RU2383014C1 - Extraction-voltamperometric method for detection of zinc, cadmium, lead and copper in natural waters - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: method consists in the fact that metals together with mercury are extracted into laminating system water-antipyrine-salicyl-sulphonic acid-potassium thiocyanate, lower phase of ionic organic liquid is formed, aliquot of which is applied on surface of graphite electrode, which is placed into three-electrode cell with 0.1 mole/l KSCN, without delay mercury accumulation potential is supplied -1.4 V for 30 seconds, and then in mode of anode reaming voltamperogram is requistered in square-wave mode within the limits of -1.20 to +0.20 V in the form of peaks of anode oxidation of zinc, cadmium, lead, copper.

EFFECT: invention makes it possible to detect low concentrations of zinc, cadmium, lead and copper, which makes it possible to perform monitoring in natural waters.

1 dwg, 2 tbl

Description

The invention relates to the field of analytical chemistry of environmental objects and is directed to the development of means of analytical control of ecosystem parameters and multi-element background monitoring of natural waters and aquatic ecosystems.

There is a method of electrochemical determination of zinc, cadmium, lead and copper in natural, drinking, purified waste water [E.A. Zakharova, G. B. Slepchenko. Quantitative chemical analysis of water for the content of zinc, cadmium, lead and copper by inversion voltammetry. MVI 08-47 / 008. Tomsk TPU 1995].

The technique (analog) is based on inversion-voltammetric measurements with the accumulation of zinc, cadmium, lead and copper on the indicator electrode (mercury-film, mercury-graphite) at a given negative electrolysis potential of 1.4 V relative to the silver chloride reference electrode. The process of electric dissolution of elements from the electrode surface and registration of analytical signals (anode peaks) on the voltammogram are carried out with a stepwise scan of the potential of 100 mV / s in the anode direction from -1.2 V to +0.15 V relative to the electrode. The potentials of the maxima of the recorded anode peaks (analytical signals) of zinc, cadmium, lead and copper against the background of formic acid are respectively equal to (-0.9 ± 0.1) V; (-0.6 ± 0.1) V; (-0.4 ± 0.1) V; (-0.05 ± 0.10) B.

Dissolved oxygen is removed with an inert gas.

Preparation of the indicator mercury-film or mercury-graphite electrode before the analytical procedure includes the stage of formation of the mercury film on the surface of the corresponding working (silver, graphite) electrode. In the case of natural waters and other liquid samples, the analogue requires a long, about one hour, laborious preparation of an analytical sample, for example, ultraviolet radiation (UV) with a photocatalyst - titanium dioxide. UV can take up to two hours, depending on the presence of organic matter in the sample. The presence in milligrams of organic matter in waters adversely affects the analytical signals of elements due to its sorption on the surface of a mercury film or graphite.

Sorption of organic matter reduces the limiting diffusion currents of elements, on the one hand, and shifts the maximum currents of the peaks of the elements due to the complexing effect of the dissolved organic matter on the determined elements. To decompose excess organic matter in water, physical effects are used: temperature, ultrasound, and others, or chemical, for example, acid treatment of the dry residue of water after evaporation. This kind of pretreatment of water samples lengthens the analytical procedure.

The decisive disadvantage of the analogue is the difficulty in determining zinc and low concentrations of less than 0.01 μg / l of lead and cadmium, for example, in a snow water filtrate (table 1).

The lack of analogue, which consists in the sorption of organic matter from natural waters to the surface of the indicator electrode, the claimed extraction voltammetric method for determining zinc, cadmium, lead and copper in natural waters makes it an advantage.

