Polarographic Determination of Microquantities of Titanium in Perchloric Acid and Its Salts

Yu. A. Karbainov, N. K. Trotsenko, S. I. Voyush

(Recommended by the scientific seminar of the Department of Analytical Chemistry)

As far as we know, the literature describes only one method for the polarographic determination of titanium in sodium and ammonium perchlorates and in perchloric acid [1]. This method is based on preliminary binding of titanium into a complex of composition \(C_{22}H_{14}N_6O_{14}Ti\) or \(C_{10}H_{10}TiBr_2\), followed by its determination under appropriate conditions [1]. The method is extremely laborious (especially the very process of synthesizing titanium complexes), and therefore is unlikely to find wide application in analytical practice. In connection with this, the objective of our work was to develop a simple method for determining \(10^{-4}\%\) and higher titanium in perchloric acid and its salts. To solve this problem, it was first necessary to select an indifferent electrolyte that could be used as a supporting solution for the polarographic determination of titanium and that would at the same time give stable, reproducible results. For this purpose, a whole series of supporting solutions already studied by other authors [2–4] was tested. In particular, a solution of \(2\,M\) \(CH_3COONa + 0.1\,M\) Trilon B [3] was studied in detail. Our data concerning the dependence of the height of the polarographic wave of titanium on the pH of the solution confirm the literature data; however, reproducible results for titanium were not obtained here.

Other supporting solutions [2, 4, 5] were also tested, but for one reason or another they proved unsuitable in our case.

It is known from the literature [3] that titanium readily forms complexes with Trilon B, and these complexes are readily reduced at a mercury electrode. On the other hand, Trilon B forms fairly stable complexes with other metals (Cu, Mn, Fe), which are not reduced at a mercury electrode under ordinary conditions and thus, although they accompany titanium in perchlorates, do not interfere with its determination.

In our work this possibility was thoroughly investigated. In particular, the behavior of titanium was studied against the background of \(0.5\,M\) \(NaClO_4 + 0.05\,M\) Trilon B \(+ 0.02\,M\) gelatin; \(0.5\,M\) \(NH_4ClO_4 + 0.05\,M\) Trilon B \(+ 0.02\,M\) gelatin; \(0.5\,M\) \(HClO_4 + 0.05\,M\) Trilon B \(+ 0.02\,M\) gelatin. The latter was used to suppress maxima. In all these mixtures we obtained titanium waves. As a rule, two waves appear, apparently corresponding to the transitions [4] \(Ti^{4+} \rightarrow Ti^{3+}\), \(Ti^{3+} \rightarrow Ti^{2+}\). Their half-wave potentials are respectively equal to \(-0.55\) and \(-1.3\); \(-0.47\) and \(-1.28\); \(-0.34\) and \(-1.11\) V.

Unfortunately, stable results were not obtained here either.

Based on the results of our studies, as well as [4], a solution of oxalic acid in sulfuric acid, at various concentrations of both components in the mixture, can be successfully used as the supporting solution in the polarographic determination of titanium.

In order to accelerate sample analyses, the possibility of determining titanium directly in \(\mathrm{NaClO_4}\), \(\mathrm{NH_4ClO_4}\), and \(\mathrm{HClO_4}\) was first investigated. The procedure for determining titanium in this case was as follows: an appropriate weighed portion of sodium or ammonium perchlorate salt, or perchloric acid (for \(\mathrm{NaClO_4}\), for example, equal to \(6\ \mathrm{g}\)) was mixed with \(0.9\ \mathrm{g}\) of \(\mathrm{H_2C_2O_4}\) and \(5\ \mathrm{ml}\) of \(\mathrm{H_2SO_4}\) (\(d=1.84\)) in a beaker of 150–200 ml capacity. The mixture was diluted with bidistilled water to 100 ml and thoroughly stirred until a homogeneous medium was obtained, after which 5 drops of a 20% solution of hydrochloric acid hydrazine were added to it (for reduction of \(\mathrm{Fe^{3+}}\) to \(\mathrm{Fe^{2+}}\)), and the mixture was heated almost to boiling. The solution was then cooled, and, for analysis, 5 ml of its total volume was taken and polarographed. The advantage of this procedure is the speed of the analysis (the duration of two parallel and one blank sample is no more than an hour), the simplicity of the apparatus arrangement, and good reproducibility of the results. Its disadvantage is undoubtedly the low sensitivity (\(10^{-3}\%\) titanium). In order to increase the sensitivity of titanium determination, we proceeded by separating it from the matrix. For this purpose, a 20% \(\mathrm{NaOH}\) solution was used [4]. The final determination of titanium was still carried out in an oxalate supporting medium. The sensitivity of this method was \(1\cdot10^{-4}\%\); thus, it is an order of magnitude higher than in the preceding procedure. The reproducibility of the analytical results is fairly high (relative error no more than 10–15%).

