Proceedings
of the Tomsk Order of the Red Banner of Labor Polytechnic
Institute named after S. M. Kirov

Vol. 174 1971

Photometric Determination of Traces of Aluminum in Titanium Dioxide

V. A. Malevannyi, Yu. L. Lelchuk*)

(Presented by the Department of Analytical Chemistry)

At the present time, for the colorimetric determination of aluminum, a number of authors use a new reagent—xylenol orange. This reagent was first used by Otomo Makoto [1] for the deter-

Table 1

Summary of literature data on the conditions for the determination of aluminum with xylenol orange

*) K. I. Banokina and V. A. Shumina took part in carrying out the experimental work.

determination of aluminum in pure salts and somewhat later by Budishinskii for the determination of aluminum in uranium compounds [2].

Both authors give contradictory characteristics of the xylenol orange complex with aluminum and nonmatching recommendations for the conditions under which the reaction should be carried out. The articles by Tikhonov [3] and Akhmedi [4] also did not clarify this question. All the recommendations of these authors are summarized in Table 1. Taking this discrepancy into account, we carried out a study of the optimal conditions for determining aluminum with xylenol orange.

The following solutions and equipment were used in the work: xylenol orange from the firm “Chemapol” (ČSSR), concentration \(10^{-3}\) M. The stock aluminum solution was prepared from metallic aluminum containing 99.9% of the main substance. The titer of the standard solution was established by the gravimetric ammonia method. Diluted solutions with a concentration of \(10^{-3}\) M were prepared on the day of use by appropriate dilution of the stock standard solution. Buffer solutions were prepared according to Babko’s prescription [5]. Control checks of pH were carried out using a “Multoscope 5” (Laboratorní přístroje, ČSSR) with a glass electrode. The optical density of the solutions was measured on an SF-10 spectrophotometer.

Experimental Part

To select the optimal acidity of the solutions, the dependence of the optical density of solutions of the xylenol orange—aluminum complex was studied in the pH range 1–11 at a 10-fold excess of reagent. Figure 1 shows the dependence of the optical density of the aluminum complex with xylenol orange on the pH of the solution. Aluminum concentration \(10^{-3}\) M, reagent concentration \(10^{-2}\) M, solution volume 25 ml, layer thickness 50 mm, \(\lambda = 556\) mµ, reference solution—the reagent solution.

As is evident from the graph, the optical density reaches its maximum at \(\mathrm{pH} = 3.3 \pm 0.2\). At the acidity recommended by Budishinskii [2], equal to 2.28 pH units, the optical density of the complex does not reach its maximum value, which leads to a considerable loss of sensitivity.

Fig. 1. Dependence of the optical density of the aluminum complex with xylenol orange on the pH of the solution.
\(C_{\mathrm{Al}} = 10^{-3}\) M, \(C_R = 10^{-2}\) M, \(V = 25\) ml, \(l = 50\) mm, \(\lambda = 556\) mµ, reference solution—the reagent.

Figure 2 shows the spectrophotometric curves of the reagent in acidic and alkaline media and of its complex with aluminum. Concentration of Al—\(10^{-3}\) M, reagent concentration—\(10^{-3}\) M, solution volume—25 ml, layer thickness—3 mm, solution pH—3.2, reference solution—water.

As is evident from the graph, xylenol orange possesses very high contrast \((\Delta \lambda = \lambda_{\mathrm{MeR}} - \lambda_{\mathrm{HR}})\), equal to 114 mµ. Such

the high contrast of the reagent is its undoubted advantage.

It is known from the literature that the aluminum complex with xylenol orange is formed very slowly. Our experiments showed that heating the solution to boiling sharply accelerates the attainment of maximum optical density. However, on prolonged standing the density decreases.

Table 2

Stability of a heated solution of the aluminum complex with xylenol orange over time

Fig. 2. Spectrophotometric curves of the reagent and its complex with aluminum. 1 — reagent, 2 — reagent and Al complex, 3 — Al complex. \(C_{\mathrm{Al}} = 10^{-3}\ \mathrm{M}\), \(C_R = 10^{-3}\ \mathrm{M}\), \(V = 25\ \mathrm{ml}\), \(l = 3\ \mathrm{mm}\), \(\mathrm{pH} = 3.2\). Reference solution — water.

The composition of the complex was determined by the method of Ostromyslensky–Job. Figure 3 shows the isomolar-series plot at \(\mathrm{pH} = 3.2\). The aluminum concentration was \(10^{-3}\ \mathrm{M}\), the reagent concentration \(10^{-3}\ \mathrm{M}\), the solution volume 25 ml, the layer thickness 3 mm, and the reference solution was water.

As can be seen from the plot, the ratio of the coefficients in the complex-formation reaction is \(1:1\). The complex of composition \(1:2\) at \(\mathrm{pH} = 3.3\) reported by Makoto [1] was not found by us. The molar absorptivity, calculated from the data of the isomolar series and by the saturation method, is \(3.01 \cdot 10^4\ \mathrm{mmol}^{-1}\mathrm{cm}^2\). This indicates the high sensitivity of xylenol orange toward aluminum. The relative difference of the molar absorptivities of the complex and the reagent (photometric sensitivity according to A. K. Babko) was found to be \(3 \cdot 10^2\). The molar absorptivity of the reagent at a wavelength of 555 mµ is \(10^2\ \mathrm{mmol}^{-1}\mathrm{cm}^2\). The absolute difference of the molar absorptivities \((\Delta \varepsilon)\) for xylenol orange is \(3 \cdot 10^4\). The spectrophotometric sensitivity of the reagent toward aluminum is \(0.023\ \mu\mathrm{g}/\mathrm{cm}^2\).

