Nephelometric Determination of Aluminum in the Form of Cryolite

Yu. L. LELCHUK, B. N. BESPROZVANNYKH

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

Tananaev and Talipov [1], and later Tananaev and Abilov [2], proposed using the low solubility of cryolite for the quantitative determination of aluminum. The authors showed that, in the presence of an excess of sodium fluoride, the solubility of cryolite in water decreases to extremely low limits, and that precipitation of aluminum by this precipitant is possible in the presence of ions of cobalt, nickel, zinc, manganese, cadmium, copper, beryllium, chromium, titanium and, under special conditions, also iron [1, 2]. Later, Tananaev and Lelchuk [3] established that the composition of the double salt of sodium fluoride and aluminum formed by the interaction of aluminum cations with sodium fluoride in an aqueous medium corresponds to the formula $3\mathrm{NaF}\cdot\mathrm{AlF}_3$ only with a considerable excess of precipitant, exceeding 1.4% NaF. At a lower concentration of sodium fluoride, a congruently soluble salt of composition $11\mathrm{NaF}\cdot4\mathrm{AlF}_3$ separates. The authors proposed using this salt as the precipitate and gravimetric form and developed the corresponding procedure for the gravimetric determination of aluminum [4].

Application of the method was conceived in two variants, depending on whether the separation of aluminum is intended for the purpose of weighing it, or only the separation of aluminum from other elements is envisaged, after which the precipitate is decomposed by evaporation with sulfuric acid and the aluminum is subsequently determined by accepted methods. The first variant is feasible in the determination of aluminum in the presence of ions of cobalt, nickel, zinc, copper, cadmium and chromium; the second—in all cases when, under the conditions of determination, constancy of the composition of the precipitate cannot be guaranteed, as occurs in the precipitation of aluminum in the presence of iron, titanium and manganese.

The data from the study of solubility in the ternary system $\mathrm{NaF}$—$\mathrm{AlF}_3$—$\mathrm{H}_2\mathrm{O}$ [3] were used by Tananaev and Levina for developing a titrimetric method for determining aluminum [5], and by Tananaev and Yakovlev for developing procedures for the gravimetric determination of aluminum in steels and alloys on iron and nickel bases containing chromium, molybdenum, tungsten, vanadium, zirconium, niobium and titanium [6].

The present communication is devoted to studying the possibility of using the precipitate $11\mathrm{NaF}\cdot4\mathrm{AlF}_3$ for the nephelometric determination of aluminum. The experiments were carried out with solutions of aluminum sulfate, chloride and nitrate both in the absence of iron salts and in their presence.

Experimental part

Chemically pure sodium fluoride was prepared by the method of I. V. Tananaev [7]; aluminum sulfate, nitrate, and chloride were obtained by recrystallization of commercial preparations Al₂(SO₄)₃·18H₂O, Al(NO₃)₃·9H₂O, and AlCl₃·6H₂O of analytical grade. The solutions were prepared with bidistilled water. The concentration of aluminum in the initial solutions was determined gravimetrically in the form of Al₂O₃. Precipitation of aluminum oxide hydrate was carried out at 50–60° in the presence of metanitrophenol. The titer of the standard aluminum sulfate solution was 0.0022 g/ml, and that of the sodium fluoride solution 0.0360 g/ml. Nephelometric measurements were carried out on an “NFM” nephelometer with a green light filter. Calibration curves were constructed in a coordinate system in which the amount by weight of aluminum in mg per 50 ml of the solution subjected to nephelometry was plotted on the horizontal axis, and the relative optical density of the solution on the vertical axis; the latter was understood as the negative logarithm of the light transmission, taken to base 10.

Our experiments showed that a more stable and uniform suspension of cryolite is obtained when a solution of an aluminum salt is added to the sodium fluoride solution, and not the reverse.

The use of a 1% solution of starch or gelatin as protective colloids to stabilize the suspension of 11NaF·4AlF₃ worsens the reproducibility of the nephelometric results. Ethyl alcohol, although it considerably lowers the solubility of cryolite, makes the suspension less stable, which leads to noticeable fluctuations in the relative optical density of solutions containing the same amounts of aluminum. The presence of small amounts of iron in the sulfate medium (up to 6% of the aluminum content) does not noticeably change the relative optical density of the solutions. Replacing aluminum sulfate by aluminum nitrate or chloride has no effect on the nephelometer readings.

Fig. 1. Calibration curve for the nephelometric determination of aluminum in the form of cryolite.

On the basis of the experiments carried out, we recommend the following procedure for constructing the calibration curve and for determining aluminum when the latter is present in amounts from 5 to 25 mg per 50 ml of the solution subjected to nephelometry. Into 50-ml volumetric flasks, 15 ml portions of a 3% sodium fluoride solution are introduced; to these is added a definite volume of the standard aluminum sulfate solution, after which the solution is made up to the mark with bidistilled water, mixed thoroughly for 1–2 minutes, and subjected to nephelometry with the green “ZS-1” light filter, scatterer No. 4, and an unchanged reading on the left drum of 0.5. Figure 1 gives the calibration curve obtained by us by this procedure. Table 1 gives the re—

[[unclear: beginning of sentence]] results of several control determinations of aluminum in the absence of iron and in its presence in amounts up to 6% of the aluminum content in the solution being nephelometrically measured.

Table 1

The applicability of the proposed procedure to the determination of aluminum in certain rocks was checked by us on two samples of sandy rocks, in which the iron content did not exceed 0.4%, while gravimetric determination of aluminum showed 1.37% Al in one sample and 2.30% in the other. Nephelometric determination of aluminum after decomposition of the weighed sample by fusion with soda, separation of silicic acid by dehydration, precipitation of sesquioxides, dissolution of them in hydrochloric acid, evaporation of the solution, and neutralization of the excess acid with ammonia to a weakly acidic medium showed 1.35% Al in the first sample and 2.22% in the second.

Conclusions

A procedure has been developed for the nephelometric determination of aluminum in the form of cryolite when its content in the solution being nephelometrically measured is from 5 to 25 mg. The error of the determination does not exceed 5%. Small amounts of iron in the presence of sulfate ions do not interfere with the determination.

LITERATURE

1. Tananaev I. V., Talipov Sh. T. Izv. AN SSSR, 547, 1938.
2. Tananaev I. V., Ablov S. G. Zhurn. prikladnoi khimii, XV, 1–2, 66, 1942.
3. Tananaev I. V., Lel’chuk Yu. L. Doklady AN SSSR, 3, 118, 1943.
4. Tananaev I. V., Lel’chuk Yu. L. Zhurn. anal. khimii, 2, 93, 1947.
5. Tananaev I. V., Levina M. I. Zavodskaya laboratoriya, XI, 9, 804, 1945.
6. Tananaev I. V., Yakovlev P. Ya. Zavodskaya laboratoriya, XVI, 10, 1149, 1950.
7. Tananaev I. V. Zhurn. prikladnoi khimii, VII, 5, 729, 1934.