Of the known technical solutions, the closest in purpose and technical essence to the claimed method is:

electrochemical method for determining mercury by modifying a graphite electrode with an ionic liquid (RF Patent No. 23234169) / S.V. Temerev, B.I. Petrov // 05/10/2008, BI No. 13 (prototype), consisting in the extraction of mercury from an aqueous delaminating system containing 1.5-2.0 mmol of antipyrine and 0.7-1.0 mmol of 2-aqueous sulfosalicylic acid per 1, 0 ml of analyzed water. The aqueous exfoliating system is vigorously shaken, sedimented at room temperature 25 ° C and centrifuged to form the lower phase of the ionic organic liquid, which is applied in an amount of 1 μl for modification on the surface of the graphite electrode, then it is placed in a three-electrode cell with 0.1 mol / L HCl , without delay, the potential for accumulation of mercury is -1.4 V for 30 s, and then, in the anode scan mode, a voltammogram with a peak of anodic oxidation of mercury observed at a potential in the range of 0.00-0.10 V and linearly hanging mercury concentration in ionic liquid in constant-current mode at a scan rate of 80 mV / s in the range from - 1,2 to 1.0 V.

The disadvantages of the prototype include:

- insufficient extraction of zinc, cadmium, lead and copper into the lower organic phase of the system during delamination; the degree of extraction is only 10-15%, and for copper it is the largest; by the method described in the prototype, it is possible to register only a copper signal in the region of -0.1 ... 0.0 V;

- this degree of extraction of 10-15% of the metals does not allow to register the anode signals of zinc, cadmium, lead and copper in the analyzed snow water.

Proposed invention

Extraction-voltammetric method for determination of zinc, cadmium, lead and copper in natural waters involves the extraction of metals from the liquid aqueous phase into the organic component of the water - antipyrine - sulfosalicylic acid - potassium thiocyanate system using mercury, which is introduced into the system in micro quantities. The mercury additive is introduced into the system together with anions of potassium thiocyanate and weighed portions of the reagents. Concentration of mercury occurs from a liquid sample, such as a snow water filtrate, in an “in situ” mode, that is, during the formation of the lower phase of the stratified system (table 2). Mercury is present in water as a thiocyanate complex and acts as a collector that co-precipitates thiocyanates of zinc, cadmium, lead and copper.

The difference from the prototype is the additional introduction of anions of thiocyanate in an amount of 1-2 mmol per 10 ml of analyzed water. An increase in the concentration of more than 200 mol / L leads to the precipitation of transparent crystals of variable composition due to the limited solubility of thiocyanates in the aqueous phase.

The inventive method differs from the prototype:

1) the introduction of thiocyanate into the system in the form of a potassium salt;

2) the introduction of an aliquot of a standard solution of mercury;

3) square-wave registration of voltammograms;

4) a volume of 20 μl (0.02 ml) of an aliquot of the lower phase concentrate used to modify the electrode from graphite;

5) the use of 0.1 mol / L KSCN solution as a working electrolyte.