The results obtained indicate that xylenol orange is one of the best reagents for aluminum. We were unable to determine the instability constant of the complex obtained by Babko’s method. However, this quantity can be estimated approximately on the basis of the following observation: upon addition to the solution of the complex

aqueous solution of Trilon B, the destruction of the complex proceeds very slowly. This indicates that the instability constants of the aluminum complexes with xylenol orange and with Trilon B are comparatively close and, probably, are expressed by values of one and the same order of magnitude.

Fig. 3. Determination of the composition of the complex by the Ostromyslensky—Job method. \(C_{\mathrm{Al}} = 10^{-3}\) M, \(C_{\mathrm{R}} = 10^{-3}\) M, \(V = 25\) ml, \(l = 3\) mm, pH = 3.2. Reference solution—water.

According to [6], the strength exponent of the aluminum complex with Trilon B, i.e., the negative logarithm of its dissociation constant, is 16.13. It is evident that the strength exponent of the aluminum complex with xylenol orange is also close to this value.

Literature data on the specificity of the reagent at pH 3—3.5 are given in Table 1. Divalent iron does not react with xylenol orange, which makes it possible, when necessary, to mask iron with thioglycolic acid.

Taking into account the results of the study of the properties of the aluminum complex with xylenol orange, we have developed a method for determining aluminum with this reagent in capacitor titanium dioxide. The analysis is carried out as follows: a weighed portion of titanium dioxide weighing 0.1000 g is placed in a silver or nickel crucible. The product being analyzed must first be thoroughly dried at 110°C to constant weight. To the crucible are added 3.0 g of dry potassium hydroxide; the contents are thoroughly mixed and heated on a hot plate with an open coil to remove the water present in the caustic alkali. After complete melting of all the alkali, the crucible is placed in a muffle furnace, heated for one hour at 700°C, cooled, and its contents are leached with boiling water. The resulting suspension is filtered into a 250 ml volumetric flask; the precipitate on the filter is washed 4–6 times with boiling water and discarded. The contents of the flask are cooled to room temperature.

temperature, dilute to the mark with water, and mix thoroughly.

To determine aluminum, an aliquot equal to 50 ml is taken from the resulting solution and transferred to a 250-ml volumetric flask; then 1 drop of a 0.01% solution of tropeolin OO is added, followed dropwise by concentrated hydrochloric acid until a red color appears in the solution. The excess acid is carefully neutralized with a 5% potassium hydroxide solution until the color changes to a faintly noticeable yellow. Then 50 ml of buffer solution with pH 3.2–3.3 and 10 ml of a 0.1% solution of xylenol orange are added to the flask. The solution is mixed, heated on an electric hot plate to boiling, cooled to room temperature, diluted to the mark with buffer solution, and mixed thoroughly. The resulting colored solution is photometered on an FEKN-57 photocolorimeter against the reagent blank, using a green light filter with a transmission maximum at 555 mµ. The results of the analysis are calculated from the calibration curve. The data for constructing the calibration curve are obtained under the same conditions and in accordance with the instructions supplied with the instrument.

Preparation of a buffer solution with pH 3.2–3.3

75.85 g of glycocol and 50.50 g of crystalline sodium chloride are dissolved and diluted in a volumetric flask to 1 liter with water. From the resulting 1 N glycocol solution, 90 ml is taken and transferred to another liter volumetric flask; 10 ml of 1 N HCl solution is added, the solution is diluted to the mark with distilled water, and mixed thoroughly. The pH of the resulting buffer solution is checked on a pH meter.

The accuracy of the method was tested by analyzing artificial mixtures. The data obtained are presented in Table 3.

Table 3

Results of testing the developed method on artificial mixtures

As can be seen from the results obtained, the accuracy of determining aluminum in condenser titanium dioxide corresponds to the usual accuracy of photometric analysis.

Conclusions

1. Optimal conditions have been found for the photometric determination of aluminum with xylenol orange.

2. The main analytical characteristics of the aluminum complex with xylenol orange have been studied: the molar extinction coefficient ...

...absorption, photometric and spectrophotometric sensitivity, contrast, and stability of the complex over time.

1. A method is proposed for the determination of aluminum in capacitor-grade titanium dioxide with xylenol orange.

REFERENCES

1. Otomo Makoto, Bull. Chem. Soc. Japan, 36, 809, 1963.
2. B. Budeshinskii. Journal of Analytical Chemistry, 18, 1071, 1963.
3. V. N. Tikhonov. Journal of Analytical Chemistry, 20, 9, 941—946, 1965.
4. A. K. Babko. Physicochemical Analysis of Complex Compounds in Solutions. Kiev, Publishing House of the Academy of Sciences of the Ukrainian SSR, p. 97, 1955.
5. M. K. Akhmedi, E. L. Glushchenko. Journal of Inorganic Chemistry, 10, 1, 98—102, 1965.
6. R. Prshibil. Complexones in Chemical Analysis, Moscow, Foreign Literature Publishing House, p. 63, 1960.