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

Vol. 163                    1970

Application of Anion Exchange for the Separation, Preconcentration, and Determination of Trace Iron in Certain Phosphor Production Materials

Yu. L. Lelchuk, R. D. Glukhovskaya

(Presented by the scientific seminar of the Chemical-Technological Faculty)

Recently, methods based on the use of anion exchangers have begun to be introduced into the practice of ion-exchange chromatography. Extensive investigations in this field were carried out by Kraus and co-workers [1–6]. The principal advantage of anion exchangers is their stability at high concentrations of solutions of many electrolytes. The stability of anion exchangers in concentrated acid solutions makes it possible to use them for the separation of metals that are close in their chemical properties and that form, in these solutions, complex anions with markedly different instability constants.

One of the variants of anion exchange of interest for analytical chemistry is the selective sorption and desorption of halide complexes of many metals at various concentrations of HCl. It has been established that almost all metals of the periodic system, with the exception of the alkali, alkaline-earth, and certain others (aluminum, nickel, thorium), are sorbed on the strongly basic anion exchanger Dowex-1 from HCl solutions of various concentrations, from 1 to 12 mol/l [1]. In this case, separation of metal ions is based on differences in the stability of their chloride complexes. Desorption and selective elution of metals in most cases are carried out with the same hydrochloric acid, only at a lower concentration.

The aim of the present work is to study the optimal conditions for the application of anion exchange to the separation and preconcentration of traces of iron and to achieve, on this basis, a considerable increase in the sensitivity of their photometric determination in various materials.

The sorption of chloride complexes of iron on the strongly basic anion exchanger Dowex-1 was first studied by Kraus and Moore [2]. They showed that trivalent iron is strongly sorbed from concentrated HCl solutions (the volume distribution coefficient at a hydrochloric acid concentration of 9 mol/l is 5900). Subsequently, the authors [3] proposed a method for separating iron, zinc, cobalt, and manganese based on the absorption of the chloride complexes of all these metals from a 12-molar HCl solution and the subsequent elution of manganese with a 6-molar solution, cobalt with a 4-molar solution, copper with a 2.5-molar solution, iron with a 0.5-molar solution, and zinc with a 0.005-molar solution of the same acid. Later, similar methods were developed for the separation of iron and manganese, iron and aluminum [7], iron—

iron and protactinium [5], iron and aluminum in sulfate solutions [6], iron, copper, and zinc [9]. Variants are also known for separating metals using, instead of hydrochloric acid, lithium chloride [5], aluminum chloride [8], and hydrofluoric acid [4]. In all these studies, sorption of metal complexes was carried out on strongly basic anion exchangers Dowex-1, Dowex-2, Permutit ES, and on the medium-basic anion exchanger L-150. As for the sorption of iron complexes on weakly basic anion exchangers, this question has been far from sufficiently studied. Meanwhile, most domestically produced anion exchangers possess weakly basic properties.

In our work we used the weakly basic anion exchanger AN-2F. We studied the conditions of sorption and desorption of chloride complexes of divalent and trivalent iron on this anion exchanger in order to determine the optimum conditions for separating and concentrating iron, with its subsequent photometric determination in the form of a complex with $\alpha,\alpha'$-dipyridyl, and determined the exchange capacity of the anion exchanger for iron.

The literature contains published studies on the use of the anion exchangers AN-2F and TM for the separation of small amounts of iron, lead, cobalt, nickel, phosphorus, and copper [10, 11], and of the anion exchanger TM for the separation of small amounts of iron, bismuth, lead, tin, zinc, and copper [12]. However, these studies contain no detailed information on the behavior of iron with respect to these anion exchangers. Sorption of iron on the anion exchanger AN-2F has also been studied by other authors, but some [13] did not obtain stable results, while others [9] were interested in the behavior of iron in saturated electrolyte solutions at low hydrochloric acid concentrations.

Preparation of the Chromatographic Column

The commercial anion exchanger AN-2F contains appreciable amounts of iron, and its use for separating traces of iron from various analytical objects for the purpose of concentration and their subsequent photometric determination is possible only after preliminary purification. It must have a definite degree of dispersion and be charged in the chloride form.

The anion exchanger was ground in an agate mortar, sieved through screens, and a fraction with grain size from 0.1 to 0.25 mm was selected for the column. The selected fraction was washed several times with distilled water and soaked for 24 hours in a concentrated sodium chloride solution. The anion exchanger was then washed 2–3 times with water and repeatedly with hydrochloric acid (1:4) until the wash waters gave a negative reaction for iron with ammonium rhodanide.