The implementation of the invention

To preliminarily form a liquid aqueous phase (rhodanide complexes of mercury and metals to be determined), 1.0–2.0 mmol (0.1 g) KChN grade potassium thiocyanate, grade ХC, 0.25 μmol is added to a 10 ml snow water filtrate (object of analysis) (50 μg) mercury Hg, powdered chemicals: 16-20 mmol (3 g) of pharmacopeia antipyrine (gross formula C ₁₁ H ₁₂ N ₂ O, melting point 113 ° C, molecular weight 188.23 g / mol) and 6- 10 mmol (1.5 g) of two-water sulfosalicylic acid (GOST 4478-78, gross formula C ₇ H ₆ O ₆ S × 2H ₂ O, molecular weight 254.21 g / mol). Then the glass tube is tightly closed with a stopper, shaken vigorously for 5 minutes, sedimented for 20-30 minutes at 25 ° C, where the dense lower phase of the organic component is formed. When the reagent antipyrine and organic acid (solid) are dissolved in the object of analysis (an aqueous solution containing trace amounts of mercury and metals), an acid-base interaction occurs between protonated antipyrine and anions of organic sulfonic acid. The resulting organic component, consisting of the ionic associate of an organic salt, antipyrinium sulfosalicylate and antipyrine, has an acidic medium, has a large buffer capacity and extracts mercury and metals from an acid thiocyanate solution due to the formation of a complex metal complex with an ionic and organic in nature liquid. The organic component of the exfoliating system, which is formed in the lower phase when exfoliating in the form of a yellow ionic liquid, is used to modify the graphite working electrode. The volume of the organic component (lower phase) is about 3 ml at 25 ° C. Next, “ex situ” was taken with a microdoser 20 μl (0.02 ml) of the lower organic phase and placed on the prepared surface of the graphite electrode. Before each determination, the surface of the graphite electrode is updated, polished with filter paper, degreased in 10 ml of 0.1 mol / L HCl with the addition of 0.2 ml of a 3% alkaline solution of sodium borohydride and washed in boiling distilled water to remove excess oxygen from its surface . On a dry graphite surface, 20 μl (0.02 ml) of the organic phase is applied using a microdoser, evenly distributing along the end surface of the electrode (geometric area 0.13 cm ² ). Such a film electrode is placed in a three-electrode cell with 10 ml of 0.1 mol / L KSCN as the background solution. The background electrolyte should not contain dissolved oxygen. Dissolved oxygen in the electrolyte is removed by sparging argon for 3-5 minutes. Then, without delay, a potential accumulation of mercury of -1.4 V is supplied for 30 seconds. Then, in the anode scan mode, a voltammogram is recorded in a square-wave recording mode with an anode scan potential of 80 mV / s (TA-2, Tomsk, TPU) with 0.1 mol / L KSCN solution that does not contain dissolved oxygen, and an overvoltage is applied - 1.4 V for 30 seconds relative to the silver chloride reference electrode and then the anode voltammogram is recorded in square-wave mode in the potential range from - 1.2 V to +0.2 V. Typical voltammograms are shown in the drawing. The maximums of the limiting diffusion currents are observed at approximately the same potentials characteristic of aqueous solutions (-0.800 ... -0.900 B) Zn, (-0.600 ... -0.700 B) Cd, (-0.400 ... -0.500 B) Pb, (-0.100 ... 0.100 B) Cu. The potential values correspond to aqueous solutions of electrolytes because water is the only homogenizing solvent, the data in table 2.

The inventive method has several advantages:

1) pre-forms thiocyanate complexes of heavy metal cations with anions of thiocyanate;

2) concentrates the thiocyanate complexes of metal cations from the analyzed water into the lower organic phase during its formation by coprecipitation with mercury;

3) the introduction of thiocyanate into an exfoliating system increases the degree of metal extraction to 75-85%.

The proposed method reduces the cost of the analysis due to the use of much simpler and inexpensive equipment that does not have sources of increased danger for the analyst, eliminates the stage of high-temperature decomposition of the analyte in order to atomize it. Electrochemical analyzers are easier to automate, much smaller in size and mass, and electrochemical methods make it possible to further concentrate the analyte on the surface of the working electrode in a voltammetric cycle. Field-based voltammetric analyzers allow you to control elements directly at the site of water sampling. A comparative analysis of tables 1 and 2 demonstrates the effectiveness of extraction voltammetry in relation to metals, especially in relation to zinc. Zinc using an analogue was not reliably detected in the snow filtrate.

Increasing the degree of extraction of the analyzed metals to 85%, the claimed invention ensures the achievement of a technical result in comparison with the prototype, i.e. reliably allows to determine low concentrations of zinc, cadmium, lead and copper in filtered snow water and thereby allows monitoring in natural waters (table 2).

