RU2374639C1 - Method of detecting iron (ii) - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method involves preparation of a sorbent and iron (II) solution, extraction of iron (II) from the solution using the sorbent and turning it into a complex compound on the sorbent surface, measurement of colour intensity of the surface iron (II) complex and determination of iron (II) content, where the sorbent used is silica which is successively modified with polyhexamethylene guanidine and 4,7-dimethyl-1,10- phenanthroline disulphonic acid, and colour intensity is determined from the value of diffuse reflection factor at 530 nm.

EFFECT: increased sensitivity and information content, and faster analysis.

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Description

The invention relates to the field of analytical chemistry of elements, namely to methods for determining iron, and can be used in its determination in geological materials, technological solutions, natural and industrial waters.

To determine iron in objects of various material composition, the photometric method is widely used and its modern version is the sorption-photometric method, which is characterized by significantly lower detection limits of elements and selectivity compared to the photometric method by combining preliminary sorption concentration and subsequent photometric determination of elements directly in phase sorbent. To realize the sorption-photometric determination of iron, the formation of a colored iron compound with functional groups of the sorbent and the presence of a dependence of the color intensity of the sorbent on the concentration of iron on its surface are necessary. For sorption-photometric determination of iron, N-heterocyclic bases are most often used as organic reagents: 2,2'-dipyridyl, 1,10-phenanthroline and its derivatives, which have the most selective properties with respect to iron.

A known method for the determination of iron in the form of a complex of iron (II) with 1,10-phenanthroline on polyurethane foams used as sorbents [S. G. Dmitrienko, L. N. Pyatkova, V. K. Runov. Optical sorption-molecular-spectroscopic analysis methods. Sorption of ionic associates of tris (1,10-phenanthroline) iron (II) polyurethane foams and its analytical application // Journal of Analytical Chemistry. 1996, T. 51, No. 6, S.600-604].

The method involves the following operations:

- preparation of the sorbent: polyurethane foams based on ethers and esters with a metal punch drive discs, to clean them of metal ions and organic compounds, they are washed in 0.1 M sulfuric acid for 30 minutes, then washed with water to pH 4-5 with acetone and dried on air;

- in the analyzed solution containing iron, add:

- 1 ml of 1.4 M hydroxylamine hydrochloride,

- 2 ml of 1 · 10 ^-2 M 1,10-phenanthroline,

- acetate-ammonia buffer with a pH of 5.8,

- 5 ml of 0.4 M sodium perchlorate and water to a total volume of 25 ml;

- make a disk of polyurethane foam;

- vigorously stirred for 30 minutes;

- the disk is removed, squeezed between sheets of filter paper, dried in air and the diffuse reflection spectra are recorded.

The detection limit is 0.005 μg of iron per disk.

The disadvantages of the method include the multi-stage and duration of the determination method, the complexity of obtaining the sorbent and reproducible results, the impossibility of sorption concentration of iron from large volumes of solutions, the inaccessibility of the proposed sorbents - polyurethane foams.

A known method for the determination of iron in the form of a complex of iron (II) with 1,10-phenanthroline on the surface of the KU-23 cation exchanger used as a sorbent [A.T. Pilipenko, A.V. Terletskaya, T.A. Bogoslovskaya. Determination of iron in water by solid-state spectroscopy with membrane filtration // Journal of Analytical Chemistry. 1990, T. 45, No. 8, S.1624-1629].

The method involves the following operations:

- 1 ml of a 4% solution of ascorbic acid is added to a solution of iron (III) to reduce iron (III) to iron (II);

- add 1 ml of a 0.28% solution of 1,10-phenanthroline;

- 0.2-0.3 ml of suspension of KU-23 cation exchanger with a grain size of 1-10 microns in water is introduced and intensively mixed;

- a suspension of the sorbent is filtered through a membrane filter;

- measure the coefficient of diffuse reflection.

The iron content is found according to the calibration graph built in similar conditions. The detection limit of iron is 0.125 μg per filter.

The disadvantage of this method is the duration and complexity of the determination procedure and a high detection limit.

