RU2712920C1 - Cadmium-selective electrode - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: present invention relates to electrochemical analysis methods, particularly to production of an ion-selective octadecylamine electrode for cadmium ions determination. Optimization of membrane composition of ion-selective electrode is carried out by changing mass ratios of membrane components. According to the invention, a membrane composition of an ion-selective electrode for determining cadmium ions, comprising an electrode-active component (EAC), plasticizer, polyvinyl chloride (PVC) and lipophilic additive, wherein electrode-active component (EAC) used is octadecylamine (ODA) and the composition has the following ratio of components: electrode-active component – 0.98 %, o-nitrophenyloctyl ether – 65.01 %, polyvinyl chloride – 33.01 %. To increase membrane selectivity to cadmium ions 0.1 M NaF is introduced into the analyzed solution.

EFFECT: according to the invention, the selectivity and life time of the disclosed electrode with a liquid membrane are improved to determine cadmium ions in the presence of certain nions and cations of heavy metals.

1 cl, 1 dwg, 3 tbl, 1 ex

Description

The invention relates to the field of electrochemical methods of analysis, in particular to devices for determining the activity of cadmium ions in solutions, in particular to electrodes intended for use in solutions of complex composition and is used both in the analysis of industrial wastewater and in environmental monitoring of aquatic ecosystems as well as food.

The invention can also be applied to the field of analysis of the ionic composition of aqueous solutions and liquids and can be used in the search for materials mechanically and chemically stable in solutions of complex composition with a high ionic background, intended for use as sensitive and stable elements of ion-selective electrodes (chemical sensors) for quantitative determination of the concentration of cadmium ions in aqueous solutions.

Known ion-selective electrodes with homogeneous and heterogeneous solid membranes fixed in the housing [Nikolsky BP, Materova EA Ion-selective electrodes. - L: Chemistry, 1980, p. 206], which are divided into the following groups.

1. Crystalline membrane electrodes are ion-selective electrodes in which the membrane is a crystalline material made from a single substance or a homogenized mixture. For example, a silver sulfide electrode manufactured by Orion includes a compressed polycrystalline Ag ₂ S membrane, and Crytur manufactured a single crystal Ag ₂ S. Membranes for copper, lead, and cadmium polycrystalline electrodes are obtained, as a rule, from sulfide mixtures silver and sulfide (chalcogenide) of the corresponding metal [Nikolsky BP, Materova EA Ion-selective electrodes-L .: Chemistry, 1980, p. 97-115].

A disadvantage of the known electrode is its relatively high cost associated with the use of a coarse mixture of cadmium and silver salts.

2. A cadmium-selective electrode containing a sensitive element, characterized in that as a sensitive element, a composite electrically conductive material containing ultrafine cadmium particles in a polymer matrix is used, obtained by chemical reduction of cadmium from solutions of its salts on the surface of a thermoplastic polymer granules, followed by hot by pressing. The disadvantage of the electrode is the lack of data on the effect of various d-elements on the electrode function of the cadmium-selective electrode [RF Patent No. 2498287 "Cadmium-selective electrode", priority 02/10/2011. Authors: Oboturova NP, Kornilov D.Yu., Khoroshilova S.E.].

The disadvantages of the above electrodes with solid membranes are the difficulty of manufacturing, the inability to restore the membrane after poisoning, as well as the use of salts of precious metals.

3. Legin A.V., Kirsanov D.O. et al., to determine cadmium ions, they use, as an electrode active component (EAA), 2,2'-dipyridyl-6,6-dicarboxylic acid diamide of the plasticizer, ortho-nitrophenyl octyl ether (ONFOE) is used, and cobalt chlorinated dicarbolide is used as a lipophilic additive ( CDK) and the composition has the following ratio of components: electrode-active component - 1.0-5.0% (by weight of the mixture plasticizer + PVC); lipophilic additive 0.1-2.5% (by weight of the mixture plasticizer + PVC), plasticizer + polyvinyl chloride (PVC), in a ratio of 2: 1 - up to 100% of the total membrane weight. [RF patent No. 2428683 "The composition of the membrane and onoselective electrode for the determination of cadmium ions", priority 05.27.2010. Authors: Legin A. V., Kirsanov D. O., Alyapyshev M. Yu. Et al.].

Also known are ion-selective electrodes for determining cadmium ions, based on the use of various ionophores [A. C. Stevens, H. Freiser, Anal. Chim. Acta 248 (1991) 315.; M.H. Mashhadizadeh, I. Sheikh-shoaie, S. Saeid-Nia, Electroanalysis 17 (2005) 648. A.C. Ion, E. Bakker, E. Pretsch, Anal. Chim.Acta 440 (2001).].