The anion exchanger freed from iron was washed with water to pH 5–6, once with a concentrated NaOH solution, and again with water to pH 7–8. This anion exchanger was used to fill a column 20–25 cm high and 0.8–1 cm in diameter, with an anion-exchanger bed height of 12–13 cm. Under the anion-exchanger layer a plug of glass wool, washed with hydrochloric acid, was placed. To charge the anion exchanger in the chloride form, 8–10 ml of especially pure 8 N hydrochloric acid was passed through the column at a rate of 2–3 ml/min.

Study of the Sorption and Desorption of Iron

In all our experiments we used especially pure hydrochloric acid, practically completely purified from traces of iron and other metals [14], all other reagents of chemically pure grade, and triply distilled water. In the final distillation, the water was distilled from a quartz apparatus.

The initial standard solution of trivalent iron, with a concentration of 100 µg of iron per 1 ml, was prepared from pure iron wire containing 99.77% iron. A weighed portion of the wire in a quartz beaker

dissolved with gentle heating in hydrochloric acid; the iron was oxidized to the trivalent state with perhydrol, the excess \(H_2O_2\) and traces of chlorine were removed by heating, and the solution was brought to the required volume with water in a volumetric flask. A working standard iron solution containing \(1\ \mu\text{g}\) of iron in \(1\ \text{ml}\) was prepared from the original standard solution immediately before use.

A standard solution of divalent iron was prepared from Mohr’s salt purified by recrystallization.

Iron in the initial hydrochloric acid and in eluates after the chromatographic column was determined photometrically as the complex with \(\alpha,\alpha'\)-dipyridyl, according to a procedure developed by us earlier [15]. To maintain the pH of the solution being photometered within the range 3.3–3.5, an acetate buffer mixture was used; to reduce the iron to the divalent state before binding it into the complex with \(\alpha,\alpha'\)-dipyridyl, a 10% solution of hydroxylamine hydrochloride was used.

In our first experiments we established the complete absence of iron sorption when a 0.1 N solution of specially pure HCl with a definite addition of standard iron solution was passed through AN-2F, and demonstrated the possibility of using this solution for quantitative elution of sorbed iron from the column.

In our subsequent experiments, in order to study the effect of HCl concentration on iron sorption, each time \(10\ \text{ml}\) of a solution of specially pure hydrochloric acid of the given concentration, with the addition of \(1\ \mu\text{g}\) of iron, was passed through the chromatographic column at a rate of \(2\text{–}3\ \text{ml}/\text{min}\). Before passing the solutions through, the column was washed with \(40\text{–}50\ \text{ml}\) of HCl of the same concentration without iron; after passage, with \(20\text{–}25\ \text{ml}\) of the same solution. Then, for elution of the iron, a 0.1 N HCl solution was passed through the column at the same rate. As it passed through, the color of the anion exchanger changed from dark brown for concentrated HCl to brick-red for 0.1 N HCl.

When only a thin dark-brown ring remained in the lower layer of the anion exchanger, the stopcock was closed, the lower end of the column was rinsed with water, and then \(8\text{–}10\ \text{ml}\) of eluate was collected in a \(25\ \text{ml}\) volumetric flask for photometric determination of the iron in it.

To the eluate were added 2–3 drops of phenolphthalein, concentrated ammonia until a faint pink color appeared, several drops of HCl until the solution was decolorized, \(5\ \text{ml}\) of acetate buffer solution, \(1\ \text{ml}\) of hydroxylamine hydrochloride, and the mixture was stirred and heated to \(50\text{–}60^\circ\). Then \(0.5\ \text{ml}\) of a 2% solution of \(\alpha,\alpha'\)-dipyridyl was added, the solution was mixed and allowed to stand for 15–20 min. After this the solution was diluted to the mark with water, and its optical density was measured on an FEKN-57 photoelectric colorimeter-nephelometer with a layer thickness of \(50\ \text{mm}\) and green light filter No. 4, with a transmittance maximum at \(508\ \text{m}\mu\). Triply distilled water served as the reference solution.

Simultaneously with preparation of the eluate for photometry, a blank sample was also prepared; its optical density was measured, and the necessary correction for the content of traces of iron in the reagents was introduced.

The calibration graph was constructed by the same procedure.