Closest to the proposed method in terms of technical nature and the achieved results is a method for determining iron using composite films based on silicon oxide (O. Yu. Nadzhafova, MV Drozdova, EV Nebesnaya, VB Ishchenko. Optimization and the use of composite coatings based on silicon oxide and polyvinyl sulfonic acid for sorption spectrophotometric determination of iron (II) and zinc (II) phenanthrolinates // Journal of Analytical Chemistry. 2007, V. 62, No. 12, S.1259-1266 (prototype)], providing the following opera tion:

- film synthesis: a sol solution, the synthesis of which is carried out using acid hydrolysis of triethoxysilane (TEOS) in the presence of Tween 20 nonionic surfactant and with the addition of an aqueous solution of polyvinyl sulfonic acid (PVA) in a sol: PVA volume ratio of 1: 1, is applied to glass, which are dried in air for a day, then glass with applied films is soaked in 10 ml of a mixture (7: 3) ethanol-water;

- in 50 ml of the analyzed solution containing iron, a 0.01 M solution of 1,10-phenanthroline, acetate buffer solution with a pH of 5.0-5.5;

- into the resulting solution lower the glass coated with a film for 5-60 minutes;

- the glass is removed, rinsed with double-distilled water;

- measure the optical density of the glasses at 490 nm.

The iron content is found according to the calibration graph built in similar conditions. The estimated value of the detection limit when concentrating iron from 50 ml of the solution is 0.18 mg / L.

The disadvantages of the method include a high detection limit, the duration of the preparation of modified glasses and methods for determining iron with their use, the inability to use films in a dynamic version of sorption concentration.

The technical result is to reduce the detection limit, expanding the range of determined iron contents and shortening its determination time.

The specified technical result is achieved by the fact that in the method for determining iron (II), which includes preparing a sorbent, an iron (II) solution, extracting iron (II) from a solution by a sorbent and converting it into a complex compound on the surface of the sorbent, measuring the color intensity of the surface iron complex ( II) and determination of the iron (II) content, new is that silica, sequentially modified with polyhexamethylene guanidine and 4,7-dimethyl-1,10-phenanthroline disulfonic acid, is used as the sorbent, and The color stain is evaluated by the value of the diffuse reflection coefficient at 530 nm.

The essence of the method lies in the fact that the iron (II) in solution with a pH of 4-7 is quantitatively (the degree of extraction is 99%) is recovered by sorbent - silica sequentially modified with polyhexamethylene guanidine and 4,7-dimethyl-1,10-phenanthroline disulfonic acid. Processing silica with an aqueous solution of polyhexamethylene guanidine allows it to be firmly fixed on the surface of the silica due to the formation of hydrogen bonds between the amino groups of polyhexamethylene guanidine and surface hydroxyl groups. Subsequent treatment with an aqueous solution of 4,7-dimethyl-1,10-phenanthroline disulfonic acid allows it to be firmly fixed on the surface of silica-modified polyhexamethylene guanidine due to the electrostatic interaction of the sulfo groups of the reagent with the protonated amino groups of polyhexamethylene guanidine, which are not involved in the formation of hydrogen bonds with the surface hydroxyl (silanesole) groups. The sorption of iron (II) in the static mode proceeds quickly - the time to establish sorption equilibrium does not exceed 5 minutes. During sorption, a red complex of iron (II) with 4,7-dimethyl-1,10-phenanthroline disulfonic acid is formed on the surface of the sorbent, having a maximum in the diffuse reflection spectrum located at 530 nm.

The method is as follows.

To 10 g of silica was added 100 ml of a 1% solution of polyhexamethylene guanidine, stirred for 5 minutes, the silica gel was separated from the solution by decantation and washed with distilled water. Then, the silica treated with polyhexamethylene guanidine is treated with a 0.04% aqueous solution of 4,7-dimethyl-1,10-phenanthroline disulfonic acid, stirred vigorously for 5 minutes, the sorbent is separated from the solution by decantation, washed with distilled water, and dried in air.