For carrying out analyses of perchloric acid samples and its salts by this method, the following reagents were used: sulfuric acid (\(d=1.84\)), chemically pure grade; sodium hydroxide—chemically pure or analytical grade; oxalic acid—chemically pure; hydrochloric acid hydrazine—analytical grade.

A standard titanium solution—\(100\ \mu\mathrm{g}/\mathrm{ml}\)—was prepared as follows: an initial weighed portion of titanium dioxide equal to \(0.0167\ \mathrm{g}\) was fused with a minimal amount of potassium pyrosulfate in a quartz or platinum beaker; after cooling, the contents were dissolved in 100 ml of \(0.5\ \mathrm{M}\ \mathrm{H_2SO_4}\).

Procedure for preparing the supporting solution: \(1.8\ \mathrm{g}\) of oxalic acid was mixed in a 200 ml volumetric flask with 10 ml of \(\mathrm{H_2SO_4}\) (\(d=1.84\)) and diluted with bidistilled water to the mark. Composition of the resulting mixture: \(0.1\ \mathrm{M}\ \mathrm{H_2C_2O_4}\) in \(0.5\ \mathrm{M}\ \mathrm{H_2SO_4}\).

Procedure for analysis for the determination of \(10^{-4}\%\) and higher titanium in perchloric acid and its salts

An appropriate weighed portion of perchloric acid, or sodium perchlorate, or ammonium perchlorate, equal to \(6\ \mathrm{g}\), is heated with 20 ml of 20% \(\mathrm{NaOH}\) to \(50^\circ\mathrm{C}\). To the heated mixture, 15–20 ml of \(\mathrm{H_2O}\) is added, and the temperature is brought to \(100^\circ\mathrm{C}\). After 10–15 min the titanium hydroxide precipitate is filtered off; the contents of the filter are thoroughly washed with water (until neutral reaction in the wash waters) and dissolved in 10 ml of \(0.1\ \mathrm{M}\ \mathrm{H_2C_2O_4}\) in \(0.5\ \mathrm{M}\ \mathrm{H_2SO_4}\). To the resulting solution, 5 drops of a 20% hydrochloric acid hydrazine solution are added, and it is boiled for 1–1.5 min. After cooling, the solution is transferred into a cell for polarography and, after ten minutes’ purging with nitrogen, is analyzed in the potential range \(-0.2\) to \(-0.7\ \mathrm{V}\).

Using this procedure, analyses were carried out on factory samples of sodium and ammonium perchlorate salts and of perchloric acid. In all these samples the titanium content was evidently below \(10^{-4}\%\), and it was not detected on the polarograms.

The procedure was checked for completeness of separation of titanium in the form of the hydroxide. In these studies the procedure described above was used, with the sole difference that a known addition of a standard titanium solution, equal to \(20\ \mu\text{g}\), was introduced into the analyzed mixture obtained after complete dissolution of the corresponding 6 g sample, before separating the titanium as the hydroxide. The results of these experiments show that the completeness of separation of titanium by this method is 93.4%.

In conclusion, we note that this procedure for determining \(10^{-4}\%\) and higher titanium in perchloric acid and its salts has been introduced at the plant, and a certificate of its implementation has been obtained.

REFERENCES

1. G. Wilkinson, P. Pauson, J. Birmingham, F. Gotton. J. Am. Chem. Soc., 20, 1012 (1953).
2. Wang Zhi-zhen, RZhKhim. 6, 19108 (1959).
3. T. A. Kryukova, S. I. Sinyakova, T. V. Aref’eva. Polarographic Analysis, Moscow, 1959.
4. Bivers, R. Breyer, J. Electroanal. Chem., 1, 39 (1959).
5. D. I. Kurbatov. Journal of Analytical Chemistry, 6, 743 (1950).