RU2374638C1 - Method of detecting cobalt (ii) - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method involves preparation of a sorbent and cobalt (II) solution, extraction of cobalt (II) from the solution using the sorbent and its detection in a complex compound on the surface of the sorbent, measurement of colour intensity of the surface cobalt (II) complex and determination of cobalt (II) content, where the sorbent used is silica which is successively modified with polyhexamethylene guanidine and nitroso-R-salt, and colour intensity is determined from value of diffuse reflection factor at 510 nm.

EFFECT: increased sensitivity and faster analysis.

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Description

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

To determine cobalt in objects of different material composition and state of aggregation, the sorption-photometric method is widely used, based on the determination of an element directly in the sorbent phase, characterized by low detection limits and high selectivity due to sorption concentration of the element being determined and its separation from interfering solution components. To implement the sorption-photometric determination of cobalt, the formation of a colored cobalt compound with functional groups of the sorbent and the dependence of the color intensity of the sorbent on the concentration of cobalt on its surface are necessary.

There is a method of sorption-photometric determination of cobalt (Ivanov V.M., Kuznetsova O.V. Immobilized 4- (2-thiazolylazo) resorcinol as an analytical reagent. Test reactions for cobalt, palladium and uranium (IV) // Journal of Analytical Chemistry. 1995. T. 50. No. 5. P.498-504], based on the separation of cobalt from solutions by a sorbent - silica gel chemically modified with 4- (2-thiazolylazo) resorcinol (SG-TAP) and its determination directly on the surface of the sorbent by the photometric method .

The method involves the following operations:

- a cobalt (II) solution was introduced into a graduated tube with a capacity of 15 ml;

- 5 ml of buffer solution was added to create a pH of 6.2 and water to a total volume of 15 ml;

- 0.3 g of sorbent SG-TAP was added and mixed vigorously;

- the sorbent was separated from the solution by decantation, dried in air, and the cobalt content on the surface of the sorbent was determined by the photometric method.

The linearity range of the calibration graph is maintained in the range of 2-15 μg cobalt in 15 ml or, accordingly, 2-15 μg per 0.3 g of sorbent.

The disadvantages of the method include a high detection limit, a narrow range of detectable contents.

Closest to the proposed method in technical essence and the achieved results is a method for determining cobalt using silica chemically modified with triphenylphosphonium groups [Dyachenko NA, Trofimchuk AK, Sukhan VV Sorption of cobalt in the form of a complex with nitroso-P-salt of silica gel with grafted triphenylphosphonium groups and its subsequent determination in the sorbent phase // Journal of Analytical Chemistry. 2002.V. 57. No. 11. S.1202-1205 (prototype)], providing for the following operations:

- introduction into a solution containing cobalt of a reagent - nitroso-R-salt;

- bringing the pH of the solution to 7-8.5 using KOH;

- the addition of silica modified with triphenylphosphonium groups;

- sorption for 15 minutes;

- separation, washing and drying of the sorbent;

- measurement of the diffuse reflection coefficient at 520 nm.

This method will allow you to determine the concentration of cobalt in the range of 0.5-10 μg in 200 ml using a weighed sorbent of 0.3 g. The lower limit of the determined cobalt contents is 2.5 μg / L.

The disadvantages of the method include a narrow range of detectable contents, multi-stage determination, a narrow pH range, inaccessibility of the proposed sorbent - silica, chemically modified with triphenylphosphonium groups.

The technical result is to reduce the detection limit, expanding the range of detectable contents.

The specified technical result is achieved by the fact that in the method for determining cobalt (II), which includes preparing the sorbent, cobalt (II) solution, extracting cobalt (II) from the solution by the sorbent and converting it into a complex compound on the surface of the sorbent, measuring the color of the surface complex of cobalt (II ) and determination of cobalt (II) content, new is that silica, sequentially modified with polyhexamethylene guanidine and 1-nitroso-2-naphthol-3,6-disulfonic acid (nitroso-P salt), is used as the sorbent the color intensity is estimated by the value of the coefficient of diffuse reflection at 510 nm.

The essence of the method lies in the fact that cobalt (II), which is in solution in the pH range of 5-9, is quantitatively (the degree of extraction is 99%) extracted with silica sequentially modified polyhexamethylene guanidine and nitroso-P-salt. 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 nitroso-P salt allows it to be firmly fixed on the surface of silica-modified polyguanidine due to the electrostatic interaction of sulfo groups of the reagent with protonated amino groups of polyhexamethylene guanidine, which are not involved in the formation of hydrogen bonds with surface silanol groups of silica.

Sorption of cobalt (II) in the static mode proceeds quickly - the time to establish sorption equilibrium does not exceed 5 minutes. In the process of complexation of cobalt (II) with a nitroso-P salt, a complex colored in red-brown is formed on the surface of the sorbent, having a maximum in the absorption spectrum at 510 nm.