In the test solution with a pH of 4-7 containing iron (III, II), add 1 ml of 0.01 M hydroxylamine solution to reduce iron (III) to iron (II), add 0.1 g of sorbent - silica, sequentially modified with polyhexamethylene guanidine and 4,7-dimethyl-1,10-phenanthroline disulfonic acid, stirred for 5 min, the sorbent was separated from the solution by decantation, placed in a fluoroplastic cuvette and the diffuse reflection coefficient was measured at 530 nm. The iron content is found according to the calibration graph built in the conditions of determination. The detection limit is 0.02 μg of iron per 0.1 g of sorbent. This amount of iron is the minimum concentration that can be recorded on this sample of sorbent on existing devices relative to the background signal, regardless of the method of concentration of iron (static or dynamic mode). The use of a dynamic sorption regime allows concentrating iron on the used sorbent mass from large volumes of solutions. So, when sorption of iron from 10 ml of the solution, the relative detection limit, iron is 2 · 10 ^-3 μg / ml, and when sorption from 1 liter of solution - 2 · 10 ^-5 μg / ml (2 · 10 ^-5 mg / l) , which is more than 1000 times less than the detection limit achieved by the method - the prototype. Thus, the absolute iron content, determined by the proposed method, in an arbitrary volume of the solution should be at least 0.02 μg. The linearity of the calibration graph is maintained up to 6 μg of iron per 0.1 g of sorbent.

Example 1 (prototype). 1 ml of acetate buffer solution, 0.5 ml of 0.01 M solution of 1,10-phenanthroline are poured into a solution containing 5.0 μg of iron (II), glass with a film is applied for 10 minutes. The glass is removed, washed with double distillate and the absorbance is measured at 490 nm. The iron content is found according to the calibration graph built in similar conditions. Found 4.7 ± 0.5 mcg.

Example 2 (the proposed method). To 10 ml of a solution with a pH of 4-7 containing 0.5 μg of iron, 1 ml of 0.1 M hydroxylamine hydrochloride is introduced, a sorbent is added - silica, sequentially modified with polyhexamethylene guanidine, and 4,7-dimethyl-1,10-phenanthroline disulfonic acid, intensively stirred for 5 min, the sorbent is separated from the solution by decantation, placed in a fluoroplastic cuvette and the diffuse reflectance at 530 nm is measured.

The amount of iron is found by the calibration graph, built in similar conditions. Found 0.51 ± 0.02 mcg.

Example 3 (the proposed method). To 10 ml of a solution with a pH of 4-7 containing 5.0 μg of iron, 1 ml of 0.1 M hydroxylamine hydrochloride is introduced, a sorbent - silica modified with polyhexamethylene guanidine and 4,7-dimethyl-1,10-phenanthroline disulfonic acid is introduced, intensively mixed in for 5 min, the sorbent is separated from the solution by decantation, placed in a fluoroplastic cell and the diffuse reflection coefficient is measured at 530 nm.

The amount of iron is found by the calibration graph, built in similar conditions. Found 4.98 ± 0.04 mcg.

Example 4 (the proposed method). In a 500 ml aqueous solution with a pH of 4-7 containing 0.5 μg of iron, 1 ml of 0.1 M hydroxylamine hydrochloride is introduced and passed through a chromatographic column containing 0.1 g of sorbent at a rate of 5 ml / min. The sorbent is taken out, placed in a fluoroplastic cell and the diffuse reflection coefficient is measured at 530 nm.

The amount of iron is found by the calibration graph, built in similar conditions. Found 0.48 ± 0.03 mcg.

The method is characterized by high sensitivity, ease of implementation and does not require the use of expensive equipment, harmful substances and inaccessible sorbents - chemically modified silicas. The simplicity of the synthesis of the sorbent makes it possible to obtain it in almost any laboratory, including in the field. The use of silica modified with polyhexamethylene guanidine and 4,7-dimethyl-1,10-phenanthroline disulfonic acid can reduce the relative detection limit of iron, expand the range of detectable concentrations in the sorbent phase in comparison with the prototype, and significantly (more than 10 times) reduce the analysis time. The dependence of the color intensity of the sorbent on the concentration of iron on its surface allows us to use this method as a test method for determining iron, including when using it in the field.

Claims (1)

A method for determining iron (II), including the preparation of a sorbent and a solution of iron (II), extracting iron (II) from a solution by a sorbent and converting it into a complex compound on the surface of the sorbent, measuring the color intensity of the surface complex of iron (II) and determining the content of iron (II ), characterized in that silica, sequentially modified with polyhexamethylene guanidine and 4,7-dimethyl-1,10-phenanthroline disulfonic acid, is used as the sorbent, and the color intensity is estimated by the diffusion coefficient th reflection at 530 nm.