The disadvantage of these electrodes is the insufficiently high selectivity to cadmium ions in the presence of other transition metals, in particular silver, copper, mercury, iron, insufficient selectivity in the presence of sodium ions (selectivity not better than 10 ^-3 ), as well as low chemical stability of materials, not allowing measurements of cadmium in acidic environments.

4. The closest to the claimed are a liquid electrode [V.K. Gupta, M. Al. Khayat, A.K. Singha, M.K. PalNanoleveldetec-tionofCd (II) usingpoly (vinylchloride) basedmembranesofSchiffbasesAnalyti-caChimicaActa 634 (2009) 36-43]

Known membrane cadmium selective electrode [VK Gupta, M.Al. Khayat, AK Singha, MK PalNanoleveldetectionofCd (II) usingpo-ly (vinylchloride) basedmembranesofSchiffbasesAnalyticaChimicaActa 634 (2009) 36-43], intended for use as a sensitive element of a cadmium selective electrode, which is closest to the proposed invention. In the known composition of the membrane, in order to increase the selectivity of the membrane to cadmium ions in the presence of some heavy metals, the composition of the membrane includes the electroactive component, which are used, Schiff bases, namely 2.2 '- (1Z, 1'Z) - (1E , 1'E) - (1,2-phenylenebis (methan-1-yl-1-ylidene)) bis (azaan-1-yl-1-ylidene) bis (methylene) bis (azaan-1-yl-1- ylidene) bis (methan-1-yl-1-ylidene) diphenol (L1) and 4.4 '- (1E, 1'E) - (butan-1,4-diylbis (azaan-1-yl-1-ylidene) ) bis (methan-1-yl-1-ylidene) dinaft-1-ol (L2). The use of a membrane of this composition can improve the selectivity (coe selectivity coefficient in the presence of most metal ions up to 10 ^-3 -10 ^-4 ) and to achieve a fast sensor response to cadmium ions in the pH range from 2 to 8.5. The disadvantage of the known membrane composition is the insufficient selectivity in the presence of sodium ions, often found in real environments, in particular in sea water, in a high concentration (about 0.5 mol / l), which does not allow the use of an electrode (membrane) in sea water, hydrolysis at pH 6-8.5, as well as insufficient chemical stability such electro s and the impossibility of working with them at a pH below 2.

The objective of the invention is to increase the selectivity and lifetime of the proposed electrode with a liquid membrane for the determination of cadmium ions in the presence of some anions and cations of heavy metals.

The problem is solved by the creation of an ion-selective electrode for the determination of cadmium ions, including a body and a membrane based on octadecylamine (ODE).

Example: 1.

An ion-selective electrode membrane was prepared by dissolving selected weighed samples of all starting materials of the highest possible purity (polyvinyl chloride, plasticizer, electrode-active component, lipophilic additive) in a freshly distilled organic solvent, such as acetone, followed by thorough mixing until the solution was completely homogenized.

The ionophore was selected by determining the lipophilicity values of various nitrogen- and sulfur-containing organic compounds. As is known, the high lipophilicity of the ionophore of a PVC plasticized membrane limits its release into solution, which contributes to the completeness of the electrode function, as well as the long life of the ISE.

The technical result of the invention includes a significant increase in membrane properties: stability, selectivity (selectivity) to cadmium ions in the presence of certain anions and cations, in particular halides, d-elements, as well as an increase in the selectivity to detectable cadmium ions in the presence of a 0.1 M NaI solution .

The specified technical result is achieved in that ortho-nitrophenyl octyl ether (o-NFOE) is used as a plasticizer, and octadecylamine (ODA) is used as an EAA and lipophilic additive, with the following structure:

а в качестве пластификатора о- ОНФОЭ.

and as a plasticizer, o-ONFOE.

The inventive membrane composition of the ion-selective electrode for the determination of cadmium ions, containing an electrode active component (EAA), a plasticizer, polyvinyl chloride (PVC) and a lipophilic additive, as an EAA contains octadecylamine ODA and has the following ratio of components: electrode active component 1.0% (by weight of the electrode plasticizer + PVC mixtures); lipophilic additive 0.1-2.5% (by weight of the mixture plasticizer + PVC), plasticizer + polyvinyl chloride (PVC) in a ratio of 2: 1 - up to 100% of the total membrane weight.

The process of preparation of the membrane consisted in the initial dissolution of the electrode-active substance in the plasticizer. Then PVC is added, while the plasticizer envelops the particles of the PVC powder, preventing them from sticking together when dissolved. The solvent tetrahydrofuran (THF) is added to the prepared mass. The bucks are tightly corked with a lid and shaken vigorously until the mixture components are completely dissolved.

