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

Vol. 163　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1970

PURIFICATION OF HYDROCHLORIC ACID FROM TRACES OF IRON

Yu. L. Lelchuk, R. D. Glukhovskaya

(Presented by the scientific seminar of the Faculty of Chemical Technology)

The problem of methods for determining microimpurities in high-purity materials is among the most urgent problems of modern analytical chemistry. New technology is continuously increasing the requirements for the sensitivity of these methods, and whereas quite recently methods allowing determination of \(10^{-4}\)—\(10^{-5}\%\) impurities were considered highly sensitive, now in a number of cases methods are required for determining \(10^{-6}\)—\(10^{-8}\%\) and less of impurities in certain objects. For carrying out determinations of microimpurities with such sensitivity, ordinary reagents of any grade produced by industry are unsuitable without additional careful purification. For such determinations, reagents are often needed in which the content of the elements to be determined has practically been brought to zero.

We have developed procedures for determining \(10^{-5}\)—\(10^{-7}\%\) iron in certain objects used by industry for the production of phosphors. In connection with these studies, the conditions were investigated for obtaining hydrochloric acid especially pure with respect to iron and suitable for such determinations.

According to the requirements of GOST 3118-46 [1], the maximum permissible impurities of iron in hydrochloric acid of the grade “pure” are \(5 \cdot 10^{-4}\%\), analytical grade \(1 \cdot 10^{-4}\%\), chemically pure \(5 \cdot 10^{-5}\%\); the maximum permissible impurities of heavy metals of the hydrogen sulfide group, calculated as lead, in acid of the grade “pure” are \(1 \cdot 10^{-3}\%\), analytical grade \(5 \cdot 10^{-4}\%\), chemically pure \(2 \cdot 10^{-4}\%\).

Sendel [2] considers that by distilling acid in a distillation apparatus made of Pyrex glass, a product can be obtained sufficiently pure for work connected with the determination of microimpurities. Our experiments showed that the iron content in hydrochloric acid can be reduced to \(10^{-7}\)—\(10^{-8}\%\) only by distilling it 4–5 times from a quartz apparatus.

The literature [2] also contains indications that very pure hydrochloric acid and ammonium hydroxide can be obtained by isothermal distillation of these compounds, i.e., by saturating especially pure water with vapors of HCl or NH\(_3\), evolved at ordinary temperature by commercial concentrated preparations of these compounds in a tightly closed vessel. Hydrochloric acid purified in this way is often called isopiestic.

We carried out experiments in order to determine the optimal conditions for obtaining such acid and the degree of its purity. The content of traces

iron in this acid was determined by a highly sensitive sorption–photometric method developed by us [3], based on the separation and concentration of iron in the form of the chloride complexes \([\mathrm{FeCl}_6]^{\prime\prime\prime}\) and \([\mathrm{FeCl}_6]^{\prime\prime\prime\prime}\) on the anion exchanger AN-2F in the chloro form, followed by its photometric determination in the form of the \(\mathrm{Fe}^{2+}\) complex with \(\alpha,\alpha'\)-dipyridyl.

In our experiments we adhered to the following procedure for purifying hydrochloric acid by isothermal distillation: commercial concentrated hydrochloric acid is poured, below the level of the porcelain support, into a large, thoroughly washed desiccator with a ground-glass lid. A wide polyethylene dish or 2–3 wide, low polyethylene beakers filled with water distilled three times are placed on the support. Quartz vessels may be used instead of polyethylene vessels. The desiccator is tightly closed with the lid and left standing for 2–3 days.

With such contact of especially pure water with concentrated hydrochloric acid, owing to isothermal distillation, the water is gradually saturated with hydrogen chloride vapor. The normality of the isopiestic hydrochloric acid obtained in this way is approximately equal to ten. Table 1 gives the experimental data we obtained on the dependence of the concentration of the acid obtained on the saturation time.

Commercial hydrochloric acid of any grade may be used for isothermal distillation.

Table 2 gives the data we obtained on the content of traces of iron in various samples of isopiestic hydrochloric acid.

The data presented show that the acid obtained in this way is not sufficiently pure for use in analysis in the determination of traces of iron in various objects with a sensitivity of \(10^{-6}\)—\(10^{-8}\%\).

Table 1

Dependence of the concentration of isopiestic hydrochloric acid on the saturation time

Table 2

Iron content in some samples of isopiestic hydrochloric acid

By repeated isothermal distillation, provided that the glass desiccator and the porcelain support in it are replaced by a suitable polyethylene vessel with a support made of polyethylene or fluoroplastic, purer hydrochloric acid can be obtained.

Especially pure hydrochloric acid, practically completely purified from traces of iron, was obtained by passing eight- to nine-normal isopiestic hydrochloric acid through an ion-exchange column filled with the weakly basic anion exchanger AN-2F in the Cl form, at an effluent rate of 2–3 ml/min. In this process the acid is also purified of copper,

of cobalt, manganese, zinc, and many other heavy metals of the hydrogen sulfide group.

Such chromatographic purification also makes it possible to obtain sufficiently pure hydrochloric acid, suitable for analytical work on the determination of iron microimpurities in various objects with sufficiently high sensitivity, from commercial hydrochloric acid of any grade without preliminary azeotropic distillation, if before passing it through the chromatographic column it is diluted with water to approximately eight- to nine-normal concentration and the column is filled with a sufficient amount of anion exchanger providing the required exchange capacity. Figure 1 shows a form of chromatographic column convenient for this purpose. The operation of such a column is easier to monitor, since in it the layer of anion exchanger practically always remains covered by the hydrochloric acid being purified.

