivity of the procedure is equal to 5·10⁻⁴%. The relative error is no more than 10–15%.
According to this procedure, as well as according to the preceding one, analyses of plant samples of L-34 phosphor were carried out. The results of the analysis and their statistical treatment are given in the second row of Table 1; the polarographic conditions are also given there.
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