To 10 g of silica was added 100 ml of a 1% solution of polyhexamethylene guanidine, stirred for 5 minutes, the silica was separated from the solution by decantation and washed with distilled water. Then, silica treated with polyhexamethylene guanidine is treated with a 0.001% solution of nitroso-P salt in water, stirred vigorously for 5 minutes, the sorbent is separated from the solution by decantation, washed with distilled water, and dried in air.

Sorbent-silica modified with polyhexamethylene guanidine and nitroso-P salt is added to the test solution with a pH of 5–9, containing cobalt (II), intensively mixed for 5 min, the sorbent is separated from the solution by decantation, placed in a fluoroplastic cell and the diffuse coefficient is measured reflection at 510 nm. The cobalt content is found according to the calibration graph built in the conditions of determination. The detection limit is 0.05 μg cobalt per 0.1 g of sorbent.

This amount of cobalt 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 cobalt (static or dynamic sorption mode). The use of a dynamic sorption regime allows one to concentrate cobalt from large volumes of solutions. So, when sorption of cobalt from 10 ml of a solution, the relative detection limit of cobalt is 5 · 10 ^-3 μg / ml, and when sorption from 1 liter of solution is 5 · 10 ^-5 μg / ml, or 5 · 10 ^-2 μg / ml, which is almost 50 times less than the detection limit achieved by the prototype method. Thus, the minimum cobalt content, determined by the proposed method, in an arbitrary volume of the solution should be at least 0.05 μg. The linearity of the calibration graph is maintained up to 5 μg per 0.1 g of sorbent.

Example 1 (prototype). In a solution with a pH of 7-8.5 containing 5 μg of cobalt (II), 1 ml of a 2.6 · 10 ^-2 M solution of nitroso-P-salt is poured, a pH of 7.0-8.5 is created with a NaOH solution, a sorbent is introduced - silica chemically modified with triphenylphosphonium groups, intensively stirred for 15 minutes The sorbent is separated from the solution by decantation, transferred to a fluoroplastic cuvette, dried to an air-dry state and the diffuse reflection coefficient is measured at 520 nm. The cobalt content is found according to the calibration graph built in similar conditions. Found 4.8 ± 0.3 μg.

Example 2 (the proposed method). To 10 ml of a solution with a pH of 5–9, containing 0.1 μg of cobalt, a sorbent - silica, sequentially modified with polyhexamethylene guanidine and nitroso-P-salt is added, intensively mixed for 5 min, the sorbent is separated from the solution by decantation, placed in a fluoroplastic cell and measure the coefficient of diffuse reflection at 510 nm. The amount of cobalt is found according to the calibration graph built in similar conditions. Found 0.098 ± 0.005 μg.

Example 3 (the proposed method). Sorbent - silica, sequentially modified with polyhexamethylene guanidine and nitroso-P-salt is added to 10 ml of a solution with a pH of 5-9 containing 5.0 μg of cobalt, intensively mixed for 5 min, the sorbent is separated from the solution by decantation, placed in a fluoroplastic cell and measure the coefficient of diffuse reflection at 510 nm. The amount of cobalt is found according to the calibration graph built in similar conditions. Found 4.97 ± 0.05 μg.

Example 4 (the proposed method). 500 ml of an aqueous solution with a pH of 5-9 containing 0.5 μg of cobalt is passed through a chromatographic column containing 0.1 g of sorbent at a rate of 5 ml / min. The sorbent is removed, placed in a fluoroplastic cell and the diffuse reflectance is measured at 510 nm.

The amount of cobalt is found according to the calibration graph built in similar conditions. Found 0.49 ± 0.02 mcg.

The method is characterized by high sensitivity, ease of implementation and does not require the use of expensive equipment and harmful substances and inaccessible sorbents - chemically modified silica gels.

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 gel modified with polyhexamethylene guanidine and nitroso-R salt can significantly reduce the detection limit of cobalt directly on the surface of the sorbent, expand the range of detectable concentrations in the sorbent phase in comparison with the prototype.

The increase in the color intensity of the sorbent with increasing concentration of cobalt on its surface allows you to use this method as a test method for determining cobalt, suitable for use in the field.

Claims (1)

A method for determining cobalt (II), including the preparation of a sorbent and a cobalt (II) solution, extracting cobalt (II) from a solution with a sorbent and converting it to a complex compound on the surface of the sorbent, measuring the color intensity of the surface complex of cobalt (II) and determining the content of cobalt (II ), characterized in that silica, sequentially modified with polyhexamethylene guanidine and nitroso-P salt, is used as the sorbent, and the color intensity is estimated by the value of the diffuse reflection coefficient p and 510 nm.