Fig. 1. Chromatographic column for purifying hydrochloric acid from traces of iron

The method of chromatographic purification of hydrochloric acid is based on the properties, known from the literature [4—8], of cations of iron, cobalt, bismuth, copper, zinc, and certain other heavy metals, at the appropriate concentration of hydrochloric acid, to pass into stable complex chloride anions capable, under certain conditions, of being quantitatively sorbed by certain anion exchangers. According to the literature [4], the composition of the chloride complexes is: \([\mathrm{FeCl}_6]^{\prime\prime\prime}\), \([\mathrm{FeCl}_6]^{\prime\prime\prime\prime}\), \([\mathrm{CoCl}_6]^{\prime\prime\prime}\), \([\mathrm{BiCl}_4]^{\prime}\), \([\mathrm{CuCl}_4]^{\prime\prime}\), \([\mathrm{ZnCl}_4]^{\prime\prime}\), etc.

The use of foreign anion exchangers for purifying hydrochloric acid from iron is reported in [6]. We used the weakly basic anion exchanger AN-2F, produced by our industry. For this anion exchanger in the chloride form we studied in detail the optimum conditions for sorption and desorption of iron.

Below, in Table 3, are literature data [8] on the concentration of hydrochloric acid above which the corresponding ions of heavy metals form stable complex chloride anions. The data we obtained on sorption and desorption of iron by the AN-2F anion exchanger in the chloride form from hydrochloric acid solutions of various concentrations are described in [3].

From our data it is evident that quantitative sorption of the chloride complex of trivalent iron on the AN-2F anion exchanger begins only at a hydrochloric acid concentration of 6 N and higher. However, in determining what concentration of HCl should be passed through the anion exchanger, it is also necessary to take into account the possible presence in the acid of divalent iron and cobalt ions. The former forms a stable chloride complex only in 7.5-normal acid, the latter in 6.5-normal acid and higher.

Our data also show that, for elution and periodic purification of the chromatographic column from sorbed iron, a diluted solution of hydrochloric acid with an HCl concentration not higher than 3 N may be used. Experiments carried out by us showed that, for this purpose, a 0.1 N solution of hydrochloric acid can be used successfully.

In accordance with the data obtained, in the procedure we propose for chromatographic purification of hydrochloric acid from traces of iron, it is recommended that the concentration of hydrochloric acid passed through the column be maintained within the limits of 8–9 normal; the rate of outflow of the purified acid from the column should be approximately 2–3 ml/min; and, in order to clean the column of sorbed iron, 40–50 ml of 0.1 normal HCl solution should periodically be passed through it at the same rate.

Commercial anion exchanger contains appreciable amounts of iron. It must have a certain degree of dispersion, be purified of iron, and be charged in the chloride form.

Table 3

Concentration of HCl above which the ions of certain heavy metals form stable chloride complex anions

Preparation of the anion-exchange column. The anion exchanger AN-2F is ground in an agate mortar, sieved through sieves, and a fraction with grain size from 0.1 to 0.25 mm is selected. The selected fraction is washed several times with distilled water and soaked for 24 hours in a concentrated sodium chloride solution. The anion exchanger is then washed 2–3 times with water and repeatedly with a hot HCl solution (1:4)

until the wash waters give a negative reaction for iron with ammonium rhodanide.

The anion exchanger washed free of iron is rinsed with distilled water until the pH of the wash waters becomes approximately 5–6; it is then washed once with a concentrated NaOH solution and again with water to pH 7–8. The prepared anion exchanger is loaded into a column 25 cm high and 0.8–1 cm in diameter. The height of the anion-exchanger layer is 12–13 cm. Beneath the anion-exchanger layer is placed a plug of glass wool washed with hydrochloric acid.

For additional conversion of the anion exchanger into the chloride form, 8–10 ml of specially purified approximately 8-normal hydrochloric acid is passed through the column at a rate of 2–3 ml/min, and the column is ready for use.

Student A. N. Artsimovich took part in carrying out the experimental work.

Conclusions

The conditions for purifying hydrochloric acid from traces of iron by distillation, isothermal distillation, and chromatographically with the aid of the anion exchanger AN-2F in the chloride form have been studied. It is shown that the purest acid, practically completely purified of traces of iron, is obtained after chromatographic purification.

LITERATURE

1. Yu. V. Karyakin, I. I. Angelov. Pure Chemical Reagents. Goskhimizdat, 282, 1955.

2. E. B. Sendel. Colorimetric Methods for Determining Traces of Metals. Mir Publishing House, 23, 1964.

3. R. D. Glukhovskaya, Yu. L. Lelchuk. Application of anion exchange for the separation and concentration of traces of iron. Izvestiya TPI, vol. 163, 146, 1970.

4. V. V. Nekrasov. Course of General Chemistry. Goskhimizdat, 1960.

5. K. A. Kraus, G. E. Moore. J. Am. Chem. Soc., 72, 5792, 1950.

6. A. C. Reents, F. H. Kahler. Ind. Eng. Chem., 47, 75, 1955.

7. K. M. Saldadze, A. B. Pashkov, V. S. Titov. New High-Molecular-Weight Ion-Exchange Compounds. Goskhimizdat, 19, 1960.

8. D. Jentzsch, J. Frotscher. Z. Anal. Chem., 144, No. 1, 1, 1955.

9. V. V. Stepin, V. I. Ponosov, E. V. Silaeva. Chromatography, Its Theory and Application. Publishing House of the Academy of Sciences of the USSR, 230, 1960.