Table 1 and Fig. 1 present the data obtained by us on two chromatographic columns on the dependence of the sorption of \(Fe^{2+}\) and \(Fe^{3+}\) on the concentration of HCl. These data show that quantitative sorption of \(Fe^{2+}\) occurs from 8–9 N acid, and of \(Fe^{3+}\) from 6–10 N acid. At HCl concentrations below 3 N, both divalent and trivalent iron are not retained by the anion exchanger. On the basis of these data, in the method we developed for separation and concentration of iron for its photometric determination in hydrochloric acid, in phosphor L-34 and in materials used ...

used for its production, it is recommended that, before passage through AN-2F, the solution to be analyzed be brought to a concentration of about 8–9 N in HCl, and that the sorbed iron be eluted with a 0.1 N solution of this acid.

Table 1

Sorption of iron on AN-2F from HCl solutions.
In all experiments, 1 µg of iron was added to the acid of each concentration

Figure 2 gives the results of experiments to determine the amount of 0.1 N HCl required for complete elution of the sorbed iron. In these experiments, 0.1 N HCl was passed through the anion exchanger with 5 µg of sorbed iron at a rate of 2–3 ml/min. The eluate was collected in 3-ml portions. The graph shows the iron content in each portion of eluate. The first 6 ml of acid do not wash out the iron complex. Under their action there occurs only movement of the sorbed iron toward the lower layer of the anion exchanger. The next 6–8 ml portion practically completely elutes the iron.

Fig. 1. Sorption of iron on the anion exchanger AN-2F: 1—Fe³⁺, 2—Fe²⁺

Fig. 2. Elution of iron with 0.1 N HCl solution: 1—Fe³⁺, 2—Fe²⁺. Amount of iron sorbed on AN-2F: 5 µg

Table 2 gives the data we obtained in checking the accuracy of the proposed method for the separation, preconcentration, and determination of traces of iron using a standard iron solution. For comparison

Table 2

Checking the accuracy of the method using a standard iron solution

data were obtained by replacing chromatographic preconcentration of iron with removal of HCl by evaporation at 90–95°. As is evident from these data, the relative error of the determination does not exceed ±3%.

The experiments carried out showed that any concentrations of Ca ions and \(\mathrm{PO_4^{\prime\prime\prime}}\) do not interfere with the separation and preconcentration of iron on AN-2F and its subsequent photometric determination in calcium carbonate and ammonium and calcium hydrogen phosphates, since neither Ca cations nor \(\mathrm{PO_4^{\prime\prime\prime}}\) anions are sorbed by the anion exchanger at 8–9 N concentration of the solution with respect to HCl. Figure 3 gives the sorption curve of phosphate ions. Calcium cations are not sorbed by the anion exchanger at any HCl concentration up to 10N.

Fig. 3. Sorption of phosphate ion on the anion exchanger AN-2F from HCl solutions. Through the column, with each portion of a given HCl concentration, 0.18 g of \(\mathrm{PO_4^{\prime\prime\prime}}\) was passed.

Experiments on determining the exchange capacity of AN-2F showed that the dynamic exchange capacity for \(\mathrm{Fe^{2+}}\) in 8N HCl is 0.030 mg per 1 g of dry anion exchanger; for \(\mathrm{Fe^{3+}}\) it is 0.720 mg/g; in 9N HCl, both for \(\mathrm{Fe^{2+}}\) and for \(\mathrm{Fe^{3+}}\), it is 0.075 mg/g. The static exchange capacity in 8N HCl for \(\mathrm{Fe^{2+}}\) is 0.52 mg/g, for \(\mathrm{Fe^{3+}}\) 1.12 mg/g; in 9N HCl for \(\mathrm{Fe^{2+}}\) it is 1.12 mg/g, and for \(\mathrm{Fe^{3+}}\) 2.00 mg/g.

In a subsequent communication we shall present the methods we have developed for determining traces of iron in certain objects, where a significant increase in the sensitivity of the determination is achieved by separating and concentrating iron on AN-2F. Student A. N. Artsimovich took part in carrying out the experimental work.

Conclusions

The sorption and desorption of iron on the weakly basic anion exchanger AN-2F in the Cl-form have been studied. It has been shown that quantitative sorption of iron is achieved from an 8–9 N HCl solution, and desorption—with a 0.1 N solution of the same acid. On the basis of the data obtained, a method has been developed for separating and concentrating iron when determining it in a number of objects from phosphor production. The developed method makes it possible to substantially increase the sensitivity of determining traces of iron in these objects.

References

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15. Yu. L. Lelchuk, P. V. Kristalev, L. L. Skripova, L. B. Kristaleva. Chemical Reagents Industry. IREA Information Bulletin, No. 4, 109, 1964.