In the preparation of membrane compositions, 2 ml portions were taken and poured into borosilicate glass rings with a diameter of 28, 24 mm and left for 1-2 days until THF was completely dissolved. To avoid rapid evaporation of the latter, Teflon cups were covered with filter paper.

Production of electrodes: membrane disks were cut out from the prepared “mother” membrane with a cork drill, which were then glued to the ends of the PVC tubes. Glue is a 13% (by weight) solution of PVC in tetrahydrofuran. Electrodes made in this way were left for a day until the tetrahydrofuran was completely evaporated. As the inner solution was poured a solution of 0.1 M [CdI _3] ^- and the resulting electrodes were immersed in a 0.1 M solution of [CdI _3] ^- to soak for 3-5 days, changing the solvent every day to remove native plasticizer (if present) to soften and tube swelling.

Next, a membrane composition was prepared by dissolving the electrode-active component in a plasticizer. By experimental selection of the optimal ratios of plasticizer and acetone, which served as a solvent for EAA, it was found that the optimal solvent volume is 1.5 ml. The resulting composition was applied to a soft PVC tube and left to dry completely.

5 cm long PVC tubes (cells) were cut out, the finished membrane was lowered there, and the ends were sealed on both sides with a previously prepared solid polymer, which was prepared by dissolving PVC in tetrahydrofuran. A central hole was made on the cells and non-plasticized PVC tubes were sealed there, through which the solution must pass in order to fill it from the inside.

Before starting the measurements the membrane was kept in 0.1 M solution of [CdI _3] ^- within a few days. The solution was poured into the internal and external cavity of the cell and washed periodically.

It is well known that not only the nature of the ionophore, but also the membrane composition has a significant effect on the sensitivity, as well as the selectivity and linearity of ISE. In order to optimize the composition of the membranes of the cadmium selective electrode, membranes were made by varying the amount of ionophore. The composition of the membranes is presented in table 1.

The electrodes numbered 3 with an octadecylamine concentration of 50 mM showed the steepness of the electrode function, which is close to the theoretical value for a singly charged ion. Based on the optimized composition of the cadmium selective electrode membrane, a flow cell was manufactured, the membrane of which has the composition (in wt.%): ODA - 0.98; o-NFEE-65.01; PVC-33.01.

The effect of the background electrolyte on the electrode potential-cadmium selective electrode with an optimized membrane composition was studied (Table 2.)

The linear range of Cd-SE functioning is 7.1 × 10 ^-3 -7.8 × 10 ^-5 M. The lower detection limit (1 in the figure) of cadmium ions was 7.8 × 10 ^-5 (8.74 μg / ml) of FIG. 1. The steepness of the electrode function is -59.8 mV / dec., The response time is 15-20 s.

The selectivity of the electrode with respect to some cations and anions accompanying in the studied object was studied. By the method of bionic potentials, the potentiometric selectivity coefficients of Cd-SC are determined. The interfering effect of the tested heavy metals forming iodide complexes in the objects of analysis was eliminated by the introduction of 0.1 M NaF. The table shows potentiometric coefficients before masking the interfering components (K ^{sweat, 1} ) and after masking (K ^{sweat, 2} ) - Table 3.

Thus, the potentiometric selectivity coefficient for anions and cations in the presence of 0.1 M NaF varies in the range 1⋅10 ^-3 -9⋅10 ^-3 , which confirms the high selectivity with respect to cadmium ions.

The authors:

1. Tataeva Sarizhat Dzhabrailovna - candidate of chemical sciences, prof. DGU

Date of birth: 07.25.1950

Tel: 89064472100

2. Magomedov Kurban Eduardovich - research engineer, DGU.

Date of birth: 12.12.1987

Tel: 89604202827

Claims (2)

1. The composition of the membrane of the ion-selective electrode for determining cadmium ions containing an electrode-active component (EAA), a plasticizer, polyvinyl chloride (PVC) and a lipophilic additive, characterized in that octadecylamine (ODA) is used as the electrode-active component (EAA) and the composition has the following ratio of components : electrode active component — 0.98%, o-nitrophenyl octyl ether — 65.01%, polyvinyl chloride — 33.01%. 2. The composition according to p. 1, characterized in that octadecylamine in cationic form with triiodocadmatanion forms an ionic associate, and interfering cations of proven heavy metals forming iodide complexes in the objects of analysis are eliminated by the introduction of 0.1 M NaF.

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