Table 1 The results of the determination of HM in melt water by the IVA method using the EWG (recommendation MVI 08-47 / 008. Tomsk. TPU. 1995) No. p / p according to table 2 The concentration of TM, μg / l Cu Pb Cd one <1 19.0 <0.01 2 <1 14.6 0.04 3 42 <0.01 <0.01 four 43 <0.01 <0.01 5 <1 10.1 <0.01 6 36 <0.01 <0.01 7 <1 6.6 <0.01 8 <1 4.0 <0.01 9 <1 3.8 0.01 10 84 <0.01 <0.01 eleven 41 <0.01 <0.01 12 45 <0.01 <0.01 13 57 3.2 0.06 fourteen 39 <0.01 <0.01 fifteen 31 8.1 <0.01 16 <1 36.1 0.05 17 53 <0.01 <0.01 eighteen <1 4.9 <0.01 19 42 <0.01 <0.01 twenty 47 2.0 0.08 21 <1 16,4 0.08 22 39 <0.01 <0.01 23 49 <0.01 <0.01 24 76 76,2 <0.01

table 2 The results of the determination of TM in the extract by the claimed extraction voltammetric method for determining zinc, cadmium, lead and copper in natural waters No. of solution Sample number The concentration of TM, μg / l Cu <Cu> ± ε _0.95 Pb <Pb> ± ε _0.95 Cd <Cd> ± ε _0.95 Zn <Zn> ± ε _0.95 one one 32 33 ± 4 3.9 3.4 ± 0.3 <0.01 <0.01 <2.0 <2,3 2 36 3,1 <0.01 <2,3 3 thirty 3,1 <0.01 <2.5 four 38 3.6 <0.01 <2,3 2 5 thirty 36 ± 5 3.4 3.6 ± 0.2 <0.01 <0.01 2.7 2.6 ± 0.2 6 39 3.8 <0.01 2.6 7 32 3,7 <0.01 2.6 8 43 3.4 <0.01 2,4 3 9 41 43 ± 2 4.6 4.6 ± 0.2 <0.01 <0.01 3.2 3.2 ± 0.1 10 45 4.9 <0.01 3.2 eleven 42 4.0 <0.01 3,1 12 46 4.7 <0.01 3.2 four 13 58 53 ± 4 6.6 6.4 ± 0.3 0.02 0.02 ± 0.005 4.4 4.3 ± 0.2 fourteen 54 6.2 0.02 4.4 fifteen 51 6.7 0.02 4.3 16 51 6.3 0.02 4.2 5 17 60 63 ± 3 5.5 5.5 ± 0.4 0.02 0.02 ± 0.005 4,5 4.4 ± 0.1 eighteen 74 5,0 0.02 4.3 19 72 5.7 0.02 4.4 twenty 47 5.8 0.02 4.3 6 21 46 52 ± 6 5.1 5.3 ± 0.2 0.01 0.01 ± 0.005 3.8 3.9 ± 0.3 22 49 5,4 0.01 4.0 23 48 5,0 0.01 3.9 24 66 5,6 0.01 3.9 Note: Extract with a volume of 20 μl of a delaminating system (H ₂ O-HAnt-HSSA) with KSCN complexing agent. HAnt - 2,3-dimethyl-1-phenyl-3-pyrazolin-5-one (antipyrine); HSSA - dihydrate sulfosalicylic acid.

Claims (1)

Extraction-voltammetric method for determination of zinc, cadmium, lead and copper in natural waters, which involves the extraction of metals from the liquid aqueous phase into the organic component of the delaminating system: antipyrine - sulfosalicylic acid and determining the metal concentrate on a graphite electrode, characterized in that for the quantitative determination of zinc , cadmium, lead and copper to 10 ml of an aqueous solution of the analyzed liquid add 16-20 mmol of antipyrine, 6-10 mmol of sulfosalicylic acid, 1-2 mmol of potassium thiocyanate KSCN and 0.25 μmol of mercury, the lower phase of the ionic organic liquid is formed, then an aliquot of 20 μl is taken and applied for modification to the surface of a graphite electrode, which is placed in a three-electrode cell with 0.1 mol / L KSCN, and then, in the anode scan mode, a voltammogram is recorded in square -wave mode in the range of -1.20 to +0.20 V with peaks of anodic oxidation of zinc, cadmium, lead, copper.